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Researchers show combining torrefaction pre-treatment with catalytic fast pyrolysis improves quality of bio-oil

18 November 2012

Srinivasan
Average of product distribution from the fast pyrolysis of pine without and with torrefaction pretreatment. Srinivasan et al. Click to enlarge.

Researchers at Auburn University and North Carolina State University have shown that using a simple pretreatment process—torrefaction—improves the quality of bio-oil produced via catalytic fast pyrolysis.

In a paper published in the ACS journal Energy & Fuels, they reported that the combination of the torrefaction pre-treatment and shape-selective catalyst (H+ZSM-5) resulted in highly deoxygenated liquid product that was favorable for aromatic hydrocarbons. The total carbon yield from catalytic pyrolysis of torrefied biomass was 1.45 times the total carbon yield from catalytic pyrolysis of untreated pine.

Fast pyrolysis is a thermal process that rapidly heats biomass to around 500 °C, then quickly cools the volatile products. While fast pyrolysis has been shown to be an effective process to convert solid biomass to a liquid (bio-oil) with a short residence time, the bio-oil has to be upgraded to be able to be used as a liquid transportation fuel.

Among its problematic characteristics are a high oxygen content (approximately 35 wt % dry basis), resulting in instability, low heating value, and immiscibility with the current hydrocarbon fuels. Furthermore, the researchers noted, bio-oil undergoes a chemical transformation during storage to attain thermodynamic equilibrium leading to changes in molecular weight distribution and viscosity of the oil. These disadvantages hinder bio-oil from becoming a feasible solution as an alternate liquid fuel.

Two main approaches have been examined for upgrading: cracking and hydro-deoxygenation. Cracking involves reacting biomass or pyrolytic vapors over certain shape selective catalysts to remove oxygen. Hydrodeoxygenation uses high-pressure hydrogen in combination with desulfurization catalyst for the elimination of oxygen.

Recently, a simple thermal pretreatment process, torrefaction, has been applied to improve the properties of biomass. Torrefaction is a thermochemical process that occurs around 200−300 °C in the absence of oxygen. During torrefaction, biomass undergoes partial decomposition with the release of volatiles, which results in overall mass loss. Furthermore, the fibrous structure of the biomass is lost as a result of torrefaction mainly because of the decomposition of hemicellulose and depolymerization of cellulose.

In addition, torrefaction results in a significant loss of oxygen from the biomass, which in turn reduces the oxygen content, increasing the calorific value of pretreated biomass while decreasing the energy required for grinding. The grinding energy for untreated pine chips and forest residues could be as high as 237 kWh/t compared to 23 kWh/t for similar biomass when torrefied.

...Most of the previous studies have examined the effect of torrefaction temperature and reaction time on the physicochemical properties of biomass. A handful of studies have focused on the influence of torrefied biomass on the quality of the liquid fuel. However, none of them have focused on the combined effect of catalyst and pretreatment on the bio-oil quality. Therefore, the main objective of this paper was to study the effect of catalyst and torrefaction on hydrocarbons yield using fast pyrolysis process.

—Srinivasan et al.

Aromatic hydrocarbons were significantly produced as a result of torrefaction, and temperature and catalyst enhanced their production. Among the findings were that torrefaction resulted in more of lignin derivatives—guaiacols, phenols—and less of holocellulose derivatives. The presence of catalyst resulted in the formation of naphthalenes due to its size selectivity.

Resources

  • Vaishnavi Srinivasan, Sushil Adhikari, Shyamsundar Ayalur Chattanathan, and Sunkyu Park (2012) Catalytic Pyrolysis of Torrefied Biomass for Hydrocarbons Production. Energy & Fuels doi: 10.1021/ef301469t

November 18, 2012 in Biomass, Fuels | Permalink | Comments (4) | TrackBack (0)

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Comments

The whole is greater than the sum of its parts.

This could be even bigger than it sounds, because there are possible sources of nearly-free heat in the temperature range required for torrefaction.  Not near major sources of biomass, though.

Sounds very promising. Torrefaction and grinding of torrefied biomass are low tech. They can be done near the point of origin of the biomass. The result has a higher mass density, with a much higher volumetric energy density. The ground product is well suited for automated handling and transport.

This overcomes one of the biggest economic hurdles for biomass.

The transport issue is substantially one of bulk, and raw torrefaction does not shrink the volume much.  This might work best as part of a multi-step process at the site where the biomass is produced (sawmills?), where the torrefaction, grinding and liquefaction can be done in sequence and only the high-density liquid product is transported.

Torrefaction + grinding gives a sort of sawdust that's similar in density to chipped branches, but super dry and higher in energy density. It's also denser than baled hay and much denser than loose branches. More important, though, is that it can be blown into containers. Also, it can be heaped into stockpiles that won't rot, as long as they're covered.

You wouldn't want to ship it hundreds of miles for further processing, but a regional co-op serving farms within a 40-mile radius would be easy.

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