The variability in both inorganic and organic properties of biomass fuels is large. This paper discusses combustion-driven transformations and deposition of inorganic material found in solid fuels, with a focus on the formation of deposits and their properties. A small number of mechanisms is used to describe both the transformations and deposition. The discussion below outlines this mechanistic approach to describing the fate of inorganic material in solid fuels with a particular focus on the mechanisms of ash deposition. This mechanistic approach has the potential of embracing a large range of fuel variations, combustor types, and operating conditions without the need of developing extensive databases or testing procedures for each new situation. The approach has been successfully demonstrated for coal combustion, and examples from coal experiments will be used as illustrations. The same methodology and logic can be applied to biomass combustion. A comparison of coal and biomass is briefly presented, including the chemical structures and the modes of occurrence of inorganic material in the fuels. The major mechanisms of ash deposition during combustion of coal and biomass are related to the types of inorganic material in the fuel and the combustion conditions. The effects of fuel (biomass or coal) characteristics and combustor operating conditions on ash deposit properties such as tenacity, emissivity, thermal conductivity, morphology, strength, chemical composition, viscosity, and rate of growth are discussed. A mechanistic model describing ash deposition in solid-fuel combustors is presented and used to postulate characteristics of ash deposits formed in biomass combustors.

Ash deposition during biomass and coal combustion: A mechanistic approach

Ash formation and deposition in coal and biomass fired combustion systems: Progress and challenges in the field of ash particle sticking and rebound behavior

Utilizing biomass and waste for power production—a decade of contributing to the understanding, interpretation and analysis of deposits and corrosion products

Shedding of ash deposits

Modeling of ash deposition in large-scale combustion facilities burning pulverized coal

Deposition of bituminous coal ash on an isolated heat exchanger tube: Effects of coal properties on deposit growth

Ash deposits formed at moderate temperatures (1100-1300 K) in the convective sections of utility boilers firing coals containing lignitic ash are associated with the formation of sticky particles following condensation of sodium sulfate from the vapor phase at 1200-1300 K The deposits are enriched in calcium, due to the abundance of calcium among the particles impacting and sticking and the low melting temperature of mixtures of calcium, magnesium, and sodium sulfates Because the formation of these deposits occurs only in a limited range of gas temperatures, approximately from the dew point to the melting temperature of sodium sulfate, they are not observed during experiments in which deposition is accelerated by increasing the temperature to values typical of furnace exit gas or higher

Fouling of convection heat exchangers by lignitic coal ash

Deux modeles sont etudies pour la generation et les depots des cendres (encrassements) pendant la combustion du charbon pulverise. Ces modeles permettent d'obtenir la granulometrie et la composition minerale des cendres volantes et de prevoir la formation d'encrassement. Ils sont verifies experimentalement dans une chambre de combustion

Generation and deposition of fly ash in the combustion of pulverised coal

Characteristics times were estimated for the heating, vaporization, ignition, and combustion of 10 to 100 μm coal-water slurry droplets under conditions measured in 1 MW turbulent diffusion flames with high swirl. Agglomeration of the coal particles in a droplet was assumed to result in a coal particle size identical to the atomized droplet size. The locations of the ignition and combustion processes along the flame axis were determined from droplet trajectories based on estimates of the initial droplet velocity and measured gas velocities. Gas temperature and oxygen profiles recalculated using the predicted spatial distribution of the combustion processes, an estimate of the droplet size distribution, and the properties of the externally recirculated combustion products, were consistent with the measurements. The droplet velocity leaving the atomizer, and the gas velocity in the region of the fuel spray have important effects on the spatial distribution of ignition and combustion through their determination of the droplet trajectories. The presence of 32 to 36 wt% water in a slurry droplet results in an increase of about 50% over the distance required for ignition of an equivalent coal particle under the conditions which existed in the flames.

Ignition and combustion of coal-water slurry in a confined turbulent diffusion flame

A substantial fraction of the fixed carbon feed appears as char fines entrained in the flue gas from bubbling atmospheric pressure fluidized bed combustion (AFBC) of bituminous coal. Incomplete conversion of these fines usually represents the most significant combustibles loss from such a system. The production of the fines is thought to be a consequence of the removal of material, loss of integrity, and fragmentation associated with combustion of larger char particles. Both experimental and theoretical results suggest that there is a particular value of the porosity at which fragmentation occurs. The available data on bituminous coal char indicate that only about 30 to 50% burnout is needed to reach this critical porosity. The balance of the char goes to fine particles, which may be elutriated from a fluidized bed. The division of fixed carbon between burned char and fines is influenced by bed temperature, excess air, and superficial velocity only to the extent that the initial porosity and porosity at fragmentation depend upon these conditions. This concept was used as the basis of a model for carbon conversion in AFBC. The model was tested by comparson with bed carbon loads measured during combustion of Kentucky No. 9 coal in a 0.6×0.6 m bed. Comparison of predictions based on alternative approximations indicated there was relatively little combustion of char fines in the bed; most were carried unburned into the freeboard region. Total carbon conversion (fixed plus volatile) at the top of the bed was estimated to have been only 56±9%. Low carbon conversion is seen to be the consequence of char fines production by fragmentation during combustion of larger particles, the relatively short residence time of char fines in the bed, and low temperature. Combustion of fines in the freeboard must account for a significant fraction of the overall carbon conversion in an efficient unit. More attention should be given to freeboard residence time and temperature in designing for improved combustion efficiency.

Peter M. Walsh

Papers

Deposition of bituminous coal ash on an isolated heat exchanger tube: Effects of coal properties on deposit growth

Fouling of convection heat exchangers by lignitic coal ash

Generation and deposition of fly ash in the combustion of pulverised coal

Ignition and combustion of coal-water slurry in a confined turbulent diffusion flame

The production and loss of char fines during fluidized bed combustion of a high volatile bituminous coal