Coal combustion combustors

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). 

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). 

Anthony, E. J. Wang, J., Capturing C02 in coal-fired combustors using CaO-based sorbents. 5th International Symposium on Coal Combustion, Nanjing, 2003. 

A semi-industrial pilot plant has been developed in which air-borne ultrasound has been applied to the reduction of particle emissions in coal combustion fumes [62]. The installation basically consists of an acoustic agglomeration chamber with a rectangular cross-section, driven by four high-power and highly directional acoustic transducers operating at 10 and/or 20 kHz, and an electrostatic precipitator (ESP). In the experiments, a fluidised bed coal combustor was used as fume generator with fume flow rates up to about 2000 m /h, gas temperatures of about 150 °C. and mass concentrations in the range 1-5 gm. The acoustic filter reduced fine particle emissions by about 40 %. 

However, 2,3-benzofuran was detected in emissions from a Swedish floor finish used on domestic flooring (van Netten et al. 1988), and in emissions from the pyrolysis of silk (Junk and Ford 1980), and in combustor flue gas emissions from fluidized-bed coal combustion at a concentration of 900 ng/g (Hunt et al. 1982). Exhaust produced by an automobile burning simple hydrocarbon fuels contained 2,3-benzofuran at concentrations ranging from less than 0.1 to 2.8 ppm (Seizinger and Dimitriades 1972), but an analysis of air in a highway tunnel in use by both diesel- and gasoline-powered vehicles indicated no... 

Although the data presented here are limited to a single coal burned in two combustor operating modes, several important observations can be made about the fine particles generated by pulverized coal combustion. The major constituents of the very small nucleation generated particles vary with combustion conditions. High flame temperatures lead to the volatilization of refractory ash species such as silica and alumina, probably by means of reactions which produce volatile reduced species such as SiO or Al. At lower flame temperatures which minimize these reactions other ash species dominate the fine particles. Because the major constitutents of the fine particles are relatively refractory, nucleation is expected to occur early in the combustion process. More volatile species which condense at lower temperatures may also form new particles or may condense on the surfaces of the existing particles. Both mechanisms will lead to substantial enrichment of the very small particles with the volatile species, as was observed for zinc. 

Table XVI shows the lead and cadmium content of one of the Pittsburgh seam coals used in the coal combustion study, and Table XVII shows the results obtained when the coal was combusted in the two experimental furnaces. Compared with mercury, a greater amount of both the cadmium and the lead were retained by the ash in both combustors.

Pulverized coal combustion systems are most commonly used in power plants. In pulverized coal combustion, temperatures typically reach around 1480 °C at atmospheric pressure. In the past couple of decades, fluidized bed combustion (FBC) technologies have been commercialized. These combustors often use limestone bed materials to capture sulfur gases. They operate at about 880 °C and usually at atmospheric pressure (Smoot and Smith, 1985), 38. 

Power generation plants such as the steam plant, the gas turbine plant, and combined cycle plants require the combustion of a fossil fuel. Now, combustion is a chemical reaction of fuel with an oxidant (usually oxygen), and it makes sense to examine the combustion process more closely and analyze its thermodynamic efficiency. This means that we will examine the furnace/combustor of Figures 9.8, 9.10, and 9.12. We will examine coal and gas combustion at the level needed for thermodynamic analysis, after discussing some commonly used coal combustion processes. 

The combustion of coal in a fixed bed (e.g., stokers) is the oldest and most common method of coal combustion. In recent decades, however, the fixed beds have lost some of their popularity due to the increased use of fluidized bed and suspended bed combustors [2,7],... 

Conceptually speaking, the technology for combustors is similar to that used for coal combustion (see Chapter 9) if solid biomass fuels are used. Typically used configurations for biomass combustion include [41] (1) pile-burned, (2) stoker-fired, (3) suspension-fired, and (4) fluidized-bed combustors. 

Pressurized FBC may make possible the use of coal in turbocharged combustors and combined cycle power systems. Success is dependent on reducing the erosive and corrosive character of hot, pressurized flue gas generated from coal combustion before expanding it through the system s gas turbine. The problems to be overcome are manifold. The usual requirement for reducing particulate emissions to environmentally acceptable levels is supplemented with stringent... 

Fluidized-bed combustors are complex chemical reactors, a fact often overlooked because their primary purpose is to generate steam and electricity. The reactions of importance in fluidized beds include those relating to coal combustion,... 

In coal combustion in China, both fundamental research and industrial experience have resulted in improved understanding of CFB combustion. It is reported that more than 100 commercial CFB coal combustors have been... 

Intelligent design of circulating fluidized bed boilers depends on sufficient understanding of the physico-chemical hydrodynamics occurring in the combustors, such as chemical kinetics of coal combustion and pollutant formation, hydrodynamics of gas-solid two phase flow, mixing of gas and solids, distribution of heat released, and heat transfer between immersed... 

Figure 42 shows that the availability of calcium for desulfurization in coal combustion in a bubbling fluidized bed decreases with increasing particle diameter and is less than 20% for particles larger than 1 mm. On the other hand, fly ash collected from flue gas shows a calcium availability of not more than 10%, possibly due to its short residence time in the combustor. 

Particulate emissions ranged from 9 mg per dry standard cubic meter (mg/dsm ) of the flue gas volume (corrected to 12% CO2) for wood offcuts to 191 mg/dsm for SRF E. nitens. Following the lowest value of 9 mg/dsm coal combustion produced the second lowest particulate emission of 40 mg/dsm This variation was associated with the density of the fuels being fired, and the overall proportion of fmes in the feedstock as fed into the combustor. However, wood offcuts with sawdust was an exception for its relatively low density and high proportion of fine particles. 

A design strategy for a CFB combustor for biomass was presented and illustrated for a 2SOO kg/hr combustor. The main operating parameters are the solids flux and gas velocity in the riser of the CFB. Isothermal conditions will be guaranteed by circulating inert sand particles of 300 pm at a solids flux of 20 kg/m s. The gas velocity in the riser is determined at 7.9 m/s, exceeding the transport velocity (-6.6 m/s) of the inert sand bed by 20%. About 2% of the diar will be lost in the ash stream. The net combustion capacity of the riser is equal to 4.1 M Wih per m of the riser, comparable with a coal CFB combustor. 

Rees, D. "An Assessment of Pulverized Coal Fired Combustor Performance" DOE Combustion Contractors Mtg., Pittsburgh, Pa., March 1981. 

Two versions of new burner design concepts were developed. Due to proprietary reasons, details of the new burners carmot be disclosed here. Each design was incorporated into a computational grid that also included the details of the combustor. Simulations of oxy-coal combustion and NO formation were performed to determine the flame and NO characteristics for each design. In addition, two different combustor sizes were also investigated using the model to quantify wall effects. 

The use of additives was initially confined to the combustion of fuel oils, presumably because the system lent itself to the injection of suitable chemicals and compounds. In more recent times the use of additives has been applied to coal combustion and at present treatment of flue gases derived from waste combustion is being developed. For instance fluidised bed combustors for waste incineration are very suitable for fouling mitigation using additives. The mixing effect of the... 

Data for validating pulverized coal combustion predictions requires accurate information for the reactor parameters shown in Table VI. Data measured in the combustion chamber typically include (1) locally measured values of the gaseous flow field velocity, temperature, and species composition, (2) coal particle burnout, number density, velocity, temperature, and composition, and (3) wall temperatures and heat fluxes. Evaluation should include comparisons with measurements from a wide variety of combustors and furnaces that range in scale from very small laboratory combustors (0.01-0.5 MW) and industrial furnaces (1-10 MW) to large utility boilers (up to 1000 MW). 

For example, coal combustion may be achieved using pulverized coal in entrained systems or as sized particles in fixed or slowly moving beds larger pieces may, in certain instances, also be used. In the case of the fixed- or slowly moving bed combustor, it is usual to employ a mechanical stoker to feed the coal and a grate to support the coal particles as well as to admit air for the combustion process. With regard to the pulverized systems, coal that has been crushed to ca. 200 mesh is carried into the system entrained by the air. 

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