Simulation of Char Dust Combustion inside a Pyroscrubber Downstream of a Petroleum Coke Calcining Kiln

A pyroscrubber is a furnace used in the petroleum coke calcining industry to recover energy from the carbonaceous contents, including char dust and hydrocarbon volatiles of the exhaust gas from the calcination kiln. The combusted hot gases are used to generate steam and produce electricity, so it is important to optimize the pyroscrubber performance to produce high-grade combusted gases to generate steam but with minimal emissions. A previous study employed the locally-homogeneous flow (LHF) model to study rhe means to improve combustion efficiency and reduce emissions. In the LHF model, the inter-phase exchange rates of mass, momentum and energy are assumed to be infinitely fast, so the dispersed phase (char dust) can be simplified as the gas phase, and the complex two-phase flow is then treated as a single-phase flow. In this study, LHF model is replaced with a solid particle combustion model by incorporating both finite-rate heterogeneous and homogeneous combustion processes. Results reveal that the particle combustion model generates much higher local flame temperature (2200K) than in LHF model (1800K). All char particles are burned before or in the high-bay area. Total energy output of the case with particle combustion model is 92% of the LHF model. Furthermore, motivated by the potential energy saving from removing the air blower power supply, this study further investigates the possible benefit of running the pyroscrubber with the ventilation doors open. Three cases with different combinations of air injections and door opening have been studied. Results show that the gas flow is stably stratified with a large amount of the entrained cold air moving at the bottom of the chamber and the hot combusted gas moving through the top. With bottom doors completely open, sufficient air can be drawn into the pyroscrubber without the need of blowing air in, but the combustion gases will be overcooled making this practice unfavorable from the energy saving point of view. INTRODUCTION Petroleum coke (petcoke) is usually calcined in a gasfired rotary kiln or rotary hearth at high temperatures, around 1,200 to 1,350°C (2,192 to 2,462°F), 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. The product is hard, dense carbon (calcined petcoke) with low hydrogen content and good electrical conductivity. These properties along with the low metals and ash contents make calcined petcoke the best material currently available for making carbon anodes for smelting of alumina to aluminum [1]. An extensive discussion of various properties of green and calcined petroleum cokes for aluminum anode grade carbon production can be found in the study of Lee et al. [2]. A pyroscrubber is namely a furnace burning carbon particles in a stream of waste gases, particularly from a petcoke calcination kiln or hearth. The schematic of the calcining process for petcoke 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, and 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]. Since the hot gases exhausted from the pyroscrubber are used to generate steam and produce electricity, it is essential to keep the energy quality of the gases high (ie. at high temperature) but with low emissions. Control of NOx emission is one of the major considerations in the design of a pyroscrubber. NOx emissions cause serious health issues, ranging from bronchitis to altered immune system function. NOx also contributes significantly to environmental problems such as acid rain and ozone depletion. NOx emission consists of mostly nitric oxide (NO). It also contains nitrogen oxide (NO2) and nitrous oxide (N2O). The quantity of NOx formed depends on the three T’s: Temperature, Time, and Turbulence. NOx control technologies currently used within the industry can be grouped into two categories i.e. combustion modifications and