OCR Text |
Show Theoretically, a stoichiometric mixture of fuel and air should result in complete combustion without any CO or 0^ being present in the products. In practice, however, complete combustion is not achieved without supply ing an excess amount of combustion air. While excess air is necessary for complete combustion of the fuel, it adds significantly to the heat loss associated with the flue gases. Large amounts of heat energy released in the combustion process simply go to heating the excess air which is expelled into the atmosphere. On the other hand, incomplete combustion occurs when an adequate amount of excess air is not supplied. Such a condition usually results in CO generation which represents energy loss associated with incomplete combustion of the fuel. By operating at a theoretical point where combustible energy losses and effluent energy losses are minimized, maximum combustion efficiency results. In practice, the excess air is kept to a minimum level necessary for stable firing while CO is monitored. Maximum combustion efficiency is usually achieved when CO generation is within 150 to 250 ppm. Accurate measurements of CO and excess 0^ precisely indicate the firing conditions. Results will be presented for No. 2 fuel oil mixed with each of the coals presented in Tables 2 and 3. EMPIRICAL RESULTS A systematic series of tests were conducted during which the excess stack 0~ was varied and measurements of stack CO were obtained for 50 weight percent fuel mixtures of each of the coals and No. 2 fuel oil. The results of these tests are presented in Figure 4. It should be mentioned that each test was conducted using the sonic burner, each stoichiometry was obtained with constant air flows and variable fuel flowrates and all measurements are obtained on a dry volumetric basis. Considering the data presented in Figure 4 a number of conclusions may be drawn. First, the fuel mixture which behaves most like the No. 2 fuel oil alone is that composed oi the No. 2 fue] oi] with North Dakota lignite. Note that the peak CO value with this mixture did not occur until the excess stack 0„ was Lowered to 0.5 percent. Even at this value (0.5 percent excess stack 0„) smoking was not observed. Consider now the data of the fuel mixture comprised of No. 2 fuel oil and Pennsylvania parent coal. At an excess stack 0? value of 2.0 13 |