Study On Burning Mixed Fuel Of High Sulfur Petroleum Coke And Oil Shale In CFB Boiler

The output of petroleum coke will reach 1.04 million t/a after completion and operation of the 20 million t/a oil product upgrading and revamping project of SINOPEC CORP. Maoming COMPANY (SINOPEC Maoming) in 2013. Based upon the property of petroleum coke, the dominant outlet of high sulfur petroleum coke is boiler fuel. However, as the market for high sulfur coke is instable, it is hard to find suitable regular customers. Some small metallurgical plants and earth kilns in the community, due to shabby combustion equipment and lack of desulfurization facilities, have been causing great environmental pollutions by the carbon dioxide.SINOPEC Maoming's 2x410 t/h CFB boilers were completed and put into operation at the end of 2005. According to the design, fuels of the two CFB boilers were 100% soft coal or 70% soft coal plus 30% petroleum coke. If calculated based on full-load operation, the two CFB boilers consume 180 K t/a by petroleum coke as 30% of the mixed combustion fuel, which, however, only accounts for 17.3% of SINOPEC Maoming's petroleum coke output. Therefore, it has become an urgent subject for the refinery to find a stable and effective environment-friendly way out for the high sulfur coke through mixed combustion to improve the quantity of mixed combustion of petroleum coke by taking advantage of CFB structural advantages.This paper argues that fuel properties are the critical factors impacting designated CFB boilers'safety, economics and environment-friendly operation by study of CFB boilers' construction features and such influencing factors as the boiler operation safety, environmental requirements and economic indicators. On the basis of theoretical research, in combination with the physical and chemical analysis data about different mixture of high sulfur petroleum coke and oil shale as well as the desulfurizer lime stone and by way of pilot research on mixed combustion of mixed fuels of high sulfur petroleum coke and oil shale at different blending ratios on 1MWt/h CFBC test boiler of Xi'an Thermal Power Research Institute, the feasibility of combusting mixed fuels of high sulfur petroleum coke and oil shale on CFB boilers was preliminarily determined and it was proposed for the first time to burn mixed fuels of high sulfur petroleum coke and oil shale on industrial CFB boilers. On the basis of semi-industrial pilot-scale test and through industrial test of combustion of mixed fuels of high sulfur petroleum coke and oil shale on SINOPEC Maoming's two 410 t/h physical CFB boilers, analysis and study were conducted for problems existing in the course of mixed combustion, reasonable control parameters and recommendations for technical modification of equipment were proposed for long-term combustion of mixed fuels of high sulfur petroleum coke and oil shale. SINOPEC Maoming was the first company completing the industrial test of combusting mixed fuels of high sulfur petroleum coke and oil shale on CFB boilers. Combining various study results, we drew the conclusion that combustion of mixed fuels of high sulfur petroleum coke and oil shale on industrial CFB is safe and cost-effective for designated CFB boilers under the condition of meeting environmental requirements through capacity verification of the slag cooling system and lime stone system, control of appropriate blending ratio of high sulfur and oil shale after modifications. The paper contains the following:The second part of this paper briefly describes CFB boilers'construction features, expounds factors impacting the safe and environment-friendly operation of CFB boilers and discusses the impact of fuel properties on CFB boilers'operation. It is pointed out in the paper that the fuel properties have greater impact on safe operation of CFB boilers. In addition, it is expressly indicated that the boilers'fuel hardness, ash melting point, sulfur content, moisture content, heat value and ash content must be within the design values. It also proves from the perspective of fume pollutant generation mechanism in the paper that the fuel properties are the only controllable factor affecting compliance discharge of boiler pollutants and the fuel's heat value, sulfur content and ash content were the critical indicators of control. It is argued through calculations of the boiler efficiency in the paper that the fuel quality is the only controllable factor for cost-effective operation of CFB.The third part of this paper analyzes the physical and chemical data of high sulfur petroleum coke, oil shale and lime stone, studied the lower heating value, element components, total sulfur, particle size distribution, density, ash melting point, ash content, ignition and burnout properties, grind ability, erosive wear, ability of self-desulfurization and particle size distribution features of high sulfur petroleum coke, oil shale and their mixture as well as the particle size distribution features, reaction capacity and activity of lime stone. Analysis reveals that some individual indicators of single sample of the petroleum coke and oil shale have exceeded the coal classification standards for power generation boiler. They are not applicable for combustion of single sample. However, indices of the mixture of such two fuels at certain ratios are close to those of conventional fuels of the coal fired power plant. Ideal effects can be achieved by combustion of mixed fuels blended with high sulfur petroleum coke and oil shale at appropriate ratio on CFB boilers.The fourth part of this paper analyzes and studies the pilot test data related to combustion of mixture of petroleum coke and oil shale on 1 MWt/h CFB testing boiler, the desulfurization, discharge and cinder property. The research data shows that combustion of mixture of high sulfur petroleum coke and oil shale on CFB under the conditions of testing boiler could meet the requirements of safety, environmental friendliness and cost-effectiveness. The main basis includes:CFB boilers adopt the mode of oil gun ignition under bed, in which the mixed fuel could successfully burn when the temperature goes higher than 420℃, the testing fuel can be stably combusted when the bed temperature is controlled between 840℃and 900℃; the content of ash flammables is 1.91%-5.70%, the content of bottom cinder flammables 0.96%-2.42%, combustion efficiency 95.7%-99.05%, the contents of fly ash 65.71%-83.90%; in the case of Ca/S=2.4 and T=877.5℃, the discharge concentration of SO2 is 175.8-491.3mg/m3(standard,6%O2) and desulfurization efficiency 95.1-98.3%; the discharge concentration of NOx is 321.7 mg/m3-625.1mg/m3 (standard,6%O2); the discharge concentration of CO is 32.45 mg/ m3-85.96mg/m3, which also changes with variations of such factors as the excess air coefficient and hearth temperature.The fifth part of this paper analyzes and studies the industrial test data related to combustion of mixed fuels of high sulfur petroleum coke and oil shale on SINOPEC Maoming's 410 t/h CFB boilers. The research data reveals that combustion of mixture of high sulfur petroleum coke and oil shale on designated industrial CFB boilers can meet the requirements of safety, environment and cost-effectiveness. The main basis includes:when 75% petroleum coke and 25% oil shale are combusted in industrial CFB boilers, the boilers are operated in a continuous and stable manner and the bed temperature maintains at about 830℃; the average carbon content in fly ash is 11.5%, the average carbon content in bottom cinder 4.3%, the content of fly ash about 35% and the boiler's heat efficiency 90.89%. The optimum operation parameters are that the oxygen content of operation fume discharge is controlled at 3.6%, the fluidized air rate under bed is controlled at about 50% and the pressure of air chamber is maintained above 11 kPa. Under the conditions of adequate lime stone capacity, the boiler's concentration of SO2 discharge is less than 200mg/Nm3 and the desulfurization efficiency 95%. The concentration of NOx discharge is 101.6mg/Nm3-132.9mg/Nm3, reaching the discharge requirements of below 650 mg/m3(standard,6%O2) as specified by GB13223-2003; Under the conditions of molar ratio of calcium to sulfur 3.0 and bed temperature of 830℃, the resistance of fly ash ratio at 150℃is 3.1×1011Ω·cm, much less than that of normal bunker coal; the fly ash abrasion index of physical boiler was 13.56% and 14.32%, indicating an average abrasion of abrasion property.