Rate of char combustion

Large uncertainties exist in the apparent and intrinsic rate of char combustion. Estimates of the rate of char combustion within the range of uncertainties can result in over designed or under designed boilers. A review (2) has shown that one of the major uncertainties in the rate of char combustion is the catalytic influence of mineral matter. This paper reviews the available information on the catalytic effects of mineral matter and concludes that many coals contain sufficient sodium and calcium to increase the rate of char combustion by a factor of 100 at low temperatures. However, insufficient information is available to assess the influences of catalysis by mineral matter at combustion temperatures. 

The rate of coal combustion can, in some instances, influence the design of boilers. The intrinsic rate of char combustion is, however, poorly known. Figure 1 shows that estimated intrinsic rate of char combustion can vary by a factor of 10 (1 ). The largest uncertainties are (2) ... 

Mineral matter contained in the coal could influence the rate of char combustion by blocking part of the coal surface or by catalytically increasing the rate of combustion. Figure 2 shows that the measured rate of combustion of purified nonporous graphites is uncertain by less than a factor of three. This is a small difference compared to the spread in the overall rate data and suggests that some of the scatter in the measured rates of coal combustion is caused by the mineral matter in the coal. 

Experiments have shown that small amounts of certain metals can accelerate the rate of char combustion (4 9). A number of anions and cations have been shown to accelerate the combustion of carbons at concentrations of 10 to 1000 ppm. Table II shows the relative influence on the combustion rate of various salts added as solutions to purified graphite. Relatively small amounts of metals can accelerate the rate of combustion by many orders of magnitude. To effectively catalyze the combustion rate of coal, the metal which accelerates the rate must be distributed on nearly the molecular level, and be present in sufficient concentration to accelerate the rate. The range of relative acceleration of the combustion rate by different metals is shown in Figure 3. These estimates are made... 

Minerals present in coal can accelerate the rate of char combustion by as much as a factor of 100. Molecularly attached sodium and calcium compounds are likely to increase the char combustion rate substantially at low temperatures. However, information is not available and work should be undertaken to assess the acceleration of char combustion by minerals at the temperatures of combustion. 

The mathematical model for char combustion described in the previous two sections is applicable to a bed of constant volume, i.e., to a fluidized bed of fixed height, Hq, and having a constant cross-sectional area, Aq. The constant bed height is maintained by an overflow pipe. For this type of combustor operating for a given feed rate of char and limestone particles of known size distributions, the model presented here can predict the following ... 

The burning rate of chars at a pressure of 1 atm for petroleum coke and the different coal rank has been compared by Sergeant and Smith [27]. The collected experimental data show that the char burning rates expressed in (kg m- s ) is slightly lower for the same particle temperature, for petroleum coke against anthracite however the behavior of both coke residue (petroleum coke) and char (anthracite) is similar. The burning profile technique is a method for predicting the relative combustion characteristics of fuels. 

Coal char combustion Phenomenological aspect In the regime of "low temperature," the chemical reaction rate is slow compared with the diffusion through the pores, because the O2 completely penetrates the char matrix. In this case the rate controlling the regime of char combustion is kinetically limited. 

Mechanisms and Rates of Combustion. AH soHd fuels and wastes bum according to a general global mechanism (Fig. 2). The soHd particle is first heated. FoHowing heating, the particle dries as the moisture bound in the pore stmcture and on the surface of the particle evaporates. Only after moisture evolution does pyrolysis initiate to any great extent. The pyrolysis process is foHowed by char oxidation, which completes the process. 

Because heat of combustion of the volatile product is not the same as that of whole wood, one cannot estimate heat release rate based on mass loss rate as can be done with ideal fuels such as gases, liquids, and some noncharring solid materials. Thus, measuring heat release rate rather than mass loss rate is appropriate for wood and charring materials. Several bench-scale calorimeters have been developed to measure heat release rate of materials (1,11,12,13). 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

The heat accumulation in the bed surface layer causes the ignition of the char combustion process. The heat is supplied from the over-fire process (see Figure 58C). When the char combustion process commenced, the macroscopic ignition front sustains itself with heat from the exothemic oxidation reactions. Large amounts of the heat released by the char combustion zone are also conducted and radiated away both upwards and downwards in the bed. The downward propagation rate of the macroscopic ignition front is controlled by several factors, such as biofuel moisture content, primary air rate and air temperature [33]. The temperature of the macroscopic propagating char combustion zone is around 1000-1200°C in batch-bed combustion of solid biofuels [38,41]. 

---