Investigation of Combustion and Thermal-flow inside a Petroleum Coke Rotary Calcining Kiln with Potential Energy Saving Considerations

Calcined coke is a competitive material for making carbon anodes for smelting of alumina to aluminum. Calcining is an energy intensive industry and a significant amount of heat is exhausted in the calcining process. Efficiently managing this energy resource is tied to the profit margin and survivability of a calcining plant. To help improve the energy efficiency and reduce natural gas consumption of the calcining process, a 3-D computational model is developed to gain insight of the thermalflow and combustion behavior in the calciner. Comprehensive models are employed to simulate the moving petcoke bed with a uniform distribution of moisture evaporation, devolatilization, and coke fines entrainment rate with a conjugate radiationconvection-conduction calculation. The following parametric studies are conducted: rotation angles, tertiary air injection angles, devolatilization zone length, discharge end gas extractions without injecting natural gas, variations of coke bed properties (thermal conductivity and heat capacity), and coke bed sliding speed. A total of 19 cases have been simulated. The results of studying the effect of tertiary air injection angles show that employing 15° tertiary air injection angle provides the best calcining condition than using 30° and 45° injection angles by achieving a higher coke bed temperature and less coke fines entrainment and attrition rate. In an attempt to reduce natural gas consumption, employing gas extraction at the discharge end successfully draws the hot combustion gas from the tertiary air zone towards the discharge end without burning natural gas. The coke bed temperature between 6 and 21 m from the discharge end is successfully raised from 10 to 100 K, but discharge end temperature is reduced 150 K without burning natural gas. The extracted gas at 1,000 K is too low to be returned to the kiln, but it could be used to preheat the tertiary air. INTRODUCTION Petroleum coke (or petcoke) is a carbonaceous solid derived from petroleum refinery cracking process. Calcination is the process of heating a substance to a high temperature, but below its melting or fusing point, to bring about thermal decomposition or a phase transition in its physical or chemical constitution. Petcoke is usually calcined in a gas-fired rotary kiln or rotary hearth at high temperatures, around 1,200 to 1,350 °C, to remove moisture, drive off volatile matters, increase the density of the coke structure, increase physical strength, and increase the electrical conductivity of the material [1]. Figure 1 shows the schematic of petcoke calcinations with tertiary air in a rotary kiln. Petcoke is fed from the feed end at the end of heat up zone and moves counter current of the hot combustion gas which is coming from the discharge end located at calcined coke zone. Tertiary air injections at calcining zone provide a combustion boost to utilize the volatiles coming off the petcoke and generate the heat required for the calcining process.