Promoting effect of biomass ash additives on high-temperature gasification of petroleum coke: Reactivity and kinetic analysis

The effect of biomass ash (rice straw ash (RSA) and cotton straw ash (CSA)) on the gasification reactivity and the evolution of physicochemical structure of petcoke char was investigated. The catalytic effect of CSA was significantly higher than that of RSA, and the catalytic effect of biomass ash was enhanced at lower gasification temperature and for higher blending ratio of biomass ash. The promoting effect of biomass ash was related to the increase of active AAEM content, the decrease of order degree of carbon structure and the development of surface structure in char gasification after biomass ash addition, which was more significant for CSA, at lower temperature and for higher blending ratio. Moreover, the shrinking core model was suitable for char gasification, and the additions of RSA and CSA reduced the activation energy of petcoke char gasification from 199.84 kJ mol⁻¹ to 159.85 kJ mol⁻¹ and 62.75 kJ mol⁻¹, respectively.     

Synergistic effects during co-pyrolysis of biomass and plastic: Gas,tar, soot,char products and thermogravimetric study

Synergistic effects of biomass and plastic co-pyrolysis on gas, tar, soot and char production and pyrolysis kinetics were studied using a fixed-bed reactor and a thermogravimetric analyzer, respectively. These pyrolysis products' yields and compositions were measured during the individual pyrolysis of biomass and plastic at 800–1100 °C, and synergistic effects were explored under non-sooty (900 °C) and sooty (1100 °C) conditions. Results shows that the soot starts to form from tar at 900–1000 °C for both biomass and plastic and that the soot from plastic pyrolysis is of greater yield and size than the biomass pyrolysis. Under non-sooty conditions, the synergistic effect of co-pyrolysis results in higher char yields but lower tar yields, while under sooty conditions co-pyrolysis inhibits the gas and soot formation, resulting in higher tar yields and different soot morphologies. The synergistic effect observed by the thermogravimetric analysis agrees with that in a fixed-bed reactor.     

Co-gasification reactivity of petroleum coke with coal and coal liquefaction residue

In this work, experimental study on co-gasification of petroleum coke with coal and coal liquefaction residue (CLR) under CO₂ atmosphere was investigated in a temperature-programmed reactor. The effect of ashes from coal/CLR and metallic oxide on gasification reactivity of petroleum coke was also investigated. The calculated and experimental gasification conversion was compared to analysis the synergetic effect during co-gasification. The results indicated that the petroleum coke has a much lower reactivity than that of coal/CLR chars, while it can be greatly improved by co-gasifying with coal/CLR. The synergetic effect on co-gasification of petroleum coke with coal/CLR was presented as the progress of co-gasification reaction. It was found that the synergetic effect can be enhanced with increasing coal proportion. However, as the increase of coal rank, the synergetic effect gradually weakened during the co-gasification process. In the selected coal/CLR samples, the low rank coal and CLR exhibited a better effect when co-gasifying with petroleum coke. The difference of synergetic effect was probably attributed to the catalytic effect of mineral matters in different coal/CLR. According to composition analyses of various ashes and the co-gasification reactivity of samples, it can be concluded that high content of active components such as Ca- and Fe-in coal/CLR was beneficial to co-gasifying with petroleum coke, while high content of inert Si- and Al- components in coal tended to resist the reaction.     

From coal towards renewables: Catalytic/synergistic effects during steam co-gasification of switchgrass and coal in a pilot-scale bubbling fluidized bed

Recent environmental sharp curbs on fossil fuel energy systems such as coal power plants due to their greenhouse gas emissions have compelled industries to include renewable fuels. Biomass/coal co-gasification could provide a transition from energy production based on fossil fuels to renewables. A low-ash coal and switchgrass rich in potassium were selected on the basis of previous thermogravimetric studies to steam co-gasify 50:50 wt% coal:switchgrass mixtures in a pilot scale bubbling fluidized bed reactor with silica sand as the bed material at ∼800 and 860 °C and 1 atm. With the switchgrass added to coal, the hydrogen and cold gas efficiencies, gas yield and HHV of the product gas were enhanced remarkably relative to single-fuel gasification. The product gas tar yield also decreased considerably due to decomposition of tar catalyzed by switchgrass alkali and alkaline earth metals. Switchgrass ash therefore can act as inexpensive natural catalysts for steam gasification and assist in operating at lower temperatures without being penalized by an increase in product tar yield. An equilibrium model over-predicted hydrogen and under-predicted methane concentrations. However, an empirically kinetically-modified model was able to predict the product gas compositions accurately.     

Co-pyrolysis and co-gasification of biomass and polyethylene: Thermal behaviors,volatile products and characteristics of their residues

Co-pyrolysis and co-gasification of biomass and plastics could be a promising method to alleviate environmental pollution and provide renewable energy. In this paper, co-pyrolysis and co-gasification of eucalyptus wood (EW) or rice straw (RS) with polyethylene (PE) were investigated by a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer (TG-FTIR) and a scanning electron microscope coupled with energy-dispersive spectroscopy (SEM/EDS). Results showed that the pyrolysis behaviors were characterized by two stages. The first stage was the decomposition of EW and RS, and the second stage primarily consisted of the degradation of PE. The gasification exhibited a third stage for the reaction of products with CO₂. A synergistic effect was presented in the pyrolysis and gasification of biomass with PE, and it could have a positive effect on the decomposition of biomass. Compared to individual pyrolysis and gasification, co-pyrolysis and co-gasification generated no new substances, but the yield of some products was changed in these processes. In co-pyrolysis, the decomposition of biomass was promoted. In co-gasification, the production of CH₄, CO and oxygenated compounds was inhibited, while the reaction to generate H₂O was promoted. Gasification and the addition of PE both increased the carbon content and reduced the oxygen content of volatile products. Additionally, more metal elements could be deposited in residues when PE was added.     

Roles of AAEMs in catalytic reforming of biomass pyrolysis tar and coke accumulation characteristics over biochar surface for H2 production

Coke formation is a significant challenge in catalytic tar reforming. AAEMs are essential in the conversion and decomposition of tar catalyzed by biochar. In this paper, four biochar catalysts with different K and Ca contents were prepared by acid washing and loading, and the coke accumulation characteristics in catalytic tar reforming at 650 °C were investigated using a single-stage reaction system. The gas-liquid-solid products were characterized by GC-MS, Raman, N₂ adsorption, FTIR and TG. The results suggest that K-loaded biochar has a maximum tar reforming capacity of 94.9%, while H-form biochar has a tar removal efficiency of only 27.8%. The micropore area in biochar is considerably reduced and the average pore size is increased after coke deposition. While K-loaded biochar retains the highest micropore area, it also exhibits a smaller increase in average pore size. The loading of K/Ca affects the growth structure of the coke, resulting in an increased number of O-containing structures in it. The coke on the Raw biochar surface is mainly small aromatic ring structures and aliphatic structures, thus increasing the intensity of the vibrational peaks corresponding to aromatic = C–H and aliphatic C–H on it. The coke on K-loaded biochar has a large proportion of aliphatic structures, which also contributes to the reduced graphitization of it after reforming. The AAEMs-free biochar surface preferentially removes tar components carrying O-containing groups. K-loaded biochar preferentially catalyzes the reforming of mono-aromatic ring components in tar. Ca-loaded biochar preferentially removes the mono-aromatic ring components, while being less selective for the removal of tar components containing hydroxyl groups and polyaromatic ring components. The loading of K/Ca promotes the dehydrogenation of the tar fraction during reforming, while only K catalyzes the deoxygenation of tar components. H-form biochar has no appreciable catalytic activity on CH₄ cracking. AAEMs have a catalytic activity on CH₄ cracking. K is particularly effective in improving tar conversion and hydrogen production of biochar.     

Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass

Torrefaction is a thermal pretreatment process for biomass where raw biomass is heated in the temperatures of 200-300 °C under an inert or nitrogen atmosphere. The main constituents contained in biomass include hemicellulose, cellulose and lignin; therefore, the thermal decomposition characteristics of these constituents play a crucial role in determining the performance of torrefaction of lignocellulosic materials. To gain a fundamental insight into biomass torrefaction, five basic constituents, including hemicellulose, cellulose, lignin, xylan and dextran, were individually torrefied in a thermogravimetry. Two pure materials, xylose and glucose, were torrefied as well for comparison. Three torrefaction temperatures of 230, 260 and 290 °C, corresponding to light, mild and severe torrefactions, were taken into account. The experiments suggested the weight losses of the tested samples could be classified into three groups; they consisted of a weakly active reaction, a moderately active reaction and a strongly active reaction, depending on the natures of the tested materials. Co-torrefactions of the blend of hemicellulose, cellulose and lignin at the three torrefaction temperatures were also examined. The weight losses of the blend were very close to those from the linear superposition of the individual samples, suggesting that no synergistic effect from the co-torrefactions was exhibited.     

Co‐gasification performance of coal and petroleum coke blends in a pilot‐scale pressurized entrained‐flow gasifier

Co‐gasification performance of coal and petroleum coke (petcoke) blends in a pilot‐scale pressurized entrained‐flow gasifier was studied experimentally. Two different coals, including a subbituminous coal (Coal A) and a bituminous coal (Coal B), individually blended with a petcoke in the gasifier were considered. The experimental results suggested that, when the petcoke was mixed with Coal A over 70%, the slagging problem, which could shorten the operational period due to high ash content in the coal, was improved. It was found that increasing O₂/C tended to decrease the syngas concentration and better operational conditions of O₂/C were between 0.6 and 0.65 Nm³ kg^?1. For the blends of Coal B and the petcoke, the slagging problem was encountered no more, as a result of low ash content in both Coal B and the petcoke. The better co‐gasification performance could be achieved if the blending ratio of the two fuels was 50%, perhaps resulting from the synergistic effect of the blends. With the aforementioned blending ratio, the optimal condition of O₂/C was located at around 0.65 Nm³ kg^?1. The co‐gasification was also simulated using Aspen Plus. It revealed that the simulation could provide a useful insight into the practical operation of co‐gasification. Copyright © 2011 John Wiley & Sons, Ltd.     

Numerical simulation of the synergistic effects of unwanted combustion enhancement by the C3H2F3Br and C2HF5 blends

Owing to the restriction of using Halon 1301 (CF₃Br) for fire suppression, several alternatives to Halon 1301 have been developed, including C₂HF₅ (HFC-125), C₃H₂F₃Br (2-BTP), and C₆F₁₂O (Novec1230). However, in the Federal Aviation Administration (FAA) Aerosol Can Explosion Test (FAA-ACET), it was found that these alternatives did not suppress lean flames at sub-inert concentrations, but promoted combustion, eventually leading to overpressure. Therefore, they have not been successfully applied in aircraft cargo compartments. Herein, different blend ratios of C₃H₂F₃Br and C₂HF₅ were used to explore their inhibitory effects on combustion enhancement under lean combustion conditions. A chemical kinetic model was developed and validated using a one-dimensional free-propagation flame simulator. The laminar burning velocity predicted by the model was consistent with the experimental results. The adiabatic flame temperature and overall reaction rate were determined using thermodynamic equilibrium calculations and perfectly stirred reactor (PSR) simulations. By comparing the blend inhibitors with different blend ratios, it was found that the blend of C₃H₂F₃Br and C₂HF₅ at blend ratios of 25/75 and 50/50 effectively reduced the total heat release and system reactivity. In addition, the blend inhibitor not only weakened the fuel properties of C₂HF₅, but also further enhanced the bromine-catalysed radical recombination cycle. Notably, a new reaction occurred when C₃H₂F₃Br and C₂HF₅ were blended into the FAA-ACET chamber: Br + CHF₂CF₃ = HBr + CF₃-CF₂, indicating that the Br atoms promoted the decomposition of C₂HF₅.     

The intrinsic reactivity of coal char conversion compared under different conditions of O2/CO2, O2/H2O and air atmospheres

The reasons for the intrinsic reactivity differences in coal char conversion under an O₂/H₂O atmosphere compared with that under an O₂/CO₂ or O₂/N₂ atmosphere have been investigated in a thermogravimetric analyzer by a simple variable activation energy (SVAE) method combined with an adsorption/desorption reaction mechanism. The results show that only CO₂ or H₂O chemisorption occurred in the non-isothermal experiments, not gasification; however, the intrinsic reaction rate (IRR) of coal char conversion at the same O₂ concentration still increases in an orderly manner under O₂/CO₂, O₂/N₂ and O₂/H₂O atmospheres. This result is due to the different chemisorption mechanisms of CO₂ and H₂O, namely, the production of C(CO), C(OH) and C(H) from CO₂ and H₂O chemisorption. At the same O₂ concentration, the trends and magnitudes of variable activation energies for coal char combustion under O₂/CO₂ and O₂/N₂ atmospheres are similar, while they are very different from those under O₂/H₂O conditions. Therefore, CO₂ has little influence on the reactivity, while H₂O changes the reactivity. In addition, according to the developed reaction mechanism, it is concluded that the SVAE method contributes to the characteristic intrinsic reactivity of coal char conversion under different atmospheres.     

Main issues of the impact of tar,H2S,HCl and alkali metal from biomass-gasification derived syngas on the SOFC anode and the related gas cleaning technologies for feeding a SOFC system: A review

Biomass gasification has acquired considerable interest as a sustainable and environmentally friendly way to produce heat, hydrogen or electricity from agro-industrial wastes or other kinds of biomass. A very effective solution for the achievement of high electrical efficiency (up to 55%) is the integration of biomass gasification with solid oxide fuel cell (SOFC) technology, including the necessary gas cleanup to avoid degradation of the SOFC. For this reason, this paper first shows in detail the risk and the impact of carbon deposition, exposure to tar, hydrogen sulphide, hydrogen chloride and alkali metals on SOFC anode and then, considering the tolerance limit for inorganic and organic contaminants (<1 ppmv for H₂S, HCl and alkali and <10 ppmv for tar), offers an overview of the most relevant and effective technologies to remove these contaminants and to feed safely a SOFC system.     

Chemical effects of CO2 addition to oxidizer and fuel streams on flame structure in H2–O2 counterflow diffusion flames

Numerical simulation of CO₂ addition effects to fuel and oxidizer streams on flame structure has been conducted with detailed chemistry in H₂–O₂ diffusion flames of a counterflow configuration. An artificial species, which displaces added CO₂ in the fuel- and oxidizer-sides and has the same thermochemical, transport, and radiation properties to that of added CO₂, is introduced to extract pure chemical effects in flame structure. Chemical effects due to thermal dissociation of added CO₂ causes the reduction flame temperature in addition to some thermal effects. The reason why flame temperature due to chemical effects is larger in cases of CO₂ addition to oxidizer stream is well explained though a defined characteristic strain rate. The produced CO is responsible for the reaction, CO₂+H=CO+OH and takes its origin from chemical effects due to thermal dissociation. It is also found that the behavior of produced CO mole fraction is closely related to added CO₂ mole fraction, maximum H mole fraction and its position, and maximum flame temperature and its position. Copyright © 2003 John Wiley & Sons, Ltd.     

Impacts of char structure evolution and inherent alkali and alkaline earth metallic species catalysis on reactivity during the coal char gasification with CO2/H2O

In this work, we studied the effects of char structural evolution and alkali and alkaline earth metallic species (AAEMs) catalysis on the reactivity during the char gasification with CO₂, H₂O, and their mixture. The gasified chars with different carbon conversion levels were prepared, and their physicochemical structures were characterized via nitrogen adsorption and FT‐Raman techniques. The concentrations of AAEMs in different modes were obtained by the sequential chemical extraction method. The reactivities of the raw and gasified chars were analyzed by the thermogravimetric analysis. The gasification atmospheres had varied effects on the physicochemical structure of coal char. The gasified char obtained in the CO₂ atmosphere had a lower aromatic condensation degree compared with that obtained in the H₂O atmosphere, irrespective of the temperature. The impact of the atmospheres on the specific surface area of the char varied with the temperature because H₂O and CO₂ have different routes of development of pore structure with coal char. A large specific surface area facilitates the exposure and dispersion of more AAEMs on the surface of the channel, which is conducive to their contact with the gasification agent to play the catalytic role. Thus, the reactivity of the gasified char is well correlated with its specific surface area at different gasification temperatures. In the absence of AAEMs, the chemical structure of coal char becomes the dominant factor affecting the reactivity.     

Identifying the roles of MFe2O4 (M=Cu,Ba, Ni,and Co) in the chemical looping reforming of char,pyrolysis gas and tar resulting from biomass pyrolysis

Biomass chemical looping gasification (BCLG), which employs oxygen carriers (OCs) as the gasification agent, is drawing more attention for its low cost and environmental friendliness. However, the complex products of biomass pyrolysis and the reactions between OCs and the pyrolysis products constrain its development. In this study, MFe₂O₄ (M = Cu, Ba, Ni and Co) ferrites synthesized via the sol-gel method were investigated as OCs in BCLG for hydrogen-rich syngas production. The properties of the as-prepared and spent OCs were characterized by X-ray diffraction (XRD), H₂-temperature programmed reduction (TPR), scanning electron microscopy (SEM), and automatic surface area porosimetry (BET). The three-phase products (char, pyrolysis gas and toluene) derived from biomass pyrolysis were employed as the reactants to investigate the reactivity of the ferrites. Then, BCLG experiments using biomass were conducted on the four ferrites to further determine their performance. The characterization results suggested that the four ferrites are all attractive for the chemical looping process, exhibiting good oxygen transferability and wide distributions of metal cations because of their metal synergistic effects in the spine structure. Reactions with pyrolysis gas and biomass char indicated that BaFe₂O₄ has a higher reactivity via a solid-solid reaction but a lower reactivity with pyrolysis gas, which make it very favorable for the production of hydrogen-rich syngas. Furthermore, BaFe₂O₄ showed excellent performance for toluene catalytic cracking with small amounts of carbon deposition. The synergetic effects between Ba and Fe metals considerably enhanced selective oxidation to produce 26.72% more H₂ than CoFe₂O₄ and 13.79% more H₂ than NiFe₂O₄ and CuFe₂O₄ for biomass gasification. The hydrogen yield produced by BaFe₂O₄ with the assistance of steam for biomass gasification can reach 41.8 mol/kg of biomass.     

Effect of biomass leaching on H2 production,ash and tar behavior during high temperature steam gasification (HTSG) process

The effect of biomass water leaching on H₂ production, as well as, prediction of ash thermal behavior and formation of biomass tar during high temperature steam gasification (HTSG) of olive kernel is the main aim of the present work. Within this study raw olive kernel samples (OK₁, OK₂) and a pre-treated one by water leaching (LOK₂) were examined with regard to their ash fouling propensity and tar concentration in the gaseous phase. Two temperatures (T = 850 and 950 °C) and a constant steam to biomass ratio (S/B = 1.28) were chosen in order to perform the steam gasification experiments. Results indicated that considering the samples' ash thermal behavior, it seemed that water leaching improved the fusibility behavior of olive kernel; however, it proved that water leaching does not favour tar steam reforming, while at the same time decreases the H₂ yield in gas product under air gasification conditions, due to possible loss of the catalytic effect of ash with water leaching.     

The effects of Fe2O3 catalyst on the conversion of organic matter and bio-fuel production during pyrolysis of sewage sludge

In this paper, the pyrolysis treatment of sewage sludge is studied in a fixed bed reactor at temperatures range of 400–600 °C. Meanwhile, the catalytic effect of Fe₂O₃ on the characteristics of the resulting gases, bio-oil and bio-char are also investigated. The experimental results indicate that the yields of gases and bio-oil respectively increase from 8.69 wt% and 32.54 wt% to 11.62 wt% and 38.74 wt%, and the char yield decreases from 58.77 wt% to 49.64 wt% during Fe-embedded sewage sludge pyrolysis when Fe₂O₃ is added equal to 5% in the dried sewage sludge. Meanwhile, Fe₂O₃ promotes the CO and H₂ formation and inhibits the CH₄ formation, while it exhibits no significantly effect on the composition of the bio-oil. Moreover, the bio-oil should be direct combustion for power generation due it contains higher oxygenated hydrocarbons. In addition, the bio-char exhibits good desulfurization activity.