Environ. Sci. Technol. 1999, 33, 4508-4513
A Simple Two-Reactor Method for Predicting Distribution of Trace Elements in Combustion Systems KRISTOFFER SANDELIN* AND RAINER BACKMAN Combustion Chemistry Research Group, A˙ bo Akademi University, Turku, Finland
Utilization of fossil fuels in energy production results in emissions of pollutants. Development in combustion technology has, however, during recent years focused on decreasing emissions of particulates, sulfur, and nitrogen oxides. In addition, combustion also includes a potential risk for emission of trace metals. Due to the fact that many trace elements have been identified as having a negative effect on human health and the natural environment, they have received special attention from a regulatory point of view. This paper describes a simple method for predicting the fate of eight trace elements (As, Cd, Hg, Ni, Pb, Se, V, and Zn) in combustion processes. Using the fuel composition and power plant operating conditions, the trace element concentrations in various ashes and the flue gas are determined. The method, which is based on a global equilibrium approach, describes the overall behavior of the eight trace elements successfully. Distribution/ partitioning of the trace elements between the bottom ash, fly ash, and the flue gases is described and compared to measured data from a full-scale coal-fired power plant burning bituminous coal. The outcome from the prediction is actual concentrations of trace elements in main power plant streams.
1. Introduction Coal is the most important fuel in production of energy in larger stationary units. Besides main and ash forming elements, the fuel also contains trace elements in low concentrations, below 1000 ppmw. Many of these trace elements have been identified as having adverse effects on human health and the natural environment. Generally, coal contains considerable concentrations of many toxic elements, which have been enriched many times as compared to the mean concentrations of the elements in the earth’s crust. The concentrations of trace elements in coal vary both regionally and locally, depending on the processes that took place before, during, and after the formation of the coal. Typical concentrations of eight trace elements in coal are given in Table 1. Coal combustion is to be considered involving a significant risk of emitting pollutants to the environment, and the release of potentially toxic metals has recently received keen attention by regulatory authorities. The 1990 Clean Air Act Amendments, of the U.S. Environmental Protection Agency, have targeted 11 trace elements (As, Be, Cd, Co, Cr, Hg, Ni, Mn, Pb, Sb, and Se) for potential control according to “Maximum * Corresponding author phone: 358 2 2154 036; fax: 358 2 2154 780; e-mail:
[email protected]. 4508
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 24, 1999
TABLE 1. Typical Concentrations (ppm) of Some Trace Elements in Coal (1) element
typical concn
range
element
typical concn
range
arsenic cadmium mercury nickel
10 0.5 0.1 20
0.5-80 0.1-3 0.02-1 0.5-50
lead selenium vanadium zinc
40 1 40 50
2-80 0.2-1.5 2-100 5-300
Achievable Control Technology”. In addition, regulations concerning trace element emissions for Cd, Hg, and Pb have been agreed upon within the United Nations Economic Commission for Europe in A˙ rhus, Denmark, 1998. Control of trace element emissions is, however, not a simple task, since the chemistry of coal combustion is characterized by complexity. The process includes practically all the elements in the periodic table, except the noble gases, and the fate of trace elements in coal combustion systems is governed by a row of chemical and physical processes that are not yet clarified. Linak and Wendt (2) give an extensive review of problems associated to toxic metal emissions from combustion. In their paper they thoroughly review problems such as regulation, occurrence, and partitioning. They also provide an overview of mechanisms and aerosol dynamics and discuss capture of toxic metals. A more recent review on the subject is given in Biswas and Wu (3) that focus on control of toxic metal emissions using sorbents. In their work, the pathway for transformation of a single metal is described by three main categories: (a) release into the gas phase in the combustor, which depends on the form the element enters the combustion process, (b) transformation of the element, that is dependent on the combustion environment and the chemical reactions that take place, and (c) aerosol dynamics, that is dependent on aerosol formation, growth through condensation, and coagulation. The fate of a single element in a combustion system is to a large extent governed by its mode of occurrence in the fuel, i.e., how the element originally is included in the fuel (4). Basically the occurrence of an element in the fuel may be divided into three different categories (2): (a) included mineral matter that consists of inorganic material existing as crystalline or glassy structures trapped in the fuel matrix, (b) excluded mineral matter that consists of distinct mineral grains separate from the fuel matrix, and (c) inherent mineral matter that by definition is individual atoms that are either chemically or physically bound to the organic fuel matrix. Low-rank coals generally have a higher proportion of “organically bound” trace elements than higher-rank coals (5). The key difference between lignite and bituminous coal has been attributed to greater association of the trace elements with carboxylic and phenolic groups in low-rank coals (6). In the report by Davidson and Clarke (4), however, it is stated that the methods of determining organic association are indirect and that, even if the method of analysis indicates that the element is associated to the organic matrix, there would still be no firm evidence for trace elements being organically bound. Davidson and Clarke (4) indicate that most of the evidence points to trace elements being part of the mineral matter, with the organic/inorganic difference being mainly a reflection of the particle size. This leads to the conclusion that trace elements, generally, are occurring in smaller “mineral particles” in younger coals. The degree of volatilization and the chemical speciation, form of occurrence, will affect the transformation (i.e., 10.1021/es990243v CCC: $18.00
1999 American Chemical Society Published on Web 11/05/1999
condensation, physical adsorption/chemisorption, and chemical reactions on fly ash particles) and aerosol formation (i.e., nucleation, growth through condensation, and coagulation) of the trace elements. The transformation of trace elements by chemical reactions is dependent on thermochemical, mass-transfer, and kinetic effects. The ash matrix and sorbents (such as calcium, silicon, or aluminum based sorbents) may capture many trace elements. The capture efficiency is, however, both element- and sorbent-specific (7) and may depend strongly on the temperature (8). Other parameters, such as chlorine content in the gas phase, also affect the capture efficiency. In understanding the fundamental chemical effects on the distribution of trace elements in combustion systems, equilibrium studies have proved to be of great value. Several equilibrium studies related to trace metals in combustion systems have previously been done. Mojtahedi et al. (9) was one of the first to report speciation and partial pressures of metals in coal combustion and gasification processes by global equilibrium analysis. Equilibrium studies have in addition to coal combustion been applied in understanding the fate of trace metals in incineration furnaces (10) and combustion of biomasses (11). The fundamental work by Frandsen (12) gives a comprehensive description of the equilibrium chemistry of trace elements (As, B, Be, Cd, Co, Cr, Ga, Ge, Hg, Ni, P, Pb, Sb, Se, Sn, Ti, V, and Zn) at typical coal combustion and gasification conditions. The work by Frandsen (12) excludes, however, interactions with the ash matrix of the fuel. A similar modeling approach to the one presented in this paper has been presented by Rizeq et al. (13). Their model is, however, essentially more complex and includes several submodels for taking into account the particle entrainment, aerosol dynamics, and particle capture of the combustion system. Their equilibrium approach is, however, more simplistic than the description presented in this paper. Partitioning is, in contrast to global equilibrium of the combustion system, based on calculating the saturated vapor pressure under anticipated operating conditions, and the important ash-forming components iron, potassium, magnesium, and sodium are excluded from their list of components. Equilibrium studies, however, have several limitations since they neither take into account any effects of local conditions nor heat transfer during pyrolysis, mass transfer, chemical kinetics, and surface phenomenon nor physical changes of the fuel during heat up. In addition, equilibrium predictions are very sensitive to the input data. All relevant species that occur in the combustion system must be taken into account, and the thermochemical data must necessarily be correct and consistent. Besides problems with the species included, consistency and availability of equilibrium data, contrasting results may arise from different solution techniques and convergence criteria (14). In the paper by Frandsen et al. (14), four thermodynamic packages (MINGTSYS, NASA-CET89, FACT, and SOLGASMIX) were used to compare the equilibrium speciation of As, Cd, Cr, Hg, Ni, Pb, and Se at typical combustion conditions. In their study they used a list of chemical species to be considered and the elemental composition of the combustion system. Main components, calcium, and one trace element were considered at a time. The greatest differences were found to be due to the presence or absence of chemical species in the calculations. Differences due to thermodynamic data for individual species were also significant. Equilibrium assumption is still a sound approximation, and it is a valuable tool in providing a first step in understanding the chemical processes in combustion systems (2, 12). It has been shown that many of the potentially toxic trace elements are enriched in the submicrometer aerosol fraction
FIGURE 1. A schematic picture of a typical modern power plant. Figure shows the main parts: (a) boiler, (b) electrostatic precipitator, ESP, for particulate removal, (c) scrubber unit, FGD, for flue gas desulfurization, and (d) stack. of the flue gas (15). As particles/aerosols of this size are difficult to capture with conventional particulate removal systems, they may form a potential health risk. It would hence be essential to restrict or suppress the size fraction in the submicrometer mode, and fundamental understanding of the aerosol dynamics is therefore most important in designing environment-friendly processes. Basically, formation of aerosols includes the initial step of (a) nucleation, when the particle is formed from a supersaturated gas phase. The newly formed particle then continues to grow, and new material will condense on the particle surface. This mechanism is called (b) condensation. Finally, particle to particle collisions, followed by adhesion, may result in (c) coagulation that reduces the total number of particles in the flue gas. Simulation of aerosol dynamics requires, despite simplifications and approximations, large amounts of CPU time. The methods can be classified as moment, continuous, and sectional ones. In predicting all mechanisms (nucleation, condensation, and coagulation), only sectional methods can be applied (3). The sectional representation has, however, a problem of “numerical diffusion” that may result in serious errors. One of the most difficult tasks in describing aerosol dynamics is to correctly simulate the nucleation rates since classic nucleation theories have shown to be inaccurate in prediction of metallic species. In the review by Biswas and Wu (3) it is concluded that considerable work is necessary to accurately predict formation rates of metallic species in high-temperature environments. Despite these facts, the particle size is an important parameter that governs the capture characteristics in particulate control devices, and aerosol models are, hence, most important in predicting the behavior of trace elements in combustion systems.
2. Simple Reactor Method A schematic picture of a typical modern power plant is shown in Figure 1. The main outgoing streams of the power plant are in the figure marked with arrows, and the temperatures shown are typical for pulverized coal-fired combustion systems. The combustion temperature is usually around 1300-1500 °C, and the temperature 1100 °C, shown in the upper part of the boiler, may be considered a mean value for the whole furnace. A typical temperature in an electrostatic precipitator is 130 °C, and a temperature of 50 °C is common for wet scrubbers. In this study, a simple method for describing the fate of eight trace elements in a full-scale power plant was developed. The method uses two imagined reactors where the chemical environment is predicted with an extensive equilibrium model (16). The structure of the simple reactor method is shown schematically in Figure 2. The first reactor imitates conditions prevailing in the boiler. The fuel is introduced together with an appropriate amount of air and burns at a mean temperature. This reactor generates an equilibrium VOL. 33, NO. 24, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4509
FIGURE 2. Schematic picture showing the structure of the simple reactor method. ash and a flue gas. Based on the ratio of bottom ash to fly ash of the specific plant, a fraction of the condensed phases is withdrawn from the system as an “equilibrium bottom ash”. The rest of the generated ash is, together with the equilibrium flue gas, sent to the second reactor where the chemistry before the particulate control system is predicted. In this reactor the remaining equilibrium ash reacts with the flue gas, and an “equilibrium fly ash” is generated. The main part of the equilibrium fly ash is withdrawn, but a small fraction of the fly ash is assumed to pass through the particulate control system. The amount and composition of the dust that passes through the particulate control device is predicted based on the particulate removal efficiency of the plant and the equilibrium prediction of the second reactor. The following information about the power plant is necessary for estimation of the trace metal distribution with the simple reactor method: (1) fuel composition, (2) air/fuel ratio, λ, (3) furnace mean temperature, (4) bottom ash/fly ash ratio, and (5) particulate removal efficiency. The output from the prediction is the composition of the bottom ash, fly ash, and flue gas. Composition of the dust particles is predicted based on the particulate removal efficiency and the equilibrium prediction of the second reactor.
3. Equilibrium Description The chemical composition in the reactors of the model is estimated by global equilibrium analysis, minimizing Gibbs free energy. In this study, chemical equilibrium was determined utilizing the nonstoichiometric approach. The computer program “ChemSage” (17) was used for the purpose. Equilibrium predictions require, in addition to a reliable Gibbs free energy minimization algorithm, reliable and consistent thermodynamic constants. In this study, the Scientific Group Thermodata Europe database for pure substances (18), SGTE database, was used. It contains equilibrium constants for reactions among the main components (C-O-H-S-N-Cl), ash forming elements (Al-CaFe-K-Mg-Na-Si), and the trace elements (As-Cd-HgNi-Pb-Se-V-Zn). In all, the description includes 21 elements, one single mixture phase including 304 gaseous species, and 393 condensed phases. All species were taken into account simultaneously, except for gaseous cadmium hydroxide, Cd(OH)2(g), that is believed to have an inconsistent numerical value (16). Gaseous cadmium hydroxide was not found in other databases, and since data for this species is classified according to the lowest reliability in the SGTE database, it was omitted in our prediction. All solid phases were used within temperature limits of the data except for 2CaO*V2O5 alone that was extrapolated from 1015 °C (1288 K). Data for many gaseous species were, however, extrapolated. Table S1 (Supporting Information) shows a complete list of the chemical species included in this study. Figure 3 shows an example of equilibrium predictions in the studied combustion system. The figure shows equilibrium distribution of arsenic-containing species as a function of temperature when burning bituminous coal at an air excess ratio of 1.2. Investigated temperature range was 25-1600 °C, 4510
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 24, 1999
FIGURE 3. Distribution of arsenic-containing species when burning bituminous coal at an air excess ratio of 1.2.
TABLE 2. Characteristics of the Bituminous Coal Mixture component C Oa H S N Cl Al2O3 CaO As Cd Hg Ni a
component Main Components, wt % 75.5 Fe2O3 5.1 K2O 5.02 MgO 0.98 Na2O 1.53 SiO2 0.13 TiO2 + PO4 2.19 sum on dry basis 0.31 moisture Trace Elements, ppmw 8.6 Pb 0.05 Se 0.08 V 6.8 Zn
1.07 0.28 0.15 0.04 6.31 0.24 98.85 10.0 6.7 1.8 26 14
Calculated.
and total pressure was set to 1 bar. The equilibrium distribution of the element was calculated in 64 steps, with a temperature interval of 25 °C that might give the curve a somewhat jagged appearance.
4. Fuel Composition and Operating Conditions The combustion system that was studied is a typical pulverized coal-fired system as described in the literature (15). An extensive 3-day measuring campaign was carried out during 1986 at the 600 MWe unit of the Gelderland power plant. The boiler was equipped with 36 wall-fired burners, and the combustion temperature was about 1400 °C. The plant was equipped with an electrostatic precipitator, for particulate control efficiency of over 99.7%, and a flue gas desulfurization plant, for control of sulfur emissions. During the measuring campaign, the fuel consisted of a bituminous blend that was pulverized to a particle size of less than 100 µm. The caloric value was 29 MJkg-1 and the ash content about 12%. The fuel contained about 1% sulfur. Fuel composition as given in the literature (15) was used as such, and the moisture content of the fuel was assumed to be 10%. Characteristics of the fuel are given in Table 2. Partitioning between bottom ash and fly ash was not known exactly, but it was estimated to be about 12% bottom ash and 88% fly ash (15), and we used these figures in predicting the composition of the bottom ash, fly ash, and the flue gas. The mean combustion temperature in the first reactor was set to 1100
°C, and the temperature in the second reactor was set to 400 °C, to correspond to a typical temperature in the flue gas channel downstream the super heaters. At this temperature the chemical reactions are considered “frozen”, or slow enough, and it is assumed that they do not have time enough to react further in the electrostatic precipitator. The mean combustion temperature is purely empirical and was chosen based on the behavior of the trace metals in the Gelderland power plant.
5. Enrichment of Trace Elements When burning a fuel, the residual ash generally contains the same elements that were present in the fuel. Ash-forming elements are, however, in an ideal case, enriched by the inverse of the ash content of the fuel. The enrichment of one single element is still dependent on the combustion system as well as on the fuel type and the particular element of interest. Description of enrichment and distribution of elements in combustion systems may be conducted in various ways. One way is to describe the flows of the power plant streams and the concentrations of the specific flows. Based on this information it may, however, be difficult to get an overview of how elements are distributed (particularly if several power plants and fuels are to be compared). It might, therefore, be convenient to use the overall partitioning of elements within the streams of the power plant as in the paper of Rizeq et al. (13). This description gives a clear view of how elements are distributed within the streams of the power plant, but in order to get an idea of how elements are enriched in the ash, the distribution of ash in the specific plant has to be known. To describe the enrichment of an element Meij introduced, the term “relative enrichment”(RE)
RE )
TABLE 3. Measured and Predicted Enrichment Factors for Eight Trace Elements for a Pulverized Coal-Fired Power Plant Burning Bituminous Coal RE factor element As Cd Hg Ni Pb Se V Zn
measd bottom ash
predicted bottom asha
measd fly ash
predicted fly asha
