Simulation of Combustion and Thermal-flow inside a Pyroscrubber

A pyroscrubber is the device used in petroleum coke Calcining industry to oxidize the carbonaceous contents, including hydrocarbon volatiles of the exhaust gas from the calcination kiln, so as to recover energy to produce electricity and leave no more than small traces of unburned volatiles, solid carbon, ashes, or emissions (e.g. CO, NOx and SOx) in the flue gas finally discharged. Motivated by the need to maximize the energy recovery and reduce pollutant emission from the pyroscrubber, a 3-D computational model is developed to simulate the combustion and thermal-flow phenomena inside the pyroscrubber to help seek means to reduce emissions and increase energy density for downstream power generation. CFD model validation is achieved by comparing baseline case results with the plant measurement data of temperature and NOx emission. The simulation results show that the specially designed high-bay wall structure generates a strong mixing zone forcing combustion to happen at an earlier stage and helps to efficiently utilize the main chamber space. A well balanced amount of excess air is favorable in generating more energy output and lowering NOx emission. Incomplete combustion with sub-stoichiometric air cuts NOx emission, but leads to less total energy output, lowers gas temperature and increased CO emission. A multistage burning strategy is introduced and studied and results show it successfully cuts emission without compromising energy (electricity power) output. INTRODUCTION A pyroscrubber is namely a furnace burning carbon particles in a stream of waste gas, particularly from a petroleum coke calcination kiln or hearth. The schematic of the calcining process for petroleum coke is shown in Fig. 1. The combusted hot gases from the pyroscrubber are ducted through a boiler to produce steam that is used to generate electricity through steam turbines. A pyroscrubber typically comprises of a U-shaped combustion chamber having a first passage arranged parallel with (preferably above) a second passage, so there is a reversal in gas flow direction between the two passages. The main function of the pyroscrubber is to oxidize the carbonaceous contents, including hydrocarbon volatiles in the exhaust gas from the calcination kiln, so as to recover energy from the waste stream and leave no more than small traces of unburned volatiles, solid carbon, ashes, or emissions (e.g. CO, NOx and SOx) in the flue gas finally discharged [3]. Pyroscrubbers of different designs have been employed worldwide to compete for more efficient and cleaner combustion of exhaust gases from coke calcination kilns or hearths. But very limited literature can be found about pyroscrubber design and studies from public resources.