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.     

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.     

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%.     

Power generation using coir-pith and wood derived producer gas in diesel engines

Partial combustion of biomass in the gasifier generates producer gas that can be used for heating purposes and as supplementary or sole fuel in internal combustion engines. In this study, the potential of coir-pith and wood chips as the feedstock for gasifier is analyzed. The performance of the gasifier–engine system is analyzed by running the engine for various producer gas–air flow ratios and at different load conditions. The system is experimentally optimized with respect to maximum diesel savings and lower emissions in the dual fuel mode operation while using coir-pith and wood chips separately. The performance and emission characteristics of the dual fuel engine are compared with that of diesel engine at different load conditions. Specific energy consumption in the dual fuel mode of operation is found to be in the higher side at all load conditions. The brake thermal efficiency of the engine while using wood chips in the dual mode operation is higher than that of coir-pith. The CO emission is higher in the case of dual fuel mode of operation as compared to that of diesel mode. In the dual fuel mode of operation, the higher diesel savings is achieved while using wood chips as compared to that of coir-pith. The comparison of the performance and emission characteristics of the dual fuel engine with diesel engine is also described.     

气化工艺特性对IGCC效率的影响

影响煤气化联合循环总效率的五个主要因素为煤转化率、燃气轮机循环,蒸汽循环,热旁路比率和发电效率。其中以煤转化率影响最大,它将包括产品气的显热和剩余蒸汽。在任何气化装置中气化炉是最主要单元并极大地影响气化性能。为改善气化效率,所有第二代气化方法均已引入加压操作。本文讨论了高温操作、生严能力放大、负荷跟踪能力、加煤方式、气化剂和煤气净化,同时提出仍待开发的某些值得注意的问题。     

Some process fundamentals of biomass gasification in dual fluidized bed

The dual fluidised bed gasification technology is prospective because it produces high caloric product gas free of N₂ dilution even when air is used to generate the gasification-required endothermic heat via in situ combustion. This study is devoted to providing the necessary process fundamentals for development of a bubbling fluidized bed (BFB) biomass gasifier coupled to a pneumatic transported riser (PTR) char combustor. In a steam-blown fluidized bed of silica sand, gasification of 1.0 g biomass, a kind of dried coffee grounds containing about 10 wt.% water, in batch format clarified first the characteristics of fuel pyrolysis (at 1073 K) under the conditions simulating that prevailing in the gasifier intended to develop. The result shown that via pyrolysis more than 60% of fuel carbon and up to 75% of fuel mass could be converted into product gas, while the simultaneously formed char was about 22% of fuel mass. With all of these data as the known input, a process simulation using the software package ASPEN then revealed that the considered dual bed gasification plant, i.e. a BFB gasifier + a PTR combustor, is able to sustain its independent heat and mass balances to allow cold gas efficiencies higher than 75%, given that the fuel has suitable water contents and the heat carried with the product gas from the gasifier and with the flue gas from the char combustor is efficiently recovered inside the plant. In a dual fluidized bed pilot gasification facility simulating the gasification plant for development, the article finally demonstrated experimentally that the necessary reaction time for fuel, i.e. the explicit residence time of fuel particles inside the BFB gasifier computed according to a plug granular flow assumption, can be lower than 160 s. The results shown that varying the residence time from 160 to 1200 s only slightly increased the gasification efficiency, but the reaction time available in the PTR, say, about 3 s in our case, was too short to assure the finish even of fuel pyrolysis.     

Co-gasification of coal and wood in a dual fluidized bed gasifier

In the last decade the reduction of CO₂ emissions from fossil fuels became a worldwide topic. Co-gasification of coal and wood provides an opportunity to combine the advantages of the well-researched usage of fossil fuels such as coal with CO₂-neutral biomass. Gasification itself is a technology with many advantages. The producer gas can be used in many ways; for electric power generation in a gas engine or gas turbine, for Fischer-Tropsch synthesis of liquid fuels and also for production of gaseous products such as synthetic natural gas (bio SNG). Moreover, the use of the producer gas in fuel cells is under investigation. The mixture of coal and wood leads to the opportunity to choose the gas composition as best befits the desired process. Within this study the focus of investigation was of gasification of coal and wood in various ratios and the resulting changes in producer gas composition. Co-gasification of coal and wood leads to linear producer gas composition changes with linear changing load ratios (coal/wood). Hydrogen concentrations rise with increasing coal ratio, while CO concentrations decrease. Due to the lower sulfur and nitrogen content of wood, levels of the impurities NH₃ and H₂S in the producer gas fall with decreasing coal ratio. It is also shown that the majority of sulfur is released in the gasification zone and, therefore, no further cleaning of the flue gas is necessary. All mixture ratios, from 100 energy% to 0 energy% coal, performed well in the 100 kW dual fluidized bed gasifier. Although the gasifier was originally designed for wood, an addition of coal as fuel in industrial sized plants based on the same technology should pose no problems.     

Identification of reaction zones in a commercial Sasol-Lurgi fixed bed dry bottom gasifier operating on North Dakota lignite

The Sasol-Lurgi fixed bed dry bottom gasification technology has the biggest market share in the world with 101 gasifiers in operation. To be able to further improve the technology and also to optimise the operating plants, it is important that the fundamentals of the process are understood. The main objective of this study was to determine the reaction zones occurring in the Sasol-Lurgi fixed bed dry bottom (S-L FBDB) gasifier operating on North Dakota lignite. A Turn-Out sampling method and subsequent chemical analyses of the gasifier fuel bed samples was used to determine the reaction zones occurring in the commercial MK IV, S-L FBDB gasifier operating on North Dakota lignite. The reaction zones were further compared with the same reactor operating on bituminous coal.Based on the results obtained from this study it was found that about two thirds of the gasifier volume was used for drying and de-volatilising the lignite thus leaving only about a third of the reactor volume for gasification and combustion. Nonetheless, due to the high reactivity of the lignite, the char was consumed within a third of the remaining gasifier volume. Clear overlaps between the reaction zones were observed in the gasifiers thus confirming the gradual transition from one reaction zone to another as reported in literature. Due to the high moisture content in the lignite, the pyrolysis zone in the gasifiers operating on North Dakota lignite occurred lower/deeper in the gasifier fuel bed as compared to the same gasifier operating on South African bituminous coal from the Highveld coalfield. All the other reaction zones in the gasifier operating on bituminous coal were also higher in the bed compared to the lignite operation. This can therefore explain the higher gas outlet temperatures for the S-L FBDB gasifiers operating on higher rank coals when compared to the gasifiers operating on lignite. The fact that the entire reactor volume was utilized for drying, de-volatilisation, gasification and combustion with carbon conversion of >98% makes the S-L FBDB gasifier very suitable for lignite gasification.     

隔板式内循环气化炉的设计和运行

介绍了一种新型的隔板式内循环气化炉的设计和运行。通过在气化炉的中间设置隔板,可使流化床气化炉的床层形成2个流化速度不等的流化区域,在高速区没有反应的原料或炭粒,在床层上部随流化介质流入低速区,在低速区内进行气化反应;未反应完的炭粒再经底部流回高速区,进一步进行气化反应,形成一个内部循环,从而达到延长原料在炉内的反应时间、增强气化效率的效果。实验结果表明:这种气化炉反应温度在790—850℃时,气化效果最好,产气率为1.6—1.9m3/kg,气体热值为5340kJ/m3(木质类原料)或4880kJ/m3(稻壳),气化效率可达70%。     

EQUILIBRIUM MODELING OF A DOWNDRAFT GASIFIER I—OVERALL GASIFIER

A model, based on the thermodynamic equilibrium of the C-H-O-Inert system and mass and energy balances, has been applied to the air-blown downdraft gasification of wood. The model predicts the temperature, gas composition and char yield at the exit of the gasifier for a specified set of heat loss and input condition. A parametric study has been conducted to simulate the influences of the air/feed mass ratio and moisture/feed mass ratio on the gasifier's performance. The model predictions are compared with a comprehensive set of experimental data obtained from the gasification of wood in a commercial-scale downdraft gasifier; the air/feed ratios range from 1.1 to 2.1 and the moisture/feed ratios range from 0.05 to 0.3. The predicted trends for variations in the operating parameters are in general agreement with the experimental data. The results of comparisons also indicate that the performance of the wood gasifier can be approximated reasonably well by the equilibrium model.     

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.     

Gasification of sewage sludge using a throated downdraft gasifier and uncertainty analysis

The most important objectives to gasify sewage sludge are to produce a clean gas of acceptable composition for synthesis or combustion, and to convert this solid resource into combustible-clean gas at high efficiency. The experiments of the gasification were conducted using a 5 kWe-throated downdraft gasifier. It was concluded that sewage sludge can be gasified to produce low-quality combustible gas, and would be an acceptable alternative source to fossil fuels for the production of the clean energy. It is suggested that the downdraft gasifier should be operated at 3.69–3.71 kg/h±1.43% of the feed rate, at 2.28–2.34 N m³/kg±1.84% of the air fuel ratio, around 497.74–514 N m³/m² h±1.50% of specific gasification rate and around 93.64–94.15%±1.92% of turndown ratio in order to achieve good quality gas and to avoid clinker formation at the throat of the gasifier because of high ash content of sludge. The thermal efficiency was calculated as between 39% and 40% at the optimum operation levels given above.     

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.     

Numerical simulation of coal gasification in entrained flow coal gasifier

This paper presents modeling of a coal gasification reaction, and prediction of gasification performance for an entrained flow coal gasifier. The purposes of this study are to develop an evaluation technique for design and performance optimization of coal gasifiers using a numerical simulation technique, and to confirm the validity of the model. The coal gasification model suggested in this paper is composed of a pyrolysis model, char gasification model, and gas phase reaction model. A numerical simulation with the coal gasification model is performed on the CRIEPI 2 tons/day (T/D) research scale coal gasifier. Influence of the air ratio on gasification performance, such as a per pass carbon conversion efficiency, amount of product char, a heating value of the product gas, and cold gas efficiency is presented with regard to the 2 T/D gasifier. Gas temperature distribution and product gas composition are also presented. A comparison between the calculation and experimental data shows that most features of the gasification performance were identified accurately by the numerical simulation, confirming the validity of the current model.     

Gasification kinetics of coals and wood

The chemical reactivity and kinetics of nine Canadian coal samples ranking from lignite to semianthracite and one wood sample were examined in a fixed gasifier in the presence of air and steam at 950–1000°C. The reactivity of the coal and wood samples decrease with an increase in carbon content, but increase with increasing oxygen content of the parent coal. The reaction velocity decreases with an increase in carbon content of the coal. The reaction mechanism based on the shrinking core model for the present gasification has been found to be chemical reaction controlled for the coal-steam-air system and ash-layer diffusion controlled for the wood-steam-air system. The present reaction system favors the water gas shift reaction based on the chemical composition of the product gas from the gasification.     

A new experimental Continuous Fixed Bed Reactor to characterise wood char gasification

Two-stage fixed bed gasification is one of the most promising technologies for low and medium energy production from biomass. In industrial processes, control and optimisation is often based on constructor know-how rather than on an understanding of the mechanisms involved. We present a new original tool, the Continuous Fixed Bed Reactor (CFiBR), which was specifically designed and built to enable a fine understanding of the limiting stage of a gasifier: the char bed gasification zone. The reactor, the instrumentation, the operating procedure and set-up tests are described in detail. The potential of the reactor is demonstrated through the characterisation of the gasification of a continuous wood char bed. Temperature profiles and gas concentrations along the 65 cm bed were established and showed that the most reactive zone was the first 10 cm of the char bed. Accurate energy and mass balances provided relevant information regarding the contributions of the main reactions involved in the fixed char bed gasification process.     

煤气化过程的模型和模拟与优化操作

介绍了煤气化过程的模型和煤气化过程采用机理模型的理由，固定床煤气化过程机理模型的建立以及模拟计算的结果，并探讨了固定床水煤气化炉和流化床水煤气炉制气过程优化操作参数的确定。开发的数学模型已用于制气炉的模拟计算，与实测数据比较符合，由气化过程的数学模拟气化过程不同条件下各种参数的变化规律，进而可得出气化过程的优化操作条件，其确定过程比试验法安全，省时，省料。     

碎煤加压固定床气化技术进展

介绍了碎煤加压固定床气化技术的发展历程、工艺特点,以及典型的Mark-Ⅳ型Lurgi气化技术的工业应用;对现代煤化工可选用的主流气化技术进行了简要分析,指出了碎煤加压固定床气化技术在褐煤气化方面具有的比较优势。最后对BRICC内径100mm小型固定床加压气化试验装置进行了介绍,该试验装置在煤样试烧评价、取得煤样气化特性数据以指导煤化工项目设计中具有重要作用。     

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%).     

Gasification of coal and PET in fluidized bed reactor

Blended fuel comprising 23 wt.% polyethyleneterephthalate (PET) and 77 wt.% brown coal was gasified in an atmospheric fluidized bed gasifier of laboratory-scale. The gasification agent was composed of 10 vol.% O₂ in bulk of nitrogen. Thermal and texture analyses were carried out to determine the basic properties of the fuel components. The influence of experimental conditions, such as the fluidized bed and freeboard temperatures on major and minor gas components and tar content, as well as features of the blended fuel gasification in comparison with the single coal gasification, were studied. In the case of coal with PET gasification, only the fluidized bed temperature showed significant influence on CO, CO₂, CH₄ and H₂ content in the producer gas, whereas the effect of the freeboard temperature was insignificant. In single coal gasification both temperatures had considerable and almost the same influence. The content of minor components, such as ethane, ethylene, acetylene and benzene, was found to be more dependent on the freeboard temperature than on the fluidized bed temperature. It was observed that the higher the freeboard temperatures get, the lower is the concentration of the minor components, with the exception of acetylene. The absolute contents of almost all minor and tar components were approximately three times higher in blended fuel gasification than that in single coal gasification. Finally, partition of carbon (char) and selected metals into bottom and cyclone ash in gasification of both fuels is discussed.