Gasification and co-gasification of biomass wastes: Effect of the biomass origin and the gasifier operating conditions

Air gasification of different biomass fuels, including forestry (pinus pinaster pruning) and agricultural (grapevine and olive tree pruning) wastes as well as industry wastes (sawdust and marc of grape), has been carried out in a circulating flow gasifier in order to evaluate the potential of using these types of biomass in the same equipment, thus providing higher operation flexibility and minimizing the effect of seasonal fuel supply variations. The potential of using biomass as an additional supporting fuel in coal fuelled power plants has also been evaluated through tests involving mixtures of biomass and coal–coke, the coke being a typical waste of oil companies. The effect of the main gasifier operating conditions, such as the relative biomass/air ratio and the reaction temperature, has been analysed to establish the conditions allowing higher gasification efficiency, carbon conversion and/or fuel constituents (CO, H₂ and CH₄) concentration and production. Results of the work encourage the combined use of the different biomass fuels without significant modifications in the installation, although agricultural wastes (grapevine and olive pruning) could to lead to more efficient gasification processes. These latter wastes appear as interesting fuels to generate a producer gas to be used in internal combustion engines or gas turbines (high gasification efficiency and gas yield), while sawdust could be a very adequate fuel to produce a H₂-rich gas (with interest for fuel cells) due to its highest reactivity. The influence of the reaction temperature on the gasification characteristics was not as significant as that of the biomass/air ratio, although the H₂ concentration increased with increasing temperature.     

Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant

Treatment of plastic waste by gasification in fluidized bed with air using dolomite as tar cracking catalyst has been studied. The gasifier has a 1 m high bed zone (diameter of 9.2 cm) followed by a 1 m high freeboard (diameter of 15.4 cm). The feedstock is composed of blends of plastic waste with pine wood sawdust and coal at flow rates of 1–4 kg/h. Operating variables studied were gasifier bed temperature (750–880 °C), equivalence ratio (0.30–0.46), feedstock composition and the influence of secondary air insertion in freeboard. Product distribution includes gas and char yields, gas composition (H₂, CO, CO₂, CH₄, light hydrocarbons), heating value and tar content in the flue gas. As a result, a gas with a medium hydrogen content (up to 15% dry basis) and low tar content (less than 0.5 g/m_n³) is obtained.     

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 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.     

Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor

Seven mixtures of coals, plastics and wood have been pelletized and fed into a pre-pilot scale fluidized bed gasifier in order to investigate the main aspects of the co-gasification of these materials. The main components of the obtained syngas (CO, H₂, CO₂, N₂, CH₄, C_nH_m) were measured by means of on-line analyzers and a gas cromatograph. The performance of the gasifier was evaluated on the basis of syngas composition, carbon conversion efficiency, energy content of syngas, cold gas efficiency and yield of undesired by-products (tar and soot-like particulate). The results of a first series of experimental tests showed the effect of gas fluidizing velocity and that of equivalence ratio on the main performance parameters for a specific coal-plastics mixture. A second series of tests has been carried out by changing the mixture composition keeping fixed the gas velocity and equivalence ratio. The presence of wood and coal in the mixture with plastics contributed to reduce the tar production even though it is accompanied by a lower syngas specific energy.     

Performance optimization of two-staged gasification system for woody biomass

The performance of a small-scale two-staged gasification system is reported. In this system wood chips are gasified with a fixed bed gasifier and then tar in the produced gas is reformed in a non-catalytic reformer, finally the production gas is used to generate electricity. In this system, the gasifying agents are high temperature air and steam supplied into the gasifier and the reformer. This paper reports on optimum gasification air ratio (defined as the ratio of the oxygen mole supplied into the gasifier to the oxygen mole required for complete combustion of biomass), reforming air ratio (defined as the ratio of the oxygen mole supplied in the reformer to the oxygen mole required for the complete combustion of biomass) and steam ratio (defined as the ratio of the steam mole supplied into the gasifier to the carbon mole in biomass supplied into the gasifier) for producing required gas supplied into a dual-fueled diesel engine. The results showed that, under optimum conditions, the higher heating value of the reformed gas was 3.9 MJ/m³_N; the cold gas efficiency (defined as the ratio of HHV reformed gas × reformed gas flow rate to HHV biomass × biomass feed rate) of the gasification system was 66%, and the gross thermal efficiency of the overall system was 27%.     

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     

Calculation of the conditions to get less than 2 g tar/mn3 in a fluidized bed biomass gasifier

The experimental conditions under which a fluidized bed biomass gasifier can generate a gas with a tar content below 2 g/m_n³ are analyzed by using and developing the model recently published for those gasifiers by Corella and Sanz [Fuel Process. Techn. 2005, 86, 1021–1053]. The analyzed experimental conditions were: the equivalence ratio, the partitioning of the air, between the primary and secondary flows, the location (height) of the inlet of the secondary air flow, the biomass moisture and the biomass flow rate. Results from the modelling work are presented for a given CFB biomass gasifier of commercial size. Some of these results are also being checked in a CFB biomass gasifier at small pilot plant scale. To obtain a gasification gas with a very low tar content the two most important experimental conditions are a high value for the equivalence ratio and a good in-gasifier material which determines the values of the kinetic constants of the reactions involved in the network at the gasifier.     

A novel biomass air gasification process for producing tar-free higher heating value fuel gas

Biomass is a promising sustainable energy source. A tar-free fuel gas can be obtained in a properly designed biomass gasification process. In the current study, a tar-free biomass gasification process by air was proposed. This concept was demonstrated on a lab-scale fluidized bed using sawdust under autothermic conditions. This lab-scale model gasifier combined two individual regions of pyrolysis, gasification, and combustion of biomass in one reactor, in which the primary air stream and the biomass feedstock were introduced into the gasifier from the bottom and the top of the gasifier respectively to prevent the biomass pyrolysis product from burning out. The biomass was initially pyrolyzed and the produced char was partially gasified in the upper reduction region of the reactor, and further, char residue was combusted at the bottom region of the reactor in an oxidization atmosphere. An assisting fuel gas and second air were injected into the upper region of the reactor to maintain elevated temperature. The tar in the flue gas entered the upper region of the reactor and was decomposed under the elevated temperature and certain residence time. This study indicated that under the optimum operating conditions, a fuel gas could be produced with a production rate of about 3.0 Nm³/kg biomass and heating value of about 5000 kJ/Nm³. The concentration of hydrogen, carbon monoxide and methane in the fuel gas produced were 9.27%, 9.25% and 4.21%, respectively. The tar formation could be efficiently controlled below 10 mg/Nm³. The system carbon conversion and cold gasification efficiency reached above 87.1% and 56.9%, respectively. In addition, the investigation of energy balance for the scale-up of the proposed biomass gasification process showed that the heat loss could be recovered by approximately 23% of total energy input. Thus, partial fuel gas that was produced could be re-circulated and used to meet need of energy input to maintain the elevated temperature at the upper region of reactor for tar decomposition. It was predicted the heating value of product fuel gas would be 8000 kJ/Nm³ if the system was scaled up.     

Axial gas profiles in a bubbling fluidised bed biomass gasifier

A 200 mm laboratory-scale atmospheric bubbling fluidised bed reactor has been used to obtain experimental data for the air/steam gasification of eucalyptus red gum wood chips and commercial wood pellets. The unique feature of this gasifier is the ability to examine the variations to axial gas composition along the bed height. At present no such data is available in the literature for biomass gasification. Gasification tests were performed using beds of; silica sand, char or clay to determine the effect of bed type on the gas composition. The behaviour of the major gas species (CO, H₂, CO₂) were observed to be strongly influenced by the water–gas shift reaction within the freeboard of the gasifier resulting in the exit gas being relatively similar in composition as compared to the in-bed variations. These small differences in gas composition for all bed types tested are the result of the achievement of equilibrium in the water–gas shift reaction. The influence of bed type exerted the most impact on the C₂–C₃ emissions (tar proxy) with the char bed found to best aid in their breakdown and to limit the amount of hydrocarbons surviving into the freeboard. The reduction of iron oxide (Fe₂O₃) content in the clay to a more reactive form of magnetite Fe₃O₄ by CO and H₂ in the product gas resulted in the clay bed to also exhibit a reduction in C₂–C₃ emissions compared to silica sand but less then char. The clay bed produced the highest calorific values for the producer gas. However, operation of the clay bed above 800 °C exhibits the potential for over reduction to form iron with subsequent agglomeration of the bed. Changing the fuel type to a biomass pellet resulted in higher emissions of C₁–C₃ hydrocarbons and in part its contribution is the result of primary particle fragmentation during screw feed conveying to the bed. Feeder location and bed design (conical or cylindrical) also exhibit an influence on hydrocarbon emissions.     

Effect of CO<Subscript>2</Subscript> addition on lignite gasification in a CFB reactor: A pilot-scale study

The addition of carbon dioxide to the gasification media during lignite gasification is introduced. The paper presents thermodynamic grounds of CO₂ enhanced gasification using a simplified equilibrium model. Experimental tests conducted using a pilot-scale circulating fluidized bed gasifier are discussed. Detailed analysis of the CO₂/C ratio on process conditions, namely on the process gas composition, lower heating value and H₂/CO ratio, is provided. Process gas composition implies that the gas is suitable for heat and power generation. Alternatively, CO₂ enhanced gasification could be considered as a carbon capture and utilization technology when external, renewable heat supply to the process is used. The results thus obtained are the initial step toward development of the CO₂ enhanced gasification process.     

生物质下吸式气化炉气化制备富氢燃气实验研究

以制取富氢燃气为目标,在自热式下吸式气化炉反应器内,进行了生物质下吸式气化炉富氧/水蒸气及空气气化的制氢特性研究。实验结果表明,与空气气化相比,富氧/水蒸气气化可显著提高氢产率和产气热值。在实验条件范围内,最大氢产率达到45.16 g/kg;最大低位热值达到11.11 MJ/m3。在富氧/水蒸气气化条件下,燃气中H2+CO体积分数达到63.27%—72.56%,高于空气气化条件下的52.19%—63.31%。富氧/水蒸气气化条件下的H2/CO体积比比值为0.70—0.90,低于空气气化条件下的1.06—1.27。实验结果证实:生物质下吸式气化炉富氧/水蒸气气化是一种有效的制取可再生氢源的工艺路线。     

Two dimensional numerical computation of a circulating fluidized bed biomass gasifier

A two dimensional model for an atmospheric CFB biomass gasifier has been developed which uses the particle based approach and integrates and simultaneously predicts the hydrodynamic and gasification aspects. Tar conversion is taken into account in the model. The model calculates the axial and radial distribution of syngas mole fraction and temperature both for bottom and upper zones. The proposed model addresses both hydrodynamic parameters and reaction kinetic modeling. Results are compared with and validated against experimental data from a pilot scale air blown CFB gasifier which uses different types of biomass fuels given in the literature. Developed model efficiently simulates the radial and axial profiles of the bed temperature and H₂, CO, CO₂ and CH₄ volumetric fractions and tar concentration versus gasifier temperature. The minimum error of comparisons is about 1% and the maximum error is less than 25%.     

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.     

Experimental investigation of a 125 kW twin-fire fixed bed gasification pilot plant and comparison to the results of a 2 MW combined heat and power plant (CHP)

Fixed bed biomass gasification is a promising technology to produce heat and power from a renewable energy source. A twin-fire fixed bed gasifier based CHP plant was realized in the year 2003 in Wr. Neustadt, Austria. Wood chips are used as fuel, which are dried and sieved before being gasified to a low calorific gas of about 5.8 MJ/Nm³_dry. Before the clean gas is fed into a gas engine a cyclone and a RME (rapemethylester)/H₂O quench system followed by a wet electrostatic precipitator (ESP) is used for gas cleaning. The CHP plant has a fuel power of 2 MW_th and an electric output of 550 kW_el. As scale up and optimization tool a hot test rig with a capacity of 125 kW_th was built. Basic parameters like the type of wood chips, power and air distribution were varied to investigate the effect on gas composition, tar content in the producer gas and carbon content in the ash. Additionally a temperature profile over the height of the 125 kW hot test rig was measured. Furthermore, the results from the hot test rig are discussed and compared with the results from the 2 MW_th demonstration plant.     

Experimental investigation and modelling study of long stick wood gasification in a top lit updraft fixed bed gasifier

We present results of an experimental study on long stick wood gasification, in an attempt to reduce wood gathering for gasification. This paper presents the results from experiments and the analyses of gasification using sticks with length 68 cm and diameter 6 cm. The moisture content of the wood was 25%. This top lit updraft gasifier operates with 180 W of blower power air supply to produce 9–10 kW of thermal energy, an energy yield of 50/1. Results were obtained for various flow conditions with airflow rates ranging from 25 to 45 m³/h. For modelling, the flaming pyrolysis time for long stick wood in the gasifier is calculated to be 2.1 min. The length of the flaming pyrolysis zone and char gasification zone is found to be 37 cm and 36 cm respectively. The turn down ratio for the gasification is around 2. The rate of feed was between 9 and 10 kg/h and the gasifier operated continuously for 5 h in two runs to study the gasifier reliability. The performance studies in specific gasification rate, equivalence ratio, turn down ratio, superficial velocity, airflow, and gas flow are analyzed. The temperature ranged from 1185 K in the combustion zone to 400 K in the drying zone. The gas and airflows can be converted to the Air/Fuel equivalence ratio, the most important aspect of gasifier operation. The equivalence ratio shows operation in a combustion mode (6.3) at start up; in a flaming pyrolysis mode (1.2) for the middle part of the run; and in the charcoal gasification mode (5.7) at the end of the run. Measurement of the equivalence ratio is a simple way of analyzing the behaviour of the gasifier. From the results of present investigation, it is revealed that the top lit updraft gasifier is more suitable for long stick wood as feed when compared to conventional updraft gasifier.     

高效能两段组合式煤气化过程热态试验

针对现有气流床气化技术在显热回收方面的不足,华东理工大学洁净煤技术研究所创新性开发煤基两段组合式气化工艺。在所建立的两段组合式煤气化炉热态试验装置上,考察了二段处理煤量和一段出口煤气组成对出口煤气热值、有效气浓度、二段碳转化率、水蒸气和二氧化碳转化率的影响。试验结果表明此气化工艺能有效利用一段炉煤气中的显热,提高气化炉出口煤气热值;二段适宜加入褐煤量为1400g,是一段处理量的10%;二段加煤量过多会降低二段煤层反应温度和促使焦油的生成;随着一段气化炉出口煤气所含水蒸气、CO2等气化剂浓度的增加,其对显热回收的作用就更明显;该工艺能减少CO2排放,具有良好的环境效益。     

High-performance oxygen transport membrane reactors integrated with IGCC for carbon capture

Precombustion carbon capture is an effective strategy to reduce large-scale CO₂ emissions, which is mainly used in the area of integrated gasification combined cycle (IGCC) power plants. Oxygen transport membranes (OTMs) were suggested as the air separation unit to produce high purity oxygen for the gasifier. However, the improvement in efficiency was limited. Here, a new IGCC process is reported based on a robust OTM reactor, where the OTM reactor is used behind the coal gasifier. This IGCC-OTM process fulfills syngas oxidation, H₂ production, and carbon capture in one unit, thus a significant decrease of the energy penalty is expectable. The membrane reactor does not use noble metal components, and exhibits high hydrogen production rates, high hydrogen separation factor (10³–10⁴), and stable performance in a gas mixture mimicking real syngas compositions from a coal gasifier with H₂S concentrations up to 1,000 ppm.     

Effect of micropores diffusion on kinetics of CH4 decomposition over a wood-derived carbon catalyst

In order to optimise hydrogen production from biomass gasification, catalytic conversion of methane contained in a surrogate biomass syngas (CH₄ 14%; CO 19%; CO₂ 14%; H₂ 16%; H₂O 30%; N₂ 7%) is investigated over a fixed bed of porous wood char as a function of temperature (800–1000 °C) and space time (1.6–6.2 min g L⁻¹). Determination of Thiele modulus evidences a change of kinetic regime from chemically- to diffusion-controlled when the temperature increases; this finding is particularly relevant when porous chars having an average pore width of 1 nm are used as catalysts. Mass diffusion transfers are accounted for by a model introducing an internal effectiveness factor. Knudsen diffusion in micropores is shown to limit the conversion rate of methane per unit mass of catalyst, and explains why such a rate is not proportional to the BET surface area, especially when the latter is higher than typically 300 m²/g. It is concluded that diffusion limitations in micropores should be taken into account, otherwise underestimated activation energy and intrinsic kinetic constant are obtained in some experimental conditions.