Influence of pressure and CO2 in fluidized bed gasification of waste biomasses

Mateusz Szul,Krzysztof Głód,Tomasz Iluk

doi:10.1007/s13399-020-00840-9

Abstract

An autothermal fluidized bed reactor was used to research the influence of pressure (0–2 barg) on the gasification process of different types of biomasses. The tested feedstocks were bark and lignin while softwood pellet was used as a reference fuel. A mixture of O2/CO2/H2O was used as a gasification agent. The impact of the application of CO2 on the yield of H2 in product gas was determined. Resulting product gas was characterized by a high content of CO which makes its use for applications based on chemical synthesis very difficult without extensive upgrading or supply of H2 from external sources. CO2 proved to improve carbon conversion efficiency (CCE) of the gasification process and to be an option for its chemical sequestration (negative carbon footprint). A slight modification of conventional indices used to evaluate efficiencies of gasification systems (CCE and water/carbon ratio) was proposed, to take into account the impact of the additional source of carbon fed into the reactor. The increase of system pressure led to changes in the composition of the product gas in line with predictions of Le Chatelier’s principle. The influence was predominantly visible in higher yields of CH4 and lower overall production of product gas. For higher hydrocarbons (CxHy), the trend was unclear. A set of stable gasification parameters were achieved for each pressure level and a standard gasification temperature of 850 °C, except for gasification of lignin performed at 2 barg. A proposed explanation for the problem is the combined effect of the increasing concentration of ash in the fluidized bed and its low characteristic melting temperatures. Due to the obtained experimental findings, a new ash agglomeration index was formulated.

Highlights

Pressurized gasification is the state-of-art technology for highly efficient production of chemical intermediates or fuels from primary energy sources
Gasification temperature of 850 °C, measured as the highest recorded in bed, was set as common for all the tested biomasses
Similar fluidization conditions were sought. This mode of design of fluidized bed (FB) experiments has given good comparability of results obtained from tests conducted at a very broad spectrum of gasifying agent compositions. This method is especially proposed for future research on pressure gasification with the use of CO2

Summary

Introduction

Pressurized gasification is the state-of-art technology for highly efficient production of chemical intermediates or fuels from primary energy sources. The impulse for the development of pressure systems for gasification of biomass has been recognized by many influential agencies. For renewable production of fuels and chemicals, the International Renewable Energy Agency and the Internal Energy Agency have prioritized the following goals: 1) To develop BtL routes for the production of biodiesel and dimethyl ether (DME) from black liquor gasification; 2) To maturate pressurized gasification plants to produce bio-synthetic natural gas (bio-SNG), as in the Bio2G project; 3) To study hybrid biochemical and thermochemical conversion routes. The first two goals are directly related to the development of pressurized biomass gasification systems [1]. From the principle for thermodynamics, gasification of biogenic feedstocks at higher operating pressures can be beneficial from several perspectives. Biomass gasification at elevated pressures provides higher reaction efficiencies and kinetics [2,3,4].

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