Effect of leaching on the ash behavior of wheat straw and olive residue during fluidized bed combustion

Fluidized bed combustion has been proven to be an attractive method for the conversion of agroresidues to energy offering economical and environmental benefits. The low melting point ash of agroresidues cause a number of problems e.g., sintering, agglomeration, deposition, etc., which consist the main obstacles for economical and viable application of this conversion method. Leaching that is considered to be a low cost pretreatment technique for the elimination of ash related problems in biomass boilers studied here. The produced results clearly demonstrate that leaching could help significantly to reduce the ash related problems caused during the operation of fluidized bed combustors with biomass.     

Experimental Study on Coal Multi-generation in Dual Fluidized Beds

An atmospheric test system of dual fluidized beds for coal multi-generation was built.One bubbling fluidized bedis for gasification and a circulating fluidized bed for combustion.The two beds are combined with two valves:one valve to send high temperature ash from combustion bed to the gasification bed and another valve to sendchar and ash from gasification bed to combustion bed.Experiments on Shenhua coal multi-generation were madeat temperatures from 1112 K to 1191 K in the dual fluidized beds.The temperatures of the combustor are stableand the char combustion efficiency is about 98%.Increasing air/coal ratio to the fluidized bed leads to theincrease of temperature and gasification efficiency.The maximum gasification efficiency is 36.7% and thecalorific value of fuel gas is 10.7 MJ/Nm3.The tar yield in this work is 1.5%,much lower than that of pyrolysis.Carbon conversion efficiency to fuel gas and flue gas is about 90%.     

Experimental investigation on co-gasification of coffee husk and sawdust in a bubbling fluidised bed gasifier

Energy can be extracted from biomass through gasification. The gasification process is influenced by the physico-chemical nature of the biomass selected for gasification. Ash content and composition of the biomasses are likely to affect the gasification process. Clinker formation in the reactor bed caused by melting and agglomeration of ashes will affect the gasification process in fluidised bed gasifiers. The agglomeration tendency of the biomass is examined by carrying out the Energy Dispersive X-ray Spectroscopy analysis on biomass ash to identify the presence of elements like potassium and sodium responsible for agglomeration. Experimental investigations on the gasification of coffee husk revealed that coffee husk is prone to agglomeration even though the hydrogen yield is more. However, gasification of saw dust is not vulnerable to agglomeration. Co-gasification of coffee husk with sawdust (which is less prone to agglomeration) is investigated experimentally.     

灰粘熔排渣技术在内循环流化床垃圾焚烧中的应用

提出将粘熔排渣技术应用于内循环流化床垃圾焚烧处理.从垃圾灰渣化学组分等角度分析其必要性和可行性.采用树脂颗粒作为床料,微晶蜡颗粒为低熔点物质,在非均匀布风内循环流化床内进行热态团聚模拟试验.试验表明：床内温度分区、团聚过程区域化是灰粘熔排渣技术的重要保证,内循环颗粒流动有利于排渣顺利进行,缩短粘性物质在床内的停留时间.图4表1参15     

Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO₂ is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO₂ capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH₄ concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO₂ capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO₂ capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.     

Preliminary results on the ash behavior of peach stones during fluidized bed gasification: evaluation of fractionation and leaching as pre-treatments

Peach stones comprise a valuable agroindustrial by-product that is available in many countries of the World and especially in the Mediterranean region. A number of important advantages such as its high energy value, the low ash content in combination with the absence of transportation costs due to the fact that is produced in agro-industries, make peach stones an ideal fuel for energy production via gasification.Gasification tests were performed in a lab-scale fluidized bed gasifier in order to study the behavior of peach stones and especially its ash during the gasification process. Apart from the tests with the initial peach stone samples, gasification tests were performed using peach stones that had been pre-treated using two different methods fractionation and leaching. Pre-treatments used in order to study their effect on the beneficiation of the materials ash and on the avoidance of ash-related problems such as deposition, agglomeration and corrosion during the gasification process.A water-cooled steel tube placed vertical to the flow of the gasification gases was used in order to collect samples of ash deposits that were analyzed using SEM-EDX analysis techniques in order to assess the effect of the pre-treatment techniques on the peach stones ash behavior.The produced results showed that peach stones can be used as gasification feedstock without significant ash problems. Fractionation resulted in a deterioration of the ash behavior of the material, increasing the amounts of alkali metals and chlorine included in its ash, while leaching showed a positive effect but to a moderate extent.     

Case studies––Problem solving in fluidized bed waste fuel incineration

Fluidized bed combustion technology has been widely used as the new, flexible, multi-fuel boiler for waste combustion and energy recovery from low grade fuels. However, problems such as low thermal efficiency, high emissions, bed agglomeration etc. are still encountered in the operation of fluidized beds. Valuable experiences were gained from two case studies recently conducted regarding wastes combustion in industrial scale fluidized beds.In the first case, the performance of a fluidized bed combustor for energy recovery from oil sludge was evaluated during the commissioning trials. Apart from the sludge characterization and bed material analysis, the combustion efficiency, solid flow balance and on stack emission of CO, SO_x and NO_x were investigated, as well as the fluidization quality. Although the system was operated with good combustion efficiency (>99.9%), sulfur dioxide emission (>1000 ppm) was found to be substantially higher than the allowable discharge limit. It was recommended to increase the limestone feed rate in order to meet the SO₂ emission standard, and subsequently, installation of a cyclone is suggested to remove the potentially significant increase in ash and fine particles.The second case study focused on the bed agglomeration observed in a fluidized bed incinerator where a burning blend of three wastes (i.e. carbon soot, biosludge and fuel oil) is involved. To understand the mechanisms and related chemistry, several analytical approaches are employed to identify the bed materials (fresh sand and degrader sand) and the clinkers formed from full scale incinerator tests. The formation of clinker is believed to follow the mechanism of partial melting and/or reactive liquid sintering. The effects of temperature and blending ratio are tested in a muffle furnace. Carbon soot is believed to be more susceptible than the other two fuels. Thermodynamic multi-phase multi-component equilibrium (TPCE) calculations predict that the main low melting point species are predominant under the oxidizing condition, suggesting that reducing conditions might be favorable to restrain bed agglomeration. This study provides valuable information for better understanding of the chemistry related to clinker formation; it also helps in developing methods for control and possible elimination of the bed agglomeration problem for the design fuels.     

Steam gasification of coal cokes in an internally circulating fluidized bed of thermal storage material for solar thermochemical processes

A laboratory-scale prototype windowed internally circulating fluidized-bed reactor made of quartz sand and coal coke particles was investigated for steam gasification using concentrated Xe-light radiation as the energy source. The quartz sand was used as a chemically inert bed material for the fluidized bed, while the coal coke particles functioned as the reacting particles for the endothermic gasification reaction. The advantages of using quartz sand as the bed material for the directly irradiated gasification reactor are as follows: (1) The bed height is maintained at a constant level during the gasification. (2) The quartz sand functions as a thermal transfer/storage medium inside the reactor. The gasification performances such as the production rates of CO, H₂, and CO₂; carbon conversion; and light-to-chemical energy conversion were evaluated for the fluidized-bed reactor with a thermal transfer/storage medium (quartz sand). The effects of using the bed material (quartz sand) on the gasification performance are described in this paper.     

Gasification of lignocellulosic biomass in fluidized beds for renewable energy development: A review

A literature review on gasification of lignocellulosic biomass in various types of fluidized bed gasifiers is presented. The effect of several process parameters such as catalytic bed material, bed temperature and gasifying agent on the performance of the gasifier and quality of the producer gas is discussed. Based on the priorities of researchers, the optimum values of various desired outputs in the gasification process including improved producer gas composition, enhanced LHV, less tar and char content, high gas yield and enhanced carbon conversion and cold gas efficiency have been reported. The characteristics and performance of different fluidized bed gasifiers were assessed and the obtained results from the literature have been extensively reviewed. Survey of literature revealed that several industrial biomass gasification plants using fluidized beds are currently conducting in various countries. However, more research and development of technology should be devoted to this field to enhance the economical feasibility of this process for future exploitations.     

Ash effects during combustion of lignite/biomass blends in fluidized bed

Aiming at investigating the role of minerals in evaluating co-firing applications of low rank coals and biomass materials, agricultural residues characteristic of the Mediterranean countries, one lignite and their blends with biomass proportions up to 20% wt, were burned in a lab-scale fluidized bed facility. Fly ashes and bed material were characterized in terms of mineralogical, chemical and morphological analyses and the slagging/fouling and agglomeration propensities were determined.The results showed that combustion of each fuel alone could provoke medium or high deposition problems. Combustion of raw fuels produced fly ashes rich in Ca, Si and Fe minerals, as well as K and Na minerals in the case of biomass samples. However, blending of the fuels resulted in a reduction of Ca, Fe, K and Na, while an increase of Si and Al elements in the fly ashes as compared to lignite combustion, suggesting lower deposition and corrosion problems in boilers firing these mixtures. The use of bauxite as an additive enriched bottom ash in calcium compounds. Under the conditions of the combustion tests, no signs of ash deposition or bed agglomeration were noticed.     

Fundamental design of a continuous biomass gasification process using a supercritical water fluidized bed

A novel biomass gasification (first stage of hydrogen production from biomass) process using a supercritical water fluidized bed was proposed and the fundamental design of the process was conducted. The flow rate was determined by evaluating the minimum fluidization velocity and terminal velocity of alumina particles enabling fluidization with the thermodynamic properties of supercritical water. Three cases were examined: a bubbling fluidized bed in which water was used mainly as the fluidized medium and biomass were added for gasification, a bubbling fluidized bed fluidized by biomass slurry feed alone, and a fast fluidized bed fluidized by biomass slurry feed alone. According to calculations of the residence time and thermal efficiency assuming heat recovery with a heat exchanger efficiency of 0.75, the bubbling fluidized bed fluidized by biomass slurry alone was appropriate for continuous biomass gasification using a fluidized bed.     

生物质内循环流化床气化炉结焦特性

分析了近年来生物质流化床气化炉的结焦研究的状况,总结了影响生物质流化床气化过程中结焦的因素和确定结焦时最小流化速度的几类计算方法。分析了辽宁营口160kW气化装置结焦过程中运行参数的变化情况,给出防止结焦发生的一些措施。     

Modeling biomass gasification in circulating fluidized beds

Detailed review of existing models resulted in the development of a new mathematical model to study biomass gasification in a circulating fluidized bed. Hydrodynamics as well as chemical reaction kinetics were considered to predict the overall performance of a biomass gasification process. The fluidized bed was divided into two distinct sections: a) a dense region at the bottom of the bed where biomass undergoes mainly heterogeneous reactions and b) a dilute region at the top where most of homogeneous reactions occur in gas phase. Each section was divided into a number of small cells, over which mass and energy balances were applied. A number of homogeneous and heterogeneous reactions were considered in the model. Mass transfer resistance was considered negligible since the reactions were under kinetic control due to good gas–solid mixing. The model is capable of predicting the bed temperature distribution along the gasifier, the concentration and distribution of each species in the vertical direction of the bed, the composition and heating value of produced gas, the gasification efficiency, the overall carbon conversion and the produced gas production rate. The modeling and simulation results were in good agreement with published data.     

Physicochemical properties and gasification reactivity of the ultrafine semi-char derived from a bench-scale fluidized bed gasifier

Zhundong coalfield is the largest intact coalfield worldwide and fluidized bed gasification has been considered as a promising way to achieve its clean and efficient utilization.The purpose of this study is to investigate the physieochemical properties and gasification reactivity of the ultrafine semi-char,derived from a bench-scale fluidized bed gasifier,using Zhundong coal as fuel.The results obtained are as follows.In comparison to the raw coal,the carbon and ash content of the semi-char increase after partial gasification,but the ash fusion temperatures of them show no significant difference.Particularly,76.53％ of the sodium in the feed coal has released to the gas phase after fluidized bed gasification.The chemical compositions of the semi-char are closely related to its particle size,attributable to the distinctly different natures of diverse elements.The semi-char exhibits a higher graphitization degree,higher BET surface area,and richer meso-and macropores,which results in superior gasification reactivity than the coal char.The chemical reactivity of the semi-char is significantly improved by an increased gasification temperature,which suggests the necessity of regasification of the semi-char at a higher temperature.Consequently,it will be considered feasible that these carbons in the semi-char from fluidized bed gasifiers are reclaimed and reused for the gasification process.     

Influence of fuel feeding positions on gasification in dual fluidized bed gasifiers

An in-bed and an on-bed feeding system are implemented in a dual fluidized bed gasifier in order to investigate the influence of the fuel feeding position on the gasification process. Two bed materials, fresh and used olivine, are used because of their varying catalytic activity. The comparison of in-bed and on-bed feeding of wood pellets shows that in-bed feeding is more favorable, because lower tar concentrations are achieved and the gas composition is closer to water–gas shift equilibrium. Better mixing of bed material and fuel particles occurs with in-bed feeding. The residence time of the gas phase in the fluidized bed is longer in the case of in-bed feeding, and therefore better performance of the gasifier is achieved. Sufficient residence time of the fuel in the bubbling bed is important when a less active bed material is used. More active bed material is capable of compensating for the shorter residence time of the gas phase in contact with bed material during on-bed feeding.     

Metals flow analysis applied to the hydrogen production by catalytic gasification of plastics

Gasification processes can play a key role in the clean energy production from fossil and non-conventional fuels because of its transformation in a synthetic gas (syngas) rich in hydrogen and carbon monoxide. Fluidized bed reactors, both bubbling and circulating, allow to use a catalytic or reacting solid material in partial or total substitution of the bed in order to promote the dehydrogenation of the fuel. Depending on the operating conditions of reactor and of fuel type, the catalyst has a strong influence on the hydrogen yield; otherwise, the effect of catalyst add to a fluidized bed can decrease during the experimental run due to its progressive deactivation. The experimental tests demonstrated that the utilization of olivine as bed material, recognized in the literature as a good catalyst for gasification process of biomass and plastics, improves the dehydrogenation of the recycled polyethylene used as fuel by producing a hydrogen-rich syngas and a coke layer on the bed particles. The tests also indicated that the olivine was not capable to catalyse the dehydrogenation process for long time because of loss of metals diffused and linked to the carbonaceous solid (coke). A Substance Flow Analysis has been applied to the experimental data in order to evaluate the Hydrogen Recovery Efficiency for the gasification tests and to follow the repartition of metals (Fe, Ni, Mg) in the input/output streams and in the bed. The limited duration of the steady state regime as observed during the tests with olivine indicated a progressive reduction of the catalytic action correlated to the loss of metals. In other words, the use of olivine allows to obtain very high hydrogen concentration in the producer gas but for a limited time and without the possibility to recover its catalytic capacity by thermal or mechanical regeneration due to the progressive loss of metals responsible of polymer dehydrogenation. This experimental result is strictly correlated to the utilization of polyethylene as fuel. This evidence has been confirmed by analysis on olivine and fines elutriated by the reactor and showed, in this work, by a material balance on metals.     

Small‐scale production of synthetic natural gas by allothermal biomass gasification

The gasification of biomass can be coupled to a downstream methanation process that produces synthetic natural gas (SNG). This enables the distribution of bioenergy in the existing natural gas grid. A process model is developed for the small‐scale production of SNG with the use of the software package Aspen Plus (Aspen Technology, Inc., Burlington, MA, USA). The gasification is based on an indirect gasifier with a thermal input of 500 kW. The gasification system consists of a fluidized bed reformer and a fluidized bed combustor that are interconnected via heat pipes. The subsequent methanation is modeled by a fluidized bed reactor. Different stages of process integration between the endothermic gasification and exothermic combustion and methanation are considered. With increasing process integration, the conversion efficiency from biomass to SNG increases. A conversion efficiency from biomass to SNG of 73.9% on a lower heating value basis is feasible with the best integrated system. The SNG produced in the simulation meets the quality requirements for injection into the natural gas grid. Copyright © 2012 John Wiley & Sons, Ltd.     

生物质流化床气化技术应用研究现状

按所得产品不同,可将生物质气化技术分为制氢、发电和合成液体燃料3大类。文章介绍了生物质流化床水蒸气气化制氢、催化气化制氢和超临界水气化制氢的工艺特点;分析了生物质流化床气化发电的技术、经济可行性;简述了生物质流化床气化合成液体燃料的研究现状;指出气化产出气化学当量比调变、焦油去除问题和合成气净化是生物质流化床气化技术应用的主要瓶颈,认为定向气化是今后研究的主要方向。     

A proposed scheme for coal fired combined cycle and its concise performance

For IGCC, the primary investment is too high due to the demand of high gasification efficiency. For PFBCC, the thermal efficiency is too low due to the relatively low turbine inlet temperature and the hot working medium of the gas turbine is not easy to clean. A new scheme is proposed for coal fired combined cycle to overcome the main drawbacks of IGCC and PFBCC. The research targets are developing a new cycle construction of coal fired combines cycle to raise the efficiency and reduce the primary investment. Actually, the new scheme is a synthesis of some existing proposals. It adopts partial gasification to reduce the primary investment of the gasification equipment. The un-gasified surplus solid is then feed to a pressurized fluidized bed boiler, but adopting Curtiss Wright type external combustion to lower the ash content in the working medium. The gas fuel from the partial gasifier is combusted in a top combustor to further increase the working medium temperature. An extremely concise performance estimation method for the new scheme and its equations is proposed in order to easily understand the basic physical meaning of the new system. Some typical calculations based on this concise method are given. Then, a more detailed computation is accomplished with Aspen Plus code. The basic feasibility of this scheme is proven to be favorable. The efficiency is higher than the existing coal fired IGCC plants. The advantage of the new scheme comes from the better utilization of coal energy. Almost all the energy of coal is first utilized in the top cycle, and then the bottom cycle, just like the gas fueled combined cycle does. The primary investment is lower than the ordinary IGCC due to the lack of air separation unit and the adoption of partial gasification. The ash content is much lower than that of the existing PFBCC plants. If no any harmful ash in working medium is required, the atmospheric fluidized bed can be applied rather than the pressurized fluidized bed. A similar proposal with atmospheric fluidized bed and its performance estimation are also given. However, its efficiency will be lower than the pressurized fluidized bed scheme.     

Biomass gasification as the first hot step in clean syngas production process – gas quality optimization and primary tar reduction measures in a 100 kW thermal input steam–oxygen blown CFB gasifier

Syngas production based on biomass gasification is an attractive, feasible alternative to fossil fuel feedstock for the production of transportation fuels. However, the product gas from biomass gasification must be cleaned and tailored to comply with strict syngas quality requirements, as it consists of a wide variety of major and minor components and impurities. The characterization of such species is important to determine downstream gas treatment steps, and to assess the efficiency of the gasification process.This paper gives an overview of the results obtained during experiments on steam–oxygen gasification of biomass using 100 kW maximal thermal input circulating fluidized bed gasifier (CFBG) that have been performed at Delft University of Technology during the CHRISGAS project. The unit is also equipped with a high-temperature ceramic gas filter and downstream reactors for upgrading of the gas.In the experiments biomass types of both woody and agricultural origin have been used. They were represented by clean wood, demolition wood, an energy crop species (miscanthus) and a true residue (Dutch straw), respectively. Moreover, different bed materials have been applied, namely quartz sand, treated and untreated olivine and magnesite. During the experiments extensive measurements of gas composition have been carried out throughout the integrated test rig. The gas characterization included major gas components as well as certain minor species and tar.The results show that with the use of magnesite as bed material, remarkable increases of hydrogen yield were attained, as compared to sand or olivine; up to a volume fraction of almost 40% (dry, nitrogen-free basis). Also the H₂:CO ratio increased from values near or lower than 1 to 2.3–2.6. This is near the values needed, for e.g., Fischer–Tropsch diesel production, indicating a potential for simplification of the gas upgrading. Furthermore, by using magnesite tar content of the raw gas was reduced to values near 2 g m^?3 (STP). Moreover, magnesite complied with the expectation to have a positive impact on agglomeration prevention for the agricultural fuels containing alkali and chlorine in the ash. The kind of olivine applied during the experiments did not yield the expected tar reduction; the measured tar concentration was even higher than when quartz sand was used as bed material. Finally kaolin proved to be an effective additive to counteract the agglomeration when fuels with high alkali content in the ash are gasified using bed material that is rich in silica, as it is the case with quartz sand and olivine.