(430c) Optimal Composition of Biogas for Methanol Production Via Dry Reforming

Biogas is an interesting fuel that is produced from anaerobic digestion of organic wastes. Its composition is mainly methane, with a certain amount of carbon dioxide and other species in smaller quantities like H₂S or NH₃ [1].Therefore, it can be considered as a source of methane. Methane or natural gas has been used as energy source or as fuel, but it has also been used as a raw material for the production of syngas. There are a number of alternatives to process methane. We can use steam to reform methane, an endothermic process. Alternatively, oxygen can be used to partially oxidize the methane. Autoreforming combines both, steam reforming and partial oxidation, so that the process is adiabatic. Lately, another technology has been added to the portfolio, dry reforming using carbon dioxide. It is also an endothermic process with a lower yield to hydrogen. The hydrogen to CO ratio varies widely depending on the method used. This ratio is key for the proper production of different species. For instance, the production of methanol requires a H₂:CO ratio of 2, the production of FT-liquids suggests values from 1 to 2 depending on the length of the hydrocarbon chain and the operating conditions at the reactor. The production of ethanol and DME needs a H₂:CO ratio of 1 [2,3].

Biogas already has CO₂ within its composition. However, the syngas produced is expected to show a low ratio H₂:CO. To tune this ratio we can follow different alternatives. Biogas composition is variable and can be adjusted. Furthermore, traditionally dry reforming of methane has been combined with other reforming modes [4-6].

In this paper, we present a mathematical optimization approach for the optimal dry reforming of biogas for the production of methanol. The raw biogas is cleaned up before reforming. We allow for the use of steam and variable biogas composition to increase the hydrogen content of the syngas. Part of the biogas is used to provide energy for the process. Next, the unreacted hydrocarbons and CO₂ are removed. Subsequently, its composition may be adjusted (using either water gas shift reaction or pressure swift adsorption if there is an excess of hydrogen). Finally, methanol is synthesized. The problem is formulated as an NLP with simultaneous heat integration for the optimal biogas composition and methanol production. Two objective functions are considered: a simplified production cost and we proposed an environmental one based on carbon footprint considering the contributions to CO₂ emissions due to the water usage as cooling agent, raw material or steam production and CO₂ consumption and release across the process.

Biogas is expected to have around 50-52% of CH₄ and 45-47% of CO₂, depending on the objective. The production cost of methanol is $1.7/gal, for a plant size that uses 10% of the potential biogas to be produced in Madrid (Spain), with an investment of $46 MM. The production cost is becoming competitive with the current price of methanol ($0.9/gal) with the advantage of its sustainability. The results obtained for both optimizations show a negative carbon footprint, creating a net capture of CO₂ while producing a valuable intermediate for chemicals or fuels.

References.

[1] Japaraju, P., Rintala, J. 17 - Generation of heat and power from biogas for stationary applications: boilers, gas engines and turbines, combined heat and power (CHP) plants and fuel cells 404-427, In The Biogas Hand book 2013

[2] Martín, M., Grossmann, I.E., On the systematic synthesis of sustainable biorefineries Ind. Eng. Chem. Res. 2013, 52 (9), 3044-3064

[3] Peral, E., Martín, M. Optimal production of DME from switchgrass based syngas via direct synthesis. Ind. Eng. Chem. Res., 2015, 54, 7464-7475 [4] Lim, Y.; Lee, C.J:; Su Jeong, Y.; Song, I-H.; Lee, C.J.; Han, C. Optimal Design and Decision for Combined Steam Reforming Process with Dry Methane Reforming to Reuse CO₂ as a Raw Material Ind. Eng. Chem. Res. 2012, 51, 4982â??4989

[5] Vernon, P. D. F.; Green, M. L. H.; Cheetham, A. K.; Ashcroft, A. T. Partial oxidation of methane to synthesis gas, and carbon dioxide as an oxidising agent for methane conversion. Catal. Today 1992, 13 (2â??3), 417â??426.

[6] Song, C.; Pan, W. Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO ratios. Catal. Today. 2004, 98 (4), 463â??484.