Kalyan Annamalai

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.q 2001 Published by Elsevier Science Ltd.

Interactive processes in gasification and combustion. Part I: Liquid drop arrays and clouds

Abstract A comprehensive review is presented of the interactive transport processes in the gasification and combustion of a cloud of drops and solid particles. The review is divided into two parts. Part I is concerned with the interactive processes for arrays, streams and clouds of drops while Part II presents a review of interactive processes for solid particles clouds. Isolated drop evaporation and combustion are briefly reviewed, as they relate to the interactive processes followed by approximate criteria for interactive combustion. The literature review reveals three distinctive types of studies: Arrays, streams and clouds in the areas of evaporation, ignition and combustion. An integrated approach starting from arrays to clouds and evaporation to combustion is presented. Array problems, analyzed with bispherical coordinates, method of images (MOI) and point source method (PSM) are reviewed. The results are briefly presented. Comparison of results between the different techniques and between the theory and experiment are given. A few of the array were interpreted with the “partial mass fraction” (analogous to partial pressure in thermodynamics) concept introduced in this review article. Convective effects on the array results are discussed. The point source resulting from the array studies are extended to the cloud problems by using the averaging technique for a statistically distributed cloud. The cloud problems of evaporation in confined and unconfined volumes, ignition and combustion are then reviewed. The relation of the results to spray combustion modeling is briefly discussed.

A theory on transition of ignition phase of coal particles

Abstract A century-long held notion was that the ignition of coal particles always occurs in the gas phase, as in a totally pyrolysing material. Recent experimental results disprove such a sequence, at least for small bituminous coal particles. Coals undergo only partial pyrolysis and hence ignition could occur in one of two possible modes: (i) exothermic gasification due to heterogeneous oxidation (regime I), and (ii) endothermic pyrolysis and subsequent oxidation of the pyrolysate in the gas phase (regime II). A steady-state ignition theory is developed for partially pyrolysing solid particles. The theory is formulated on an asymptotic basis: regime I is assumed to proceed independent of regime II. For regime I, Semenovs thermal theory is used to obtain the heterogeneous ignition temperature (HIT) and the solutions reveal that an increase in particle size and oxygen concentration results in decrease of HIT. For regime II, an adiabatic criterion is invoked to define the gas ignition temperature (GIT). The solutions reveal that a decrease in particle size and an increase in oxygen concentration increases the GIT. Superimposing the results for HIT and GIT, the conditions for transition of ignition phase (TIP) are found. The Pittsburgh seam bituminous particles of size below 350 μm may not undergo gas phase ignition, confirming recent experimental results.

Interactive processes in gasification and combustion. II: Isolated carbon, coal and porous char particles

Most combustion systems use dense sprays of liquid fuels or pulverized fuel suspension for combustion. Thus the interactive processes between the drops/particles play a crucial role in the ignition and combustion behavior and pollutant formation and destruction. A comprehensive review is presented of the interactive transport processes in the gasification and combustion of a cloud of drops and solid particles. The review originally intended to be in two parts is now divided into three parts. Part I (Prog. Energy Combust. Sci., 18, 1992) is concerned with the interactive processes for arrays, streams and clouds of drops, Part II presents a review of isolated coal/char particles as an introductory material to interactive processes and finally Part III deals with the interactive processes for solid particle arrays, streams and clouds. While drops are totally gasifying, coal is a partially gasifying solid which yields char on gasification. As such there exists separate studies for the pyrolysis of coal and combustion of char particles. The kinetics modeling of pyrolysis, oxidation of volatiles and CO, carbon reactions, heterogeneous and homogeneous ignition and combustion of carbon and coal are reviewed. Because of strong analogy of the group ignition and combustion to porous char ignition and combustion, the literature on porous char combustion is also included. In order to maintain the continuity, new results are presented on the internal ignition of porous char particles using a Frank-Kamenetskii type of analysis. The ignition and combustion results of porous char are later used in Part III of the review which deals with group ignition and combustion of char clouds.

The transient ignition of isolated coal particle

A transient model for ignition of a coal particle is presented. Criteria for heterogeneous and homogeneous ignition for transient model are proposed. Using the inflection condition for particle temperature, the criterion for heterogeneous ignition is reduced in terms of critical mass loss rate. Ignition for smaller particles is primarily heterogeneous (HI), followed by secondary homogeneous ignition (SGI). Primary ignition is homogeneous (GI) at high gas temperatures and for larger particles. The boundary separating homogeneous ignition from heterogeneous ignition is estimated using the transient model, which changes with ambient temperature and oxygen concentration. The SGI is obtained for particle diameters below 400 μm at ambient gas temperature of 1500 K and oxygen concentration of 0.23. The ignition sequence of HI followed by SGI is also revealed by our recent video imaging of particle ignition. The heterogeneous ignition temperature (Tp,HI) of the particle is inversely related to particle diameter and oxygen concentration. While the minimum ambient gas temperature (T∞,GI) required for homogeneous ignition shows a slight decrease with an increase in oxygen concentration, the local homogeneous ignition temperature (Tg,SG) does not appear to change with oxygen concentration. The results are compared with those predicted with the steady state model of other researchers.

FIXED-BED GASIFICATION OF FEEDLOT MANURE AND POULTRY LITTER BIOMASS

The U.S. cattle industry is a

Technical Notes: Estimation of Gross Heating Values of Biomass Fuels

175 billion industry with an estimated 100 million cattle. About 10 million head of these cattle are in feedlots producing harvestable manure. At the same time, the U.S. poultry industry is the world’s largest producer and exporter of poultry meat. Not surprisingly, one outcome is the production of a large quantity of manure byproducts, with approximately 60 million tons of dry harvestable animal manure produced annually from confined livestock and poultry. This article describes a method of extracting energy from feedlot manure or poultry litter biomass either individually or combined with each other. High-ash (approximately 45% dry weight basis) feedlot biomass (HFB) and poultry litter biomass (HLB) were gasified in a 10 kW (thermal) fixed-bed, counter-current atmospheric pressure gasifier to generate a mixture of combustible gases that could be further burned to generate heat. This article discusses the effect of the biomass particle size on the composition of the product gas leaving the gasifier, the temperature profiles in the fixed bed, and the ash fusion of HFB and HLB during gasification. Air-blown gasification of the biomass fuels yielded a low-Btu gas with a higher heating value of 4.4 ±0.4 MJ/m3 and an average product gas composition (dry basis) of H2: 5.8 ±1.7%, CO: 27.6 ±3.6%, CH4: 1.0 ±0.5%, CO2: 6.7 ±4.3%, and N2: 59.0 ±7.1%. The overall average equivalence ratio was 2.82 ±0.43 on a dry ash-free basis. The experimental results also show that high-alkaline content fuels, such as HLB (Na2O + K2O = 16.7% of ash), can be gasified by blending with lower-alkaline content fuels, such as HFB (6.0%), to reduce agglomeration in the fuel bed without significantly affecting the heating value of the product gas. The gasification of HLB and HFB yields a low-Btu gas that can be combusted to generate heat for steam or power generation. The process has potential for reducing transportation costs for traditional cropland-based manure application in some regions.

Ignition and combustion of coal particle streams

ABSTRACT EXPERIMENTAL data reported in the literature for gross heating values of various types of biomass were correlated with heating values that were estimated using the Boie equation. This equation was originally developed to estimate the gross heating values of fossil fuels from their respective compositions of carbon, hydrogen, oxygen, nitrogen, and sulfur. The estimated heating values agreed within 50% of the experimental data for 47 types of plants and crop residues and within 8% for cattle feedlot manure of normal quality. This correlation makes it possible to accurately predict the gross heating values of most biomass fuels of known ultimate composition and ash content.

Co-firing of coal and cattle feedlot biomass (FB) Fuels, Part III: fouling results from a 500,000 BTU/h pilot plant scale boiler burner☆

Abstract This paper presents a model for group combustion of a cylindrical stream of coal particles and preliminary experimental data. Results are obtained for transient ignition and combustion behaviour. It is found that homogeneous ignition (ignition of volatiles) occurs for a dense cloud, while heterogeneous ignition (ignition of carbon) occurs for a dilute cloud. Once ignited, the flame moves towards the cloud, reaches the cloud surface and then bifurcates into two flames, called the inner and outer flames. The inner flame propagates into an unburnt volatile-air mixture in the cloud, while the outer diffusion flame moves away from the cloud. The inner flame propagates at the laminar burning velocity and rapidly heats the particles. There is only a diffusion flame outside the cloud after all the oxygen in the cloud is consumed. The flame moves toward the cloud as the volatiles are depleted. The cloud, with the volatiles exhausted and starved of oxygen, burns slowly, controlled by diffusion into the cloud rather than to each particle in the cloud. Qualitative comparisons are given with preliminary experimental data obtained using a digital imaging system.

Group combustion of char/carbon particles

Abstract Part I of the paper presented a methodology for fuel collection, fuel characteristics of the FB, its relation to ration fed, and the change in fuel characteristics and volatile oxides due to composting. The bench scale experiments with 30 kWt (100,000 BTU/h) facility revealed better combustion for coal: FB blends (90:10) than for coal alone and the NOx emission were slightly less with the blend (Part II). Part III concerns with larger-scale (pilot plant) experiments conducted at the 150 kWt (150,0000 BTU/h) Combustion and Environmental Research Facility (CERF) of the National Energy Technology Laboratory (NETL). Only fouling part of the results are reported in part III. The 90:10 coal:FB blend resulted in almost twice the ash output compared to coal and ash deposits on heat exchanger tubes that were more difficult to remove than baseline coal ash deposits. The increased fouling behavior with blend is probably due to the higher ash loading and ash composition of FB.