Combustion of coffee husks

Combustion mechanisms of two types of coffee husks have been studied using single particle combustion techniques as well as combustion in a pilot-scale fluidised bed facility (FBC), 150 mm in diameter and 9 m high. Through measurements of weight-loss and particle temperatures, the processes of drying, devolatilisation and combustion of coffee husks were studied. Axial temperature profiles in the FBC were also measured during stationary combustion conditions to analyse the location of volatile release and combustion as a function of fuel feeding mode. Finally the problems of ash sintering were analysed. The results showed that devolatilisation of coffee husks (65–72% volatile matter, raw mass) starts at a low temperature range of 170–200°C and takes place rapidly. During fuel feeding using a non water-cooled system, pyrolysis of the husks took place in the feeder tube leading to blockage and non-uniform fuel flow. Measurements of axial temperature profiles showed that during under-bed feeding, the bed and freeboard temperatures were more or less the same, whereas for over-bed feeding, freeboard temperatures were much higher, indicating significant combustion of the volatiles in the freeboard. A major problem observed during the combustion of coffee husks was ash sintering and bed agglomeration. This is due to the low melting temperature of the ash, which is attributed to the high contents of K₂O (36–38%) of the coffee husks.     

生物质与煤混燃灰熔融结渣特性的实验研究

通过对生物质燃料（锯末、玉米秸和麦秸）与煤混燃灰化学成分和熔融温度的测定,利用灰分的碱酸比B／A、硅比G、硅铝比S／A、积灰沾污特性指数Hw、磨损特性指数日。等判别指数对生物质纯燃、与煤混燃时的结渣、积灰和磨损特性进行了研究和分析。结果表明,生物质灰都具有结渣倾向,麦秸灰具有严重的积灰倾向,玉米秸灰和锯末灰有易积灰倾向。生物质灰的磨损倾向都较轻微。随着生物质与煤混燃比例的增加,结渣有加重趋势。灰中酸性氧化物和碱性氧化物的含量会直接影响灰的熔融温度。     

Shedding of ash deposits

Ash deposits formed during fuel thermal conversion and located on furnace walls and on convective pass tubes, may seriously inhibit the transfer of heat to the working fluid and hence reduce the overall process efficiency. Combustion of biomass causes formation of large quantities of troublesome ash deposits which contain significant concentrations of alkali, and earth-alkali metals. The specific composition of biomass deposits give different characteristics as compared to coal ash deposits, i.e. different physical significance of the deposition mechanisms, lower melting temperatures, etc. Low melting temperatures make straw ashes especially troublesome, since their stickiness is higher at lower temperatures, compared to coal ashes. Increased stickiness will eventually lead to a higher collection efficiency of incoming ash particles, meaning that the deposit may grow even faster.     

生物质燃料成份对生物质流化床燃烧的影响

基于流化床的生物质燃烧技术应用日益广泛.生物质燃料流化床的缺点是容易产生床体结焦.灰的组分和生物质燃料中的硫、氯是影响流化床锅炉烧结倾向、锅炉污染速率、灰沉积过程、结焦和过热器腐蚀的主要因素.以灰成分为基础划分生物质燃料,可分为具有显著的不同燃烧特性的3类.在实践的基础上,阐述了各类生物质燃料及其灰分特性,以及在流化床...     

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.     

秸秆类生物质燃烧特性的研究

利用热重分析仪对江苏宿迁地区的玉米秆、稻秆和麦秆三种生物质的燃烧特性进行了分析，测定了生物质的灰熔点和灰组成，用XRD和TEM表征了生物质灰的物相结构和形貌。研究结果表明，三种生物质的燃烧规律基本一致，燃烧过程可分为四个阶段：干燥过程，热解过程，晶型转变过程和熔融过程；三种生物质中，玉米秆灰熔点最高。灰量最少且碱金属含量最低；生物质灰为形态各异的纳米颗粒。     

Ash deposition behaviors during combustion of raw and water washed biomass fuels

Biomass-fired boilers have the tendency to suffer from severe problems of fouling and slagging due to the high potassium content of biomass fuel. The troublesome potassium, however, can be removed efficiently by water washing pretreatment. In this study, the ash deposition behaviors during combustion of raw and water washed biomass fuels were investigated by a one-dimensional furnace and a deposition probe. Two biomass fuels (corn stalk and wheat straw) were used, and deposition mass, deposition efficiency, composition and morphology of the deposit were studied. The ash deposition while firing raw biomass exhibits a “fast?slow?fast?slow” trend with the sampling time. After water washing, the deposition mass decreases dramatically, and the deposition efficiency reduces gradually as the sampling time increases. The analyses of elemental composition, morphology and chemical composition on the deposit from raw biomass imply that the condensation/thermophoresis is quite significant in the earlier deposition stage, whereas the chemical reaction is remarkable in the later stage. After water washing, the potassium content of the deposit decreases significantly. Morphology and chemical composition analyses indicate that the deposit from water washed biomass ascribes to the physical accumulation of non-viscous fly ash particles. The deposition mass can easily approach a maximum value. The ash fusion temperatures of deposits increase remarkably after water washing. In addition, ash deposition mechanisms during biomass combustion are discussed.     

Influence of phosphorus on ash fouling deposition of hydrothermal carbonization sewage sludge fuel via drop tube furnace combustion experiments

Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases.     

生物质链条炉床层燃烧建模及一次风影响分析

基于多孔介质非热平衡的方法,考虑了床层高度的变化及颗粒内部温度梯度的影响,建立了一维非稳态燃烧模型来模拟炉排上移动床层的生物质燃烧。模拟计算结果与实验值对比分析表明,总体上数值计算结果与实验数据吻合较好。通过对不同一次风参数下床层燃烧的模拟结果分析得到,随着一次风风量的增加,床层剩余质量先减小后增大;在燃烧前期,床层出口气体温度上升速度减慢,挥发分析出速率降低,焦炭燃烧速率增大;在燃烧中期,床层出口气体温度先上升后下降,焦炭燃烧速率下降。一次风风温相比于一次风风量对床层燃烧影响较小,增大一次风风温可以提高挥发分析出速率,降低床层出口气体温度和床层剩余质量。     

马蹄形火焰玻璃熔窑燃烧空间流动与传热的数值模拟

对马蹄形火焰玻璃窑炉燃烧空间内的流动、燃烧及辐射传热等过程进行了数值模拟研究,得到了炉内燃烧空间的速度场、温度场、组分浓度分布及燃烧空间向玻璃液面传递的热流分布。探讨了燃烧空间入口的进气角度对炉内温度场和向玻璃面传递的热流的影响,模拟结果表明,当入口的进气角度在5°～10°之间时,传热效果较好。     

Assessment of the rice husk lean-combustion in a bubbling fluidized bed for the production of amorphous silica-rich ash

Rice husk lean-combustion in a bubbling and atmospheric fluidized bed reactor (FBR) of 0.3 m diameter with expansion to 0.4 m in the freeboard zone and 3 m height was investigated. Experiment design - response surface methodology (RSM) - is used to evaluate both excess air and normal fluidizing velocity influence (independent and controllable variables), in the combustion efficiency (carbon transformation), bed and freeboard temperature and silica content in the ashes. Hot gases emissions (CO₂, CO and NO_x), crystallographic structure and morphology of the ash are also shown. A cold fluidization study is also presented. The values implemented in the equipment operation, excess air in the range of 40-125% and normal fluidization velocities (0.13-0.15 Nm/s) show that the values near the lower limit, encourage bed temperatures around 750 °C with higher carbon transformation efficiencies around 98%. However, this condition deteriorated the amorphous potential of silica present in the ash. An opposite behavior was evidenced at the upper limit of the excess air. This thermochemical process in this type of reactor shows the technical feasibility to valorize RH producing hot gases and an amorphous siliceous raw material.     

生物质成型燃料循环流化床燃烧试验研究

在0.15 MW循环流化床试验台上,进行了玉米秸秆和苹果树枝成型燃料燃烧特性以及排放特性的试验研究.试验结果表明,玉米秸秆成型燃料和苹果树枝成型燃料在循环流化床中能够稳定燃烧,燃烧效率达到96.8%;尾气中HCl的排放质量浓度较高,SO2的排放质量浓度随着生物质的硫含量增大而增大;使用选择性炉渣作为床料同时加入黏土作为添加剂的方法,能够有效抑制玉米秸秆成型燃料和苹果树枝成型燃料燃烧过程中床料的黏结,黏土对循环流化床的物料循环流化起到了稳定作用.     

混合生物质颗粒燃料的燃烧特性

从生物质燃料的灰熔融性和热值两个关键性的特性指标着手,以玉米秸秆和水稻稻壳作为主要原料,用试验优化设计和分析的方法 ,寻求混合生物质颗粒燃料的优化配方,提高混合生物质颗粒的软化温度ST>1 400℃,有效地解决了玉米秸秆颗粒结渣问题。对混合生物质颗粒进行了热重试验研究和燃烧机理、动力学特性分析,结果表明:混合生物质颗粒具有易着火和单峰值热解特性,燃烧性能良好。此研究为改善单一成分生物质燃料的燃烧性能,推广利用生物质能提供指导性建议。     

Modeling of multiphase combustion and deposit formation in a biomass-fed furnace

A comprehensive computational model for biomass combustion is presented, featuring a solid phase combustion model, a fluid dynamic model for the gas phase, and a solid particle transport and deposit formation model. The submodel developed to track particle trajectories is briefly outlined. The model is implemented on the Finite Element code XENIOS++, and a test case is considered of a furnace burning wooden biomass chips added with water and inert material; a dedicated flamelet library is worked out to model combustion. Results underline the capability of the code to predict combustion conditions and, in particular, the growth rates of deposits of different particle size over the furnace walls, as well as the most critical locations for particle deposition.     

横火焰玻璃窑炉燃烧空间流动与传热的数值模拟

对横火焰玻璃窑炉燃烧空间内的流动、燃烧及辐射传热等过程进行了数值模拟研究,建立了玻璃窑炉燃烧空间内的综合数学模型,给出了诸控制方程的统一的数值解法,得到了炉内燃烧空间的速度场、温度场、组分浓度分布及燃烧空间向玻璃液面传递的热流分布。     

Experimental study on Egyptian biomass combustion in circulating fluidized bed

The present study investigates the combustion of four kinds of biomass in a circulating fluidized bed. The combustion chamber is a steel cylinder with 145 mm inner diameter and 2 m height. Tests were conducted on wheat straw, sawdust-wood, cottonseed burs, and corncobs. Excess air was varied for each fuel. Temperature, heat flux and gas emissions were measured along the combustion chamber and at the chimney inlet. Results showed that sawdust-wood produces the highest values of CO emissions (about 3000 mg/Nm³). On the other hand, cottonseed burs produce the lowest values of CO emissions (about 250 mg/Nm³). The SO₂ emissions were very low in all tests (less than 20 mg/Nm³). The lowest emission value occurred at an excess air ratio (EA) of 1.24 except for cottonseed burs where it was 1.4.     

A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags

Gasification or combustion of coal and biomass is the most important form of power generation today. However, the use of coal/biomass at high temperatures has an inherent problem related to the ash generated. The formation of ash leads to a problematic phenomenon called slagging. Slagging is the accumulation of molten ash on the walls of the furnace, gasifier, or boiler and is detrimental as it reduces the heat transfer rate, and the combustion/gasification rate of unburnt carbon, causes mechanical failure, high-temperature corrosion and on occasions, superheater explosions. To improve the gasifier/combustor facility, it is very important to understand the key ash properties, slag characteristics, viscosity and critical viscosity temperature. This paper reviews the content, compositions, and melting characteristics of ashes in differently ranked coal and biomass, and discusses the formation mechanism, characteristics, and structure of slag. In particular, this paper focuses on low-rank coal and biomass that have been receiving increased attention recently. Besides, it reviews the available methodologies and formulae for slag viscosity measurement/prediction and summarizes the current limitations and potential applications. Moreover, it discusses the slagging behavior of different ranks of coal and biomass by examining the applicability of the current viscosity measurement methods to these fuels, and the viscosity prediction models and factors that affect the slag viscosity. This review shows that the existing viscosity models and slagging indices can only satisfactorily predict the viscosity and slagging propensity of high-rank coals but cannot predict the slagging propensity and slag viscosity of low-rank coal, and especially biomass ashes, even if they are limited to a particular composition only. Thus, there is a critical need for the development of an index, or a model or even a measurement method, which can predict/measure the slagging propensity and slag viscosity correctly for all low-rank coal and biomass ashes.     

Experimental investigation on the kinetics of biomass combustion in vertical tube reactor

The paper presents results of experimental investigation performed in order to examine kinetics of loose biomass combustion in vertical tube reactor. The investigation conducted included continuous measurement of the fuel mass loss rate, with two biomass combustion models (piston and batch model) proposed, each relying on appropriate theoretical postulates. Results obtained indicated that piston combustion model had shown better agreement between theoretical and experimental data and was therefore used to further analyse effects of excess-air on the combustion kinetics, as well as associated effects of flue gas recirculation. Recirculation of cold flue gases is used to lower peak temperature inside the furnace, as well as to reduce a zone where ash melting problems may potentially occur. During the investigation performed, effects of flue gas recirculation on the combustion process were simulated by simultaneously injecting nitrogen and air flows into the furnace. This was deemed appropriate to simulate real-life conditions prevailing in the furnace with gas recirculation. Experiments were conducted on specially designed and constructed apparatus that enabled kinetic parameters to be determined for the combustion of different types of biomass. Results obtained have indicated that quantity of air affects kinetics of biomass combustion and that increased recirculation leads to reduced biomass reaction rate. The same conclusion was reached based on the results of experiments conducted with two different types of agro-biomass, namely wheat straw and corn stalks, which are most commonly used for energy generation. Results achieved are deemed particularly important when it comes to design of new plants that utilize cigarette type combustion system, but also for development of numerical models used to simulate combustion of biomass bales, with special emphasis placed on the impact of recirculation gases on the combustion kinetics.     

Effect of varying fuel types on oxy‐combustion performance

In search for clean energy solutions in a global warming era, oxy‐fuel combustion systems are promising. In the study, combustion products are calculated, and exergy analysis is done using the proposed multifeature equilibrium combustion model. And the results obtained for oxy‐combustion of different fuels at various oxygen fractions are given in comparison with conventional combustion. For validation, the model results are compared with popular combustion calculation tools, GASEQ and CEA. Effect of oxygen content on oxy‐combustion exergy analysis is calculated, also considering changes in equivalence ratio and combustion chamber inlet temperature. Moreover, indicating parameters for combustion performance, temperature ratio, chemical exergy, physical exergy, total specific exergy, and exergy destruction are utilized in the calculations elaborately. Changes in combustion product mole fractions are explained for rich and lean combustion regions. And also, specific exergy results are presented. In terms of exergy destruction, oxy‐combustion is more advantageous than conventional combustion. It has been shown that exergy destruction in combustion process with conventional air is approximately 1.5 times higher compared with 21% oxy‐combustion, both at different equivalence ratios and at different combustion chamber inlet temperatures. Nowadays, environment‐friendly, clean energy production systems are growing in numbers. In this concept, exergetic analyses of combustion for different fuels and greener natural gas, compared with diesel, gasoline, and methanol, are given in comparison. Considering four fuel types, advantageous and disadvantageous cases are presented for oxy‐combustion at different oxygen fractions and conventional combustion. As a result, diesel fuel is more advantageous than the other three fuel types, in terms of temperature ratio and exergy. Natural gas combustion appears to be disadvantageous in terms of specific exergy and temperature ratio, but it is the most advantageous in terms of exergy destruction. Consequently, distinctive comparison is done for oxy‐combustion and conventional combustion, determining positive and negative effects for different fuels.     

Effects of primary air temperature on emissions of a gas turbine fired by liquefied spruce wood

Innovative lignocellulosic biofuel, obtained through solvolysis of spruce wood in multifunctional alcohols was tested in an experimental turbine engine with different primary air temperatures. Variation of primary air temperature was used to emulate two types of microgas turbine generators – fully recuperated and simple cycle setups resulting in different temperatures of combustion chamber intake air. Results indicate that different temperatures, velocities, and flow conditions in primary zone of combustion chamber strongly influence on droplet penetration depth and rate of mixture formation as well as emissions formation. For the innovative lignocellulosic biofuel emission trends of CO and THC were found to be significantly higher in simple cycle mode, whereas no influence of operation mode was observed for baseline diesel fuel. NO_x emissions of innovative biofuel generally increased in regenerative cycle mode and also became sensitive on turbine inlet temperature, with similar trends being observed with diesel fuel. In the case of innovative fuel, deposits of char and polymerized fuel were observed on the combustor walls after operation in simple cycle mode, whereas in regenerative cycle mode, only small amounts of ash deposits were found on hot path surfaces, indicating beneficial influence of high primary air temperatures on combustion efficiency of innovative fuel.