Low-tar,highly burnable gas production from the gasification of pelletized palm empty fruit bunch

Gasification is an attractive method to convert lignocellulosic biomass into a combustible gas mixture for electricity and power generation. To control the tar concentration in the produced gas to be within the allowable limit of downstream applications, it is important for a gasification system to be integrated with a tar removal process. In this study, an integrated gasification system consisting of a downdraft gasifier and a secondary catalytic tar-cracking reactor was designed and tested for the gasification of pelletized oil palm empty fruit bunch. To further purify the producer gas, the system was also integrated with a cyclone, a water scrubber, and a carbon-bed filter. Biomass was fed at a rate of 5 kg/h, while the air equivalence ratio (ER) and the gasification temperature were set at 0.1 and 800°C, respectively. In total, 5 kg of the specially developed low-cost Fe/activated carbons (AC) catalyst was used in the hot gas catalytic tar-cracking reactor. Results indicate that our integrated gasification system was able to produce a clean burnable gas with a lower heating value (LHV) of 9.05 MJ/Nm³, carbon conversion efficiency (CCE) of 79.4%, cold gas efficiency (CGE) of 89.9%, and H₂ and CH₄ concentrations of 29.5% and 10.3%, respectively. The final outlet gas was found to only contain 32.5 mg/Nm³ of tar, thus making it suitable for internal combustion engine (ICE) application.     

Tar removal from the producer gas of a small scale downdraft gasifier using a fatty acid based,wet packed-bed scrubber

Small scale gasification combined heat and power (CHP) systems offer an alternative to diesel fuelled generators for power generation in remote communities and industrial sites. Tar and particulates in the producer-gas can damage the internal combustion engine generator and increase operation and maintenance costs. In this work, we present a novel trickle-bed scrubber using filtered waste cooking oil as a cost effective and easy-to-operate gas clean-up method for a small CHP system. The performance of the trickle-bed scrubber was compared against a packed-bed filter utilizing woodchips in a 20 kW_th downdraft gasifier. Used-cooking oil was selected as the solvent and woodchips as the bed-material as these are readily available, inexpensive, and can be recycled in the gasifier as fuel. A woodchip packed-bed filter reduced the tar and particulate matter (PM) in the producer gas from gasification of spruce chips (11% moisture) from 1.6 to 1.4 g/Nm³ and from 0.16 to 0.087 g/Nm³ respectively. The trickle-bed scrubber was able to reduce the tar and PM in the producer gas from gasification of pinewood (8% moisture) from 1.38 to 0.28 g/Nm³, and 0.209 to 0.082 g/Nm³, respectively. Tar and PM removal efficiency improved by 60% and 29% respectively. Components such as benzene, toluene, naphthalene, and biphenylene were the major tar components. After passing the trickle-bed, most tar was removed, with a preference for removal of multi-ringed aromatics and gravimetric tars. Parameters such as the tar and particulate concentration, feedstock moisture content, and feedstock source affect the performance of the gas clean-up system.     

Two-stage dual fluidized bed gasification: Its conception and application to biomass

The quoted two-stage dual fluidized bed gasification (T-DFBG) devises the use of a two-stage fluidized bed (TFB) to replace the single-stage bubbling fluidized bed gasifier involved in the normally encountered dual fluidized bed gasification (N-DFBG) systems. By feeding fuel into the lower stage of the TFB, this lower stage functions as a fuel gasifier similar to that in the N-DFBG so that the upper stage of the TFB works to upgrade the produced gas in the lower stage and meanwhile to suppress the possible elutriation of fuel particles fed into the freeboard of the lower-stage bed. The heat carrier particles (HCPs) circulated from the char combustor enter first the upper stage of the TFB to facilitate the gas upgrading reactions occurring therein, and the particles are in turn forwarded into the lower stage to provide endothermic heat for fuel pyrolysis and gasification reactions. Consequently, with T-DFBG it is hopeful to increase gasification efficiency and decrease tar content in the produced gas. This anticipation was corroborated through gasifying dry coffee grounds in two 5.0kg/h experimental setups configured according to the principles of T-DFBG and N-DFBG, respectively. In comparison with the N-DFBG case, the test according to T-DFBG increased, the fuel C conversion and cold gas efficiency by about 7% and decreased tar content in the produced gas by up to 25% under similar reaction conditions. Test results demonstrated also that all these upgrading effects via adopting T-DFBG were more pronounced when a Ca-based additive was blended into the fuel.     

Modeling of Biomass Gasification in a Downdraft Gasifier with Integrated Tar Adsorption

Thermal gasification of biomass is known for its capabilities in flexible and decentral power station applications for cogeneration. However, the product gas contains tar compounds adversely for a stable operation. Integrated tar adsorption in a subsequent cooled section is therefore an option to reduce the tar pollution. The char coal, used here as adsorption agent, is formed by biomass pyrolysis in the gasifier. A kinetic model is proposed, considering the kinetics of all main reactions as well as heat and mass transport phenomena. Results are presented for axial temperature profiles, gas compositions, and the gas purity at different air‐to‐fuel ratios. The resulting output mass flows could indicate a requirement on the adsorption capacity of at least 0.3 g g^?1 for the activated char coal.     

Gasification of rice husk in a cyclone gasifier

The experimental results of air gasification of rice husk in the cyclone gasifier were presented at the fuel rate of 20.1 kg/h. With the equivalence ratios varied in the range of 0.21–0.32, the heating value of the producer gas decreases from 6.98 MJ/Nm³ to 3.11 MJ/Nm³ and the cold gas efficiency decreases from 64% to 31%. However, the tar content in the prouder gas decreases with the increase of the equivalence ratio. The rice husk and ash were examined under a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) elemental analysis. The outer surface of the fuel particle which is of scale structure does not change basically during the gasification. The pyrolyzed gas is mainly released from the inner surface of the fuel particle. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

胜利褐煤负载镍生物质气化催化剂实验研究

为获得较低温度下生物质焦油高效裂解气化的廉价型催化剂,基于褐煤富含含氧官能团特点,通过离子交换法将镍负载到胜利褐煤上制备褐煤负载镍催化剂(Ni/SLC)。研究了溶液p H值和炭化温度对催化剂物理化学性质的影响,得到Ni/SLC催化剂最佳制备条件,最后在两段式移动床石英反应器中将催化剂用于玉米芯挥发分的催化气化,研究了催化剂对生成气产量和碳平衡的影响。结果表明:p H值为11,炭化温度为650℃时制备的Ni/SLC比表面积最大达到266.3 m2/g,镍微晶尺寸较小为5.0 nm。催化气化实验表明:650℃下Ni/SLC催化剂具有高的焦油裂解活性,气体产量高达43.9 mmol/g,相当于无催化实验气体总产量的3.3倍;水蒸气气化可将热解焦油完全气化,气体产量为85.1 mmol/g,H2产量高达61.9 mmol/g,占气体总量的72.7%,说明Ni/SLC催化剂可作为生物质催化气化制氢的潜在催化剂。     

Flaming pyrolysis model of the fixed bed cross draft long-stick wood gasifier

The future industrial development of biomass energy depends on the application of renewable energy technology in an efficient manner. Of all the competing technologies under biomass, gasifiers are considered to be one of most viable applications. The use of biomass fuel, especially biomass wastes, for distributed power production can be economically viable in many parts of the world through gasification of biomass. Since biomass, is a clean and renewable fuel, gasification gives the opportunity to convert biomass into clean fuel gas or synthesis gas for industrial uses. The preparation of feedstock for a gasifier requires time, energy and labour and this has been a setback for gasifier technology development. The present work is focused on gasification of long-stick wood as a feed material for gasifiers. This application makes reduction not only in the cost but also on the power consumption of feed material preparation. A 50 m³/h capacity gasifier was fabricated in the cross draft mode. The cross draft mode makes it possible to produce low tar content in producer gas. This cross draft mode operates with 180 W of blower supply for air to produce 10 kW of thermal output. The initial bed heights of the long-stick wood and charcoal are 58 cm and 48 cm respectively. Results were obtained for various flow conditions with air flow rates ranging from 20 to 30 m³/h. For modelling, the flaming pyrolysis time for long-stick wood in the gasifier is calculated to be 1.6 min. The length of the flaming pyrolysis zone and char gasification zone is found to be 34 cm and 30 cm respectively. The rate of feed was between 9 and 10 kg/h. Continuous operation for 5 h was used for three runs to study the performance. In this study we measured the temperature and pressure in the different zones as a function of airflow. We measured the gas flow and efficiency of the gasifier in order to determine its commercial potential for process and power industries.     

Effect of operating conditions on tar and gas composition in high temperature air/steam gasification (HTAG) of plastic containing waste

In this work, the high temperature air/steam gasification (HTAG) technique has been tested for a fuel in pellet form made from waste material of woody and plastic origin. The feedstock was gasified in an updraft fixed bed reactor by mixtures of air and steam (102 Nm³/h, 4% to 82% steam) preheated to 1400 °C, a temperature well above the fluid temperature of the feedstock. The produced gas was analyzed with respect to composition, including a detailed characterization of the tar. Lower heating values up to 9.5 MJ/Nm³ and gas yields as high as 3.4 Nm³/kg were reported, indicating the process to be highly efficient for waste-to-energy applications. The composition of the tars, suggested extensive cracking as a result of the high temperatures of the outgoing gas.     

Steam‐air blown bubbling fluidized bed biomass gasification (BFBBG): Multi‐scale models and experimental validation

During fluidized bed biomass gasification, complex gas‐solid mixing patterns and numerous chemical and physical phenomena make identification of optimal operating conditions challenging. In this work, a parametric experimental campaign was carried out alongside the development of a coupled reactor network model which successfully integrates the individually validated sub‐models to predict steady‐state reactor performance metrics and outputs. The experiments utilized an integrated gasification system consisting of an externally‐heated, bench‐scale, 4‐in., 5 kWth, fluidized bed steam/air blown gasifier fed with woody biomass equipped with a molecular beam mass spectrometer to directly measure tar species. The operating temperature (750–850°C) and air/fuel equivalence ratio (ER = 0–0.157) were independently varied to isolate their effects. Elevating temperature is shown to improve the char gasification rate and reduce tar concentrations. Air strongly impacts the composition of tar, accelerating the conversion of lighter polycyclic‐aromatic hydrocarbons into soot precursors, while also improving the overall carbon conversion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1543–1565, 2017     

Biomass tar recycling and destruction in a CFB gasifier

Conversion of biomass into producer gas by thermal gasification broadens the scope of biomass applications. Usually, tar has to be removed from the producer gas. Tar recycling within the gasification process may solve the associated waste problem and increase the system efficiency, provided tar is broken down under gasification conditions. We present results of tar recycling experiments at a Circulating Fluidised Bed gasifier. At 830 °C, from each of 15 main tar compounds 70-80% are broken down. Continuous recycling of tar would increase the tar content in raw producer gas by 50% at most and save about 3% fuel input.     

Gasification characteristics of combustible wastes in a 5 ton/day fixed bed gasifier

The gasification characteristics of combustible wastes were determined in a 5 ton/day fixed bed gasifier (1.2 m I.D. and 2.8m high). The fixed bed gasifier consisted of air compressor, oxygen tank, MFC, fixed bed gasifier, cyclone, heat exchanger, solid/gas separator, water fluidized bed reactor and blower. To capture soot or unburned carbon from the gasification reaction, solid/gas separator and water fluidized bed were used. The experiments with 10–50 hours of operation were carried out to determine the effects of bed temperature, solid/oxygen ratio and oxidant on the gas composition, calorific value and carbon conversion. The calorific values of the produced gas decreased with an increase of bed temperature because combustion reaction happened more actively. The gas composition of partial oxidation of woodchip is CO: 34.4%, H₂: 10.7%, CH₄: 6.0%, CO₂: 48.9% and that of RPF is CO: 33.9%, H₂: 26.1%, CH₄: 10.7%, CO₂: 29.2%. The average calorific values of produced gas were about 1,933 kcal/Nm³, 2,863 kcal/Nm³, respectively. The maximum calorific values were 3,100 kcal/Nm³ at RPF/oxygen ratio: 7     

整体式新型生物质气化催化反应器的实验研究

在现有生物质气化反应器及焦油处理方法的基础上,开发出一种整体式新型生物质气化催化反应器,并对该反应器进行相关的实验研究。实验研究结果表明:当木粉进料速率为6.48 g/min,空燃比RE为0.23,气化温度在500—670℃,这种整体式新型生物质气化催化反应器内有、无催化剂时对木粉气化产生的燃气中焦油的含量以及气体组分有明显影响;当采用钴与氧化钴的质量分数为20%,氧化钙的质量分数为80%的钴基催化剂作为焦油裂解催化剂,裂解温度为800℃,标态下体积空时为1.8 s的情况下,燃气中夹带的焦油可完全被催化裂解,同时燃气中的气体成分氢体积分数可从无催化剂时的15%提高到有催化剂时的35%,净提高20%。同时也对使用前后的钴基催化剂进行了XRD表征分析,发现氧化钙在生物质气化过程中具有一定的CO2捕集能力。     

Entrained flow gasification of coal: 1. Evaluation of mixing and reaction processes from local measurements

Local mixing and reaction processes were studied within a laboratory-scale, entrained coal gasifier at atmospheric pressure, using a Utah high-volatile, low-sulphur bituminous coal at a design flow rate of 24.5 kg h⁻¹. The coal-oxygen-steam feed mass ratio was 1.00:0.91:0.27. A water-quenched sample probe was used to collect radial gas and char samples at seven different axial positions in the 124 cm long reactor for the measurement of gasification products and residual char composition. The observed carbon conversion was 79 ± 3%. Coal hydrogen and oxygen were converted more rapidly and more completely than carbon. Devolatilization, which occurred very rapidly near the inlet, led to most of this carbon conversion; heterogeneous char reactions with CO₂ and steam apparently accounted for the balance. Oxygen was consumed through reaction with volatiles very quickly in the upper gasifier region. These data were used to evaluate mixing and reaction characteristics within the reactor. Agreement of measurements with predictions from a generalized two-dimensional entrained coal gasification model was good.     

Model development and validation: Co-combustion of residual char, gases and volatile fuels in the fast fluidized combustion chamber of a dual fluidized bed biomass gasifier

A one-dimensional steady state model has been developed for the combustion reactor of a dual fluidized bed biomass steam gasification system. The combustion reactor is operated as fast fluidized bed (riser) with staged air introduction (bottom, primary and secondary air). The main fuel i.e., residual biomass char (from the gasifier), is introduced together with the circulating bed material at the bottom of the riser. The riser is divided into two zones: bottom zone (modelled according to modified two phase theory) and upper zone (modelled with core-annulus approach). The model consists of sub-model for bed hydrodynamic, conversion and conservation. Biomass char is assumed to be a homogeneous matrix of C, H and O and is modelled as partially volatile fuel. The exit gas composition and the temperature profile predicted by the model are in good agreement with the measured value.     

Effect of biomass particle size and air superficial velocity on the gasification process in a downdraft fixed bed gasifier. An experimental and modelling study

A one-dimensional stationary model of biomass gasification in a fixed bed downdraft gasifier is presented in this paper. The model is based on the mass and energy conservation equations and includes the energy exchange between solid and gaseous phases, and the heat transfer by radiation from the solid particles. Different gasification sub-processes are incorporated: biomass drying, pyrolysis, oxidation of char and volatile matter, chemical reduction of H₂, CO₂ and H₂O by char, and hydrocarbon reforming. The model was validated experimentally in a small-scale gasifier by comparing the experimental temperature fields, biomass burning rates and fuel/air equivalence ratios with predicted results. A good agreement between experimental and estimated results was achieved. The model can be used as a tool to study the influence of process parameters, such as biomass particle mean diameter, air flow velocity, gasifier geometry, composition and inlet temperature of the gasifying agent and biomass type, on the process propagation velocity (flame front velocity) and its efficiency. The maximum efficiency was obtained with the smaller particle size and lower air velocity. It was a consequence of the higher fuel/air ratio in the gasifier and so the production of a gas with a higher calorific value.     

Experimental research on air staged cyclone gasification of rice husk

A novel air cyclone gasifier of rice husk has been used to obtain experimental data for air staged gasification. Three positions and five ratios of secondary air were selected to study effect of the secondary air on the temperature profile in the gasifier and quality of syngas. Temperature profile and the syngas component are found to be strongly influenced by the injection position and ratio of the secondary air. Generally, gas temperature in all conditions increased at the early stage of reaction, and then decreased in the reduction zone where reactions were endothermic. The peak temperature in the gasifier changed with the injection positions and ratios of the secondary air, which could be as high as 1056 °C. The concentration of CO₂, CO, H₂ and CH₄ increased with the secondary air while the O₂ concentration remained constant. The syngas component exhibited different laws when the secondary air ratio was changed. It was also shown that the optimum condition was that the secondary air was injected in the oxidization zone at a secondary air ratio of about 31%. Under that condition, the fuel gas production was 1.30 Nm³/kg, the low heating value of the syngas was 6.7 MJ/Nm³, the carbon conversion rate was 92.2% and the cold gas efficiency of the gasifier was 63.2%. The tar content of the syngas was also studied in this paper. It decreased from 4.4 g/m³ for gasification without the secondary air to 1.6 g/m³ for gasification with the secondary air injected in the oxidization zone.     

Destruction of anthracene using a gliding arc plasma reformer

The gasification technology for biomass conversion has a limitation for some applications, including engines and turbines, because it produces tar-containing gas. In this study, a gliding arc plasma reformer was developed to remove tar. The plasma discharge in the gliding-type reformer is based on the both non-equilibrium and equilibrium plasmas. A simulation test was conducted using anthracene, which is produced during the gasification of biomass and waste, as the representative tar substance. In the optimal condition, the anthracene decomposition efficiency was 96.1%, and the energy efficiency was 1.14 g/kWh. The higher heating value of the gas produced from the anthracene decomposition was 11,324 kJ/Nm³, and the carbon balance was 98%. The steam flow rate, power input, total gas flow rate, and input concentration change were used as variables for the test. The anthracene decomposition efficiency was 81% when the gliding arc plasma reformer was used. When steam was fed at a rate of 0.63 L/min, the decomposition efficiency was highest (96.1%) due to the creation of OH radicals. The energy efficiency was highest (2.63 g/kWh) when the total gas flow rate was 24.1 L/min. H₂, CO, and CO₂ were produced as reformed gases. At the steam injection rate of 0.37 L/min or more, carbon black did not appear. Thus, it was verified that the gliding arc plasma reformer is effective for tar reduction.     

Performance analysis of integrated biomass gasification fuel cell (BGFC) and biomass gasification combined cycle (BGCC) systems

Biomass gasification processes are more commonly integrated to gas turbine based combined heat and power (CHP) generation systems. However, efficiency can be greatly enhanced by the use of more advanced power generation technology such as solid oxide fuel cells (SOFC). The key objective of this work is to develop systematic site-wide process integration strategies, based on detailed process simulation in Aspen Plus, in view to improve heat recovery including waste heat, energy efficiency and cleaner operation, of biomass gasification fuel cell (BGFC) systems. The BGFC system considers integration of the exhaust gas as a source of steam and unreacted fuel from the SOFC to the steam gasifier, utilising biomass volatilised gases and tars, which is separately carried out from the combustion of the remaining char of the biomass in the presence of depleted air from the SOFC. The high grade process heat is utilised into direct heating of the process streams, e.g. heating of the syngas feed to the SOFC after cooling, condensation and ultra-cleaning with the Rectisol^® process, using the hot product gas from the steam gasifier and heating of air to the SOFC using exhaust gas from the char combustor. The medium to low grade process heat is extracted into excess steam and hot water generation from the BGFC site. This study presents a comprehensive comparison of energetic and emission performances between BGFC and biomass gasification combined cycle (BGCC) systems, based on a 4th generation biomass waste resource, straws. The former integrated system provides as much as twice the power, than the latter. Furthermore, the performance of the integrated BGFC system is thoroughly analysed for a range of power generations, ～100–997 kW. Increasing power generation from a BGFC system decreases its power generation efficiency (69–63%), while increasing CHP generation efficiency (80–85%).     

The fate of fuel-nitrogen during gasification of biomass in a pressurised fluidised bed gasifier

The distribution of fuel-nitrogen in gases, tar and char from gasification of biomass in a pressurised fluidised bed gasifier was investigated. Four species of biomass: birch, Salix, Miscanthus and Reed canary grass were gasified at 0.4 MPa and 900 °C. Oxygen-enriched nitrogen was used as fluidising agent. As a reference, gasification of Daw Mill coal was also carried out under the same experimental conditions. The experimental results illustrate that both the nature of the original fuels and the chemical structure of the nitrogen in the fuel have influence on the distribution of fuel-nitrogen in gases (NH₃, HCN, NO), tar and char under the employed experimental conditions. The present work also shows that the types of nitrogen heterocyclic compounds (NHCs) in the tar from different kinds of biomass are the same and the major compound is pyridine. However, the distribution of the various NHCs in the tar from the four species of biomass varies: the higher the content of fuel-nitrogen, the higher the concentration of two-ring NHCs in the tar. An effective method for extracting NHCs from the acidic absorption of the product gas was introduced in the present work. The method makes use of solid phase extraction (SPE) by a silica-based C₁₈ tube to extract the NHCs which subsequently were analysed by gas chromatography (GC) with flame ionisation detection (FID). The recovery and reproducibility of the SPE technique for NHCs is discussed.     

Effects of Chinese dolomites on tar cracking in gasification of birch

To minimize tar in the producer gas from birch gasification at 700, 750 and 800 °C, four Chinese dolomites (Zhenjiang, Nanjing, Shanxi, Anhui) and a Swedish dolomite (Sala) used as reference were studied in a laboratory-scale atmospheric fluidized bed gasifier. The gasifier was equipped with a downstream fixed catalyst bed. The results imply that all dolomites but Anhui dolomite effectively decompose tar into gases. Anhui dolomite showed a low catalytic capacity to crack tar produced at 700 and 800 °C. The influence of various ratios of steam to biomass on tar content in the producer gas after passing over dolomite was studied. The tar cracking efficiency of the dolomites did not improve significantly with the ratio of steam to biomass in the region 0.11-0.52.