Biomass Pyrolysis and Gasification – Processes and Differences

Biomass is a broad term used to describe material of recent biological origin which is renewable in the short term, i.e. less than 20 years. This includes forestry and waste wood, agricultural residues (e.g. crop residues, manures) and industrial wastes (e.g. paper mill sludge, residues from sugar mills).

Biomass Pyrolysis

Biomass pyrolysis is defined as the thermal decomposition of biomass in the absence of an oxidizing agent (air/oxygen) and occurs at temperatures in the range of 400 to 800ºC. With the addition of heat the biomass breaks down to condensable vapours, non-condensable gases (pyrolysis gas), and charcoal. In some cases a limited amount of air, not enough for gasification, may be admitted to promote the process by heat generation. The pyrolysis gas contains carbon monoxide, carbon dioxide, hydrogen, methane and higher hydrocarbons. The condensable vapours form a liquid known as bio-oil or pyrolysis liquid, which contains a wide range of oxygenated chemicals and water. All products are combustible. It is possible to some extent to influence the product mix so that one of the products is promoted (Figure 1).

Figure 1 Product distribution of different thermochemical conversion processes

Slow pyrolysis is characterised by relatively low temperatures (<450°C), low heating rates and long residence times and maximizes the charcoal yield (about 35% wt). The yield depends on the pyrolysis temperature and the composition of the starting material.

Intermediate pyrolysis is characterised by temperature around 450 to 500°C and hot vapour residence time in the order of few seconds. The solids residence time can be varied as desired. The typical product ratio is 15 to 25% wt pyrolysis gas, 20 to 30% wt char and 45 to 65% wt bio-oil.

Fast pyrolysis is characterised by very high heating rates (>1000°C/s), very short hot vapour residence time (<2 s) and rapid cooling of the products. Moderate temperatures (up to 550°C depending on the feedstock) maximize the yield of condensable vapours and therefore bio-oil (about 70% wt). High temperatures (above 600°C) maximize pyrolysis gas yields (about 80% wt).

Bio-oil can be used as a substitute for fossil fuels in boilers, turbines and diesel engines to generate heat, power or combined heat and power or as a source of chemicals. Its production is decoupled from power generation, unlike combustion and gasification. It can be upgraded to a transportation fuel such as methanol and Fischer-Tropsch fuels through synthesis gas processes.

Biomass Gasification Process

The Biomass gasification process is the conversion by partial oxidation (i.e. more oxidizing agent than for pyrolysis but less than for complete combustion) at high temperature (>800°C) of biomass into a gas. This gas, commonly called "producer gas" if the oxidizing agent is air and "syngas" if the oxidising agent is pure oxygen, contains hydrogen, carbon monoxide, carbon dioxide, methane and trace amounts of higher hydrocarbons such as ethane and ethane. Steam can be added with the oxidant to promote gasification or used as an oxidant by itself.

The oxidising agent influences the heating value of the product gas. Air gasification produces a low heating value gas suitable for boiler, engine and turbine operation, whereas oxygen or steam gasification produces a medium heating value gas suitable as synthesis gas e.g. for conversion to methanol and gasoline.

Figure 2 Applications of biomass thermochemical conversion processes

The following biomass fast pyrolysis, gasification and combustion power generation systems have been compared in terms of net electrical efficiency (Figure 3):

• Fast pyrolysis and diesel engine (FPyrEng);

• Combustion in a boiler followed by a steam cycle (Comb);

• Atmospheric gasification and dual-fuel diesel engine (GasEng);

• Pressurised gasification and gas turbine combined cycle (IGCC).

The engine-based generators are relatively efficient in smaller systems but their efficiencies do not improve much as the system capacity increases.  In contrast the Integrated Gasification Combined Cycle (IGCC) and Combustion (Comb) system efficiencies improve significantly as system capacity increases.  IGCC has a clear advantage over the other systems at the larger capacities. The Combustion system shows poor performance at small scale, but rises to approximately the same performance as the engine systems at 20 MWe.

Figure 3 System net electrical efficiencies (A.V. Bridgwater, Renewable and Sustainable Energy Reviews, 6 (2002) 181–248)

Conversion and Resource Evaluation Ltd. (CARE Ltd) provides specialist technical and economic services in the bioenergy and waste-to-energy sector. We can offer the following services:

• Design of complete bio-energy systems from feedstock reception to heat and power generation.  

• Technology surveys, reviews and feasibility studies of thermal conversion processes for energy, chemicals and derived products.

• Due diligence of thermal conversion technologies.

• Techno-economic modelling and evaluation of complete thermochemical conversion systems.

• Market evaluation of the opportunities for renewable products and technologies.  

• Advice on environmental legislation, project authorisation and compliance with emissions.

References

1.European Biomass Industry Association    http://www.eubia.org
2.C. Peacocke and S. Joseph, Notes on Terminology and Technology in Thermal Conversion, International Biochar Initiative: http://www.biochar-international.org
3.A.V. Bridgwater, A. J. Toft and J. G. Brammer, A techno-economic comparison of power production by biomass fast pyrolysis with gasification and combustion, Renewable and Sustainable Energy Reviews, 6 (2002) 181–248.