Kinetic model of biomass gasification

Biomass is a promising CO2-neutral carbon source for fuel products, but it is deficient in hydrogen for that purpose. On the other hand, hydrogen is the fuel that is most readily made from renewable power (wind, wave, solar, etc.) by the electrolysis of water, but it is also a challenge to store, transport or distribute. By augmenting biofuel synthesis gas with hydrogen from electrolysis, we can design processes that will enhance both the ability to derive liquid fuels from renewable electricity, and also the ability to convert biomass to fuels efficiently. In this paper, we discuss the example of methanol synthesis from woodchips to illustrate the potential of this approach; and we present mass and energy balances in the cases of gasification by oxygen. In the combined gasification plus electrolysis (GPE) process, the avoidance of a shift step and of CO2 elimination can contribute to more effective utilisation of the biomass, increasing conversion to methanol by up to 130%. Since land availability and the competition with food (or animal feed) crops are factors that could severely limit the contribution of biofuels to a low-carbon economy, this type of process is of particular interest. We take into account the need to operate the process over a large turn-down ratio, due to the variable power input from renewable sources. This is of particular importance when selecting the type of methanol synthesis reactor as well as the gasifier. Finally we present an economic study for the process, complemented by a sensitivity analysis.

On the use of electrolytic hydrogen from variable renewable energies for the enhanced conversion of biomass to fuels

This report summarises knowledge from the open literature on the reactivity of tars during pyrolysis and gasification of biomass. Also other mechanism of the chemical reactions involved is considered. The goal of this summary is to make the knowledge accessible not only to ECN but for a broader community and help in the development of both producer gas cleaning technology and innovative gasification processes. Gaseous biomass tars can react under inert conditions (thermal cracking) or with components in the producer gas such as H2, H2O or CO2 (gasification). The reaction rate of thermal cracking is such that high temperatures of approximately 1200°C or higher (also depending on residence time) are needed to create a producer gas with low tar concentrations. The rate of thermal cracking of tars depends on the kind of tar. The rate decreases in the series: biomass pyrolysis oils/tars > phenolic tar compounds (phenol, cresol, naphthol) > Pyrolysis tars from coal > Polycyclic aromatic tar compounds (anthracene, phenanthrene, naphthalene, benzene). The rate of thermal cracking also depends on the atmosphere in which the tars are cracked because the gas phase components H2, H2O en CO2 play a role in the cracking reactions. H2O and/or CO2 increase the decomposition rate of tars whereas H2 decreases this rate. The aromatic rings of tars can also be hydrogenated which only occurs under hydrogasification conditions at high partial pressures of H2. This leads to the production of CH4. Radical reactions are the main reactions in the mechanism of tar decomposition and the formation of methane. Radical formation is the rate-determining step in this mechanism. After radical formation, the composition of the gas phase determines what are the final products of the tar decomposition.

Tar formation in pyrolysis and gasification

Experimental analysis of biomass gasification with steam and oxygen

This paper discusses the results from experiments and analysis on the conversion of wood char with reactants of composition typical of gasification. Experimental studies on the steam-char reaction with particles of 4–15 mm diameter (do) have led to the evaluation of conversion time variation at different ambient temperatures and steam mass fractions. These are interpreted using a mathematical model validated earlier for carbon dioxide and oxygen/nitrogen mixtures but presently including the C−H2O heterogeneous chemistry and changes in average gas-phase thermal conductivity. A comparative study on the conversion process for different reactants indicates do2 dependence for the air/oxygen environment, whereas do1 and do1.2–1.3 indicate dependence in the CO2 and H2O atmospheres, respectively. The variation of temperature and reactant mass fractions through the reacting sphere are used to argue the role of diffusion and reaction in the conversion process. Based on the foregoing, the process in a characoal reactor gasifier operation is modeled. The gasification process is modeled with char reacting with different reactants as of a typical gasifier. The propagation of reaction front into the packed char bed against the airstream is modeled. The results are compared with experimental data on a model quartz charcoal reactor based gasifier with regard to propagation of the reaction front through the packed bed along with the experiments of Groeneveld's characoal gasifier.

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Wood-char gasification: Experiments and analysis on single particles and packed beds

In 1979 the US Department of Energy, Office of Alcohol Fuels, asked the Solar Energy Research Institute to develop a process for manufacturing methanol from biomass. This can be achieved by gasification of the biomass to a ''synthesis gas'' (syn-gas) (composed of primarily hydrogen and carbon monoxide) followed by catalytic conversion of the gas to methanol. The catalytic conversion of syn-gas is a well developed commercial process. There are a number of gasifiers for wood, but most of them make either a producer gas, high on nitrogen or a pyrolysis gas high in hydrocarbons. None were developed to make syn-gas. Thus the principal technical problem was to develop a gasifier to make synthesis gas from biomass. Work was performed at SERI from 1980--1985 which resulted in the development of a prototype 1 ton/day oxygen-biomass gasifier. In 1985 a program was undertaken for Congress by the US Department of Energy (DOE) to build a commercial scale (50--200 tons/day) medium energy gasifier, based on DOE or other research. A new company, Syn-Gas Inc. (SGI), research. A contract was awarded to SGI to modify the air gasifier for oxygen operation for this project. This modification allowed extended tests of the gasifier with oxygenmore » to determine the possibility of scaling up the SERI-SGI gasifier to 50--200 tons/day.« less

/pdf/fundamentals-development-and-scaleup-of-the-air-oxygen-30ucwnycbm.pdf

T.B. Reed

Papers

Fundamentals, development and scaleup of the air=oxygen stratified downdraft gasifier