Co–gasification of coal/biomass blends in 50 kWe circulating fluidized bed gasifier

This paper reports gasification of coal/biomass blends in a pilot scale (50 kWe) air-blown circulating fluidized bed gasifier. Yardsticks for gasification performance are net yield, LHV and composition and tar content of producer gas, cold gas efficiency (CGE) and carbon conversion efficiency (CCE). Net LHV decreased with increasing equivalence ratio (ER) whereas CCE and CGE increased. Max gas yield (1.91 Nm³/kg) and least tar yield (5.61 g/kg of dry fuel) was obtained for coal biomass composition of 60:40 wt% at 800 °C. Catalytic effect of alkali and alkaline earth metals in biomass enhanced char and tar conversion for coal/biomass blend of 60:40 wt% at ER = 0.29, with CGE and CCE of 44% and 84%, respectively. Gasification of 60:40 wt% coal/biomass blend with dolomite (10 wt%, in-bed) gave higher gas yield (2.11 Nm³/kg) and H₂ content (12.63 vol%) of producer gas with reduced tar content (4.3 g/kg dry fuel).     

Biomass gasification in a 100 kWth steam-oxygen blown circulating fluidized bed gasifier: Effects of operational conditions on product gas distribution and tar formation

Biomass gasification is one of the most promising technologies for converting biomass, a renewable source, into an easily transportable and usable fuel. Two woody biomass fuels Agrol and willow, and one agriculture residue Dry Distiller’s Grains with Solubles (DDGS), have been tested using an atmospheric pressure 100 kWth steam-oxygen blown circulating fluidized bed gasifier (CFB). The effects of operational conditions (e.g. steam to biomass ratio (SBR), oxygen to biomass stoichiometric ratio (ER) and gasification temperature) and bed materials on the composition distribution of the product gas and tar formation from these fuels were investigated. Experimental results show that there is a significant variation in the composition of the product gas produced. Among all the experiments, the averaged concentration of H₂ obtained from Agrol, willow and DDGS over the temperature range from 800 to 820 °C was around 24 vol.%, 28 vol.% and 20 vol.% on a N₂ free basis, respectively. A fairly high amount of H₂S (∼2300 ppmv), COS (∼200 ppmv) and trace amounts of methyl mercaptan (<3 ppmv) on a N₂ free basis were obtained from DDGS. Due to a relatively high content of K and Cl in DDGS fuel, an alkali-getter (e.g. kaolin) was added to avoid agglomeration during gasification. Higher temperatures and SBR values were favorable for increasing the mole ratio of H₂ to CO and the tar decomposition but less advantageous for the formation of CH₄. Meanwhile, higher temperatures and SBR values also led to higher gas yields, whereas a higher SBR caused a lower carbon conversion efficiency (CCE%), cold gas efficiency (CGE%) and heating values of the product gas due to a high steam content in the product gas. From solid phase adsorption (SPA) results, the total tar content obtained from Agrol was the highest at around 12.4 g/Nm³, followed by that from DDGS and willow gasification. The lowest tar content produced from Agrol, willow and DDGS using Austrian olivine (Bed 1) as bed materials was 5.7, 4.4 and 7.3 g/Nm³, values which were obtained at a temperature of 730, 820 and 730 °C, SBR of 1.52, 1.14 and 1.10, and ER of 0.36, 0.39 and 0.37, respectively.     

Effect of design and operating parameters on the gasification process of biomass in a downdraft fixed bed: An experimental study

The main objective of this paper is to study the effect of design and operating parameters, mainly reactor geometry, equivalence ratio and biomass feeding rate, on the performance of the gasification process of biomass in a three air stage continuous fixed bed downdraft reactor. The gasification of corn straw was carried out in the gasifier under atmospheric pressure, using air as gasifying agent. The results demonstrated that due to the three stage of air supply, a high and uniform temperature was achieved in the oxidation and reduction zones for better tar cracking. The designing of both the air supply system and rotating grate avoided bridging and channeling. The gas composition and tar yield were affected by the parameters including equivalence ratio (ER) and biomass feeding rate. When biomass feeding rate was 7.5 kg/h and ER was 0.25–0.27, the product gas of the gasifier attained a good condition with lower heating value (LHV) about 5400 kJ/m³ and cold gas efficiency about 65%. An increase in equivalence ratio led to higher temperature which in turn resulted in lower tar yield which was only 0.52 g/Nm³ at ER = 0.32. Increasing biomass feeding rate led to higher biomass consumption rate and process temperature. However, excessively high feeding rate was unbeneficial for biomass gasification cracking and reforming reactions, which led to a decrease in H₂ and CO concentrations and an increase in tar yield. When ER was 0.27, with an increase of biomass feeding rate from 5.8 kg/h to 9.3 kg/h, the lower heating value decreased from 5455.5 kJ/Nm³ to 5253.2 kJ/Nm³ and tar yield increased from 0.82 g/Nm³ to 2.78 g/Nm³.     

Characteristics of cyclone gasification of rice husk

The gasification characteristics of the rice husk were studied in a cyclone gasifier using air as the gasifying medium to generate the fuel gas with available heating value and less tar content. The influence of equivalence ratio on temperature profiles, composition and low heating value of the produced gas, tar content, carbon conversion and cold gas efficiency was investigated. The equivalence ratios considered in this study were 0.20–0.32. The results show that the optimal equivalence ratio is 0.29 and the maximum temperature of gasification should be lower than 1000 °C. In order to optimize the performance of the cyclone gasifier, the main body of the gasifier was lengthened and air staged gasification was carried out. The low heating value of the produced gas, carbon conversion, cold gas efficiency and tar content are 4.72 MJ/Nm³, 57.5%, 37.3% and 1.85 g/Nm³, respectively.     

Improved yield parameters in catalytic steam gasification of forestry residue; optimizing biomass feed rate and catalyst type

The catalytic gasification (900 °C) of forestry industry residue (Eucalyptus saligna) was laboratory-studied. Biomass feed rate and type and amount of catalyst were assayed for their effect on the gasified product composition and the overall energy yield of the gasification reaction. The use of a calcined dolomite catalyst resulted in a combustible gas mixture of adequate calorific power (10.65 MJ m^?3) for use as fuel, but neither the product gas composition nor the energy yield varied significantly with widely different amounts of the catalyst (2 g and 20 g). The use of NiO-loaded calcined dolomite catalysts did not affect the product gas composition significantly but led to a 30% increase in the total product gas volume and to a reduction in the rate of tar and char formation. The catalyst loaded with the smallest amount of NiO studied (0.4 wt%. Ni/Dol) led to the highest energy yield (21.50 MJ kg^?1 on a dry-wood basis) based on the use of the gasified product as fuel. The gasified product was found to have an adequate H₂/CO molar ratio and H₂ content for use as synthesis gas source and partial source of H₂.     

Experimental study on biomass gasification in a double air stage downdraft reactor

This work presents an experimental study of the gasification of a wood biomass in a moving bed downdraft reactor with two-air supply stages. This configuration is considered as primary method to improve the quality of the producer gas, regarding its tar reduction. By varying the air flow fed to the gasifier and the distribution of gasification air between stages (AR), being the controllable and measurable variables for this type of gasifiers, measuring the CO, CH₄ and H₂ gas concentrations and through a mass and energy balance, the gas yield and its power, the cold efficiency of the process and the equivalence ratio (ER), as well as other performance variables were calculated. The gasifier produces a combustible gas with a CO, CH₄ and H₂ concentrations of 19.04, 0.89 and 16.78% v respectively, at a total flow of air of 20 Nm³ h⁻¹ and an AR of 80%. For these conditions, the low heating value of the gas was 4539 kJ Nm⁻³. Results from the calculation model show a useful gas power and cold efficiency around 40 kW and 68%, respectively. The resulting ER under the referred operation condition is around 0.40. The results suggested a considerable effect of the secondary stage over the reduction of the CH₄ concentration which is associated with the decreases of the tar content in the produced gas. Under these conditions the biomass devolatilization in the pyrolysis zone gives much lighter compounds which are more easily cracked when the gas stream passes through the combustion zone.     

Enhanced trace pollutants removal efficiency and hydrogen production in rice straw gasification using hot gas cleaning system

This study investigates the enhancement of tar and trace gaseous pollutants (e.g. hydrogen sulfide (H₂S) and hydrogen chloride (HCl) removal efficiency derived from rice straw gasification using an integrated hot-gas cleaning system. A bubbling fluidized bed gasifier was used by controlling the temperature at 800 °C and equivalence ratio (ER) ranging 0.2 to 0.4. The hot gas cleaning system was operated at 250 °C and designed to combine three types of absorbents including zeolite, calcined dolomite, and activated carbon. Tar, H₂S, and HCl removal efficiency and enhanced hydrogen production were also discussed. The experimental results indicated that light fraction tar removal efficiency was higher than 90% and the overall tar removal efficiency was approximately 70%. In the case of ER 0.4, the syngas tar content was decreased from 71.88 g/Nm³ (without hot gas cleaning system) to 16.53 g/Nm³ (with hot gas cleaning system). The tar removal efficiency is nearly 77% using the hot gas cleaning system. The HCl and H₂S removal efficiency ranged from 94% to 98% and from 80.7% to 83.92%, respectively. In the case of ER 0.3 and with the hot gas cleaning system, the HCl and H₂S concentrations in cleaned syngas gas were less than 40 ppm and 100 ppm, respectively. Meanwhile, the hydrogen concentration of produced gas was also increased from 6.82% to 9.83% with hot gas cleaning system used. It means that the hot gas cleaning system can effectively remove HCl and H₂S from produced gas in gasification, but also it has good potential for improving syngas quality and enhancing gas turbine application in the future.     

Hydrogen-rich syngas production from municipal solid waste gasification through the application of central composite design: An optimization study

This study aims to investigate the influence and interaction of experimental parameters on the production of optimum H₂ and other gases (CO, CO₂, and CH₄) from gasification of municipal solid waste (MSW). Response surface method in assistance with the central composite design was employed to design the fifteen experiments to find the effect of three independent variables (i.e., temperature, equivalence ratio and residence time) on the yields of gases, char and tar. The optimum H₂ production of 41.36 mol % (15.963 mol kg-MSW⁻¹) was achieved at the conditions of 757.65 °C, 0.241, and 22.26 min for temperature, ER, and residence time respectively. In terms of syngas properties, the lower heating value and molar ratio (H₂/CO) ranged between 9.33 and 12.48 MJ/Nm³ and 0.45–0.93. The predicted model of statistical analysis indicated a good fit with experimental data. The gasification of MSW utilizing air as a gasifying agent was found to be an effective approach to recover the qualitative and quantitate products (H₂ and total gas yield) from the MSW.     

Steam and supercritical water gasification of densified canola meal fuel pellets

In this research, canola meal was densified using bio-additives including alkali lignin, glycerol, and l-proline. The fuel pellet's formulation was optimized. The best fuel pellet demonstrated relaxed density and mechanical durability of 1015 kg/m³ and 99.0%, respectively. Synchrotron-based computer tomography technique indicated that lack of water in pellet formulation resulted in a twofold increase in pellet porosity. Thermogravimetric analysis showed that ignition temperature (240 °C) and burn-out temperature (640 °C) for fuel pellet were smaller than those for coal. Impacts of process parameters were evaluated on the quality of the gas product obtained from pellet's steam gasification and hydrothermal gasification. The gasification experiments showed production of untreated syngas with a suitable range of H₂/CO molar ratio (1.3–1.6) using steam gasification. Hydro-thermal gasification produced a larger molar ratio of H₂/CO (1.8–51.2) for the gas product. Modeling of pellet's steam gasification showed an excellent agreement with experimental results of steam gasification.     

Valorisation of vegetable market wastes to gas fuel via catalytic hydrothermal processing

Residues of leek, cabbage and cauliflower from the market places as representatives of lignocellulosic biomass were processed via hydrothermal gasification to produce energy fuel. The experiments were carried out in a batch reactor at temperatures 300, 400, 500 and 600 °C and corresponding pressures varying in the range of 7.5–43 MPa. Natural mineral additives trona, dolomite and borax were used as homogenous catalysts to determine their effects on the gasification. More than 70 wt% of carbon in vegetable residue samples were detected in the gas phase after the hydrothermal gasification process at 600 °C. The addition of trona mineral further promoted the gasification reactions and as a result, less than 5 wt% carbon remained in the solid residue at the same temperature, degrading the biomass samples into gas and liquid products. The fuel gas with the highest calorific value was recorded to be 25.6 MJ/Nm³, from the hydrothermal gasification of cabbage at 600 °C, when dolomite was used as the homogeneous catalyst. The liquid products obtained in the aqueous phase were detected as organic acids, aldehydes, ketones, furfurals and phenols. The gas products were consisted of hydrogen, carbon dioxide, methane, and as minors; carbon monoxide and low molecular weight hydrocarbons (ethane, propane, etc.). Above 500 °C, all biomass samples yielded 50–55 vol% of CH₄ and H₂ while the CO₂ composition was around 40 vol% as the gas product.     

Effect of the type of gasifying agent on gas composition in a bubbling fluidized bed reactor

It is commonly accepted that gasification of coal has a high potential for a more sustainable and clean way of coal utilization. In recent years, research and development in coal gasification areas are mainly focused on the synthetic raw gas production, raw gas cleaning and, utilization of synthesis gas for different areas such as electricity, liquid fuels and chemicals productions within the concept of poly-generation applications. The most important parameter in the design phase of the gasification process is the quality of the synthetic raw gas that depends on various parameters such as gasifier reactor itself, type of gasification agent and operational conditions. In this work, coal gasification has been investigated in a laboratory scale atmospheric pressure bubbling fluidized bed reactor, with a focus on the influence of the gasification agents on the gas composition in the synthesis raw gas. Several tests were performed at continuous coal feeding of several kg/h. Gas quality (contents in H₂, CO, CO₂, CH₄, O₂) was analyzed by using online gas analyzer through experiments. Coal was crushed to a size below 1 mm. It was found that the gas produced through experiments had a maximum energy content of 5.28 MJ/Nm³ at a bed temperature of approximately 800 °C, with the equivalence ratio at 0.23 based on air as a gasification agent for the coal feedstock. Furthermore, with the addition of steam, the yield of hydrogen increases in the synthesis gas with respect to the water–gas shift reaction. It was also found that the gas produced through experiments had a maximum energy content of 9.21 MJ/Nm³ at a bed temperature range of approximately 800–950 °C, with the equivalence ratio at 0.21 based on steam and oxygen mixtures as gasification agents for the coal feedstock. The influence of gasification agents, operational conditions of gasifier, etc. on the quality of synthetic raw gas, gas production efficiency of gasifier and coal conversion ratio are discussed in details.     

Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier

The chemical looping gasification (CLG) of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carriers. The yield of gas, H₂/CO, lower heating value of syngas (LHV), conversion efficiency and performance parameters were analyzed to obtain CLG reaction characterization and optimal reaction conditions. Results showed that when O/C increased from 0.5 to 3.0, the gas production, H₂/CO, CO₂ yield and carbon conversion efficiency gradually increased, while the yield of H₂, CO and CH₄ and LHV gradually decreased. At the same time, a highest gasification efficiency was obtained when O/C was 1.5. As increasing temperature, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, while the yield of H₂, CH₄ and CO₂, H₂/CO and LHV gradually decreased. Sintering and agglomeration was obvious when the temperature was higher than 850 °C. When the reaction time increased from 10 min to 60 min, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, but the yield of H₂, H₂/CO and LHV decreased, among which 30 min was the best reaction residence time. In addition, coprecipitation was the best preparation method among several preparation methods of oxygen carrier. Finally, O/C of 1.5, 800 °C, 30 min and coprecipitation preparation method of oxygen carrier were the optimal parameters to obtain a gasification efficiency of 26.88%, H₂ content of 35.64%, syngas content of 56.40%, H₂/CO ratio of 1.72 and LHV of 12.25 MJ/Nm³.     

Operation of solid oxide fuel cell on biomass product gas with tar levels >10 g Nm−3

This work assesses experimentally the feasibility of feeding a high tar load product gas from biomass gasification to a planar solid oxide fuel cell (SOFC) for renewable electricity generation. The SOFC had a nickel gadolinium-doped ceria anode (Ni-GDC) and the gasifier was a pilot scale circulating fluidized bed, employing hot gas-cleaning to remove particulates, HCl and H₂S. The SOFC operated for several hours on either pre-reformed gas (reduced tar levels < 0.5 g Nm^?3) as well as on high tar-laden wood gas (tar levels > 10 g Nm^?3) i.e. with no pre-reforming of tars. The tests were carried out at low fuel utilization U_f of around 20% at a current density j = 130 mA cm^?2. In all cases stable continuous SOFC performance was established. Post experimental examination of the SOFC showed that the anode was not affected by carbon deposition or other impurity accumulation.     

Steam gasification of Miscanthus X Giganteus with olivine as catalyst production of syngas and analysis of tars (IR, NMR and GC/MS)

Steam gasification of Miscanthus X Giganteus (MXG) at high heating rate in a fluidised bed reactor with the use of olivine as catalyst was investigated. The effects of temperature (815-880 °C) on the yields and the compositions of syngas and tars were determined. The experimental results show that the gas yields and the content of H₂ increase with the temperature, while the yields of tar, char and the content of CO, CO₂ and CH₄ in the product gas decrease. Noteworthy is that about 1.1 m³ of dry gas (at ambient conditions) per kg of dry ash free biomass were obtained with about 46% of H₂ and 24% of CO by volume at 880 °C.The tars composition was determined by FTIR, NMR and GC/MS. The identification of different compounds shows mainly the presence of simple molecules. This may be facilitating the possibility of complete tar reforming process (hot gas cleaning), to improvement of the syngas yield and the decrease of the formation of pollutants.     

Performance evaluation of an innovative 100 kWth dual bubbling fluidized bed gasifier through two years of experimental tests: Results of the BLAZE project

In this work, the results of two years of experimental tests on an innovative dual bubbling fluidized bed gasifier are reported. These are related to the activities of the BLAZE project (Horizon 2020) for the integration of steam biomass gasification and solid oxide fuel cell. Several tests were carried out on the pilot-scale reactor at various operating conditions, and in this work the results are reported in terms of dry gas composition and yield, organic and inorganic contaminants (tar, particulate matter, H₂S). The compact design of the gasifier (a single reactor with two concentric chambers and in-situ hot gas cleaning and conditioning) reduces the heat losses and produces close to nitrogen-free syngas. Preliminary tests using a filter candle filled with conventional catalyst, installed in the freeboard of the gasifier, show that the tar content dropped to about 2 g/Nm³, and the H₂ concentration increased up to 41%_vol,dry.     

Hydrogen production from steam gasification of tableted biomass in molten eutectic carbonates

The steam gasification of tableted biomass for H₂ production in molten salts was investigated under different conditions. The results showed that the ternary molten carbonates (32 wt% Li₂CO₃, 33 wt% Na₂CO₃ and 35 wt% K₂CO₃) acted as heat medium and catalyst in the gasification process. The use of molten salts could significantly increase total gas and H₂ production and simultaneously decrease the concentrations of CO and CH₄ in the product gas, and also decrease the yield of condensable tar. The increase in gasification temperature and mass ratio of steam to biomass (S/B) was beneficial for H₂ production process. However, excessive steam contributed slightly to the increase in H₂ production and largely increased the energy consumption. The optimal S/B ratio was found to be 1.0. The feedstock after tabletting could completely immersed in molten salts, which improved the contact between biomass and molten salts and thus favored the biomass gasification for H₂ production. When biomass particle size was 0.25 g/piece, the yield of H₂ reached 807.53 mL/g biomass.     

100 t/a-Scale demonstration of direct dimethyl ether synthesis from corncob-derived syngas

Direct conversion of biomass-derived syngas (bio-syngas) to dimethyl ether (DME) at pilot-scale (100 t/a) was carried out via pyrolysis/gasification of corncob. The yield rate of raw bio-syngas was 40–45 Nm³/h with less than 20 mg/Nm³ of tar content when the feedrate of dried corncob was 45–50 kg/h. After absorption of O₂, S, Cl by a series of absorbers and partial removal of CO₂ by the pressure-swing adsorption (PSA) unit sequentially, the obtained bio-syngas (H₂/CO≈1) was directly synthesized to DME over Cu/Zn/Al/HZSM-5 catalyst in the fixed-bed tubular reactor. CO conversion and DME space-time yield (STY) were 67.7% and 281.2 kg/m_cat³/h respectively at 260 °C, 4.3 MPa and 3000 h^?1(GHSV, syngas hourly space velocity). Synthesis performance would be increased if the tail gas (H₂/CO > 2) was recycled to the reactor when GHSV was 650–3000 h^?1.     

Using chemical looping gasification with Fe2O3/Al2O3 oxygen carrier to produce syngas (H2+CO) from rice straw

The chemical looping gasification of rice straw using Fe₂O₃/Al₂O₃ as oxygen carrier was studied at reaction time of 5–25 min, steam-to-biomass (S/B) ratio of 2.0–4.8, reaction temperature of 750–950 °C, and oxygen carrier-to-biomass of 1.0. The gasification can be regarded completed in 20-min reaction. There exist an optimal S/B ratio of 2.8 and reaction temperature of 900 °C leading to maximum performances yielded are 1.22 Nm³/kg gas yield at 54.6% H₂+24.2% CO. The studied Fe₂O₃ oxygen carrier/rice straw is a feasible platform for syngas production from an agricultural waste.     

Hydrogen-rich gas production from biomass air and oxygen/steam gasification in a downdraft gasifier

Biomass gasification is an important method to obtain renewable hydrogen. However, this technology still stagnates in a laboratory scale because of its high-energy consumption. In order to get maximum hydrogen yield and decrease energy consumption, this study applies a self-heated downdraft gasifier as the reactor and uses char as the catalyst to study the characteristics of hydrogen production from biomass gasification. Air and oxygen/steam are utilized as the gasifying agents. The experimental results indicate that compared to biomass air gasification, biomass oxygen/steam gasification improves hydrogen yield depending on the volume of downdraft gasifier, and also nearly doubles the heating value of fuel gas. The maximum lower heating value of fuel gas reaches 11.11 MJ/N m³ for biomass oxygen/steam gasification. Over the ranges of operating conditions examined, the maximum hydrogen yield reaches 45.16 g H₂/kg biomass. For biomass oxygen/steam gasification, the content of H₂ and CO reaches 63.27–72.56%, while the content of H₂ and CO gets to 52.19–63.31% for biomass air gasification. The ratio of H₂/CO for biomass oxygen/steam gasification reaches 0.70–0.90, which is lower than that of biomass air gasification, 1.06–1.27. The experimental and comparison results prove that biomass oxygen/steam gasification in a downdraft gasifier is an effective, relatively low energy consumption technology for hydrogen-rich gas production.