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Show entrainment characteristics for both cold and hot systems. Mathematical modeling is used for a wide range of combustion and heat transfer processes including the burnout of oil, coal and coke particles, heat transfer and the residence time and concentrations of feed and product in the process. For heat transfer modeling the kiln is divided into axial slices, typically 100 mm thick, and the mixing rates, combustion heat release, and radiative effects of the gases and particles calculated within each slice to determine the radiant heat transfer to the product and walls. Convective heat transfer effects are also calculated within each slice. B y stepping the calculation through the system, a realistic estimate of the burnout, gas temperature, heat transfer and product temperatures can be obtained. The flame itself and the combustion products absorb, and emit, thermal radiation. Both gases and particulate material present in the flame contribute to the absorbing propensity of the flame. Within the flame, the chemical effects of the combustion process are secondary, since the reaction time constants are orders of magnitude faster than the diffusional mixing constants. Thus, the combustion process can be reduced, with a 'mixed is burnt' assumption controlling the rate of heat release. The mathematical model used by F C T for calculating the heat transfer from flames in rotary cement kilns takes these factors fully into account (12). 3.5 Modeling the Ash Grove kiln The first step in modeling a proposed combustion system is to reproduce the existing combustion conditions. This serves to provide confidence in the model. A number of existing conditions were modeled on the Ash Grove kiln because the original burner was initially designed to fire coal only and consisted of a straight pipe. W h e n gas became available a gas burner was added inside the coal pipe. This restricted the coal flow by approximately 5 0 % , and so higher coal flows tended to back up the coal mill. The gas burner was removed when 1 0 0 % coal firing was required. Tire derived fuel (TDF) is also burnt in the kiln and this was taken into account when modeling the kiln. When firing 50% - 100% gas with the original burner the fuel and air mixing was generally slow, indicating a long and lazy flame. The flame was generally axial along the length of the kiln which implied that the kiln aerodynamics were good, i.e. the flame shape or direction were not detrimentally affected by the air flow pattern. The "delayed" fuel and air mixing was due to low primary air momentum flow, which was insufficient to entrain all the combustion air in an efficient manner. The performance of the original burner firing coal only was also modeled. Once again, delayed fuel and air mixing was observed, but with a relatively axial flame shape. The design of the coal firing section of the new burner was carried out first. To improve the fuel and air mixing, higher primary air momentum than the original burner was required, which meant an increase in the primary air flow and/or velocity. It was not possible to make the changes recommended by the model without major changes to the coal milling plant. This was considered unfeasible because the kiln fires natural gas most of the time. 9 |
