Coal combustion

Combustion of coal produces many of the same ultimate water pollutants as combustion of petroleum does, that is, PAHs. Coal burning, however, produces greater quantities of metals, sulfur dioxide, and haloacids. Coal combustion stack emissions contain significant quantities of arsenic, mercury, selenium, copper, and tin. I25 Sulfur dioxide is ultimately converted to sulfuric acid in the air. Sulfuric acid and the haloacids (HF, HC1, 

and HI) ultimately come down as acid rains and acidify surface waters. I26l Acidified water is by itself toxic to marine life. Its effect is amplified, however, by the solubilization of metals and hydrolysis of other chemical compounds. 

During coal combustion, for example, In a power plant, the minerals associated with the coal are heated to melting temperatures and the resultant ash and fly ash can be investigated by means of Mossbauer spectroscopy, among the other spectroscopic analysis techniques. As a general rule, the decomposition of the abundant mineral species is depicted in Table 30.3. 

Fly ash samples and coarse agglomerated ash samples were obtained from the Lethabo power plant, which receives coal from the New Vaal mine (coal sample 3, Table 30.1). These samples and ash obtained after laboratory combustion at various temperatures of the coal fed to the power plant were ground to l80p,m and used as absorbers in the Mossbauer experiments. The combustion of the coal samples was done in a muffle furnace, under atmospheric 

TABLE 30.3 Summary of Mineral Changes that Occur at Various Temperatures During Combustion 

The Mossbauer spectrum of the fly ash sample obtained from the Lethabo power plant consisted of a Fe and Fe component, typical of silica glass. In addition to the glass formation, hematite (Fe203) was also observed (see Fig. 30.12). The same spectrum was observed for the agglomerated coarse ash sample, but with a slightly different Fe /Fe ratio. 

In the laboratory simulations, carried out on sample 3 from the New Vaal mine, it was clearly observed by Mossbauer spectroscopy how the amount of pyrite diminished as the combustion temperatures increased and how it altered until finally to the point where the iron was taken up in the glass and the hematite. At 200 °C, the pyrite started to change, at 400 ° C al ready 60% had been altered to hematite, and at 600 °C only 20% of the pyrite remained. At 800 °C, most of the pyrite had been transformed and above a temperature of 1000 °C the pyrite had fused completely into the glass and the oxide and thus similar products were formed in the fly ash from the power plant and the laboratory-produced ash as shown in Fig. 30.12, with Mossbauer parameters, given in Table 30.2, consistent with those found in the literature [8]. 

---

Ga.s-to-Pa.rticle Heat Transfer. Heat transfer between gas and particles is rapid because of the enormous particle surface area available. A Group A particle in a fluidized bed can be considered to have a uniform internal temperature. For Group B particles, particle temperature gradients occur in processes where rapid heat transfer occurs, such as in coal combustion. 

Environmental considerations also were reflected in coal production and consumption statistics, including regional production patterns and economic sector utilization characteristics. Average coal sulfur content, as produced, declined from 2.3% in 1973 to 1.6% in 1980 and 1.3% in 1990. Coal ash content declined similarly, from 13.1% in 1973 to 11.1% in 1980 and 9.9% in 1990. These numbers clearly reflect a trend toward utilization of coal that produces less SO2 and less flyash to capture. Emissions from coal in the 1990s were 14 x 10 t /yr of SO2 and 450 x 10 t /yr of particulates generated by coal combustion at electric utiUties. The total coal combustion emissions from all sources were only slightly higher than the emissions from electric utiUty coal utilization (6). 

Combustion. Coal combustion, not being in the strictest sense a process for the generation of gaseous synfuels, is nevertheless an important use of coal as a source of gaseous fuels. Coal combustion, an old art and probably the oldest known use of this fossil fuel, is an accumulation of complex chemical and physical phenomena. The complexity of coal itself and the variable process parameters all contribute to the overall process (8,10,47—50) (see also COLffiUSTION SCIENCE AND technology). 

A significant issue in combustors in the mid-1990s is the performance of the process in an environmentally acceptable manner through the use of either low sulfur coal or post-combustion clean-up of the flue gases. Thus there is a marked trend to more efficient methods of coal combustion and, in fact, a combustion system that is able to accept coal without the necessity of a post-combustion treatment or without emitting objectionable amounts of sulfur oxides, nitrogen oxides, and particulates is very desirable (51,52). 

The complex nature of coal as a molecular entity (2,3,24,25,35,37,53) has resulted ia the chemical explanations of coal combustion being confined to the carbon ia the system. The hydrogen and other elements have received much less attention but the system is extremely complex and the heteroatoms, eg, nitrogen, oxygen, and sulfur, exert an influence on the combustion. It is this latter that influences environmental aspects. 

For example, the conversion of nitrogen and sulfur, duting coal combustion, to the respective oxides duting combustion caimot be ignored ... 

For central station power generation the open cycle system using electrically conducting coal combustion products as the working fluid is employed. The fuel typically is pulverized coal burned directly in the MHD combustor, although in some plant designs cleaner fuels made from coal by gasification or by beneficiation have been considered (8—10) (see Fuels, synthetic). 

L. B. Heia, A. B. Phillips, and R. D. Young, "Recovery of Sulfur Dioxide from Coal Combustion Stack Gases," ia F. S. MaHette, ed. Problems and Control of A.ir Pollution, Reioliold Publishing Corporation, New York, 1955, pp. 155—169. 

The discussion of combustion fundamentals so far has focused on homogeneous systems. Heterogeneous combustion is the terminology often used to refer to the combustion of Hquids and soHds. From a technological viewpoint, combustion of Hquid hydrocarbons, mainly in sprays, and coal combustion are of greatest interest. 

Sohd fuels are burned in a variety of systems, some of which are similar to those fired by Hquid fuels. In this article the most commonly burned soHd fuel, coal, is discussed. The main coal combustion technologies are fixed-bed, eg, stokers, for the largest particles pulverized-coal for the smallest particles and fluidized-bed for medium size particles (99,100) (see Coal). 

Pulverized-Goal Firing. This is the most common technology used for coal combustion in utiUty appHcations because of the flexibiUty to use a range of coal types in a range of furnace sizes. Nevertheless, the selection of cmshing, combustion, and gas-cleanup equipment remains coal dependent (54,100,101). 

The main stages of coal combustion have different characteristic times in fluidized beds than in pulverized coal combustion. Approximate times are a few seconds for coal devolatilization, a few minutes for char burnout, several minutes for the calcination of limestone, and a few hours for the reaction of the calcined limestone with SO2. Hence, the carbon content of the bed is very low (up to 1% by weight) and the bed is 90% CaO in various stages of reaction to CaSO. About 10% of the bed s weight is made up of coal ash (91). This distribution of 90/10 limestone/coal ash is not a fixed ratio and is dependent on the ash content of the coal and its sulfur content. 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

R. H. Essenbigh, in M. A. Elliot, eds.. Fundamentals of Coal Combustion, In Chemistry of Coal Utilization, 2nd Suppl. Vol., John Wiley Sons,... 

E. J. Badin, Coal Combustion Chemistry—Correlation Fispects Elsevier, New York, 1984, Chapt. 6, p. 68. 

L. D. Smoot and D. T. Pratt, eds.. Pulverised Coal Combustion and Gassification, Plenum Press, New York, 1979. 

Fluiaized-Bea Boilers As explained in the earlier discussion of coal combustion equipment, the furnace of a fluid-bed boiler has a unique design. The system as a whole, however, consists mainly of standard equipment items, adapted to suit process requirements. The... 

The use of more efficient technologies or process changes can reduce PIC emissions. Advanced coal combustion technologies such as coal gasification and fiuidized-bed combustion are examples of cleaner processes that may lower PICs by approximately 10%. Enclosed coal crushers and grinders emit lower PM. 

Today s major emissions control methods are sorbent injection and flue gas desulfurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulfur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulfur oxide emissions. 

The rotational operation of a CFB leads to a vortex motion in the freeboard which tends to inhibit particle loss by elutriation. Because of the relatively compact nature of the CFB and the operating flexibility provided by the rotational motion, the CFB has been proposed for a variety of applications including coal combustion, flue gas desulfurization, gas combustion, coal liquefaction and food drying. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

Ash. This is a secondary source and is the by-product of coal combustion. It contains silicates. 

---