Methanol Synthesis from Simulated Bio-syngas: Experimental and Modeling Studies

Abstract

Methanol is increasingly being considered as an alternative fuel due to its potential to reduce environmental pollution and its ability to be used directly in internal combustion engines without any engine modification. This study focuses on using biomass as a source for methanol synthesis. The use of biomass for methanol synthesis has two main advantages: it addresses the air pollution caused by biomass incineration, and it generates value-added chemicals from waste biomass. In this work, the suitability and profitability of producing methanol from biomass-derived syngas (bio-syngas) were investigated. Methanol is typically produced in industries by catalytic conversion of syngas obtained from the steam methane reforming process (SMR). However, syngas obtained from biomass gasification is a relatively newer route since this process generates syngas with lower H2 and higher CO2 content.

The hydrogen content in biomass is only 5-7%, in comparison to the carbon value of 48-52%, making the choice of gasifying agent and gasifier design crucial as it determines the H2 percentage and tar content in the syngas. Thus, the simulated syngas composition obtained from oxy-steam downdraft gasification was chosen as an input for studying its effect on the final methanol yield. Thermodynamic analysis of methanol generation from both bio-syngas and SMR showed that methanol yield is sensitive to temperature, pressure, and stoichiometric number (S) in both cases. The optimized methanol yield was achieved at 61.87% for SMR-based syngas and 39.54% for bio-syngas at 483 K and 5 MPa, respectively.

Before developing an Aspen Plus® model for biomass to methanol (B2M) conversion, a kinetic-based downdraft gasification model was developed in Aspen Plus® software. The model included tar kinetics and considered the downdraft gasification process in four separate zones with major reaction kinetics. The model was validated with literature data for different feedstocks and three different gasifying agents: air, oxygen, and oxy-steam. And as for the methanol generation system, limited literature was available for methanol production via bio-syngas. This thesis includes experiments with simulated bio-syngas composition for methanol production. Methanol synthesis experiments were performed in a high-pressure reactor using commercial Cu/ZnO/Al2O3 catalysts. The experimental values were used for the validation of the Aspen Plus® methanol kinetic model. The methanol yield values were optimized for the parameters like temperature, pressure and the S for the methanol reactor setup. These optimized values of the parameter were then considered for the B2M process optimization as well.

Surrogate models were created using multi-variable analysis to predict and optimize the methanol yield value for the entire B2M process. The model predicts that the maximum achievable methanol yield was 37.77% for bio-syngas. This can be achieved at a gasification condition where the Equivalence Ratio (ER), temperature, Steam to Biomass Ratio (SBR) values were 0.2, 1173 K, and 4, respectively.

Finally, a techno-economic analysis of B2M process was done to assess the feasibility of this new alternative route in comparison to the existing natural gas reforming process for methanol synthesis. The techno-economic studies show that biomass to methanol technology can be developed in a country like India where surplus biomass is available. This process becomes economically viable at a methanol selling price of Rs 28 per litre or 0.3 Euro per litre and above a plant capacity of 2000 Tonnes per day of methanol production.