Tesis sobre el tema "Biomass gasification. Coal gasification. Thermogravimetry"

Thesis (M.S.)--West Virginia University, 2009.
Title from document title page. Document formatted into pages; contains vi, 184 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 82-84).

Thermal co-processing of coal and biomass has been increasingly focused for its environmental and economic benefits. In the present work, the experimental and kinetic study on co-pyrolysis and co-gasification of Rhenish brown coal (HKN) and wheat straw (WS) was made. The pyrolysis behavior, especially for co-pyrolysis, was investigated in a thermogravimetric analyzer (TGA) and a small fixed bed reactor (LPA). In TGA, the mass loss and reaction rate of single and blend samples were studied under various experimental conditions, and their effects on synergy effects. The synergy effects on products yield and properties of chars were studied in LPA. The kinetics of pyrolysis was obtained based on data from TGA by using the Coats-Redfern method. For gasification with CO2, a small fixed bed reactor (quartz glass reactor), equipped with an online GC to monitor the gas composition, was used. The effects of processing conditions on gasification behavior and synergy effects for mixed chars and co-pyrolysis chars were investigated. The volume reaction model (VRM), shrinking core model (SCM) and random pore model (RPM), were applied to fit the experimental data. The model best fitting the experiments was used to calculate the kinetic parameters. The reaction orders of gasification reactions with single chars are also investigated. The pyrolysis study showed that a small amount of wheat straw added to the brown coal promoted the decomposition better and showed more significant synergy effects. The synergy effects varied with increasing heating rates and pressures, especially at 40 bar. The kinetic parameters were inconsistent with experimental behavior during co-pyrolysis, since the reaction was also affected by heat transfer, contact time, particles distribution and so on. The gasification study on single chars showed that Rhenish brown coal chars had higher reactivity; chars pyrolyzed at higher temperatures showed lower reactivity; and higher gasification temperatures and CO2 partial pressures led to higher reactivity. For co-gasification process, there was no significant synergy effect for mixed chars. However, negative synergy effects (reactivity decreased compared to the calculated values based on rule of mixing) were observed for co-pyrolysis chars, caused by properties change by co-pyrolysis process. For kinetics, the reaction orders of chars ranged from 0.3 to 0.7. Only random pore model fitted most experiments at low and high temperatures. Synergy effects were also observed in kinetic parameters. The values of activation energy E and pre-exponential factor A for mixed chars and co-pyrolysis chars were lower than expected. The negative synergy effects showed the pre-exponential factor A had more effects. However, the higher reactivity of mixed chars than co-pyrolysis chars showed that the reaction was affected more by activation energy E. Therefore, only investigating E or A value was not enough. In addition, a marked compensation effect between activation energies and pre-exponential factors was found in the present study. The isokinetic temperature for the present study was 856 °C. This was close to the temperature at which the gasification reaction transforms from the chemical controlled zone to the diffusion controlled zone for most chars.

The past few years have seen an upsurge in the use of renewable biomass as a source of energy due to growing concerns over greenhouse gas emissions caused by the combustion of fossil fuels and the need for energy independence due to depleting fossil fuel resources. Although coal will continue to be a major source of energy for many years, there is still great interest in replacing part of the coal used in energy generation with renewable biomass. Combustion converts inherent chemical energy of carbonaceous feedstock to only thermal energy. On the other hand, partial oxidation processes like gasification convert chemical energy into thermal energy as well as synthesis gas which can be easily stored or transported using existing infrastructure for downstream chemical conversion to higher value specialty chemicals as well as production of heat, hydrogen, and power. Devolatilization or pyrolysis plays an important role during gasification and is considered to be the starting point for all heterogeneous gasification reactions. Pyrolysis kinetic modeling is, therefore, an important step in analyzing interactions between blended feedstocks. The thermal evolution profiles of different coal-biomass blends were investigated at various heating rates using thermogravimetric analysis. Using MATLAB, complex models for devolatilization of the blends were solved for obtaining and predicting the global kinetic parameters. Parallel first order reactions model, distributed activation energy model and matrix inversion algorithm were utilized and compared for this purpose. Using these global kinetic parameters, devolatilization rates of unknown fuel blends gasified at unknown heating rates can be accurately predicted using the matrix inversion method. A unique laboratory scale auto-thermal moving bed gasifier was also designed and constructed for studying the thermochemical conversion of coal-biomass blends. The effect of varying operating parameters was analyzed for optimizing syngas production. In addition, stable carbon isotope analysis using Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS) was used for qualitatively and quantitatively measuring individual contributions of coal and biomass feedstocks for generation of carbonaceous gases during gasification. The predictive models utilized and experimental data obtained via these methods can provide valuable information for analyzing synergistic interactions between feedstocks and also for process modeling and optimization.

With the limited reserves of fossil fuels and the environmental problems associated with their use, the world is moving towards cleaner, renewable, and sustainable sources of energy. Biomass is a promising feedstock towards attaining this goal because it is abundant, renewable, and can be considered as a carbon neutral source of energy. Syngas can be further processed to produce liquid fuels, hydrogen, high value chemicals, or it can be converted to heat and power using turbines. Most of the downstream processing of syngas occurs at high pressures, which requires cost intensive gas compression. It has been considered to be techno-economically advantageous to generate pressurized syngas by performing high-pressure gasification. Gasification utilizes high temperatures and an oxidizing gas to convert biomass to synthesis gas (syngas, a mixture of CO and H2). Most of the past studies on gasification used process conditions that did not simulate an industrial gasification operation. This work aims at understanding the chemical and physical transformations taking place during high-pressure biomass gasification at heating rates of practical significance. We have adopted an approach of breaking down the gasification process into two steps: 1) Pyrolysis or devolatalization (fast step), and 2) Char gasification (slow step). This approach allows us to understand pyrolysis and char gasification separately and also to study the effect of pyrolysis conditions on the char gasification kinetics. Alkali and alkaline earth metals in biomass are known to catalyze the gasification reaction. This potentially makes biomass feedstock a cheap source of catalyst during coal gasification. This work also explores catalytic interactions in biomass-coal blends during co-gasification of the mixed feeds. The results of this study can be divided into four parts: (a) pyrolysis of loblolly pine; (b) gasification of pine chars; (c) pyrolysis and gasification of switchgrass; (d) co-gasification of pine/switchgrass with lignite and bituminous coals.

In recent years, Integrated Gasification Combined Cycle Technology (IGCC) has become more common in clean coal power operations with carbon capture and sequestration (CCS). Great efforts have been spent on investigating ways to improve the efficiency, reduce costs, and further reduce greenhouse gas emissions. This study focuses on investigating two approaches to achieve these goals. First, replace the subcritical Rankine steam cycle with a supercritical steam cycle. Second, add different amounts of biomass as feedstock to reduce emissions. Finally, implement several types of CCS, including sweet- and sour-shift pre-combustion and post-combustion. Using the software, Thermoflow®, this study shows that utilizing biomass with coal up to 50% (wt.) can improve the efficiency, and reduce emissions: even making the plant carbon-negative when CCS is used. CCS is best administered pre-combustion using sour-shift, and supercritical steam cycles are thermally and economically better than subcritical cycles. Both capital and electricity costs have been presented.

Fluidization characteristics and co-gasification of pulverized sub-bituminous coal, hybrid poplar wood, corn stover, switchgrass, and their mixtures were investigated. Co-gasification studies were performed over temperature range from 700°C to 900°C in different media (N2, CO2, steam) using a bubbling fluidized bed reactor. In fluidization experiments, pressure drop (ΔP) observed for coal-biomass mixtures was higher than those of single coal and biomass bed materials in the complete fluidization regime. There was no systematic trend observed for minimum fluidization velocity (Umf) with increasing biomass content. However, porosity at minimum fluidization (εmf) increased with increasing biomass content. Channeling effects were observed in biomass bed materials and coal bed with 40 wt.% and 50 wt.% biomass content at low gas flowrates. The effect of coal pressure overshoot reduced with increasing biomass content. Co-gasification of coal and corn stover mixtures showed minor interactions. Synergetic effects were observed with 10 wt.% corn stover. Coal mixed with corn stover formed agglomerates during co-gasification experiments and the effect was severe with increase in corn stover content and at 900°C. Syngas (H2 + CO) concentrations obtained using CO2 as cogasification medium were higher (~78 vol.% at 700°C, ~87 vol.% at 800°C, ~93 vol.% at 900°C) than those obtained with N2 medium (~60 vol.% at 700°C, ~65 vol.% at 800°C, ~75 vol.% at 900°C). Experiments involving co-gasification of coal with poplar showed no synergetic effects. Experimental yields were identical to predicted yield. However, synergetic effects were observed on H2 production when steam was used as the co-gasification medium. Additionally, the presence of steam increased H2/CO ratio up to 2.5 with 10 wt.% hybrid poplar content. Overall, char and tar yields decreased with increasing temperature and increasing biomass content, which led to increase in product gas.

This thesis presents research on the co-gasification characteristic of biomass and coal, and mathematical modelling of the co-gasification process in two main parts: i) experimental investigation and mathematical modelling of reaction kinetics of steam gasification of single char particles of pure coal, pure biomass, and blended coal and biomass; and ii) Experimental investigation and mathematical modelling of gasification characteristics of biomass, coal and their blends in pilot scale gasifiers. From the char reactivity study, the instinct difference in gasification characteristics of the two chars has been explained and reactivity of blended char can be predicted. In the pilot scale gasifier study, effects of blending ratio in feedstock and operating conditions on co-gasification of biomass and coal were investigated.

Thesis (M.S.)--West Virginia University, 2009.
Title from document title page. Document formatted into pages; contains v, 126 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 59-69).

Gasification of coal or biomass can produce hydrogen rich synthetic gas (syngas) for use in fuel cells, liquid fuels or chemicals. While coal gasification is well established, biomass gasifiers have been hindered by costs and difficulties such as tar and ash deposition. Ultra-Superheated Steam (USS) has been proposed as an economical method to maximise gasification temperatures and hydrogen yields. A novel entrained flow USS gasification system showed promise with coal in a previous investigation. The main objectives were to investigate how a USS gasification system produced high hydrogen yields and feedstock conversion within a short residence time. Secondly, apply the system to biomass gasification for sustainable hydrogen production. The principle tasks were to identify the factors affecting the product composition, and experimentally compare the conversion and yields from coal and biomass materials. Numerical software was used to investigate gas and particle behaviour inside the burner. Coal and a unique high ash softwood char were successfully gasified. Char yielded up to 34.9%mol H2 and 25.1%mol CO in the dry gas, demonstrating higher conversion and yields than coal despite lower feedstock heating value and feeding rates. Biomass ash was considered to catalyse char conversion. No detrimental effect was observed from ash deposition, which was dry and easily removed. A fluid model mapped temperature distribution, showing good correlation with validation measurements and supporting the observation that wall temperature greatly affected particle conversion. Particle residence times were inversely proportional to particle diameter and density. High ash biochar showed greater conversion than coal. Economic analysis revealed the system would be most competitive on an existing site with available feedstocks and steam. A longer reactor would increase time for homogeneous reactions to play a greater role. With further development this technology has potential to produce hydrogen competitively on a commercial scale.

In this study, efforts ensued to increase our knowledge of fluidization and gasification behavior of Geldart A particles using CFD. An extensive Eulerian-Eulerian numerical study was executed and simulations were compared and validated with experiments conducted at Utah State University. In order to improve numerical predictions using an Eulerian-Eulerian model, drag models were assessed to determine if they were suitable for fine particles classified as Geldart A. The results proved that if static regions of mass in fluidized beds are neglected, most drag models work well with Geldart A particles. The most reliable drag model for both single and binary mixtures was proved to be the Gidaspow-blend model. In order to capture the overshoot of pressure in homogeneous fluidization regions, a new modeling technique was proposed that modified the definition of the critical velocity in the Ergun correlation. The new modeling technique showed promising results for predicting fluidization behavior of fine particles. The fluidization behavior of three different mixtures of coal and poplar wood were studied. Although results indicated good mixing characteristics for all mixtures, there was a tendency for better mixing with higher percentages of poplar wood. In this study, efforts continued to model co-gasification of coal and biomass. Comparing the coal gasification of large (Geldart B) and fine (Geldart A) particles showed that using finer particles had a pronounced effect on gas yields where CO mass fraction increased, although H2 and CH4 mass fraction slightly decreased. The gas yields of coal gasification with fine particles were also compared using three different gasification agents. Modeling the co-gasification of coal-switchgrass of both fine particles of Geldart A and larger particles of Geldart B showed that there is not a synergetic effect in terms of gas yields of H2 and CH4. The gas yields of CO, however, showed a significant increase during co-gasification. The effects of gasification temperature on gas yields were also investigated.
Ph. D.

An increasing emphasis on mitigating global climate change (global warming) over the last few decades has created interest in a broad range of sustainable or alternative energy systems to replace fossil fuel combustion. Biomass, when harvested responsibly, is a renewable fuel with many uses in replacing fossil fuels. Cofiring biomass with coal in traditional large-scale coal power plants represents one of the lowest risk, least costly, near-term methods of CO2 mitigation. Simultaneously, it is one of the most efficient and inexpensive uses of biomass. Alternatively, biomass can be transformed into useful products through gasification to produce clean syngas for highly efficient gas turbines, or feedstock to produce light gases, fuels, chemicals or other products. A large portion of this investigation focused on the effect of cofiring biomass on the near burner region of a commercial coal flame. This research included first-of-their-kind field measurements of flame structure and particle properties in front of a full-scale burner fired with biomass and coal, including measurements of particle size and composition, gas velocity, composition, and temperature in the near-burner region of multiple cofired flames in a 350 MWe full-scale power plant in Studstrup, Denmark. A novel sampling and analysis technique was developed enabling the estimation of the fraction of biomass in the flow as a function of position and the burnout of biomass and coal particles separately. These data show that biomass particles do not follow gas stream lines to the same extent that coal particles do. This is consistent with the larger sizes, slower heating and reaction rates, and higher momentum of biomass particles. This research also includes first-of-their-kind single particle continuous measurements of particle mass, surface and internal temperature, size, shape, during biomass pyrolysis and gasification. The single particle measurements provided among the most highly resolved and repeatable biomass gasification results reported to date for wood, switchgrass and corn stover. All three samples showed greater gasification reactivity to H2O than to CO2. The experiments included results in both reactants individually and combined. One of the most important findings of this work was the experimental confirmation that as the char particles gasify, their ash fractions increase and reaction rates decrease on both an intrinsic and external surface area basis. The analyses in this work show that this decrease in burnout quantitatively corresponds to the change in the predicted fraction of the surface that is ash and does not reflect any change in organic reactivity. Reaction rate parameters suitable for relatively simple power-law models based on external surface area describe all the data reasonably well.

Gasification is a process used to convert any carbonaceous species through heterogeneous reaction to obtain the desired gaseous products of H2 and CO which are used to make chemicals, liquid transportation fuels, and power. Both pyrolysis and heterogeneous gasification occur in commercial entrained-flow gasifiers at pressures from 4 to 65 atm with local gas temperatures as high as 2000 °C. Many gasification studies have been performed at moderate temperatures, heating rates, and pressures. In this work, both pyrolysis and char gasification experiments were performed on coal, petroleum coke, and biomass at conditions pertinent to commercial entrained-flow gasifiers. Rapid biomass pyrolysis experiments were performed at atmospheric pressure in an entrained-flow reactor for sawdust, switchgrass, corn stover, and straw mostly using a peak gas temperature of 1163 K at particle residence times ranging from 34 to 113 ms. Biomass pyrolysis was modeled using the Chemical Percolation Devolatilization model assuming that biomass pyrolysis occurs as a weighted average of its individual components (cellulose, hemicellulose, and lignin). Thermal cracking of biomass tar into light gas was included using a first-order model with kinetic parameters regressed in the current study. Char gasification rates were measured for biomass, petroleum coke, and coal in a pressurized entrained-flow reactor at high heating-rate conditions at total pressures between 10 and 15 atm. Peak centerline gas temperatures were between 1611 and 1879 K. The range of particle residence times used in the gasification experiments was 42 to 275 ms. The CO2 gasification rates of biomass and petroleum coke chars were measured at conditions where the reaction environment consisted of approximately 40 and 90 mol% CO2. Steam gasification rates of coal char were measured at conditions where the maximum H2O concentration was 8.6 mol%. Measured data was used to regress apparent kinetic parameters for a first-order model that describes char conversion. The measured char gasification rates were far from the film-diffusion limit, and are pertinent for pulverized particles where no internal particle temperature gradients are important. The modeling and measured data of char gasification rates in this research will aid in the design and efficient operation of commercial entrained-flow gasifiers, as well as provide validation for both existing and future models at a wide range of temperatures and pressures at high heating-rate conditions.

Thesis (PhD)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: Gasification provides a proven alternative to the dependence on petroleum for the production of high value products such as liquid fuels and chemicals. Syngas, the main product from gasification can be converted to fuels and chemicals via a number of possible synthesis processes. Coal and natural gas are currently the main feedstock used for syngas production. In South Africa (SA), Sasol operates the largest commercial coal-to-liquids conversion process in the world, based on updraft fixed bed gasification of low grade coal to syngas. Co-utilizing alternative and more sustainable feedstock (such as biomass and wastes) with coal in existing coal-based plants offers a realistic approach to reducing the costs and risks associated with setting up dedicated biomass conversion plants. An experimental and modelling investigation was performed to assess the impacts of co-gasifying two of the most commonly available agricultural wastes in SA (sugarcane bagasse and corn residue) with typical low grade SA coals, on the main products of updraft fixed bed gasification, i.e. liquid condensates and syngas. Condensates are produced in the pyrolysis section of the updraft gasifier, whereas syngas is a result of residual char conversion. An experimental set-up that simulates the pyrolysis section of the gasifier was employed to investigate the yield and composition of devolatilized products at industrially relevant conditions of 26 bars and 400-600°C. The results show that about 15 wt% of coal and 70 wt% of biomass are devolatilized during the pyrolysis process. The biomass derived condensates were determined to comprise of significantly higher quantities of oxygenates such as organic acids, phenols, ketones, and alcohols, whereas coal derived hydrocarbon condensates were dominated by polycyclic aromatic hydrocarbons, creosotes and phenols. Results of investigation into the influence of coal-biomass feedstock mix ratio on yields of products from pyrolysis show limited evidence of non-additive or synergistic behaviour on the overall distribution of solid, liquid and gas yields. On the other hand, in terms of the distribution of specific liquid phase hydrocarbons, there was significant evidence in favour of non-additive pyrolysis behaviour, as indicated by the non-additive yield distribution of specific chemicals. Synergistic trends could also be observed in the thermogravimetric (TGA) study of pyrolysis under kinetically controlled non-isothermal conditions. Model free and model fitting kinetic analysis of the TGA data revealed activation energies ranging between 94-212 kJ mol-1 for the biomass fuels and 147-377 kJ mol-1 for coal. Synergistic interactions may be linked to the increased presence of hydrogen in biomass fuels which partially saturates free radicals formed during earlier stages of devolatilization, thereby preventing secondary recombination reactions that would have produced chars, allowing for the increased formation of volatile species instead. Analysis of char obtained from the co-pyrolysis experiments revealed that the fixed carbon and volatile content of the blended chars is is proportional to the percentage of biomass and coal in the mixture. CO2 reactivity experiments on the chars showed that the addition of biomass to coal did not impose any kinetic limitation on the gasification of blended chars. The blended chars decomposed at approximately the same rate as when coal was gasified alone, even at higher biomass concentrations in the original feedstock blend. Based on these observations, a semi-empirical equilibrium based simulation of syngas production for co-gasification of coalbiomass blends at various mix ratios was developed using ASPEN Plus. The model showed that H2/CO ratio was relatively unaffected by biomass addition to the coal fuel mix, whereas syngas heating value and thermal efficiency were negatively affected. Subsequent evaluation of the production cost of syngas at biomass inputs ranging between 0-20 wt% of coal reflected the significant additional cost of pretreating biomass (3.3% of total capital investment). This resulted in co-gasification derived syngas production costs of ZAR146/tonne (ZAR12.6/GJ) at 80:20 coalbiomass feedstock ratio, compared to a baseline (coal only) cost of ZAR130/tonne (ZAR10.7/GJ). Sensitivity analysis that varied biomass costs from ZAR0 ZAR470 revealed that syngas production costs from co-gasification remained significantly higher than baseline costs, even at low to zero prices of the biomass feedstock. This remained the case even after taking account of a carbon tax of up to ZAR117/tCO2. However, for range of carbon tax values suggested by the SA treasury (ZAR70 tCO2 to ZAR200 tCO2), the avoided carbon tax due to co-feeding biomass can offset between 40-96% of the specific retrofitting cost at 80:20 coal-biomass feedstock mass ratio. In summary, this dissertation has showed that in addition to the widely recognized problems of ash fouling and sintering, co-feeding of biomass in existing coal based updraft gasification plants poses some challenges in terms of impacts on condensates and syngas quality, and production costs. Further research is required to investigate the potential in ameliorating some of these impacts by developing new high value product streams (such as acetic acid) from the significant fraction of condensates derived from biomass.
AFRIKAANSE OPSOMMING: Vergassing bied 'n beproefde alternatief vir die afhanklikheid van petroleum vir die produksie van hoë waarde produkte soos vloeibare brandstof en chemikalieë. Sintese gas, die belangrikste produk van vergassing, kan omgeskakel word na brandstof en chemikalieë deur 'n aantal moontlike sintese prosesse. Steenkool en aardgas is tans die belangrikste grondstowwe wat gebruik word vir sintese gas produksie. In Suid-Afrika (SA) bedryf Sasol die grootste kommersiële steenkool-totvloeistof omskakelingsproses in die wêreld, gebaseer op stygstroom vastebed vergassing van laegraadse steenkool na sintese gas. Die gebruik van alternatiewe en meer volhoubare grondstowwe (soos biomassa en afval) saam met steenkool in die bestaande steenkool-gebaseerde aanlegte bied 'n realistiese benadering tot die vermindering van die koste en risiko's wat verband hou met die oprigting van toegewyde biomassa omskakelingsaanlegte. 'n Eksperimentele en modelleringsondersoek is uitgevoer om die impak van gesamentlike vergassing van twee van die mees algemeen beskikbare landbouafvalprodukte in Suid-Afrika (suikerriet bagasse en mieliereste) met tipiese laegraadse SA steenkool op die vernaamste produkte van stygstroom vastebed vergassing, dws vloeistof kondensate en sintese gas, te evalueer. Kondensate word geproduseer in die piroliese gedeelte van die stygstroomvergasser, terwyl sintese gas 'n resultaat is van die omskakeling van oorblywende houtskool. 'n Eksperimentele opstelling wat die piroliese gedeelte van die vergasser simuleer is gebruik om die opbrengs en die samestelling van produkte waarvan die vlugtige komponente verwyder is by industrie relevante toestande van 26 bar en 400-600°C te ondersoek. Die resultate toon dat ongeveer 15% (massabasis) van die steenkool en 70% (massabasis) van die biomassa verlore gaan aan vlugtige komponente tydens die piroliese proses. Daar is vasgestel dat die kondensate afkomstig van biomassa uit aansienlik hoër hoeveelhede suurstofryke verbindings soos organiese sure, fenole, ketone, en alkohole bestaan, terwyl koolwaterstofkondensate afkomstig uit steenkool oorwegend bectaan uit polisikliese aromatise verbindings, kreosote en fenole. Die resultate van die ondersoek na die invloed van die verhouding van steenkool tot biomassa grondstof op piroliese opbrengste toon beperkte bewyse van nie-toevoegende of sinergistiese gedrag op die algehele verspreiding van soliede, vloeistof en gas opbrengste. Aan die ander kant, in terme van die verspreiding van spesifieke vloeibare fase koolwaterstowwe, was daar beduidende bewyse ten gunste van 'n sinergistiese piroliese gedrag. Sinergistiese tendense is ook waargeneem in die termogravimetriese (TGA) studie van piroliese onder kineties beheerde nieisotermiese toestande. Modelvrye en modelpassende kinetiese analise van die TGA data het aan die lig gebring dat aktiveringsenergieë wissel tussen 94-212 kJ mol-1 vir biomassa brandstof en 147-377 kJ mol-1 vir steenkool. Ontleding van die houtskool verkry uit die gesamentlike piroliese eksperimente het aan die lig gebring dat die onmiddellike kenmerke van die gemengde houtskool die geweegde gemiddelde van die individuele waardes vir steenkool en biomassa benader. CO2 reaktiwiteitseksperimente op die houtskool het getoon dat die byvoeging van biomassa by steenkool nie enige kinetiese beperking op die vergassing van gemengde houtskool plaas nie. Die gemengde houtskool ontbind teen ongeveer dieselfde tempo as wanneer steenkool alleen vergas is, selfs teen hoër biomassa konsentrasies in die oorspronklike grondstofmengsel. Op grond van hierdie waarnemings is 'n semi-empiriese ewewig-gebaseerde simulasie van sintese gas produksie vir gesamentlike vergassing van steenkool-biomassa-mengsels vir verskeie mengverhoudings ontwikkel met behulp van Aspen Plus. Die model het getoon dat die H2/CO verhouding relatief min geraak is deur biomassa by die steenkool brandstofmengsel te voeg, terwyl sintese gas se verhittingswaarde en termiese doeltreffendheid negatief geraak is. Daaropvolgende evaluering van die produksiekoste van sintese gas vir biomassa insette wat wissel tussen 0-20% (massabasis) van die hoeveelheid steenkool het die aansienlike addisionele koste van die vooraf behandeling van biomassa (3.3% van die totale kapitale belegging) gereflekteer. Dit het gelei tot 'n produksiekoste van ZAR146/ton (ZAR12.6/GJ) vir sintese gas afkomstig uit gesamentlike-vergassing van 'n 80:20 steebkool-biomassa grondstof mengesl, in vergelyking met 'n basislyn (steenkool) koste van ZAR130/ton (ZAR10.7/GJ). Sensitiwiteitsanalise wat biomassa koste van ZAR0 - ZAR470 gevarieër het, het aan die lig gebring dat sintese gas produksiekoste van gesamentlike vergassing aansienlik hoër bly as die basislyn koste, selfs teen 'n lae of nul prys van biomassa grondstof. Dit bly die geval selfs nadat koolstof belasting van tot ZAR117/tCO2 in ag geneem is. In opsomming het hierdie verhandeling getoon dat, bykomend tot die wyd-erkende probleme van as besoedeling en sintering, die gesamentlike gebruik van biomassa in bestaande steenkool stygstroom vergassingsaanlegte groot uitdagings inhou in terme van die impak op die kwaliteit van kondensate en sintese gas, asook produksiekoste. Verdere navorsing is nodig om die potensiaal te ondersoek vir die verbetering van sommige van hierdie impakte deur die ontwikkeling van nuwe hoë waarde produkstrome (soos asynsuur) uit die beduidende breukdeel van kondensate wat verkry word uit biomassa.

Thesis (M.S.)--Chemical Engineering, Georgia Institute of Technology, 2005.
Jones, Christopher, Committee Member ; White, David, Committee Member ; Teja, Amyn, Committee Member ; Realff, Matthew, Committee Chair. Includes bibliographical references.

Today’s problems with emissions of green house gases, land filling of waste and depletion of the oil reserves calls for new energy systems based on alternative fuels like biomass and waste. Gasification is an attractive technology for the use of such solid fuels. Conventional gasification, in the vast majority of cases, uses in-reactor heat release from combustion of part of the feedstock, possibly coupled with a limited preheating of the agent, to obtain the necessary temperatures in the gasifier bed. During recent years, a new gasification technology, using highly preheated gasification agents (> 1273 K), has been developed. The extra heat brought into the process by the high temperature agent reduces the amount of feedstock that has to be oxidized to supply the necessary heat and the use of highly preheated agents has previously proven to have several positive effects on the fuel gas quality.In difference to the previous work on gasification with highly preheated agents, this thesis primarily focuses on the fundamental aspects namely, mass conversion, heating and ignition. It starts by considering single fuel particles or thin beds of fuel particles inserted into highly preheated agents. Mass conversion, heating and ignition are reported in function of the temperature and oxygen concentration of the agent and formulas for the prediction of ignition time and ignition mechanism are developed. The perspective is then widened to include the whole gasifier bed. Simulations of fixed bed batch gasification using highly preheated agents are performed with a mathematical model and used to study how the high agent temperature influences the mass conversion, devolatilisation front rate and the temperature distribution in the fixed fuel bed. Further, the gas quality and gasification efficiency are studied by means of large scale experiment. Ultimately, a thermodynamic analysis of the whole autothermal gasification system, including both a regenerative preheating system and the gasifier, is made.The particle study reports results from experiments with wood and coal and agents consisting of mixtures of nitrogen and oxygen in various proportions. It is shown that an increase in agent temperature from 873 K to 1273 K make the conversion process faster, mostly due to an early onset of the devolatilisation (fast drying) but also due to an increased devolatilisation rate (at least in the case of wood). The time to ignition also decreases significantly, particularly so between 873 and 1073 K. Further, it is shown that the higher the agent temperature, the more pronounced was also the tendency of the coal particles to heat significantly faster in oxygen diluted conditions (5,10 and 21% oxygen) than in inert (0% oxygen) or oxygen rich conditions (30, 50, 80 and 100% oxygen). An increase in agent temperature is also shown to reduce the dependency of the process on the oxygen concentration, at least in diluted conditions (5-21% oxygen). The results also indicate that for coal an increase in the oxygen concentration, specifically in the region above the atmospheric concentration, leads to a decreased dependency on the agent temperature. It is finally shown in the experiments with agent temperatures of 1073 and 1273 K that a flame is promptly formed even in very low concentrations of oxygen.The gasifier study reports results from simulation of batch air gasification and experiments in both batch and continuous up-draft fixed bed gasifier with wood and waste derived fuel and air and mixtures of air and steam. It is shown that the conversion process is faster the higher the air temperature. In particular somewhere between air temperatures of 623 K and 803 K the process behaviour changes. In fact, the devolatilisation rate is significantly increased in this region while it increases less sharply with air temperature below and above this temperature window. The temperature distribution in the bed shows less sharp gradients at high temperature (> 803 K) than at low temperatures (< 623 K). It is also showed experimentally and in fairly large scale that the use of highly preheated air for the gasification of biomass and waste derived fuels can produce - in continuous mode – relatively high yields of product syngas with relatively high fractions of combustible gases and probably also low content of tar. The efficiency of the gasification under these conditions, even when the extra heat input in the preheated agent is considered in the computation of the gasification efficiency, is shown to be comparable to that of conventional gasification techniques. The results also shows that with the use of steam in the agent, the content of hydrogen can be further increased with respect to gasification with only preheated air.In base of the results of the particle study and the gasifier study it is shown that a there exists two regimes of operation in function of the agent temperature, separated by the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature. The value of this temperature depend on material properties and the kinetics of the reaction, thus also on the oxygen concentration. When agent temperatures below the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature are used, the drying and devolatilisation are mainly controlled by the heat released by reactions. The heating of the fuel particles and their devolatilisation are relatively slow and the devolatilisation rate is highly oxygen dependent. In a fixed bed, the devolatilisation front rate is low and the bed is characterised by significant temperature gradients.When the agent temperature is higher than the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature, the drying and devolatilisation are mainly controlled by the convective heat transfer from the preheated agent and the released volatiles ignite very fast even in diluted conditions. This results in very efficient heat transfer to the fuel particles. In the fixed fuel bed the process is characterized by a high devolatilisation front rate. Thus, the temperature gradients in the bed are significantly reduced and the gasification can be said to be thermally homogeneous. Thanks to high rates of heat transfer and mass conversion, the heating value of the dry produced syngas is high with high concentrations of combustible species. The ignition of the volatiles and the high temperatures all along the bed presumably contributes to the reduction of the tar content even in up-draft configurations. The high temperatures also allows for operation with reduced air – to – fuel ratios which further increased the value of the produced gas (thanks to less dilution by nitrogen).The system study presents a concept for an autothermal system including both preheating and gasification. Results from a thermodynamic analysis of such a system are reported. Autothermal operation of a thermally homogeneous gasifier is possible only in a twin component system in which the gasifier is coupled to a preheating system able to reach preheating temperatures well above the minimum agent temperature to guarantee spontaneous ignition regardless of the particle temperature. It is shown that to reach certain temperature levels of the gasification air, heat exchange between product gas and air is not enough and the preheating system has to improve the temperatures involved, for example by burning part of the produced gas in a regenerative preheater. Further, it is shown that in comparison to gasifier without such a system for additional preheating, the autothermal Thermally Homogeneous Gasification system has the ability to significantly improve the gas quality (in terms of heating value of the dry gas) without losing energy- or exergy efficiency to an appreciable extent.
QC 20100903

The objective of this thesis is to experimentally investigate the performance of steam gasification of chars of pure coal (lignite, sub-bituminous), pure biomass (radiata pine, eucalyptus nitens) and their blends. The influences of gasification temperature, types of coal and biomass, coal-biomass blending ratio, alkali and alkaline earth metal (AAEM) in lignite, on specific gasification characteristics (producer gas composition and yield, char reactivity) were studied. In addition, synergistic effects in co-gasification of coal-biomass blend char were also investigated. This project is in accordance with objectives of the BISGAS Consortium. In this study, experiments were performed in a bench-scale gasifier at gasification temperatures of 850°C, 900°C and 950°C, respectively. Two types of coals (lignite and sub-bituminous) and two kinds of biomass (radiata pine and eucalyptus nitens) from New Zealand were selected as sample fuels. From these raw materials, the chars with coal-to-biomass blending ratios of 0:100 (pure coal), 20:80, 50:50, 80:20 and 100:0 (pure biomass), which were derived through the devolatilization at temperature of 900°C for 7 minutes, were gasified with steam as gasification agent. During the gasification tests, the producer gas composition and gas production were continuously analysed using a Micro gas chromatograph. When the gas production was undetectable, the gasification process was assumed to be completed and the gasification time was recorded. The gasification producer gas consisted of three main gas components: hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2). The results from gasification of chars of individual solid fuels (coal or biomass) confirmed that biomass char gasification was faster than coal char gasification. The influences of gasification temperatures were shown as: when gasification temperature increased, the H2 yield increased in coal char gasification but decreased in biomass char gasification. In the meantime, CO yields increased while CO2 yields decreased in both coal char and biomass char gasification. In addition, the char reactivity of all the pure fuel samples increased with elevated gasification temperatures. The results from co-gasification of coal-biomass blend char exhibited that the syngas production rate, which is defined as the total gas production divided by the gasification completion time, was enhanced by an increase in gasification temperatures as well as an increase in the biomass proportion in the blend. The AAEM species played a significant catalytic role in both gasification of pure coal chars and co-gasification of coal-biomass blend chars. The presence of AAEM increased the producer gas yield and enhanced the char reactivity. The positive synergistic effects of the coal-biomass blending char on syngas production rate only existed in the co-gasification of lignite-eucalyptus nitens blend chars. The other blend chars showed either insignificant synergistic effects or negative effects on the syngas production rate.

Coal fired power stations produce vast amounts of harmful products that may affect our health and environment. Co-gasification of coal and biomass could be a solution to this issue as an emerging technology. Biomass may reduce emissions significantly and it may contribute to reducing capital operational cost while providing high gas yields. This research tests the co-gasification of coal and biomass blended chars. Coal and biomass were both prepared. Coal Illinois No #6 was prepared as coal semi-char and coal-char while Jatropha biomass was torrefied at six different temperatures ranging from [200-300] ºC. The co-gasification experiments was conducted in a fixed-bed reactor. A gasification temperature was 900 ºC and a constant flow rate of 100 mL/min. Carbon conversion, maximum char reactivity, products yield and amount of hydrogen produced were evaluated and studied based on data obtained from the G.C. Additionally, weight of bed material and ash leftover weight from gasification process were significantly contributed in calculating the carbon conversion percentages.

Biomass gasification has attracted the interest of researchers because it produces zero carbon to the atmosphere. This technology does not only produce syngas but also the byproducts which can be used for various application depending on quality.The study conducted at Melani village in Alice in the Eastern Cape of South Africa was aimed at investigating the possible applications of the gasification byproducts instead of being thrown away. Pine wood was employed as the parent feedstock material for the gasifier. Biomass gasification by-products were then collected for further analysis. The studied by-products included tar(condensate), char, soot and resin. These materials were also blended to produce strong materials.The essence of the blending was to generate ideal material that is strong but light at the same time.The elemental analysis of the samples performed by CHNS analyser revealed that carbon element is in large quantities in all samples. The FTIR spectra showed almost similar results for all the studied samples, since the samples are end products of lignocellulosegasification. SEM gave the sticky images of resin as well as porous char structures. Char showed a higher heating value of 35.37MJ/Kg when compared to other by-products samples.

Este trabalho tem como objetivo a identificação da melhor rota para a produção de combustíveis e derivados sintéticos através do desenvolvimento e aplicação de uma ferramenta metodológica desenvolvida tendo como base ferramentas da qualidade: diagrama de afinidade, diagrama de relações e matriz causa-efeito. Estes diagramas foram adaptados para a análise e discussão dos fatores positivos e negativos de cada item da tríade considerada: insumo-processo-produto. A partir desta análise foram criadas as matrizes de causa-efeito, também separadas em fatores positivos e negativos para os insumos: gás natural (GN), biomassa e carvão mineral; para os processos: produção de gás de síntese (syngas) a partir do GN, gaseificação do carvão e a gaseificação da biomassa; e para os produtos: óleo lubrificante, óleo diesel, nafta, metanol e amônia. A análise destas matrizes causa-efeito gerou a matriz final, denominada matriz saldo, que permitiu a seleção da rota mais adequada para a produção de combustíveis e derivados sintéticos. Dentre os insumos estudados, o gás natural apresentou evidentes vantagens e, consequentemente, o processo a ser utilizado deve ser a produção do syngas a partir do GN, e dentre os produtos o metanol apresentou maiores benefícios para ser produzido.
This paper aims to present to identify of the best route for the production of fuels and synthetic derivatives through the development and application of a methodological tool based on quality tools: affinity diagram, relations diagram and matrices cause-effect. The diagrams have been adapted for the analysis and discussion of positive and negative factors of each item of the triad considered: feedstock-process-product. From the analysis, matrices of cause and effect were created and also, separated into positive and negative factors for the inputs: natural gas (NG), biomass and coal; for the processes: production of synthesis gas (syngas) from GN, coal gasification and biomass gasification; and for the products: lubricating oil, diesel fuel, naphtha, methanol and ammonia. The analysis of cause-effect matrices generated the final matrix, named balance matrix, which allowed the selection of the most suitable route for the production of fuels and synthetic derivatives. Among the input studied, NG presented remarkable advantages among the others. Therefore, the process to be used should be the production of syngas from NG. Among the products considered, methanol showed the best benefits to be produced.

Die Korrosion des Feuerfestwerkstoffs durch flüssige Schlacke stellte in ausgemauerten Vergasern die Hauptschädigung dar. Daher werden derzeit hoch chromhaltige Keramiken eingesetzt, die eine geringe Löslichkeit in (alumo-)silikatischen Schmelzen aufweisen. Jedoch weisen auch diese Werkstoffe nur geringe Standzeiten von 3 bis 24 Monaten auf, sind zudem sehr teuer und müssen nach ihrem Einsatz z. T. als giftiger Sondermüll (Chrom-VI-Verbindungen) entsorgt werden. Mögliche Alternativen stellen korund- und spinellbasierte Keramiken mit optimierten Zusammensetzungen dar. Diese werden erfolgreich in verschiedenen Bereichen der Stahlherstellung eingesetzt. Jedoch unterscheiden sich CaO-reiche und SiO2-arme Schlacken der Stahlherstellung deutlich von denen aus der Kohle- oder Biomassevergasung. Die Korrosionsmechanismen beim Angriff von Brennstoffschlacken auf korund- und spinellbasierte Werkstoffe sind hierbei nicht hinreichend erforscht. In der vorliegenden Arbeit wurden die Mechanismen der Korrosion einer sauren, einer intermediären und einer basischen Schlacke an derartigen Al2O3-haltigen Werkstoffen untersucht. Die Charakterisierung der Brennstoffaschen sowie Kurzzeitkorrosionstests an verschiedenen korund- und spinellbasierten Keramiken unter reduzierender Atmosphäre dienten als Grundlage für die darauffolgende detaillierte Analyse des geschädigten Bereichs mittels Röntgenbeugung und elektronenmikroskopischer Methoden. In Kombination mit thermochemischen Betrachtungen in (quasi-)ternären Phasendiagrammen, konnten die Korrosion sowie die wirkenden Mechanismen beschrieben werden. Die drei Testaschen wiesen ein unterschiedliches Korrosionsverhalten auf, wobei nur eine geringe Anzahl an Mineralphasen gebildet wurde. Die saure Asche (SA) führte, abhängig von der getesteten Keramik, zu einer starken oder geringen Schädigung. Im Gegensatz dazu zeigte die intermediäre Asche (IA) durchweg eine tiefe Infiltration der Probekörper, einhergehend mit deutlichen Lösungs- und Kristallisationserscheinungen (v. a. Anorthit, CaAl2Si2O8). Die basische Asche (BA) drang in alle Werkstoffe nur bis in eine geringe Tiefe ein, führte aber im Infiltrationsbereich zu einer starken Korrosion durch die Kristallisation von Gehlenit (Ca2Al2SiO7), gefolgt von Hibonit (CaAl12O19). Die Auswertung der Schädigung verdeutlichte, dass die Korrosion der korund- und spinellbasiertern Keramiken, neben ihrem Mikrogefüge, v. a. durch die chemische Zusammensetzung der Aschen/Schlacken bestimmt wird. Durch die Betrachtung der Lösungs- und Kristallisationsvorgänge im isothermen Schnitt des SiO2-CaO-Al2O3-(Na2O/K2O)-Phasendiagramms konnten die experimentellen Ergebnisse bestätigt und die Korrosionsmechanismen beschrieben werden. Durch das Lösen von Al2O3 aus der keramischen Matrix reichert sich die Schlacke so lange an dieser Komponente an, bis die Schlackezusammensetzung das Stabilitätsfeld von Korund (α-Al2O3) erreicht und die Korrosion zum Erliegen kommt. Die tiefe Infiltration und starke Korrosion von IA konnte durch einen weiten zu durchquerenden Bereich alleiniger Flüssigphase erklärt werden. Dabei konnte die Schlacke sehr viel Al2O3 aus der keramischen Matrix lösen, bis sich erste kristalline Phasen bildeten. Im Gegensatz dazu führte bei BA das frühe Ausscheiden von Gehlenit zu einer starken Viskositätserhöhung und zu einem raschen Verbrauch der flüssigen Schlacke, was die weitere Infiltration in die Keramik verhinderte. Die Zusammensetzung von SA bedingte ein sofortiges Ausscheiden von Anorthit und ein frühes Erreichen des Korundstabilitätsfeldes. Unterschiede im Korrosionsverhalten von SA zwischen den verschiedenen Keramiken konnten auf die Verfügbarkeit von Al2O3 und das Vorhandensein von sinterinduzierten Calciumaluminat- oder Glasphasen in der Matrix zurückgeführt werden. Die Aufklärung der Korrosionsmechanismen bei chromfreien, Al2O3-haltigen Werkstoffen zeigte, dass neben einer optimierten keramischen Mikrostruktur die Aschechemie angepasst werden kann, um die Infiltration von Schlacke wirksam zu mindern. Im technischen Prozess kann dies durch die Zugabe von „Flussmitteln“ (Kalkstein, CaCO3 bzw. Sand, SiO2) oder durch geschicktes Mischen von Brennstoffen erreicht werden. Langzeitauslagerungen über 150 h und der Test eines Werkstoffs im SFGT-Flugstromvergaser bestätigten die Ergebnisse der Kurzzeittests
The corrosion of the refractory lining is the most crucial wear mechanism in slurry fed gasifiers. Due to the low solubility of Cr2O3 in (alumino-)silicate melts, high chromia bricks are state of the art, yet they are expensive, offer a non-satisfying service life of 3 to 24 months, and may need to be disposed of as toxic waste (chromium-VI compounds). Possible alternative materials are alumina or spinel based refractories with optimized composition, as used in various steel making processes. However, steel ladle slags, rich in CaO and poor in SiO2, differ significantly from coal or biomass slags of the gasification process. The corrosion mechanisms of coal or biomass slag attacks onto alumina and spinel based refractories are not investigated in detail yet. In the present work, the corrosion mechanisms of an acidic, an intermediate, and a basic slag against such Al2O3-containing materials have been studied. Ash analyses and short-term corrosion tests in reducing atmosphere were used as a basis for the description of the corrosion. Wear areas were investigated by means of X-ray diffraction and electron microscopy methods. In combination with thermochemical investigations using (quasi-)ternary phase diagrams it was possible to characterize the corrosion processes. The three ashes revealed a different corrosion behavior, with only few mineral phases being formed. Depending on the tested ceramic, the acidic ash (SA) led to a strong or light damage, respectively. In contrast, the intermediate ash (IA) exhibited a deep infiltration and intense corrosion in terms of dissolution and crystallization (mainly anorthite, CaAl2Si2O8) throughout all tested specimens. The basic ash (BA) infiltrated all samples only to a shallow depth. Nevertheless, a strong corrosion due to the massive crystallization of gehlenite (Ca2Al2SiO7), followed by hibonite (CaAl12O19), was observed. The evaluation of the occurring wear pointed up that the corrosion of alumina and spinel based refractories mainly depends on their micro structure and especially on the composition of the ash/slag. Considering the dissolution and crystallization processes in the isothermal section of the SiO2-CaO-Al2O3-(Na2O/K2O) phase diagram, the experimental findings could be confirmed and the corrosion mechanisms described. By dissolution of Al2O3 from the ceramic matrix the slag is enriched in this component until its composition reaches the stability field of corundum (α-Al2O3), which causes corrosion to stop. The deep infiltration and strong corrosion of IA could be explained by a wide range of pure liquid phase which needed to be traversed on its corrosion path in the ternary phase diagram. This enabled IA to dissolve a lot of Al2O3 before the first crystalline phases were formed. For BA, in contrast, the rapid crystallization of gehlenite increased the slag viscosity and led to a consumption of the remaining melt, which in turn stopped further infiltration. The composition of SA is located in the stability field of anorthite plus liquid, forcing the direct precipitation of this mineral. This, and the proximity to the corundum stability field, led to shallow infiltration in some cases. For other tested samples the infiltration and corrosion was stronger, showing that for SA the availability of Al2O3 and the presence of calcium aluminates or glass phases in the ceramic matrix played an essential role. The elucidation of the corrosion mechanisms of chromia-free, Al2O3-containing refractories showed that, besides an optimized micro structure, the adjustment of the slag chemistry is a further option to minimize infiltration. In the technical processes fluxes (lime stone, CaCO3 or quarz sand, SiO2) can be added or blends of different feed stocks can be used. Long-term exposure tests for 150 h and a field trial of one sample in the SFGT entrained-flow gasifier confirmed the findings of the short-term corrosion tests

The present study mainly focus on two parts: one was the optimization of FactSage calculation, compared with HT-XRD measurements on mineral matter behaviors during the heating of coal and blend ashes from 500 °C to 1000 °C in reducing atmosphere. The aim was to obtain the optimized input parameters and options for FactSage calculation, and the outputs should be as close as possible to HT-XRD results. The other was the application of FactSage on ash melting behaviors. Since the maximum temperature of HT-XRD measurement in laboratory was 1000 °C in reducing atmosphere, the optimized FactSage was applied to investigate the ash melting behaviors in temperature range between 600 °C and 1600 °C for coal, biomass and their blends. The FactSage calculation was optimized by investigations of several input parameters and options including the mass ratio of reactant gas amount to ash sample, solution species and compound solid species. The results obtained from the optimized calculation were much better to fit the mineral transformations measured by HT-XRD. However, there were still some differences between the results from optimized FactSage calculations and HT-XRD measurements. This is mainly due to the amorphous substances which occurred as solid phases and liquid slag in FactSage outputs but cannot be detected by HT-XRD. Besides, several factors, such as the diffusion, particle size distribution and so on, affect the actual measurements greatly but been neglected in thermodynamic calculations, which enhance the distinctions. In addition, the effects of atmosphere were investigated and the differences of mineral matter behaviors were mainly embodied in sulphur-rich minerals, iron-rich minerals and amorphous substance. For application of FactSage on ash melting behaviors, AFTs tests for coal, biomass and their blends were adopted, and the results were well investigated by ash chemical components analyzed by XRF and also equilibrium phases calculated by FactSage. Hemispheric temperature and flowing temperature were mainly dependent on the high melting point substances at high temperature, such as free CaO in HKN and SWC, SiO2 in WS and KOL. The sintering temperature was largely affected by alkali oxides, which could combine with other oxides to form low melting point substances. For blended ashes, AFTs of the blended ash of HKN and WS shown a V shape with WS addition mass ratio rising, and the minimum values of AFTs appeared at 50 wt.% WS addition. AFTs of KOL changed in a small scale when mixed with WS, due to their similar ash composition (high in SiO2). As the SWC ash contents is much less than HKN and KOL, it did not affect the AFTs much when blended with coals. Moreover, the biomass addition affection on the blended ashes AFTs were also well illustrated by the liquid phases mass fraction and also the mineral matter transformations calculated by FactSage.

博士
國立成功大學
機械工程學系碩博士班
101
Gasification plays an important role in the development of clean coal technology. In addition, not only coal but also other materials can be employed as feedstocks in gasification. Biomass is an important source of renewable energy in the world. It can also be gasified with coal through co-gasification. Therefore, the objective of this study is to seek appropriate operations for gasification process, the present study develops a numerical method to predict coal gasification phenomena in an entrained-flow gasifier. Particular emphasis is placed on the influence of injection pattern upon synthesis gas (syngas) production. The parameter of steam/coal ratio is also taken into account to evaluate its impact on hydrogen generation. With oxygen injected from the center inlet and coal from the middle ring inlet of the reactor, the operating pattern gives the best performance of coal gasification where the carbon conversion (CC) and coal gas efficiency (CGE) are 89 and 72%, respectively. Increasing steam into the reactor reduces CC and less CO is generated. For the results of gasaification by different feeds, the obtained results suggest that in all cases, the carbon conversions of the three fuels are higher than 90%. However, the cold gasification efficiency for raw bamboo is low, mainly due to the relatively lower calorific value of the material. In the case of the torrefied bamboo fuel, the gasification performance is enhanced significantly and is quite similar to the coal gasification under the same conditions. It appears that the optimum oxygen-to-fuel mass flow ratios for the gasification of raw bamboo, torrefied bamboo, and coal are 0.9, 0.7, and 0.7, and their equivalence ratios are 0.692, 0.434, and 0.357, respectively. On the other hand, Gasification is a very complex thermal conversion process. The result of gasification is influenced by many factors. Hence, the present study was conducted to optimize the gasification process in an entrained-flow gasifier through the application of the Taguchi method. Results suggest that the optimum conditions are a wall temperature of 1500 K, an O/F ratio of 0.6, coal feed type and a gasifier pressure of 3 MPa. The influence strength order of each control condition is feed type＞O/F ratio＞wall temperature＞pressure. The value of the S/N ratio for the optimum case is 13.40, which is the highest value compared to other cases. The simulations suggest that the developed numerical method is able to provide an accurate prediction on syngas formation. With oxygen injected from the center inlet and coal from the middle ring inlet of the reactor, the operating pattern gives the best performance of coal gasification. Analysis of the Taguchi method was used to evaluate the calculation results. Results show that the Taguchi method is able to investigate the gasification process well.

Chars were obtained from individual fuels and blends with different blend ratios of coal, coke and biomass in Drop Tube Furnace at different temperatures. Based on TGA experimental data, it was shown that the effect of the blending ratio of biomass to other fuels on the reactivity of the co-pyrolyzed chars is more pronounced on the chars prepared at lower temperature, due to the presence of synergetic effects originating from the interaction of the two fuels. SEM images showed differences in shapes and particle size of char particles from biomass and coal/coke. These also show the agglomeration of coal and coke chars with biomass char particles at high temperatures. The agglomeration may be the reason for the non-additive behavior of the blends. BET analysis showed increase in the surface area with an increasing temperature for biomass and coal, but the trend for coke was inversely related to the temperature.
Chemical Engineering

Concentrated animal feeding operations such as cattle feedlots and dairies produce a large amount of manure, cattle biomass (CB), which may lead to land, water, and air pollution if waste handling systems and storage and treatment structures are not properly managed. However, the concentrated production of low quality CB at these feeding operations serves as a good feedstock for in situ gasification for syngas (CO and H2) production and subsequent use in power generation. A small scale (10 kW) countercurrent fixed bed gasifier was rebuilt to perform gasification studies under quasisteady state conditions using dairy biomass (DB) as feedstock and various air-steam mixtures as oxidizing sources. A DB-ash (from DB) blend and a DB-Wyoming coal blend were also studied for comparison purposes. In addition, chlorinated char was also produced via pure pyrolysis of DB using N2 and N2-steam gas mixtures. The chlorinated char is useful for enhanced capture of Hg in ESP of coal fired boilers. Two main parameters were investigated in the gasification studies with air-steam mixtures. One was the equivalence ratio ER (the ratio of stochiometric air to actual air) and the second was the steam to fuel ratio (S:F). Prior to the experimental studies, atom conservation with i) limited product species and ii) equilibrium modeling studies with a large number of product species were performed on the gasification of DB to determine suitable range of operating conditions (ER and S:F ratio). Results on bed temperature profile, gas composition (CO, CO2, H2, CH4, C2H6, and N2), gross heating value (HHV), and energy conversion efficiency (ECE) are presented. Both modeling and experimental results show that gasification under increased ER and S:F ratios tend to produce rich mixtures in H2 and CO2 but poor in CO. Increased ER produces gases with higher HHV but decreases the ECE due to higher tar and char production. Gasification of DB under the operating conditions 1.59less than0.8 yielded gas mixtures with compositions as given below: CO (4.77 - 11.73 %), H2 (13.48 - 25.45%), CO2 (11-25.2%), CH4 (0.43-1.73 %), and C2H6 (0.2- 0.69%). In general, the bed temperature profiles had peaks that ranged between 519 and 1032 degrees C for DB gasification.

Thermo-gravimetric analysis was carried out on a vitrinite-rich coal (VC), highveld grass (HG) and pine wood (PW) chars, and coal-biomass char blends of each. The analysis was carried out on combustion and gasification tests using air and CO2 respectively. The blends were modeled by the application of a distributed activation energy (DAE) based model. The DAE based model is a modification of an algorithm developed by Scott et al. for the pyrolysis of complex fuels obeying linear kinetics (Scott et al., 2006). The modified DAE model was able to derive the activation energy, , the grouped pre-exponential factor, , and the number of reactions occurring in the thermal conversion process. Furthermore, the mass fraction associated with each unique reaction was obtained. The ability to determine multiple reactions distinguishes the DAE based model as a unique and robust method for kinetics determination. The first order and the random pore reaction models (RPM) were applied to describe the reaction profiles. The conversion of all the coal and biomass blends were successfully modeled using the RPM to high accuracy. During combustion, ’s and ’s in the range of 180-255kJ/mol and 5.34E+8 to 2.80E+15 s-1m-1 were determined for the PW char. ’s and ’s in the range of 125- 138kJ/mol and 5.38E+4 to 3.94E+5 s-1m-1 were determined for the rest of the chars and blends during combustion. For gasification, ’s and ’s in the range of 222 -304kJ/mol and 5.36E+5 to 3.96E+9 s-1m-1 were determined for all the chars and blends. The structural parameters ( ) obtained lie in the range of 8.3 to 18.9. The determined during combustion were sufficient for modeling the same material during gasification. Multiple reactions were identified for most of the chars during both gasification and combustion. Kinetic analysis showed that PW char was the most reactive char, followed by the HG and VC chars respectively. For the 50:50 heat input ratio coal-biomass blends during combustion, synergetic behavior and a decrease in was observed.

Environmental regulations have historically been focused on individual emission points, facilities, or industrial sectors. However, recent and emerging regulations for greenhouse gas (GHG) emissions such as those contained in the Energy Independence and Security Act (EISA) of 2007 have introduced the concept of product life cycle limits on the emissions of transportation fuels. Thus, a complete life cycle assessment (LCA) of the transportation fuel must be completed where all emissions from field to the vehicle’s fuel tank and from tank to the vehicle’s exhaust must be assessed. However, although there have been extensive analysis of the GHG emissions associated with transportation fuels, there are substantial uncertainties associated with these estimates that can be attributed to poor data quality, inconsistent methodological choices, and model uncertainties, among others. This thesis evaluates the uncertainties present in LCA through the case study of fuel production using Fischer-Tropsch (F-T) synthesis of fuels derived from coal and biomass. Specifically, GHG emission estimates for F-T synthesis process scenarios are presented and the uncertainties in the estimates are discussed. Overall uncertainties in GHG emissions due to changes in the details of the process configurations in the F-T process can be up to 11%. This finding suggests that the details of fuel refining conditions will need to be specified in determining whether fuels meet GHG emission requirements, complicating the implementation of life cycle GHG regulations.
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Syngas consisting mainly of a mixture of carbon monoxide, hydrogen and other diluents, is an important fuel for power generation applications since it can be obtained from both biomass and coal gasification. Clean coal technologies require stable and efficient operation of syngas-fired gas turbines. The trapped vortex combustor (TVC) is a relatively new gas turbine combustor concept which shows tremendous potential in achieving stable combustion under wide operating conditions with low emissions. In the present work, combustion of low calorific value syngas in a TVC has been studied using in-situ laser diagnostic techniques and numerical modeling. Specifically, this work reports in-situ measurements of mixture fraction, OH radical concentration and velocity in a single cavity TVC, using state-of-the art laser diagnostic techniques such as Planar Laser-induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV). Numerical simulations using the unsteady Reynolds-averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) approaches have also been carried out to complement the experimental measurements. The fuel-air momentum flux ratio (MFR), where the air momentum corresponds to that entering the cavity through a specially-incorporated flow guide vane, is used to characterize the mixing. Acetone PLIF experiments show that at high MFRs, the fuel-air mixing in the cavity is very minimal and is enhanced as the MFR reduces, due to a favourable vortex formation in the cavity, which is corroborated by PIV measurements. Reacting flow PIV measurements which differ substantially from the non-reacting cases primarily because of the gas expansion due to heat release show that the vortex is displaced from the centre of the cavity towards the guide vane. The MFR was hence identified as the controlling parameter for mixing in the cavity. Quantitative OH concentration contours showed that at higher MFRs 4.5, the fuel jet and the air jet stream are separated and a flame front is formed at the interface. As the MFR is lowered to 0.3, the fuel air mixing increases and a flame front is formed at the bottom and downstream edge of the cavity where a stratified charge is present. A flame stabilization mechanism has been proposed which accounts for the wide MFRs and premixing in the mainstream as well. LES simulations using a flamelet-based combustion model were conducted to predict mean OH radical concentration and velocity along with URANS simulations using a modified Eddy dissipation concept model. The LES predictions were observed to agree closely with experimental data, and were clearly superior to the URANS predictions as expected. Performance characteristics in the form of exhaust temperature pattern factor and pollutant emissions were also measured. The NOx emissions were found to be less than 2 ppm, CO emissions below 0.2% and HC emissions below 700 ppm across various conditions. Overall, the in-situ experimental data coupled with insight from simulations and the exhaust measurements have confirmed the advantages of using the TVC as a gas turbine combustor and provided guidelines for stable and efficient operation of the combustor with syngas fuel.

Die Korrosion des Feuerfestwerkstoffs durch flüssige Schlacke stellte in ausgemauerten Vergasern die Hauptschädigung dar. Daher werden derzeit hoch chromhaltige Keramiken eingesetzt, die eine geringe Löslichkeit in (alumo-)silikatischen Schmelzen aufweisen. Jedoch weisen auch diese Werkstoffe nur geringe Standzeiten von 3 bis 24 Monaten auf, sind zudem sehr teuer und müssen nach ihrem Einsatz z. T. als giftiger Sondermüll (Chrom-VI-Verbindungen) entsorgt werden. Mögliche Alternativen stellen korund- und spinellbasierte Keramiken mit optimierten Zusammensetzungen dar. Diese werden erfolgreich in verschiedenen Bereichen der Stahlherstellung eingesetzt. Jedoch unterscheiden sich CaO-reiche und SiO2-arme Schlacken der Stahlherstellung deutlich von denen aus der Kohle- oder Biomassevergasung. Die Korrosionsmechanismen beim Angriff von Brennstoffschlacken auf korund- und spinellbasierte Werkstoffe sind hierbei nicht hinreichend erforscht. In der vorliegenden Arbeit wurden die Mechanismen der Korrosion einer sauren, einer intermediären und einer basischen Schlacke an derartigen Al2O3-haltigen Werkstoffen untersucht. Die Charakterisierung der Brennstoffaschen sowie Kurzzeitkorrosionstests an verschiedenen korund- und spinellbasierten Keramiken unter reduzierender Atmosphäre dienten als Grundlage für die darauffolgende detaillierte Analyse des geschädigten Bereichs mittels Röntgenbeugung und elektronenmikroskopischer Methoden. In Kombination mit thermochemischen Betrachtungen in (quasi-)ternären Phasendiagrammen, konnten die Korrosion sowie die wirkenden Mechanismen beschrieben werden. Die drei Testaschen wiesen ein unterschiedliches Korrosionsverhalten auf, wobei nur eine geringe Anzahl an Mineralphasen gebildet wurde. Die saure Asche (SA) führte, abhängig von der getesteten Keramik, zu einer starken oder geringen Schädigung. Im Gegensatz dazu zeigte die intermediäre Asche (IA) durchweg eine tiefe Infiltration der Probekörper, einhergehend mit deutlichen Lösungs- und Kristallisationserscheinungen (v. a. Anorthit, CaAl2Si2O8). Die basische Asche (BA) drang in alle Werkstoffe nur bis in eine geringe Tiefe ein, führte aber im Infiltrationsbereich zu einer starken Korrosion durch die Kristallisation von Gehlenit (Ca2Al2SiO7), gefolgt von Hibonit (CaAl12O19). Die Auswertung der Schädigung verdeutlichte, dass die Korrosion der korund- und spinellbasiertern Keramiken, neben ihrem Mikrogefüge, v. a. durch die chemische Zusammensetzung der Aschen/Schlacken bestimmt wird. Durch die Betrachtung der Lösungs- und Kristallisationsvorgänge im isothermen Schnitt des SiO2-CaO-Al2O3-(Na2O/K2O)-Phasendiagramms konnten die experimentellen Ergebnisse bestätigt und die Korrosionsmechanismen beschrieben werden. Durch das Lösen von Al2O3 aus der keramischen Matrix reichert sich die Schlacke so lange an dieser Komponente an, bis die Schlackezusammensetzung das Stabilitätsfeld von Korund (α-Al2O3) erreicht und die Korrosion zum Erliegen kommt. Die tiefe Infiltration und starke Korrosion von IA konnte durch einen weiten zu durchquerenden Bereich alleiniger Flüssigphase erklärt werden. Dabei konnte die Schlacke sehr viel Al2O3 aus der keramischen Matrix lösen, bis sich erste kristalline Phasen bildeten. Im Gegensatz dazu führte bei BA das frühe Ausscheiden von Gehlenit zu einer starken Viskositätserhöhung und zu einem raschen Verbrauch der flüssigen Schlacke, was die weitere Infiltration in die Keramik verhinderte. Die Zusammensetzung von SA bedingte ein sofortiges Ausscheiden von Anorthit und ein frühes Erreichen des Korundstabilitätsfeldes. Unterschiede im Korrosionsverhalten von SA zwischen den verschiedenen Keramiken konnten auf die Verfügbarkeit von Al2O3 und das Vorhandensein von sinterinduzierten Calciumaluminat- oder Glasphasen in der Matrix zurückgeführt werden. Die Aufklärung der Korrosionsmechanismen bei chromfreien, Al2O3-haltigen Werkstoffen zeigte, dass neben einer optimierten keramischen Mikrostruktur die Aschechemie angepasst werden kann, um die Infiltration von Schlacke wirksam zu mindern. Im technischen Prozess kann dies durch die Zugabe von „Flussmitteln“ (Kalkstein, CaCO3 bzw. Sand, SiO2) oder durch geschicktes Mischen von Brennstoffen erreicht werden. Langzeitauslagerungen über 150 h und der Test eines Werkstoffs im SFGT-Flugstromvergaser bestätigten die Ergebnisse der Kurzzeittests.:1 Einleitung und Zielstellung 2 Kenntnisstand 2.1 Ausmauerung in Vergasern 2.2 Werkstoffverschleiß und Korrosionstests 2.2.1 Arten des Werkstoffverschleißes 2.2.2 Korrosionstests 2.2.3 Auswertung von Korrosionstests 2.3 Chromhaltige Steine 2.4 Chromfreie Feuerfestwerkstoffe für die Vergasung und Stahlerzeugung 2.4.1 Korundbasierte Steine 2.4.2 Spinellhaltige Steine 3 Proben und Untersuchungsmethodik 3.1 Auswahl der Einsatzstoffe 3.1.1 Keramikproben 3.1.2 Testaschen/-schlacken 3.2 Charakterisierung der Einsatzstoffe und des Korrosionsverhaltens 3.2.1 Röntgenfluoreszenzanalyse 3.2.2 Ascheschmelzverhalten 3.2.3 Röntgendiffraktometrie 3.2.4 Hochtemperatur-Röntgendiffraktometrie 3.2.5 Berechnung der Hochtemperatur-Phasenentwicklung 3.2.6 Lichtmikroskopie 3.2.7 Rasterelektronenmikroskopie 3.3 Korrosionsuntersuchungen und -berechnungen 3.3.1 Kurzzeitauslagerung im thermo-optischen Messsystem 3.3.2 Langzeitauslagerung mittels Tiegeltest 3.3.3 Keramiktest im Flugstromvergaser 3.3.4 Berechnung von Phasendiagrammen 4 Ergebnisse und Diskussion 4.1 Charakterisierung der Aschen 4.1.1 Elementzusammensetzung der Aschen 4.1.2 Ascheschmelzverhalten 4.1.3 Phasenbestand der Aschen 4.1.4 Hochtemperaturverhalten der Aschen 4.2 Ergebnisse der TOMAC-Tests 4.2.1 Lichtmikroskopische Untersuchungen 4.2.2 REM/EDS-Untersuchungen 4.3 Ableitung von Korrosionsmechanismen 4.3.1 Wechselwirkungen mit der sauren Asche 4.3.2 Wechselwirkungen mit der intermediären Asche 4.3.3 Wechselwirkungen mit der basischen Asche 4.3.4 Wechselwirkungen beim Referenz-Chromstein 4.4 Schlussfolgerungen aus den TOMAC-Tests 4.4.1 Zusammenfassung der TOMAC-Ergebnisse 4.4.2 Technische Bedeutung 4.4.3 Vorschlag für eine vereinfachte Untersuchungsmethodik 4.5 Validierung der Ergebnisse 4.5.1 Werkstofftest in der Langzeitauslagerung 4.5.2 Werkstofftest im Flugstromvergaser 5 Zusammenfassung und Ausblick
The corrosion of the refractory lining is the most crucial wear mechanism in slurry fed gasifiers. Due to the low solubility of Cr2O3 in (alumino-)silicate melts, high chromia bricks are state of the art, yet they are expensive, offer a non-satisfying service life of 3 to 24 months, and may need to be disposed of as toxic waste (chromium-VI compounds). Possible alternative materials are alumina or spinel based refractories with optimized composition, as used in various steel making processes. However, steel ladle slags, rich in CaO and poor in SiO2, differ significantly from coal or biomass slags of the gasification process. The corrosion mechanisms of coal or biomass slag attacks onto alumina and spinel based refractories are not investigated in detail yet. In the present work, the corrosion mechanisms of an acidic, an intermediate, and a basic slag against such Al2O3-containing materials have been studied. Ash analyses and short-term corrosion tests in reducing atmosphere were used as a basis for the description of the corrosion. Wear areas were investigated by means of X-ray diffraction and electron microscopy methods. In combination with thermochemical investigations using (quasi-)ternary phase diagrams it was possible to characterize the corrosion processes. The three ashes revealed a different corrosion behavior, with only few mineral phases being formed. Depending on the tested ceramic, the acidic ash (SA) led to a strong or light damage, respectively. In contrast, the intermediate ash (IA) exhibited a deep infiltration and intense corrosion in terms of dissolution and crystallization (mainly anorthite, CaAl2Si2O8) throughout all tested specimens. The basic ash (BA) infiltrated all samples only to a shallow depth. Nevertheless, a strong corrosion due to the massive crystallization of gehlenite (Ca2Al2SiO7), followed by hibonite (CaAl12O19), was observed. The evaluation of the occurring wear pointed up that the corrosion of alumina and spinel based refractories mainly depends on their micro structure and especially on the composition of the ash/slag. Considering the dissolution and crystallization processes in the isothermal section of the SiO2-CaO-Al2O3-(Na2O/K2O) phase diagram, the experimental findings could be confirmed and the corrosion mechanisms described. By dissolution of Al2O3 from the ceramic matrix the slag is enriched in this component until its composition reaches the stability field of corundum (α-Al2O3), which causes corrosion to stop. The deep infiltration and strong corrosion of IA could be explained by a wide range of pure liquid phase which needed to be traversed on its corrosion path in the ternary phase diagram. This enabled IA to dissolve a lot of Al2O3 before the first crystalline phases were formed. For BA, in contrast, the rapid crystallization of gehlenite increased the slag viscosity and led to a consumption of the remaining melt, which in turn stopped further infiltration. The composition of SA is located in the stability field of anorthite plus liquid, forcing the direct precipitation of this mineral. This, and the proximity to the corundum stability field, led to shallow infiltration in some cases. For other tested samples the infiltration and corrosion was stronger, showing that for SA the availability of Al2O3 and the presence of calcium aluminates or glass phases in the ceramic matrix played an essential role. The elucidation of the corrosion mechanisms of chromia-free, Al2O3-containing refractories showed that, besides an optimized micro structure, the adjustment of the slag chemistry is a further option to minimize infiltration. In the technical processes fluxes (lime stone, CaCO3 or quarz sand, SiO2) can be added or blends of different feed stocks can be used. Long-term exposure tests for 150 h and a field trial of one sample in the SFGT entrained-flow gasifier confirmed the findings of the short-term corrosion tests.:1 Einleitung und Zielstellung 2 Kenntnisstand 2.1 Ausmauerung in Vergasern 2.2 Werkstoffverschleiß und Korrosionstests 2.2.1 Arten des Werkstoffverschleißes 2.2.2 Korrosionstests 2.2.3 Auswertung von Korrosionstests 2.3 Chromhaltige Steine 2.4 Chromfreie Feuerfestwerkstoffe für die Vergasung und Stahlerzeugung 2.4.1 Korundbasierte Steine 2.4.2 Spinellhaltige Steine 3 Proben und Untersuchungsmethodik 3.1 Auswahl der Einsatzstoffe 3.1.1 Keramikproben 3.1.2 Testaschen/-schlacken 3.2 Charakterisierung der Einsatzstoffe und des Korrosionsverhaltens 3.2.1 Röntgenfluoreszenzanalyse 3.2.2 Ascheschmelzverhalten 3.2.3 Röntgendiffraktometrie 3.2.4 Hochtemperatur-Röntgendiffraktometrie 3.2.5 Berechnung der Hochtemperatur-Phasenentwicklung 3.2.6 Lichtmikroskopie 3.2.7 Rasterelektronenmikroskopie 3.3 Korrosionsuntersuchungen und -berechnungen 3.3.1 Kurzzeitauslagerung im thermo-optischen Messsystem 3.3.2 Langzeitauslagerung mittels Tiegeltest 3.3.3 Keramiktest im Flugstromvergaser 3.3.4 Berechnung von Phasendiagrammen 4 Ergebnisse und Diskussion 4.1 Charakterisierung der Aschen 4.1.1 Elementzusammensetzung der Aschen 4.1.2 Ascheschmelzverhalten 4.1.3 Phasenbestand der Aschen 4.1.4 Hochtemperaturverhalten der Aschen 4.2 Ergebnisse der TOMAC-Tests 4.2.1 Lichtmikroskopische Untersuchungen 4.2.2 REM/EDS-Untersuchungen 4.3 Ableitung von Korrosionsmechanismen 4.3.1 Wechselwirkungen mit der sauren Asche 4.3.2 Wechselwirkungen mit der intermediären Asche 4.3.3 Wechselwirkungen mit der basischen Asche 4.3.4 Wechselwirkungen beim Referenz-Chromstein 4.4 Schlussfolgerungen aus den TOMAC-Tests 4.4.1 Zusammenfassung der TOMAC-Ergebnisse 4.4.2 Technische Bedeutung 4.4.3 Vorschlag für eine vereinfachte Untersuchungsmethodik 4.5 Validierung der Ergebnisse 4.5.1 Werkstofftest in der Langzeitauslagerung 4.5.2 Werkstofftest im Flugstromvergaser 5 Zusammenfassung und Ausblick