Waste biomass to methanol–optimisation of gasification agent to feed ratio

Abstract

Residues from agriculture and forest maintenance (lignocellulosic waste biomass) create a huge source of renewable energy and valuable materials. Gasification of this waste provides combustible gases consisting mainly of H2, CO and CO2, which can be used as syngas feed for methanol synthesis after treatment. Oxygen, steam or their mixtures can be used as gasifying agents in the biomass to methanol process. One of the most important parameters with crucial effect on gas composition and reactor temperature is the gasifying agent to feed mass flow ratio. In this work, gasification process of two samples of mixed agricultural waste, gas treatment and synthesis of methanol from the syngas produced was designed and simulated in the Aspen Plus environment. Three different cases were investigated: (1) only oxygen as gasifying agent, (2) oxygen and steam as gasifying agents without steam recycling and (3) oxygen and steam as gasifying agents with steam recycling. Samples of mixed agricultural waste consisted of corn leaves and stalks, wheat straw, barley straw, sunflowers and wood chips were characterised by proximate, ultimate and calorimetric analysis. Oxygen and steam flows were optimised to achieve maximum theoretical yield of methanol while maintaining the gasifier temperature, equal to, at least 900 °C. The best methanol to biomass ratio (MBR) of 0.43 was reached for a sample with lower moisture and ash content when both oxygen and steam were used as gasifying agents. The optimal oxygen to biomass ratio (OBR) was 0.57, and the steam to biomass ratio (SBR) was 0.89.