Primary methods in biomass gasification for gas conditioning and cleaning

Abstract

The substitution of natural gas by a renewable equivalent is an interesting option to reduce the use of fossil fuels and the accompanying greenhouse gas emissions, as well as from the point of view of security of supply. Green gas is the renewable alternative. It comprises biogas (1st generation biofuel) and SNG (2nd generation biofuel). Being a direct substitute of natural gas, green gas can be injected directly into the gas grid taking advantage of the existing infrastructure. The Dutch government has the ambition of replacing 20% of natural gas by green gas by the year 2030. With an annual consumption of around 1500 PJ, a 20% substitution corresponds to 300 PJ. The potential of biogas in the Netherlands is maximum 60 PJ due to the limited availability of suitable digestible feedstocks. To accomplish the 20% replacement in 2030 an SNG production capacity is required of at least 240 PJ. This process requires the conversion of the solid biomass into gas and the further conditioning of the gas into SNG. This project refers mainly to the development of in-bed methods (primary methods) that are primarily effective for the route biomass-to-SNG in order to increase the overall efficiency of this process. The goal is to optimize the composition of the gas generated from indirect gasification, mainly to increase the methane yield and to convert tars into valuable gases. The different approaches for in-bed measures are proper selection of operating parameters and/or the use of bed additives/catalysts along with gasifier modification. For the purpose of the experimental program a new set-up was designed and built to study the influence of the primary methods on the biomass gasification process. This was a flexible and rather simple system that allowed on-line measurements trough a mass spectrometer. Several methods were analysed along this research: temperature; gas phase residence time; carrier agent (inert, oxygen and water); bed additives like dolomite and olivine; individual biomass components (hemicellulose, cellulose and lignin); mineral matter present in the biomass; torrefaction as pre-treatment for biomass. This work has given insight into the mechanisms of biomass gasification process. Tar cracking reactions are crucial for the production of permanent gases and the main source of methane. CO occurs as the main product from tar conversion but also CH4, C2H4 and H2 are produced. Increasing temperature and residence times the tertiary compounds become an important fraction of tar. Dolomite and related compounds (like magnesite and calcite) adsorb tars into their pores and promote tar cracking reactions. These materials convert benzene and naphthalene but do not convert methane. Dolomite and related compounds also promote the water gas shift reaction towards the production of hydrogen. Iron is the key element for the activity of olivine, and the state of the iron determines the catalytic activity of olivine. Hereby the pre-treatment applied to the olivine is crucial. Oxidized olivine generates CO2 at the expense of CO and slightly improves the tar cracking reactions whereas reduced olivine promotes the formation of H2 at the expense of H2O and has a strong impact over the tar cracking reactions that leads to the formation of CO. The mineral matter present in the biomass has catalytic activity towards tar cracking but also promotes bed agglomeration which may cause an unwanted shut-down of the fluid bed gasifier. However, if dolomite is the bed material the agglomeration phenomenon is at least delayed. If the lignocellulosic biomass contains a low amount of mineral matter, its behaviour can be correlated to the behaviour of its individual components at specific experimental conditions. The experiments with torrefied birchwood show that upon the increase of the torrefaction temperature more solid carbon and less gas will be formed during the pyrolytic gasification. Methane revealed itself as the most stable compound and the most difficult one to manipulate. In the case of cellulose, methane is mainly formed from the tar cracking reactions whereas in the case of lignin a large amount of methane may be already formed from the devolatilization of the particle due to the methoxy groups present in the structure of the lignin. The mineral matter present in the biomass has the potential to promote tar cracking reactions that lead to less methane formation. Inert conditions are the most suitable for methane formation since the presence of water or oxygen leads to a decrease of this component. Among the tested bed additives dolomite was the one that revealed the most limitations in methane conversion which makes it a suitable catalyst if a high content of methane and a low content of tars are required.