CFD STUDY OF A COAL DIRECT CHEMICAL LOOPING PILOT PLANT

Abstract

Carbon emission from fossil fuel, estimated by IPCC has thrown considerable challenge for researchers and scientists in the past decade. For obvious reasons, the applications of clean technologies such as chemical looping combustion, oxy-fuel combustion, fuel cells and similar technologies are becoming an attractive proposition in foreseeable future. Traditional technologies that generate electricity from fossil fuel via combustion or gasification process generate flue gas from which separation of carbon dioxide is costly and technically cumbersome. However, the chemical looping technology, in which carbonaceous materials such as coal can be oxidize as fuel to generate pure sequestration ready carbon dioxide and heat to generate electricity appears to be a befitting solution to carbon emission problem. Abstract Various studies on gas based fuel for chemical looping combustion have been the major focus in the past decade while, solid based fuel for chemical looping combustion is relatively a new concept and very little research efforts have been directed towards this field more specifically towards CFD simulation of the complete system. The present work entitled as “CFD study of a coal direct chemical looping pilot plant” is related to the modeling of the pilot plant developed by Ohio State University, USA for coal direct chemical looping process using Iron (III) oxide on alumina support as an oxygen carrier for which experimental data are available. For this purpose, a two dimensional model of two interacting reactors (moving bed reactor and fluidized bed reactor) is developed using quadrilateral cell on Fluent 6.3.26 and Gambit 2.3.16. The present models uses Species Transport model and assumes fluid system to study volumetric reactions between gases and solids. The model takes in to account eighteen homogeneous reactions (coal Devolatilization, char gasification, oxygen carrier reductions and oxidations, char combustion) taking place inside two reactors and their inter-connecting parts. The results are verified with the published pilot plant results. Further, the verified model is used to study the suitability of Indian coal for coal direct chemical looping process and to identify the possible bottlenecks. The model predictions of the present work are in good agreement with that of the pilot plant data. The results of fuel conversion (based on dry ash free coal) of present model for sub-bituminous coal and metallurgical coke are 91.52% and 85.12% respectively, whereas, respective fuel conversion for pilot plant are in the range of 97-99% and 70-99%. Thus, the predicted fuel conversion results have a maximum error of 7.55% and 14.02%. Furthermore, the purity of CO2 ii (on dry and free nitrogen basis) in reducer exhaust stream is 89.62% and 93.56% for subbituminous coal and metallurgical coke respectively, while, the pilot plant result shows purity levels of CO2 Further, the developed model is used for studying the suitability of four different grades of coals found in the region of Asia-Pacific and Australia having considerable amount of ash and are denoted as “A”, “B”, “C” and “D”. The conversion (based on dry ash free coal) for coal “A” is 65.27% , for coal “B” is 87.82%, for coal “C” is 93.8%, and for “D” is 87.79% while purity of CO (on dry and free nitrogen basis) in reducer exhaust stream to be 99% and 99.9% respectively. 2 From the simulation study it has been identified that for coal with high ash content the consumption of coal is about 2-4.5 times than that of metallurgical coke. Further, it has been seen that the reactor bed temperature falls appreciably (5% to 40%) when high ash content coal is used. For some coals the reactor bed temperature also quenches to a limit that makes process inoperable. It has been observed that for high ash coals the exhaust CO (on dry and nitrogen free basis) in reducer exhaust is 70.27%, 80.27%, 89.2% and 90.72% respectively for coals “A”, “B”, “C” and “D”. 2 gas from fuel reactor contains small amount (3-7%) of silica which may cause problem in CO2 In addition to it, it has been found that the pressure of the chemical looping combustion process has considerable effect on fuel conversion and CO separation. 2 purity. It is thus recommended that chemical looping combustion should be operated at about 10-15 atmosphere. However, the exact pressure will be based on economic evaluation of the process.