Techno-economic Evaluation Of Coke Oven Gas To Methanol With CO₂ Recycle

Coke oven gas（COG）is a byproduct from coking process which is a vital part in Chinese industry.How to utilize the large amount of COG is becoming a tough problem especially in independent coking plants.Coke oven gas is also a hydrogen-rich resource.Normally,50% of it is combusted in the coking combustor to supply heat for coking chamber.The other part of COG is better utilized to chemicals synthesis.Methanol is the most common chemical from COG synthesis for its high potential in liquid fuel applic ation and also for its role of the important platform chemical.Although the technology of COG to methanol is industrially mature,problems like low methanol production and hydrogen waste in vent gas are still existing in the process.The cause for them is the high hydrogen to carbon（H/C）ratio in COG,which is much higher than the value for methanol synthesis.The general solution to these problems is to supplement carbon to COG to reduce H/C from 5.0 to 2.0,and there are two ways for this.The first is using purchased CO₂ and the second is addition of subsystem to produce carbon-rich gas.While both of them will generate more energy consumption and increase the system complexity.This work proposes a simple process of COG to methanol with CO₂ recycle and adopts dry reforming（DRM）as COG reforming technology.The hydrogen content in the COG is as high as 55 vol.%-60 vol.%,and hydrogen is also the product of COG reforming reaction.Separating hydrogen from COG is preferable for this technology,based on which COG without hydrogen separation（CWOHS）and COG with hydrogen separation（CWHS）processes are both modeled and simulated by software Aspen Plus.Unlike the conventional process using 50% of COG for methanol synthesis,the ratio in the new processes is set to be higher.And the fuel gas for heat supply consists of part COG and part unreacted syngas instead of 50% COG only.Oxygen combustion is selected from the point of the easines s of CO₂ capture from flue gas.The captured CO₂ is partly used for COG reforming and another part of CO₂ is recycled to combustor for temperature control.Unreacted syngas is partly recycled to methanol synthesis and partly mixed with COG as fuel gas.After the processes are optimized and the element and energy balance are obtained from the simulation and calculation,it is revealed that 1)the new processes can increase methanol output and reduce CO₂ emissions.2)The reforming unit has much influence on the energy efficiency,economic performance and carbon emissions of the systems.3)Dry reforming can devote to higher H2 utilization compared to auto-thermal reforming.4)Hydrogen separation can increase CH₄ and CO₂ conversion,reduce handling capacity of the reforming reactor and equipment investment.5)The ratio of COG for reforming to COG for combustion λ is the key parameter of the processes.As to CWOHS,the technologic performance is optimal under the condition of λ = 0.55/0.45 with the highest energy efficiency of 64.9% and highest exergy efficiency of 71.0%,while the best economic performance is under λ = 0.60/0.40.When it comes to CWHS,both technologic and economic performances present the most welcoming results when λ = 0.55/0.45 with the highest energy efficiency of 67.7%,the highest exergy efficiency of 73.9%,the lowest CO₂ emission of 0.69 t/t-methanol and the highest internal rate of return of 35.1%.This work can provide COG to methanol process with meaningful reference and guide.