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.     

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).     

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.     

Gasification and power generation characteristics of woody biomass utilizing a downdraft gasifier

Energy utilization from biomass resources has started to attract public attention as a method to reduce CO₂ emissions. In this study, the characteristics of syngas production from biomass gasification were investigated in a downdraft gasifier that was combined with a small gas engine system for power generation. Syngas temperatures from the gasifier were maintained at a level of 700-1000 °C. When the air ratio for gasification was 0.3-0.35, the low heating value of syngas was 1100-1200 kcal Nm⁻³ and the cold gas efficiency 69-72%. Tar concentration in raw syngas was around 3.9-4.4 g Nm⁻³. Syngas combustion in the gas engine after purification showed that HC concentration was below 200 ppm, and NOx concentration was below 40 ppm in the exhaust gas.     

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.     

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.     

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%.     

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.     

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.     

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.     

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.     

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.     

An experimental study on air gasification of biomass micron fuel (BMF) in a cyclone gasifier

Biomass micron fuel (BMF) produced from feedstock (energy crops, agricultural wastes, forestry residues and so on) through an efficient crushing process is a kind of powdery biomass fuel with particle size of less than 250 μm. Based on the properties of BMF, a cyclone gasifier concept has been considered in our laboratory for biomass gasification. The concept combines and integrates partial oxidation, fast pyrolysis, gasification, and tar cracking, as well as a shift reaction, with the purpose of producing a high quality of gas. In this paper, characteristics of BMF air gasification were studied in the gasifier. Without outer heat energy input, the whole process is supplied with energy produced by partial combustion of BMF in the gasifier using a hypostoichiometric amount of air. The effects of equivalence ratio (ER) and biomass particle size on gasification temperature, gas composition, gas yield, low-heating value (LHV), carbon conversion and gasification efficiency were studied. The results showed that higher ER led to higher gasification temperature and contributed to high H₂-content, but too high ER lowered fuel gas content and degraded fuel gas quality. A smaller particle was more favorable for higher gas yield, LHV, carbon conversion and gasification efficiency. And the BMF air gasification in the cyclone gasifier with the energy self-sufficiency is reliable.     

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.     

Comparison of green waste gasification performance in updraft and downdraft fixed bed gasifiers

This study aimed to investigate the gasification potential of municipal green waste in different fixed-bed gasifier configurations as updraft and downdraft. Both reactor systems were constructed from stainless-steel with a cyclone separator to increase synthesis gas yield and reduce tar production. Green waste collected from parks and gardens by Manisa Metropolitan Municipality, Turkey, was used in the experiments. After full-characterization of green waste, gasification experiments were performed above 700 °C to produce syngas with more than 40% (volumetric) H₂ and heating value around 12.54 MJ/Nm³. Dry air (DA) and pure oxygen (PO) were used as gasification agents. DA was applied with the flow-rates ranged between 0.4 and 0.05 L/min while the flow-rate of PO was 0.01 L/min. The maximum H₂ production as 45 vol% was obtained in downdraft reactor while it was about 51 vol% in updraft system. CH₄ production was obtained as higher value (app. 19 vol%) in downdraft reactor than that (13 vol%) in the updraft one. In the experiments with DA above 700 °C, the H₂/CO ratio varied between 1 and 3, and in the experiments with PO, it increased up to a maximum value of 4. The study has found a suitable set of gasification process parameters for two reactor systems. Therefore, the findings have been compared and discussed in detail.     

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.     

Biomass-based hydrogen production: A review and analysis

In this study, various processes for conversion of biomass into hydrogen gas are comprehensively reviewed in terms of two main groups, namely (i) thermo-chemical processes (pyrolysis, conventional gasification, supercritical water gasification (SCWG)), and (ii) biological conversions (fermentative hydrogen production, photosynthesis, biological water gas shift reactions (BWGS)). Biomass-based hydrogen production systems are discussed in terms of their energetic and exergetic aspects. Literature studies and potential methods are then summarized for comparison purposes. In addition, a biomass gasification process via oxygen and steam in a downdraft gasifier is exergetically studied for performance assessment as a case study. The operating conditions and strategies are really important for better performance of the system for hydrogen production. A distinct range of temperatures and pressures is used, such as that the temperatures may vary from 480 to 1400 °C, while the pressures are in the range of 0.1–50 MPa in various thermo-chemical processes reviewed. For the operating conditions considered the data for steam biomass ratio (SBR) and equivalence ratio (ER) range from 0.6 to 10 and 0.1 to 0.4, respectively. In the study considered, steam is used as the gasifying agent with a product gas heating value of about 10–15 MJ/Nm³, compared to an air gasification of biomass process with 3–6 MJ/Nm³. The exergy efficiency value for the case study system is calculated to be 56.8%, while irreversibility and improvement potential rates are found to be 670.43 and 288.28 kW, respectively. Also, exergetic fuel and product rates of the downdraft gasifier are calculated as 1572.08 and 901.64 kW, while fuel depletion and productivity lack ratios are 43% and 74.3%, respectively.     

Performance evaluation of producer gas burner for industrial application

The purpose of this study is to evaluate the performance of premixed type industrial burner with producer gas, in terms of emission, axial and radial flame temperature. The developed burner was a concentric tube type where the air supplied through a central tube, which is surrounded by another one. It having 150 kWth capacity and tested on open core downdraft type gasifier. The burner consists of swirl vane for mixing the air and producer gas, mixing tube and bluff body for flame stabilization. Swirl angle and bluff body diameter was kept constant throughout the study. Burner was evaluated with open core downdraft type gasifier. In order to understand the qualitative differences in the temperature profile and emissions during combustion of producer gas, an experiment was conducted on three-flow rate and air-fuel ratio. Study shows low NO_x and CO emission at 125 Nm³ h⁻¹ as compared to that of 75 and 100 Nm³ h⁻¹. Maximum flame temperature (753 °C) was recorded at of 10 cm axial and 10 mm radial distance.     

Hydrogen-rich syngas production via integrated configuration of pyrolysis and air gasification processes of various algal biomass: Process simulation and evaluation using Aspen Plus software

In the present study, hydrogen-rich syngas production via integrated configuration of pyrolysis and air gasification processes of different algal biomass is investigated at relevant industrial condition. A comprehensive steady state equilibrium simulation model is developed using Aspen Plus software, to investigate and evaluate the performance of pyrolysis and air gasification processes of different algal biomass (Algal waste, Chlorella vulgaris, Rhizoclonium sp and Spirogyra). The model can be used as a predictive tool for optimization of the gasifier performance. The developed process consists of three general stages including biomass drying, pyrolysis and gasification. The model validation using reported experimental results for pyrolysis of algal biomass indicated that the predicted results are in good agreement with experimental data. The effect of various operational parameters, such as gasifier temperature, gasifier pressure and air flow rate on the gas product composition and H₂/CO was investigated by sensitivity analysis of parameters. The achieved optimal operating condition to maximize the hydrogen and carbon monoxide production as the desirable products were as follows: gasifier temperature of 600 °C, gasifier pressure of 1 atm and air flow rate of 0.01 m³/h.