pyrolysis step and because of the ash content of coal. Problems directly related to coal—such as emissions, waste products, and char oxidation efficiency—are receiving far less attention than problems relating to other fuels. Much of the recent advanced research on coal-related combustion issues, notably the interaction with coal ash and the final stages of oxidation, has been conducted in the United States, principally under DOE and, to a lesser extent, National Science Foundation sponsorship. The committee noted that, in the absence of research needs and funding from other sources, DOE support is important to achieve progress in quantitative understanding of coal-related combustion and gasification issues and to identify innovative concepts for further investigation.
While still a promising area for research, gas-phase chemistry of NOx formation and destruction and of the oxidation of carbon monoxide (CO) and hydrocarbons, has advanced to the point where simplified gas kinetic models can be used in conjunction with primitive turbulence modeling as a semiquantitative design and development tool for low-emission furnaces and gas turbine combustors. However, in the case of coal, the early release of gas-phase hydrogen cyanide introduces NOx production pathways not yet quantitatively explored. Moreover, promising research opportunities still exist, including implementation of more sophisticated models. In contrast, the understanding and quantitative treatment of carbon kinetics, taking into account catalytic and physical interactions with ash and graphitization of carbon as the oxidation process proceeds, is at a relatively primitive stage. Since future innovations in coal gasifier and combustor design will depend, to a considerable extent, on quantitative understanding of the interaction between pyrolysis, carbon oxidation, and emissions, the committee noted that DOE's advanced research program for coal needs to address this issue.
The final stage of carbon oxidation is of special interest because of the observed reduction of reactivity at high conversion rates (Davis et al., in press). The long reaction times and high temperatures required for high carbon conversion will increase thermal NOx formation in the presence of excess air. The interactions involved are complex, and improved quantitative understanding of the evolution of carbon reactivity and its interaction with the physical and catalytic properties of the coal ash is needed for choice of optimum levels of carbon oxidation.
Two major advanced research opportunities were identified by the committee as a basis for improving high-performance gasification systems. In low-temperature and countercurrent fixed-bed gasification processes, escape of fuel nitrogen as ammonia can occur, resulting in the formation of additional NOx on combustion if not removed. Quantitative treatment of this problem is needed for improvement of these processes. For low-temperature gasification processes where high carbon conversion is needed, catalysis of carbon gasification by ash constituents, such as calcium, or by added catalysts remains a promising area related to future advances in gasification efficiency.