OCR Text |
Show Combustion of the char occurs in the lower furnace area either on the furnace walls or the char bed at the base of the unit. The chemical reactions and transport phenomen~ occurring in char combustion are not well understood. The measurement of the temperature of the char bed would provide one means of control of the process, but the measur~ment of. even this simple parameter has presented a significant challenge to current sensor systems. After the organIc matenal has burned from the char, the inorganic chemicals are recovered as a molten smelt that flows out o~the furnace through smelt spouts. The smelt is then dissolved in recycled water to regenerate the needed pulping chemIcals . . . During flight, some inorganic material is volatilized and carried with the exhaust gases and can end up d~posltIng on the hea~ transfer surface areas of the recovery boiler. These deposits can sinter, which make them more difficult to remove WIth soot blowers. In certain situations, the buildup may force a decrease in firing rate or even a shutdown to clean the passages (Alkaline Pulping Committee 1992). . . Overall, the detailed chemical reactions and transport phenomena associated with the combustion of black lIquor In a rec~v~ry boiler are not well understood. Computer modeling of the process has been attempted and is the fo.cus of ~n eXIstIng OIT financial assistance grant. However, the work has proven to be difficult. More research is Sh~l conSIdered necessary, and, if a more fundamental understanding of the combustion process were obtained, better boIler controls could be devised to optimize the process. Alternatively, another method of chemical recovery is the use of gasification technology. Gasification processes have been researched intensely for several years, and a foreign manufacturer has commercialized one process. Further development of other gasification technologies could be warranted to provide alternatives to the existing recovery process. Finally, in general, corrosion is a problem in both recovery boiler combustion and gasification processes. Also, research to reduce the variability and control the properties of the black liquor fuel can be a benefit to both combustion and gasification processes. 4.7.2 Research Needs Several projects are currently sponsored by DOE in the areas of black liquor, gasification, and sensor technologies. TIle following research needs are intended as outgrowths: • • • • Understanding Combustion in Black Liquor Fired Boilers. The generation of steam by combustion of fuels provides a significant amount of energy to this industry. The boilers used are similar to other industrial boilers if fired by natural gas, coal, fuel oil, wood, or biomass, for example, but are significantly different when fired by black liquor. In this regard, the pulp and paper industry has distinct combustion research needs. A more fWldamental understanding of the black liquor combustion process (reaction rates, combustion species, velocities, residence time, etc.) needs to be developed. Understanding of the combustion of black liquor is not as developed as the understanding , of the combustion of coal, for example. Gasifier Demonstration and Advanced Research. The industry has expressed the need for the development of lower-cost, safer, and more efficient alternatives to the existing chemical recovery process. It is noted that black liquor gasification has significant potential in this regard. The development of black liquor gasification can also be combined with the further development of combined cycle cogeneration systems - another need expressed by the industry. Air Emi·ssions. Air emissions from all combustion sources (especially from recovery boilers) must meet present and future regulations, including nitrogen oxides, sulfur oxides, toxic air emissions, and (particularly for tl,le kraft p~lp mill) o.dor. The. combustion air envirolUllent in the .recovery boiler can partially reduce mtrogen ?xlde ~mlsslon~, but .If changes are. m~de to the combustlOn process to optimize recovery (especially hIgher solIds finng), these emlSSlOns ca!1 be adversely affected. A. . better understanding of the fundamental combustion process can prOVIde the means for both attammg air emission standards and optimizing the operation of the recovery boiler. Sensors and Controls. Development of effective combustion control with black liquor fired fuels is needed. In particular, better measurements of temperature of the char bed and temperatures in the 22 |