Process Simulation and Assessment of Coke Oven Gas to Methanol Integrated with CO_2 Recycle

摘要

COG as a byproduct of coking industry abounds in China, half of it is burned out for heat required of coke oven, the other half is usually combusted or discharged into air which is not only a waste of the hydrogen-rich valuable resource but also causing some environmental problems. Though methanol synthesis from COG is mature and a good way for COG utilization, H_2/CO ratio of nearly 5 is too high for methanol synthesis, to which H_2/CO close to 2 is optimal. Generally, to tackle this issue, carbon-rich gas making subsystems is adopted to supply carbon resource and balance the surplus H_2. From this way, it can get more H_2 conversion and methanol output, but it will undoubtedly cause system complexity and investment increase. In this work, a newly designed COG to methanol process with CO_2 recycle was proposed and simulated by Aspen Plus software. The new system maintains the same merits, and also presents the potential in terms of reduction of system complexity, curtailment of capital investment, and decrease of energy consumption compared with the traditional carbon-supplementary method. Two projects concerning whether H_2 is separated from COG or not are presented, and they are marked by project CWOSH (COG without separation H_2) and CWSH (COG with separation H_2), respectively. In the CWOSH, COG is splits into two parts, one for dry reforming and the other for combustion, and syngas from reforming reactor is directly sent for methanol synthesis. In project CWSH, H_2 is firstly separated from the COG, and then COG is divided into two streams likewise in CWOSH. However, the separated H_2 mixed with the reformed gas to produce syngas with the proper H_2/CO for methanol synthesis in CWSH. In both projects, the unreacted syngas partially recycles back to react and the other part to be mixed with COG for combustion with oxygen. The heat from combustion is supplied to coking process and reforming reaction. High content of CO_2 in flue gas is roughly separated by a cooler, and then recycled to reform with COG, also to coke oven for temperature adjustment. Optimization of the two projects involving element utilization, energy conversion, and economy benefit is given by adjusting variables, like recycle ratio, COG reforming ratio, etc. It is revealed that 3-8% energy saving, about 50% CO_2 emission reduction and 5-10% internal rate of return (IRR) increase will be obtained in both projects, versus traditional COG to methanol process. And project CWSH will get more CO_2 emission reduction and economic benefit but lower energy saving than CWOSH.