. . . But it’s always a good idea to look at what pollutants may remain. Here are a number of possibilities
Paul F. Fennelly, Hans Klemm, and Robert R. Hall GCA/Technology Division Bedford, Mass. 0 1730
Donald F. Durocher Kimberly-Clark Corp. Lee, Mass. 01238
Fluidized-bed combustion systems operate at significantly lower temperature than conventional systems (about 1650 OF vs. 2700 OF, respectively). The solids are supported on a grid at the bottom of the boiler through which combustion air is passed at high velocities, typically 2 to 5 feet per second. The solids are held in suspension by the upward flow of the air and a quasi-fluid is created that contains many properties of a liquid. The most important liquid-like property to the boiler designer is the fact that the bed material is exceptionally well mixed and flows throughout the system without mechanical agitation. Fluidized-bed combustion systems for the production of steam and/or electricity have several advantages over conventional combustion systems. These include: High heat transfer coefficients and volumetric heat release rates will reduce the boiler size by onehalf to two-thirds or more compared to a conventional unit. Capital costs will be reduced because of the size reduction and the potential for shop fabrication instead of field construction. The use of limestone as bed material provides a means for in situ SO2 removal. The high heat transfer coefficients permit lower operating temperatures (1550-1 750 OF), which can potentially decrease NO, emissions. Questions have been raised concerning the emissions that could result at these lower operating temperatures. As a first step in answering these questions, one can conduct a “preliminary environmental assessment.” In performing a preliminary environmental assessment, one’s major role in a sense is to serve as a devil’s advocate with respect to pollutant generation. The air is to focus attention on potential environmental problems as early in the development cycle as possible. This provides maximum lead time to gather the technical data on which decisions regarding control technology or process modifications (should they be needed) can be based. It is widely known that fluidized-bed combustion (FBC) of coal results in low SO2 and NO, emissions. The idea in this feature article is to focus attention on the so-called “other” pollutants. These pollutants are divided into three generic classes: trace elements, organic compounds, and particulates.There are some limited experimental data available on trace elements in FBC; also, investigations of particulate size distribution and their chemical composition are just underway. Unfortunately,no data available on organic compounds that could be produced in coal-fired fluidized-bed combustion are yet available. Numerous compounds can be included in an initial list of conceivable pollutants. Developing a list of conceivable pollutants is not necessarily a technically sophisticated task, but is an important effort. It establishes the scope of the environmental assessment; the more comprehensive the list, the less chance there is for unexpected pollutants to escape discovery. 244
Environmental Science & Technology
Organic and other pollutants
Potential organic pollutants are those compounds that could form from the incomplete combustion of coal. One can simplistically view it as a sequential process. The particle vaporizes or volatilizes; the volatile compounds can react among themselves in a chemically reducing atmosphere. Next, they react with oxygen within the system in a diffusion flame. After devolatilization is completed, the char continues to burn. To specify the potential organic compounds that could form, the basic question is, what types of chemical species are produced during coal pyrolysis and to what extent will they survive in the reactive environment of a fluidized-bed combustor? Only two classes of hydrocarbons should be of any significance in coal combustion: small hydrocarbons (less than about three carbon atoms) and polynuclear aromatic hydrocarbons. Both could form and survive within the bed at temperatures on the order of 1500 OF. The chemical structure of coal can be viewed as a network of interconnected aromatic hydrocarbon compounds. Small hydrocarbons can form directly from cleavage of substituted alkyl groups. Polynuclear aromatic hydrocarbons can also form directly via bond cleavages in the structural network, or they can form through condensation reactions of various hydrocarbon decomposition products. Even though generally endothermic, at FBC temperatures of 1500 OF, these condensation reactions probably proceed at a significant rate. This belief is based on the fact that branched or cyclic hydrocarbons are seldom found as products of coal pyrolysis at similar temperatures. Similar arguments apply to the generation of organic nitrogen and sulfur compounds. Species such as pyridine decompose at temperatures on the order of 1000 O F to form hydrogen cyanide (HCN) and small hydrocarbons. Thiophenes and mercaptans can also decompose to form small hydrocarbons and hydrogen sulfide (H2S). Concentration estimates
For a rough estimate of the concentrations at which some of the small hydrocarbons and reduced sulfur and nitrogen compounds might exist, one can use equilibrium calculations based on free energy minimization.An upper limit can be obtained from calculations performed in conjunction with coal gasification experiments where highly reducing, fuel-rich conditions exist. For example, even with only 60% stoichiometric oxygen present, concentrations of HCN, carbonyl sulfide (COS), carbon disulfide (CS2),and the like, are less than 10 parts per million (ppm). Extrapolation of the calculations to typical operating conditions such as 20 YO excess air, in which case SO2 and NO, become the predominant sulfur and nitrogen compounds, indicates that compounds such as H2S, HCN, COS, and cyanogen ((CN)2)should be present in concentrations less than 1 ppm. Free energy minimization calculations for the more complicated polynuclear aromatic hydrocarbons are impractical.
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rrsenk te’etm ::0*1b However, to estimate the concentrations at which these types of compounds might exist in the FBC flue gas, one can use empirical correlations between a compound such as benzo[a]pyrene and methane (CH4)concentrations from measurements in conventional coal-fired combustion systems. Under normal operating conditions, about 3% O2 in the flue gas (20% excess air), the concentration of hydrocarbons (as CH4) is about 100 ppm (volume/volume or V/V). Although emissions can often vary between different fluidized-bed systems, 100 ppm provides a convenient average value. Previous measurements with conventional coal-fired systems indicate that the concentration of compounds such as benzo[a]pyrene is typically times less than the concentration of total hydrocarbons as CH4. Thus, using a reference value of 100 ppm CH4, one can infer that in a fluidized-bed system, polynuclear aromatic hydrocarbons (PAH) could exist in the flue gas at concentrations (V/V) on the order of 1 part ber billion (ppb).However, since flue gases are eventually diluted by roughly a factor of a thousand when they are emitted from the stack, ambient concentrationsof PAH near FBC facilities would more likely be on the order of 1 part per trillion. This corresponds to about 0.6 ng/m3, which is roughly comparable to the natural background concentration ranges found in rural areas. Accordingly, it seems that polynuclear aromatic hydrocarbon concentrations should not be high enough to cause problems. Recently, some coal-fired flue gases have been tested for the presence of polychlorinated biphenyls (PCB) and trace concentrations have been reported. Experience in coal combustion
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Elements of concern because they could be emitted in toxic concentrations (based on “worst case analyses.”) Elements of concern because of possible enrichment on fine particles (
