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.     

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.     

Gasification performance of olive pomace in updraft and downdraft fixed bed reactors

This study aims to investigate the gasification potential of olive pomace with using different fixed-bed gasifier systems. Olive pomace as a dried form was supplied from a chemical industry plant working on olive oil soap, located in Izmir, Türkiye. After a complete characterization of olive pomace, gasification experiments by using fixed bed reactor systems were done at three different gasifier temperatures as 700, 800 and 900 °C. As a gasification agent, dry air was used with four different flowrates (0.4, 0.2, 0.1, 0.05 L/min) while pure oxygen experiments were carried out with a flow rate of 0.01 L/min. Syngas with H₂ content of 48% and 45% (volumetric) were obtained in updraft and downdraft gasifiers, respectively, by using dried air as a gasifying agent. Heating value of syngas was around 12.4 MJ/Nm³. In the pure oxygen atmosphere, H₂ contents of the syngas were measured as 53% and 39%vol. In the updraft and downdraft gasifiers. This paper presents the research results on the olive pomace gasification study as a part of a large-scale research project and discuss them in the context of hydrogen production from the fixed bed reactors.     

Evaluation of sorbents for high temperature removal of tars,hydrogen sulphide,hydrogen chloride and ammonia from biomass-derived syngas by using Aspen Plus

Biomass gasification is a promising technology to produce secondary fuels or heat and power, offering considerable advantages over fossil fuels. An important aspect in the usage of producer gas is the removal of harmful contaminants from the raw syngas. Thus, the object of this study is the development of a simulation model for a gasifier including gas clean-up, for which a fluidized-bed gasifier for biomass-derived syngas production was considered, based on a quasi-equilibrium approach through Gibbs free energy minimisation, and including an innovative hot gas cleaning, constituted by a combination of catalyst sorbents inside the gasification reactor, catalysts in the freeboard and subsequent sorbent reactors, by using Aspen Plus software. The gas cleaning chain simulates the raw syngas clean-up for several organic and inorganic contaminants, i.e. toluene, benzene, naphthalene, hydrogen sulphide, hydrogen chloride and ammonia. The tar and inorganic contaminants final values achieved are under 1 g/Nm³ and 1 ppm respectively.     

Hydrogen gas yield and trace pollutant emission evaluation in automotive shredder residue (ASR) gasification using prepared oyster shell catalyst

This work examines the hydrogen gas yield and trace pollutants partitioning in automobile shredder residue (ASR) catalytic gasification by fixed bed and fluidized bed gasifier with controlling at equilibrium ratio (ER) 0.2, temperature 900 °C, and 5%–15% prepared catalyst addition. Oyster shell (OS) is a valuable resource due to its higher calcium content that it could prepare as a catalyst for enhancing the hydrogen production in ASR gasification. In the case of the fixed bed gasifier experiments, the highest lower heating value (LHV) and syngas production were found at 900 °C and 10% OS catalyst addition. The maximum H₂ and CO composition were 6.57% and 5.97%, respectively. The LHV of syngas was approximately 4.43 MJ/Nm³. The fluidized bed gasifier could provide a good ASR decomposition and heat transfer behavior. The syngas yield results indicated the maximum H₂ and CO composition were 12.12% and 10.59%, respectively. It was obviously showed that the syngas production and energy conversion efficiency were enhanced by applying fluidized bed gasifier. The maximum produced gas LHV was 10.77 MJ/Nm³ as well as the cold gas efficiency (CGE) of produced gas was 71.62%. On the other hand, the volatile sulfur and chlorine speciation formed in ASR gasification were mainly partitioned in the solid and/or liquid phase. It implied that tested OS catalysts could inhibit the volatile sulfur and chlorine speciation emission in the produced gas as well as enhance the produced gas quality. In summary, this research could provide basic insight into enhanced syngas production and quality in ASR catalytic gasification using the prepared OS catalyst.     

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.     

Entrained flow gasification of straw- and wood-derived pyrolysis oil in a pressurized oxygen blown gasifier

Fast pyrolysis oil can be used as a feedstock for syngas production. This approach can have certain advantages over direct biomass gasification. Pilot scale tests were performed to investigate the route from biomass via fast pyrolysis and entrained flow gasification to syngas. Wheat straw and clean pine wood were used as feedstocks; both were converted into homogeneous pyrolysis oils with very similar properties using in-situ water removal. These pyrolysis oils were subsequently gasified in a pressurized, oxygen blown entrained flow gasifier using a thermal load of 0.4 MW. At a pressure of 0.4 MPa and a lambda value of 0.4, temperatures around 1250 °C were obtained. Syngas volume fractions of 46% CO, 30% H₂ and 23% CO₂ were obtained for both pyrolysis oils. 2% of CH₄ remained in the product gas, along with 0.1% of both C₂H₂ and C₂H₄. Minor quantities of H₂S (3 vs. 23) cm³ m⁻³, COS (22 vs. 94) cm³ m⁻³ and benzene (310 vs. 532) cm³ m⁻³ were measured for wood- and straw derived pyrolysis oils respectively. A continuous 2-day gasification run with wood derived pyrolysis oil demonstrated full steady state operation. The experimental results show that pyrolysis oils from different biomass feedstocks can be processed in the same gasifier, and issues with ash composition and melting behaviour of the feedstocks are avoided by applying fast pyrolysis pre-treatment.     

Emission characteristics of vehicles fueled by hydrogen-enriched syngas under no-load condition

The emission characteristics of vehicles fueled by hydrogen-enriched syngas were investigated under the no-load condition. Syngas was produced using a fixed-bed gasification system. A Jetta car was modified and used for emission experiments with engine speed between 800 and 4800 rpm. Syngas compositions and emissions were analyzed using gas chromatography and a SPX-Tecnotest 488 exhaust gas analyzer, respectively. The results showed that syngas composition had significant effect on HC and NOx emission and air ratio (λ) was the key factor affecting CO emission. HC and NOx emissions increased significantly from 6 to 16 ppm and 139–832 ppm, respectively, when engine speed increased from 800 to 4800 rpm. CO emission did not significantly increase (lower than 0.5%) at engine speed less than 3200 rpm; at higher than 4800 rpm engine speed, CO emission increased rapidly (up to 2.7%).     

Hydrogen-rich gas production from biomass steam gasification in an updraft fixed-bed gasifier combined with a porous ceramic reformer

This paper investigates the hydrogen-rich gas produced from biomass employing an updraft gasifier with a continuous biomass feeder. A porous ceramic reformer was combined with the gasifier for producer gas reforming. The effects of gasifier temperature, equivalence ratio (ER), steam to biomass ratio (S/B), and porous ceramic reforming on the gas characteristic parameters (composition, density, yield, low heating value, and residence time, etc.) were investigated. The results show that hydrogen-rich syngas with a high calorific value was produced, in the range of 8.10–13.40 MJ/Nm³, and the hydrogen yield was in the range of 45.05–135.40 g H₂/kg biomass. A higher temperature favors the hydrogen production. With the increasing gasifier temperature varying from 800 to 950 °C, the hydrogen yield increased from 74.84 to 135.4 g H₂/kg biomass. The low heating values first increased and then decreased with the increased ER from 0 to 0.3. A steam/biomass ratio of 2.05 was found as the optimum in the all steam gasification runs. The effect of porous ceramic reforming showed the water-soluble tar produced in the porous ceramic reforming, the conversion ratio of total organic carbon (TOC) contents is between 22.61% and 50.23%, and the hydrogen concentration obviously higher than that without porous ceramic reforming.     

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.     

First and second law thermodynamic analysis of a single and dual-stage ignition biomass downdraft gasifier

The dual-stage ignition biomass downdraft gasifier is an enormous tar reduction technology as against a single-stage ignition biomass gasification. Exergetic analysis of the system guides toward a possible performance enhancement. In dual-stage gasification, around 67.76% of input exergy is destructed in the several components, while 9.16% is obtained as a useful exergy output and 24.34% is found to be as a useful energy output there. The entire unit was assessed with a progressively rising electric load from 15.24 kW to 38.86 kW. The enhanced producer gas quality comes from 57% combustible gas with a higher heating value of 6.524 MJ/Nm³ and tar content of 7 mg/Nm³ after the paper filter, whereas the biomass consumption rate is 58 kg/h at the greatest load with the grate temperature of 1310–1370 °C. The samples of exhaust gas emissions are obtained environmentally favorable. The results even described that the dual-stage ignition biomass downdraft gasifier has significantly greater energetic and exergetic efficiency as compared to the single-stage gasifier.     

Co-gasification between coal/sawdust and coal/wood pellet: A parametric study using response surface methodology

This study had compared raw biomass and pre-treated biomass co-gasified with coal with the aim of investigating the reliability of pre-treated biomass for enhancing gasification performance. Sawdust (SD) and wood pellet (palletisation form of sawdust - WP) and blends of these two feedstocks with sub-bituminous coal (CL), were gasified in an air atmosphere using an external heated fixed-bed downdraft gasifier system. Response surface methodology (RSM) incorporating the central composite design (CCD) was applied to assist the comparison of all operating variables. The three independent variables were investigated within a specific range of coal blending ratios from 25% to 75%, gasification temperature from 650 °C to 850 °C and equivalence ratio from 0.20 to 0.30 against the dependent variables, namely the H₂/CO ratio and higher heating value of the syngas (HHVsyngas). The results revealed the H₂/CO ratio and a higher heating value of the syngas of more than 1.585 and 6.072 MJ/Nm³, respectively. Findings also showed that the H₂/CO ratio in the syngas from CL/WP possessed a higher value than the CL/SD. In contrast, CL/SD possessed a higher heating value for syngas with about 1% difference compared to the CL/WP. Therefore, co-gasified coal with wood pellets could potentially be a substitute for sawdust.     

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).     

Experimental analysis of air-steam gasification of biomass with coal-bottom ash

Biomass gasification is an attractive option for producing high-quality syngas (H₂+CO) due to its environmental advantages and economic benefits. However, due to some technical problems such as tar formation, biomass gasification has not yet been able to achieve its purpose. The purpose of this work was to study the catalytic activity of coal-bottom ash for fuel gas production and tar elimination. Effect of gasification parameters including reaction temperature (700–900 °C), equivalence ratio, EQR (0.15–0.3) and steam-to-biomass ratio, SBR (0.34–1.02) and catalyst loading (5.0–13 wt %) on gas distribution, lower heating value (LHV) of gas steam, tar content, gas yield and H₂/CO ratio was studied. The tar content remarkably decreased from 3.81 g/Nm³ to 0.97 g/Nm³ by increasing char-bottom ash from 5.0 wt% to 13.0 wt%. H₂/CO significantly increased from 1.12 to 1.54 as the char-bottom ash content in the fuel increased from 5.0 wt% to 13.0 wt%.     

Thermal conversion efficiency of producing hydrogen enriched syngas from biomass steam gasification

This paper presents the results from an experimental study on the energy conversion efficiency of producing hydrogen enriched syngas through uncatalyzed steam biomass gasification. Wood pellets were gasified using a 100 kW_th fluidized bed gasifier at temperatures up to 850 °C. The syngas hydrogen concentration and cold gas efficiency were found to increase with both bed temperature and steam to biomass weight ratio, reaching a maximum of 51% and 124% respectively. The overall energy conversion to syngas (based on heating value) also increased with bed temperature but was inversely proportional to the steam to biomass ratio. The maximum energy conversion to syngas was found to be 68%. The conversion of energy to hydrogen (by heating value) increased with gasifier temperature and gas residence time, but was found to be independent of the S/B ratio. The maximum conversion of all energy sources to hydrogen was found to be 25%.     

Simulation and economic evaluation of biomass gasification with sets for heating,cooling and power production

In this work, syngas was used directly as fuel source for the renewable CCHP system, which can be producted through biomass gasification process. The advantages and limitation of entrained flow gasifier are compared, followed by discussion on the key parameters that are critical for the optimum production of syngas. Gasification agent of 450 °C temperature and 30 atm pressure has been proposed as a optical solution to a entrained flow gasifier using air as gasification agent at 0.27 ER (oxygen equivalence ratio), in that it provides a syngas of 5.665 MJ/m³ LHV and up to 77% gasification efficiency. Depending on the key parameters of gasification process, the properties of syngas produced can be varied. It is thus essential to thoroughly understand the cogeneration system to identify the suitable methods for a renewable CCHP system. These process was simulated using Aspen Plus to perform the rigorous material and energy balances. The results obtained from simulation and experiment agreed well. This paper later focused on economic evaluation of the entire process, as well as the environmental benefits. The renewable CCHP system could able to attain lower CO₂ and SO₂ emission with total energy efficiency and gas yield of 75.43% and 2.476 m³/kg respectively.     

Modeling and parametric optimization of air catalytic co-gasification of wood-oil palm fronds blend for clean syngas (H2+CO) production

Syngas production from biomass gasification is a potentially sustainable and alternative means of conventional fuels. The current challenges for biomass gasification process are biomass storage and tar contamination in syngas. Co-gasification of two biomass and use of mineral catalysts as tar reformer in downdraft gasifier is addressed the issues. The optimized and parametric study of key parameters such as temperature, biomass blending ratio, and catalyst loading were made using Response Surface Methodology (RSM) and Artificial Neural Network (ANN) on tar reduction and syngas. The maximum H₂ was produced when Portland cement used as catalyst at optimum conditions, temperature of 900 °C, catalyst-loading of 30%, and biomass blending-ratio of W52:OPF48. Higher CO was yielded from dolomite catalyst and lowest tar content obtained from limestone catalyst. Both RSM and ANN are satisfactory to validate and predict the response for each type of catalytic co-gasification of two biomass for clean syngas production.     

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.     

Air gasification of dried sewage sludge in a two-stage gasifier. Part 2: Calcined dolomite as a bed material and effect of moisture content of dried sewage sludge for the hydrogen production and tar removal

In order to produce a clean producer gas, the air gasification of dried sewage sludge was conducted in a two-stage gasifier that consisted of a bubbling fluidized bed and a tar-cracking zone. The kind and amount of bed materials, the kind of additives in the upper-reactor, and the moisture content in the sewage sludge were selected as operating variables in order to investigate their effects on the development of the producer gas characteristics. In our experiments, the gasification of a dried sewage sludge sample containing 30 wt.% of moisture with a combination of calcined dolomite as the bed material and activated carbon in the tar-cracking zone removed the most tar and produced the highest hydrogen concentration. The total tar removal efficiency and the H₂ content in the producer gas from the sample noted above reached 88.4% and 32.1 vol.%, respectively. The LHVs of all the producer gases were high with values above 7 MJ Nm⁻³.