ECN publication
Tar formation in pyrolysis and gasification
Published by: Publication date:
ECN Biomass, Coal and Environmental Research 22-6-2009
ECN report number: Document type:
ECN-E--08-087 ECN publication
Number of pages: Full text:
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This report summarises the information in the published literature on the reactivity of tars during the pyrolysis and gasification of biomass and discusses the mechanism of the reactions involved. The purpose of summarising this information is to make it accessible so that it can be used at ECN in the further development of fuel gas cleaning and innovative gasification processes such as OLGA and MILENA. The report focuses on the rate at which reactions take place between gaseous tar compounds; for the most part it ignores reactions in liquid tars and between tars and solids, as these are less relevant to the development of biomass gasification technology and there is not so much data available on them. Gaseous biomass tars can react under inert conditions (thermal cracking) or with fuel gas components such as H2, H2O and CO2 (gasification). The rate of thermal cracking is such that high temperatures are required – in the order of 1200°C or higher (also depending on the residence time at high temperature) – in order to break down enough tars so that the remaining fuel gas can be used problem-free in a downstream device such as a gas engine, gas turbine or catalytic synthesis processes. This is evident from waste processing processes where thermal cracking is used (the Thermoselect and Noell dust cloud gasification) and from the small amount of data available on the large-scale thermal tar cracker downstream from a biomass gasifier, the Creusot-Loire gasifier, which was built and run in the mid-1980s. The rate of thermal cracking depends on the type of tar being cracked and decreases in the following series: biomass pyrolysis oils/tars > phenolic tar compounds (phenol, cresol, naphthol) > coal-based pyrolysis tars > polycyclic aromatic tar compounds (anthracene, phenanthrene, naphthalene, benzene) The rate of thermal cracking also depends on the atmosphere in which the tars are cracked, as the gas phase components H2, H2O and CO2 play a role in cracking reactions in a biomass fuel gas (air, steam or oxygen gasification) or at elevated hydrogen pressures. H2O and/or CO2 increase the tar decomposition rate, whereas H2 reduces it. In hydrogasification, because of the high hydrogen pressures at high temperatures, aromatic rings may also be hydrogenated, causing higher concentrations of CH4 as well as tar decomposition. The products of thermal tar cracking are solid as well as gaseous, as it converts a small to large portion of the tars into carbon (= carbon-rich dust or soot). This has two consequences for the use of thermal cracking to remove tar in a biomass gasification plant. The first is that, if the fuel gas is used in a gas engine or turbine or in synthesis processes, the carbon needs to be removed first by means of a (bag) filter. The second is that, as a result, thermal cracking makes only part of the energy content of the tars available to the lower heating value (LHV) of the fuel gas, as the carbon will not be used to generate energy if the gas is burned in the engine or turbine or converted in the synthesis. Hydrogasification and steam gasification of biomass probably produce the same tar compounds as biomass gasification. Under hydro and steam gasification conditions high temperatures are needed to crack these tars; converting them into methane moreover requires high hydrogen and steam pressures. The mechanism of tar decomposition and methane formation involves radical reactions, and radical formation is the step in the mechanism that determines the rate. The composition of the gas phase in the second reaction step determines what the products are.

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