Coal Combustion Efficiency Research Articles

Impact of biomass char on coal char burn-out under air and oxy-fuel conditions

Although biomass co-firing is now well established in pulverised fuel (PF) combustion under conventional air-fired conditions, there is little information available on how biomass will behave in oxy-fuel firing. Using thermogravimetric analysis (TGA) and a drop tube furnace (DTF), this study examines the impact of co-firing biomass and coal under oxy-fuel conditions compared to normal air firing, with the emphasis on the potential catalytic effect of biomass-contained alkali and alkaline metals on coal char burnout. Individual chars and their blends prepared from sawdust, pinewood and a South African coal in a DTF and normal tube furnace under slow-heating conditions have been used in TGA char burnout tests. The results demonstrate that the coal/biomass char blends burned off significantly faster than predicted under both oxy-fuel and air-firing conditions, and this synergistic catalytic effect was found to be considerably more pronounced in oxy-fuel conditions. In particular, the DTF biomass/coal char blends from devolatilisation in CO2 burn off approximately two times faster than those prepared in nitrogen. To further examine the catalytic effect, the raw sawdust sample was first extracted with 5M hydrochloric (HCl) acid to remove its contained alkali and alkaline metals before the char preparation and subsequent char burnout tests. It was found that the removal of the alkali and alkaline metals led to almost complete loss of the catalytic effect as observed with the untreated sawdust derived char samples. The results indicate that biomass having relatively high contents of alkali and alkaline metals can serve as effective combustion catalysts to improve coal combustion efficiency.

The use of ilmenite as oxygen-carrier in a 500 W th Chemical-Looping Coal Combustion unit

Chemical-Looping Combustion, CLC, is a promising technology to capture CO 2 at low cost in fossil-fuelled power plants. In CLC the oxygen from air is transferred to the fuel by a solid oxygen-carrier that circulates between two interconnected fluidized-bed reactors: the fuel- and the air-reactor. This work studies the CLC technology in a 500 W th facility fuelled with bituminous coal with ilmenite as oxygen-carrier. The effect of temperature and coal particle size on coal conversion and combustion efficiency was assessed. Char gasification and combustion of both gasification products and volatile matter were evaluated. At higher temperatures, gasification and combustion reactions are promoted. Carbon capture and combustion efficiencies grow with the temperature, with faster increase at temperatures higher than 910 °C. The outgoing unburnt gases come from volatile matter that was not fully oxidized by ilmenite. Little CH 4 was measured and there were neither hydrocarbons heavier than CH 4 nor tars in the fuel-reactor outlet. At 870 °C the char conversion was 15% and reached 82% at 950 °C. The combustion efficiency in the fuel-reactor increased from 70% at 870 °C to 95% at 950 °C. The results show that ilmenite has good behavior as oxygen-carrier and that optimizing CLC with coal can lead to energy production with high CO 2 capture.

Effects of operational parameters on emission performance and combustion efficiency in small-scale CFBCs

A well-designed CFBC can burn coal with high efficiency and within acceptable levels of gaseous emissions. In this theoretical study effects of operational parameters on combustion efficiency and the pollutants emitted have been estimated using a developed dynamic 2D (two-dimensional) model for CFBCs. Model simulations have been carried out to examine the effect of different operational parameters such as excess air and gas inlet pressure and coal particle size on bed temperature, the overall CO, NO x and SO 2 emissions and combustion efficiency from a small-scale CFBC. It has been observed that increasing excess air ratio causes fluidized bed temperature decrease and CO emission increase. Coal particle size has more significant effect on CO emissions than the gas inlet pressure at the entrance to fluidized bed. Increasing excess air ratio leads to decreasing SO 2 and NO x emissions. The gas inlet pressure at the entrance to fluidized bed has a more significant effect on NO x emission than the coal particle size. Increasing excess air causes decreasing combustion efficiency. The gas inlet pressure has more pronounced effect on combustion efficiency than the coal particle size, particularly at higher excess air ratios. The developed model is also validated in terms of combustion efficiency with experimental literature data obtained from 300 kW laboratory scale test unit. The present theoretical study also confirms that CFB combustion allows clean and efficient combustion of coal.

A Study of Coal Selection for a Coal Fired Power Plant Considering Coal Property and Combustion Efficiency

A Study of Coal Selection for a Coal Fired Power Plant Considering Coal Property and Combustion Efficiency

Analysis of environmental benefits of CFB combustors via one-dimensional model

Superior environmental performance of CFB combustors is one of the prime motivations of its extensive use in industry. A well-designed CFB combustor can burn coal with high efficiency and within acceptable level of emission pollutants. The carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen oxide and char content in stack gases are the major emission pollutants in CFB combustors with respect to atmospheric environmental conditions. This paper presents a modeling study of environmental pollutions resulting from coal combustion in a CFB combustor. Using this model, the variations of these emissions along the combustor height with different operational conditions such as particle diameter, bed operational velocity and excess air are investigated. The simulation results are compared with test results obtained from the 50 kW Gazi University Heat Power Laboratory pilot scale unit and good agreement is observed. The present study proves that CFB combustion allows clean and efficient combustion of coal which is demonstrated by the fact that both experimental data and model simulation results have low and acceptable level of emission pollutants.

Simultaneous Combustion of Waste Plastics with Coal for Pulverized Coal Injection Application

A bench-scale study was conducted to investigate the effect of simultaneous cofiring of waste plastic with coal on the combustion behavior of coals for PCI (pulverized coal injection) application in a blast furnace. Two Australian coals, premixed with low- and high-density polyethylene, were combusted in a drop tube furnace at 1473 K under a range of combustion conditions. In all the tested conditions, most of the coal blends including up to 30% plastic indicated similar or marginally higher combustion efficiency compared to those of the constituent coals even though plastics were not completely combusted. In a size range up to 600 {mu}m, the combustion efficiency of coal and polyethylene blends was found be independent of the particle size of plastic used. Both linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) are shown to display similar influence on the combustion efficiency of coal blends. The effect of plastic appeared to display greater improvement on the combustion efficiency of low volatile coal compared to that of a high volatile coal blend. The study further suggested that the effect of oxygen levels of the injected air in improving the combustion efficiency of a coal-plastic blend could be more effective under fuel rich conditions.more » The study demonstrates that waste plastic can be successfully coinjected with PCI without having any adverse effect on the combustion efficiency particularly under the tested conditions. 22 refs., 12 figs., 2 tabs.« less

Plasma-aided solid fuel combustion

Plasma supported solid fuel combustion is promising technology for use in thermal power plants (TPP). The realisation of this technology comprises two main steps. The first is the execution of a numerical simulation and the second involves full-scale trials of plasma supported coal combustion through plasma-fuel systems (PFS) mounted on a TPP boiler. For both the numerical simulation and the full-scale trials, the boiler of 200 MW power of Gusinoozersk TPP (Russia) was selected. The optimization of the combustion of low-rank coals using plasma technology is described, together with the potential of this technology for the general optimization of the coal burning process. Numerical simulation and full-scale trials have enabled technological recommendations for improvement of existing conventional TPP to be made. PFS have been tested for boilers plasma start-up and flame stabilization in different countries at 27 power boilers steam productivity of 75–670 tons per hour (TPH) equipped with different type of pulverised coal burners. At PFS testing power coals of all ranks (brown, bituminous, anthracite and their mixtures) were used. Volatile content of them varied from 4 to 50%, ash—from 15 to 48% and calorific values—from 6700 to 25,100 kJ/kg. In summary, it is concluded that the developed and industrially tested PFS improve coal combustion efficiency and decrease harmful emission from pulverised coal-fired TPP.

System automation for measuring coal combustion efficiency

Monitoring the combustion process for electricity generation using coal as a primary resource is of a major concern to the pertinent industries, power generation companies in particular. The carbon content of fly ash is indicative of the combustion efficiency. The determination of this parameter is useful to characterize the efficiency of coal burning furnaces. Traditional methods such as thermo-gravimetric analysis and loss on ignition, which are based on ash collection and subsequent analysis, proved to be tediously difficult, time consuming, and costly. Thus, a new technology was inevitable and needed to monitor the process in a more efficient method, yielding a better exploitation of the resources at a lower cost. The main aim of this work is to introduce a new automated system, which can be bolted onto a furnace and work online. The system consists of three main components, namely, a laser instrument for signal acquisition, a neural network tool for training, learning and simulation, and a database system for storage and retrieval. The components have been designed, adapted and tuned for knowledge acquisition of this multi-dimensional problem. When the particles are dispersed across the test space, the instrument observed single particle counts simultaneously on the two photomultipliers. The output voltages displayed represent the intensity of horizontally and vertically polarized light, and the polarization ratio is calculated from the ratio of these voltages. It was found that the carbon-in-ash is related to the polarization ratio and the carbon mass fraction could be determined to within ± 1 per cent of the carbon content. However, if a proximate analysis is performed on the coal, the prediction could be improved to within ± 0.05 per cent using neural network simulation. The system has been tested for a range of coal ashes and proved to be efficient.

Coal combustion efficiency in CFB boiler

The carbon content in the fly ash from most Chinese circulating fluidized bed (CFB) boilers is much higher than expected, thus directly influencing the combustion efficiency. In the present paper, carbon burnout was investigated both in field tests and laboratory experiments. The effect of coal property, operation condition, gas-solid mixing, char deactivation, residence time and cyclone performance are analyzed seriatim based on large amount of experimental results. A coal index is proposed to describe the coal rank, defined by the ratio of the volatile content to the coal heat value, is a useful parameter to analyze the char burnout. The carbon content in the fly ash depends on the coal rank strongly. CFB boilers burning anthracite, which has low coal index, usually have high carbon content in the fly ash. On the contrary, the CFB boilers burning brown coal, which has high coal index, normally have low carbon content. Poor gas-solid mixing in the furnace is another important reason of the higher carbon content in the fly ash. Increasing the velocity and rigidity of the secondary air could extend the penetration depth and induce more oxygen into the furnace center. Better gas solid mixing will decrease the lean oxygen core area and increase char combustion efficiency. The fine char particles could be divided into two groups according to their reactivity. One group is “fresh” char particles with high reactivity and certain amount of volatile content. The other group of char particles has experienced sufficient combustion time both in the furnace and in the cyclone, with nearly no volatile. These “old” chars in the fly ash will be deactivated during combustion of large coal particles and have very low carbon reactivity. The generated fine inert char particles by attrition of large coal particles could not easily burn out even with the fly ash recirculation. The fraction of large coal particles in coal feed should be reduced during fuel preparation process. The cyclone efficiency controls the particle residence time in CFB loop, especially for that of the fine particles. So the cyclone efficiency, especially the cut size, will greatly influence the carbon content in the fly ash.

Alkali Sorption in Second-Generation Pressurized Fluidized-Bed Combustion

Modern second-generation pressurized fluidized-bed combustion (2nd-Gen. PFBC) combined cycle power systems are developed to increase the efficiency of coal combustion. However, combined cycle power systems require reliable hot gas cleaning. The alkali metals sodium and potassium can lead to severe damage of the gas turbine blades, because of hot corrosion. This investigation was conducted to assess the potential for the reduction of alkali metals from hot gas by different aluminosilicate sorbents, such as silica, bauxite, bentonite, and mullite, under reducing atmospheres at a temperature of 750 °C. Using a flow channel reactor, an alkali chloride-laden gas stream was passed through a bed of aluminosilicate sorbents. Qualitative and quantitative analysis of the hot gas downstream of the sorbent bed was performed using high-pressure mass spectrometry (HPMS). Thus, the influence of different gas atmospheres on the alkali sorption was determined and conclusions were drawn concerning the chemical reactions that were occurring inside the sorbent bed. Moreover, annealing experiments were performed to determine the sorption capacity of the sorbent. Subsequent scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and X-ray diffraction (XRD) analyses showed that the phases formed inside the tested sorbent materials. Sorbent capacities for sodium and potassium of up to 110 mg/g have been achieved after 192 h for bentonite. Finally, thermodynamic computations were performed to scale the results of the laboratory experiments up to the conditions prevalent in 2nd-Gen. PFBC systems. The investigations revealed the possibility of reducing the overall alkali concentration in the hot gas under 2nd-Gen. PFBC conditions to values of <50 ppbv through the use of bentonite and activated bauxite. The amount of sorbent needed for a 500 MW el power plant was estimated to be ∼100 kg/h at a hot gas temperature of 700 °C.

The Effect of BOF Slag and BF Flue Dust on Coal Combustion Efficiency

Injection into a BF of BOF slag to improve the slag formation and of BF flue dust to improve the recycling of in-plant fines has previously been tested. The effects on the PC combustion efficiency ...

Theoretical consideration of problems relating to high coal rate injection into blast furnaces

Some key problems relating to high rate pulverised coal injection into the blast furnace are discussed, including furnace permeability in the lower furnace and raw materials quality, heat compensation and theoretical flame temperature, oxygen enrichment, material and gas distribution control and so on. A new method of calculation of theoretical flame temperatures is proposed, taking into account coal combustion efficiency in the raceway. Measures that should be taken for increasing coal injection rates are discussed.

Emission characteristics of NO x and unburned carbon in fly ash during combustion of blends of bituminous/sub-bituminous coals ☆

Because of large recoverable reserves, sub-bituminous coal is anticipated to become a substitute for the bituminous coal used presently as fuel in Japan. Most sub-bituminous coal contains more than 20% moisture. In the case of the utilization of sub-bituminous coal in Japan, it is considered that, in the near future, sub-bituminous coal will be applied to conventional boilers designed for bituminous coal combustion. We have already clarified that moisture in coal causes low combustion efficiency and a high conversion ratio of fuel nitrogen to NO x compared with a coal having the same fuel ratio with little moisture, in pulverized coal combustion. In particular, oxygen consumption during combustion of sub-bituminous coal with high moisture is more delayed than that during combustion of bituminous coal. Therefore, sub-bituminous coal is utilized in blended combustion with bituminous coal. In this paper, the emission characteristics of NO x and unburned carbon in fly ash in blended combustion of sub-bituminous coal with bituminous coal is investigated using a pulverized coal combustion test furnace with a single burner (coal combustion rate: 0.1 t/h). NO x concentration in blended combustion approaches the NO x concentration in sub-bituminous coal combustion with the increase of the amounts of sub-bituminous coal. As the remaining moisture content in sub-bituminous coal increases, NO x concentration becomes higher in blended combustion. On the other hand, the unburned carbon concentration in fly ash in blended combustion is higher than that in non-blended combustion of each coal. Furthermore, unburned carbon concentration in fly ash becomes much higher as the moisture in sub-bituminous coal increases. This tendency is caused by the influence of the moisture in the sub-bituminous coal on the decrease of the combustion efficiency of bituminous coal, since moisture in sub-bituminous coal delays oxygen consumption at the burner exit and the shape of the combustion flame is diffused. When the swirl vane angle of the burner is controlled in order to suppress diffusion of the combustion flame, oxygen consumption progresses and emissions of both NO x and unburned carbon in fly ash are reduced.

A new image reconstruction method for tomographic investigation of fluidized beds

AbstractIt is important to investigate the behavior of circulating fluidized beds (CFB) for efficient coal combustion. Electrical capacitance tomography (ECT) was applied to the measurement of gas/solids distribution in square CFBs. An iterative method was used to achieve improved image reconstruction over the commonly used linear back‐projection algorithm. The solids concentration profile, temporal variation of average solids concentration, and frequency spectra, which were obtained from the ECT data, are presented.

Waste plastics as supplemental fuel in the blast furnace process: improving combustion efficiencies

The possibility of using waste plastics as a source of secondary fuel in a blast furnace has been of recent interest. The success of this process, however, will be critically dependent upon the optimization of operating systems. For instance, the supply of waste plastics must be reliable as well as economically attractive compared with conventional secondary fuels such as heavy oil, natural gas and pulverized coal. In this work, we put special importance on the improvement of the combustibility of waste plastics as a way to enhance energy efficiency in a blast furnace. As experimental variables to approach this target, the effects of plastic particle size, blast temperature, and the level of oxygen enrichment were investigated using a custom-made blast model designed to simulate a real furnace. Lastly, the combustion efficiency of the mixture of waste plastics and pulverized coal was tested. The observations made from these experiments led us to the conclusion that with the increase of both blast temperature and the level of oxygen enrichment, and with a decrease in particle size, the combustibility of waste polyethylene could be improved at a given distance from the tuyere. Also it was found that the efficiency of coal combustion decreased with the addition of plastics; however, the combustion efficiency of mixture could be comparable at a longer distance from the tuyere.

1504 フラクタル・モデルを用いた高温空気燃焼場における微粉炭燃焼特性の解析

High-Temperature Air Combustion Technology (HiCOT) of pulverized coal combustion is expected to improve the combustion efficiency of coal and to reduce NO_x emission. The objective of this research is to predict the combustion behavior of the pulverized coal in such a furnace with optimum design, which fully utilizes the advantage of HiCOT technique. Especially, char combustion behavior under high-temperature air condition is numerically calculated using the Fractal Reaction Model and compared with the experimental data. From the results of the calculation, this new reaction model in high temperature air combustion condition predicts the conversion of the pulverized coal more highly than the conventional model. Comparing with the experimental data, the new model can predict the char conversion better than the conventional model in the early stage of char combustion.

EVOLUTION LIGNITE MESOPORE STRUCTURE DURING DRYING. EFFECT OF TEMPERATURE AND HEATING TIME

The knowledge of the intrinsic pore structure of coals is significant in elucidating the kinetics of mass transport and chemical reaction that leads to design of more efficient coal combustion and conversion equipment. The results of pore structure studies of Greek lignite are reported in this work. Isothermal drying of Greek lignite samples, under vacuum, caused mesopore structure evolution despite the severe (∼50%) particle size contraction due to heating. Mesopore volume and surface area were increased as the drying temperature was raised to 200°C while further drying up to 250°C caused a mesopore volume and surface area decrease. Lignite drying at 100°C for up to 3 h resulted in a monotonic increase of the mesopore structure properties while heating for a longer period i.e., 6 h, despite a slight increase of weight loss, caused pore volume and surface area reduction. Nitrogen sorption (77 K) hysteresis data obtained for partially dried samples have been processed to deduce BET surface area and pore size distributions (PSD) by using both the Roberts and a new method based on a Corrugated Pore Structure Model (CPSM-nitrogen) methods. The latter method was applied successfully in hysteresis loop simulations and predicted pore surface areas consistent with the respective BET values. Bimodal PSD have been detected with one peak at 3 nm and the second at 20 nm while surface area varied over the range 2.98–5.30 m2/g. Dry Greek lignite has shown a higher mesopore volume than that of several American and Canadian coals of varying rank. Mesopore volume distribution of dry Greek lignite, obtained from nitrogen sorption data, agree well with those deduced from mercury penetration data corrected for coal compressibility.

00/03054 Understanding the behavior of Australian black coals in pressurized fluidized bed combustion

Ultimately, this study aims to predict the coal combustion efficiency in an industrial pressurized fluidized bed combustor (PFBC) for Australian black coals. This combustion efficiency depends predominantly upon the rate of elutriation of fine carbon particles, which is proportional to bed carbon loading in atmospheric experiments. The bed carbon loading is, in turn, dependent upon the rate of combustion of char particles within the PFBC. A novel batch-fed reactor has been designed, constructed and commissioned to enable separation and study of the mechanisms of coal devolatilization, char combustion and fine carbon particle elutriation in a PFBC and extraction of coal-specific parameters to describe these processes. The attrition and char combustion rates can only be determined experimentally and it is essential to match the environment around each coal particle, so that the results may be translated to the industrial scale. Therefore, the rig was designed for identical conditions of pressure, temperature, particle size and fluidizing velocity within the bed to those used industrially. The exhaust gas is analyzed continuously for oxygen, carbon dioxide, carbon monoxide and hydrocarbons as a function of time after coal injection, allowing separation and identification of the devolatilization and char combustion stages as well as measurement ofmore » the combustion rates. The elutriated carbon particles undergo minimal freeboard combustion and are collected in a cyclone and an in-line filter over any period of time during the experiment, for subsequent analysis. The sand bed containing the rig for collection and characterization of the partially burnt char particles. The rig is mostly computer-controlled and the design was subjected to a hazards analysis before construction. Results from the rig will be used in a mathematical model to predict the performance of the coals in industrial-scale PFBC.« less

98/04039 Investigation of coal combustion efficiency for mixed injection into BF

98/04039 Investigation of coal combustion efficiency for mixed injection into BF

A TG method of analysis for use in monitoring the efficiency of fluidized-bed coal combustion processes

A TG method developed for the analysis of reaction products in fluidized-bed coal combustion processes in which limestone is added for sulphur capture was modified and further refined. The method is now capable of determining H 2O, Ca(OH) 2, CaCO 3, carbonaceous materials, CaSO 4 and Fe 2O 3 in the samples. The viability and accuracy of the method were verified by analysing reagent grade mixtures and fluidized-bed samples from a pilot-scale fluidized-bed coal combustion process and comparing the results with those of other methods. The TG method may be used to monitor the efficiency of coal combustion and limestone utilization in fluidized-bed processes.