Engineering equations for determining coal-water fuel combustion stages

Research on the combustion of coal-water fuel has grown significantly in recent years. Many experimental and numerical calculated data are available in the scientific literature. Nowadays, a large number of different models of the combustion of coal-water fuel have been developed for numerical research. However, there is no systematic generalization of all these data in the published results.This article is an attempt to fill the gap in the field of such a generalization. Simple equations are obtained based on a generalization of experimental data on the combustion of coal-water fuel for engineering applications, as well as to verify already existing and new mathematical models being developed. Those equations are basic for calculation of each stage duration of “pure” coal-water fuel (without any additions) combustion parameters in still air; they take into account all metamorphic stages of source coal, the different water content in the range of 25–50% and mineral impurities content (ash) 0.5–60% in the temperature range of the oven medium 550–1000 °C.     

Evidence of the illegitimacy of the additive approach to the determination of the thermophysical properties of coal-water fuel with glycerol

For the fuel ignition, the thermal conductivity and heat capacity are the key properties that determine the pre-ignition behavior of the drop of the fuel. The classic monophase fuels, such as natural gas, liquid propellants, or solid one-component fuels, have been investigated for a long time; and their thermophysical properties are well known in most of the cases. Composite fuels, which have recently attracted the attention of the researchers, have complex contents. In many cases, composite fuel is a mixture of solid and liquid components in the form of a slurry. Coal-water fuel and its derivatives with different additives are examples of such type fuels. For those fuels, the thermophysical properties are usually unknown. Nowadays, researchers use simple additivity theory for the calculation of the thermophysical properties of complex fuels for the first approach. Authors of this research believe that the simple additivity approach is not correct and can lead to the wrong results in the case of the numerical research of the ignition and burning processes of such a fuel. In the present research, the thermophysical properties of coal-water fuel with glycerol additives were experimentally obtained. It was found that the coefficient of thermal conductivity increases with temperature and varies in the range of 0.45 to 0.53 W/(m·K). The heat capacity of the fuel also increases with the temperature and varies from 4.7 to 5.5 kJ/kg·K. The higher the glycerol content, the lower the thermal conductivity and heat capacity of the composite fuel in the investigated temperature range. The results confirm the failure of the approach of the additivity law usage. Neither, thermal conductivity coefficient or heat capacity of the coal-water fuel with the addition of up to 20% glycerol complies with the additivity law. Differences between real values of the thermophysical properties and calculated ones are more than 30% to 50%. Empirical expressions for calculation of the thermophysical properties of coal-water fuel with the addition of up to 20% glycerol are presented.     

Hydrothermal preparation of low-rank coal-water fuel slurries

Lignite and sub-bituminous coals are by nature high in inherent moisture and oxygen content and low in calorific heating value. As-mined low-rank coals when mixed with water generally produce slurries with low solids content and with heating values generally less than 11.6 MJ/kg. These same slurries are usually unstable and form hard-pack sediments quickly, unless chemical additives or constant agitation are added. The low heating value and poor storage and flow characteristics of these coal-water mixtures discourage the use of raw lignite and subbituminous coals for preparation of slurries for fuel purposes.Hydrothermal conditioning, in a water slurry at temperatures above 230 °C and pressures above 552 MPa, is one method that can significantly aid in the preparation of low-rank coal-water fuels. High-pressure hot-water thermal conditioning of lignites and sub-bituminous coals has been found not only to change both the chemical and physical characteristics of the coal but also to alter the coal slurry's rheological properties. These changes are controlled by process variables (i.e. temperature, residence time, particle size, and mode of processing) and result because of decarboxylation, mild pyrolysis, extraction, dehydration, and surface modification; all of which occur during hydrothermal treatment. Using the hydrothermal process, concentrated low-rank coal-water slurries with heating values approaching or exceeding the heating value of the as-mined coal have been achieved with pseudoplastic flow behavior and stability towards settling, without the use of additives.Pilot-scale studies using a 90-kg/hr process development unit (PDU) are currently under way to produce hydrothermally treated low-rank coal fuel slurries for combustion tests in a pilot-scale, slurry-fed test furnace.     

Numerical Simulation of Pressure Effects on the Gasification of Australian and Indian Coals in a Tubular Gasifier

This article presents a numerical study on the effect of pressure on the gasification performance of an entrained flow tubular gasifier for Australian and Indian coals. Gasification using a substoichiometric amount of air, with or without steam addition, is considered. The model takes into account phenomena such as devolatilization, combustion of volatiles, char combustion, and gasification. Continuous-phase conservation equations are solved in an Eulerian frame and those of the particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The numerical results obtained show that the gasification performance increases for both types of coal when the pressure is increased. Locations of devolatilization, combustion, and gasification zones inside the gasifier are analyzed using the temperature plots, devolatilization plots, and mass depletion histories of coal particles. With increase in pressure, the temperature inside the gasifier increases and also the position of maximum temperature shifts upstream. For the high-ash Indian coal, the combustion of volatiles and char and the gasification process are relatively slower than those for the low-ash Australian coal. The mole fractions of CO and H₂ are found to increase with increase in pressure, in all the cases considered. Further, the effects of pressure on overall gasification performance parameters such as carbon conversion, product gas heating value, and cold gas efficiency are also discussed for both types of coals.     

The efficiency of heat transfer through the ash deposits on the heat exchange surfaces by burning coal and coal-water fuels

The results of the numerical simulation of heat transfer from the combustion products of coal and coal-water fuels (CWF) to the internal environment. The mathematical simulation has been carried out on the sample of the pipe surfaces of the combustion chamber of the boiler unit. The change in the characteristics of heat transfer (change of thermochemical characteristics) in the conditions of formation of the ash deposits have been taken into account. According to the results of the numerical simulation, the comparative analysis of the efficiency of heat transfer has been carried out from the furnace environment to the inside pipe coolant (water, air, or water vapor) from the combustion of coal and coal-water fuels. It has been established that, in the initial period of the boiler unit operation during coal fuel combustion the efficiency of heat transfer from the combustion products of the internal environment is higher than when using CWF. The efficiency of heat transfer in CWF combustion conditions is more at large times (t ≥ 1.5 h) of the boiler unit. According to the results the numerical simulation of the temperature distributions in the system “pipeline environment — pipe wall — a layer of ash — the products of combustion” have been obtained. A significant decrease in heat flux from the combustion products to the inside pipe coolant in the case of coal combustion compared to CWF has been found. It has been proved that this is due primarily to the fact that massive and strong ash deposits are formed during coal combustion.     

浅议水煤浆锅炉的技术特点及其应用

本文从节约能源、减少污染物排放的角度出发,阐述说明水煤浆锅炉在节能环保方面的优越性。文章详细介绍了水煤浆燃料的特点;结合工程实例着重说明水煤浆锅炉燃烧系统、除尘、除硫、烟气排放的工艺流程;水煤浆锅炉在燃料消耗、污染物排放2方面与燃煤锅炉的技术经济比较分析,以及水煤浆锅炉的环保效益。     

Promising components of waste-derived slurry fuels

The paper presents the results of experimental studies of energy (calorific value, ignition delay times and threshold ignition temperatures, duration and temperature of combustion) and environmental (CO₂, NO_x and SO_x emission) characteristics of fuel slurries based on pulverized wood (sawdust), agricultural (straw), and household (cardboard) waste. Wastewater from a sewage treatment plant served as a liquid medium for fuels. Petrochemical waste and heavy oil were additives to slurries. The focus is on obtaining the maximum efficiency ratio of slurry fuel, calculated taking into account environmental, cost, energy and fire safety parameters. All slurry fuels were compared with typical coal-water slurry for all the parameters studied. A comparison was also made between slurries and traditional boiler fuels (coal, fuel oil). The relative efficiency indicator for waste-based mixtures was varied in the range of 0.93–10.92. The lowest ignition costs can be expected when burning a mixture based on straw, cardboard and oil additive (ignition temperature is about 330 °C). The volumes of potential energy generated with the active involvement of industrial waste instead of traditional coal and oil combustion are forecasted. It is predicted that with the widespread use of waste-derived slurries, about 43% of coal and oil can be saved.     

水煤浆粒度分布的分形学研究

在水煤浆燃料的制备过程中，水煤浆中煤粒的粒度分布至关重要。从分形．几何原理出发，理论解析得出了连续分布的水煤浆中煤粒的分形粒度分布公式，并与几种常用的经验型粒度分布和实际应用的粒度分布进行了比较，得出了分形是粒度分布的本质特征。研究结果表明：水煤浆的浓度取决于煤粒的最大、最小粒径和煤粒的粒度分形维数。研究得到的水煤浆中煤粒的分形粒度分布可用于指导高浓度、流动性好的水煤浆燃料的制备。     

Effects of coal characteristics to performance of a highly efficient thermal power generation system based on pressurized oxy‐fuel combustion

Because of its fuel flexibility and high efficiency, pressurized oxy‐fuel combustion has recently emerged as a promising approach for efficient carbon capture and storage. One of the important options to design the pressurized oxy‐combustion is to determine method of coal (or other solid fuels) feeding: dry feeding or wet (coal slurry) feeding as well as grade of coals. The main aim of this research is to investigate effects of coal characteristics including wet or dry feeding on the performance of thermal power plant based on the pressurized oxy‐combustion with CO₂ capture versus atmospheric oxy‐combustion. A commercial process simulation tool (gCCS: the general carbon capture and storage) was used to simulate and analyze an advanced ultra‐supercritical(A‐USC) coal power plant under pressurized and atmospheric oxy‐fuel conditions. The design concept is based on using pure oxygen as an oxidant in a pressurized system to maximize the heat recovery through process integration and to reduce the efficiency penalty because of compression and purification units. The results indicate that the pressurized case efficiency at 30 bars was greater than the atmospheric oxy‐fuel combustion (base line case) by 6.02% when using lignite coal firing. Similarly, efficiency improvements in the case of subbituminous and bituminous coals were around 3% and 2.61%, respectively. The purity of CO₂ increased from 53.4% to 94% after compression and purification. In addition, the study observed the effects of coal‐water slurry using bituminous coal under atmospheric conditions, determining that the net plant efficiency decreased by 3.7% when the water content in the slurry increased from 11.12% to 54%. Copyright © 2016 John Wiley & Sons, Ltd.     

Co-firing of coal and biomass fuel blends

This paper reviews literature on co-firing of coal with biomass fuels. Here, the term biomass includes organic matter produced as a result of photosynthesis as well as municipal, industrial and animal waste material. Brief summaries of the basic concepts involved in the combustion of coal and biomass fuels are presented. Different classes of co-firing methods are identified. Experimental results for a large variety of fuel blends and conditions are presented. Numerical studies are also discussed. Biomass and coal blend combustion is a promising combustion technology; however, significant development work is required before large-scale implementation can be realized. Issues related to successful implementation of coal biomass blend combustion are identified.     

Contribution of particle shape and surface roughness on the flotation behavior of low-ash coking coal

In this study, the influence of particle shape and surface roughness on the flotation behavior of +0.25–0.5 mm low-ash coking coal particles was investigated. The low-ash coking coal particles with different particle shape and surface roughness obtained through grinding or crushing were measured, calculated, and analyzed using an optical microscope associated with Image J software. The flotation kinetics tests were conducted in a 0.75 L XFD flotation cell with the presence of frother and in the absence of collector in order to investigate the natural floatability of the low-ash coking coal particles. The flotation kinetics constant of low-ash coking coal particles was calculated through the first-order rate equation. The experimental results illustrated that the flotation kinetics constant increased with increasing the aspect ratio and roughness, while the particle owning high roundness and circularity value led to smaller flotation kinetics constant. Finally, the quantitative contributions of particle shape and roughness of low-ash coking coal particles on flotation performance were established.     

Methods of choosing the optimal parameters for solid fuel combustion in stoker-fired boilers

Stoker-fired boilers are used for the combustion of coal and solid wastes. The most important disadvantage is their low thermal efficiency. The authors present methods of choosing the optimal rate of travel of the grid and height of the fuel layer basing on both realscale and laboratory measurements. Basing on industrial-scale experiments the authors calculated the optimal thermal efficiency and main energy losses using the least squares adjustment method. The stepwise regression method was used to correlate the main energy losses as functions of grid operating parameters. These correlations were used in the optimization method to estimate the optimal rate of travel of the grid and height of the fuel layer. The minimum retention time of the coal can be also calculated.     

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.     

Evaluation of the biomass “date stones” as a fuel in furnaces: A comparison with coal combustion

The present investigation gives insight into the potential of the biomass fuel “date stones” as an energy source for furnaces. This is accomplished by comparing combustion and heat transfer characteristics obtained in a furnace fueled with both fuels: date stones and a type of coal.The analysis of the two fuels investigated and the combustion experiments conducted reveal that date stones contain much volatile material that is released during the pyrolysis process, and ultimately burned under diffusion or partially premixed flame conditions. Heterogeneous reactions are found to be of lower importance in this case; contrasting the case of coal combustion. The results obtained highlight superiority of the date stones.     

Electrochemistry of fuel cells for transportation applications

Fuel cells are promising power sources for electric vehicles and do not suffer from the inherent limitations of efficiency, energy density, and lifetime, as encountered with all types of batteries considered for this application. The projected performance of fuel-cell vehicles is comparable to that of the internal combustion and diesel engine vehicles but with the additional advantages of higher fuel efficiency, particularly with synfuels from coal. The ideal fuel for a fuel-cell power plant for electric vehicles is methanol. This fuel is reformed to hydrogen, which combines with oxygen from the air in an acid electrolyte (phosphoric, solid polymer, or superacid) fuel cell to produce electricity. Though the phosphoric acid fuel cell is in the most advanced state of development (mainly for power generation applications), the solid polymer and superacid electrolyte fuel cells are more promising for the transportation application because of the faster oxygen reduction kinetics (and hence potential for higher power densities) and shorter start-up times. Alkaline electrolyte fuel cells can be used only with pure hydrogen (which causes a weight or energy penalty for any of the methods it can be carried on board the vehicle), but have the best potential for minimizing or eliminating noble metal requirements. The paper summarizes needed areas of research (i.e. reduction or elimination of noble metal loading, finding CO-tolerant electrocatalysts, finding less expensive solid polymer electrolytes, synthesis and elucidation of higher molecular weight superacids) to advance fuel-cell technology for vehicular applications.     

Developing coal-combustion technologies

Improved and new methods for direct coal utilization are reviewed, including the following: direct burning of pulverized coals, clean-fuel-combined-cycle systems, atmospheric pressure fluidized-bed combustion, advanced and pressurized fluidized-bed combustion, slagging combustors, coal-oil mixtures, coal-water mixtures, other coal slurries, etc. The emphasis is on research needs identified by the DOE/Coal Combustion and Applications Working Group (CCAWG) and relating to such processes as coal cleaning, slagging and fouling, environmental control systems, on-line diagnostics, and modeling.     

Developing coal-combustion technologies

Improved and new methods for direct coal utilization are reviewed, including the following: direct burning of pulverized coals, clean-fuel-combined-cycle systems, atmospheric pressure fluidized-bed combustion, advanced and pressurized fluidized-bed combustion, slagging combustors, coal-oil mixtures, coal-water mixtures, other coal slurries, etc. The emphasis is on research needs identified by the DOE/Coal Combustion and Applications Working Group (CCAWG) and relating to such processes as coal cleaning, slagging and fouling, environmental control systems, on-line diagnostics, and modeling.     

Hybrid clean energy technologies for power generation from sub-bituminous coals: a case of 250 MW unit

Major re-thinking is required on the conventional pulverized fuel conversion route of power generation wherein the ash and mineral burden in coals is transported through the entire flow passage of the boiler. For high-ash fuels, this has to be contained and the boiler must be clear of all mineral matter. The two independent clean coal candidate technologies for efficiency enhancement and emission controls – ultra-supercritical cycle (USC) and integrated gasification with combined cycle (IGCC) – both have limitations in adaptation to high-ash coals. While the USC is limited by the steam temperature up to 600°C (commercial scale) (700°C pilot scale) and boiler tube failure risks, IGCC is limited to high-quality fuels like diesel, naphtha, etc. (commercial scale) and high-grade coals (pre-commercial scale). The hybridization of the two technologies in their current form (ultra-supercritical cycle with gasification conversion) and carbon capture and storage (CCS) together with solar energy (solar thermal and solar photovoltaic) integration presents possibilities for immediate application to low-grade sub-bituminous coals to achieve the clean technology goals. The energy efficiency of the hybrid system is around 44.45%, which is of the order of the USC with pulverized coal combustion. But the predominant benefits of a clean operation override. The benefits are reduction in CO₂ generation from 0.86 to 0.70 kg/kWh and reduction in ash expelled from 0.20–0.24 to 0.12–0.18 kg/kWh besides elimination of dispersion of ash around the power station and facilitating CCS.     

水煤浆与煤粉燃烧脱硫比较

以煤代油是能源工业的发展方向。作为新型代油燃料,水煤浆有广阔的应用前景。从硫析出特点、脱硫影响因素（温度、Ｃａ／Ｓ比）以及烟尘排放等方面,研究了水煤浆燃烧脱硫与煤粉燃烧脱硫的异同。试验结果表明水煤浆燃烧脱硫优于煤粉,是值得推广的脱硫技术。