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Present regulatory requirements enforces the modification of the firing modes of existing coal-fired utility boilers and the use of coals different from those originally designed for these boilers. The reduction of SO2 and NOx emissions were the primary motivation for these changes. However, economic considerations played a major role too. Using sub-bituminous coals has become an important solution for emissions compliance due to their unique constituents and combustion characteristics; these coals are often referred to as enviro coals. Powder River Basin (PRB) Coals are classified as sub-bituminous ranked coals. Unlike higher ranked bituminous coals, which have tight pore structures that limit the amount of moisture they could hold, low rank coals, such as PRB coals, have looser pore structure and additional moisture retention capacity. PRB coals differ in many properties from those of the commonly burned bituminous coals, including low heating value, low fusion temperature, and high moisture content. However, PRB coals have low sulfur content and are relatively low cost. They can also lower NOx emission from power plants due to higher volatile content. When power plants switch from the designed coal to a PRB coal, operational challenges, including transportation, handling, storage, and combustion, were encountered. A major problem faced when using PRB coals is severe slagging and excess fouling on the heating surface. Not only is there an insulating effect from deposit, but there is a change in reflectivity of the surface. Excess furnace fouling and high reflectivity ash may cause reduction of heat transfer in the furnace, which results in higher furnace exit gas temperatures (FEGT), especially with opposite wall burners and with single backpass. Higher FEGT usually result in higher stack gas temperature and increasing in the reheater spray flow and therefore decreasing the boiler efficiency with higher heat rate of the unit. The modification of an existing unit for firing of PRB coals is confined to — and constrained by — existing equipment. All successful conversions happen when in the design phase of a project the following parameters are evaluated: (1) capacities or limitations of furnace size, (2) firing system type and arrangement, (3) heat transfer surface, (4) pulverizers, (5) sootblowers, (6) fans, and (7) airheaters. In the present study we used a comprehensive methodology to predict the behavior of three PRB coals fired in a 575MW T-fired boiler.

The present regulatory requirements enforce the modification of the firing modes of existing coal-fired utility boilers and the use of coals different from those originally designed for these boilers. The reduction in SO2 and NOx emissions was the primary motivation for these changes. Powder river basin (PRB) coals, classified as subbituminous ranked coals, can lower NOx and SOx emissions from power plants due to their high volatile content and low sulfur content, respectively. On the other hand, PRB coals have also high moisture content, low heating value, and low fusion temperature. Therefore when a power plant switches from the designed coal to a PRB coal, operational challenges were encountered. A major problem that can occur when using these coals is the severe slagging and excess fouling on the heat exchanger surfaces. Not only is there an insulating effect from deposit, but there is also a change in reflectivity of the surface. Excess furnace fouling and high reflectivity ash may cause reduction in heat transfer in the furnace, which results in higher furnace exit gas temperatures (FEGTs), especially with opposite wall burners and with a single backpass. Higher FEGTs usually result in higher stack gas temperature, increasing the reheater spray flow and therefore decreasing the boiler efficiency with a higher heat rate of the unit. A successful modification of an existing unit for firing of PRB coals requires the evaluation of the following parameters: (1) capacities or limitations of the furnace size, (2) the type and arrangement of the firing system, (3) heat transfer surface, (4) pulverizers, (5) sootblowers, (6) fans, and (7) airheaters. In the present study we used a comprehensive methodology to make this evaluation for three PRB coals to be potentially fired in a 575 MW tangential-fired boiler.

Computational examination of pulverized coal combustion in an entrained flow downer reactor exhibited the existence of two distinct combustion regions referred to as near burner combustion (NBC) and far region combustion (FRC) with lack of mediate state. Operating conditions and material properties were altered within a broad range in which the reactor’s behavior was studied. A transition mechanism between the two combustion structures was recognized and isolated to illustrate ranges of stable combustion. NBC was mainly affected by the hydrodynamics while FRC was mainly affected by the devolatilization kinetics. Non dimensional analysis of the gas solid behavior yielded Stokes values close to unity for the transition cutoff. A new methodology for the selection of a distribution representative particle diameter is presented and assumed to be applicable for various types of two phase exothermic reacting flows. The pulverized coal combustion inside the reactor was simulated using the Euler–Lagrange approach and validated against experimental results. Special attention was given to the backmix of hot gases combined with the inherent recirculation zone near the reactor’s entrance for their effects on the transition mechanism between combustion regions. Understanding these mechanisms will lead to better control over processes regarding phase continuous mixing vs short contact within entrained flow downer reactors.