WBA releases thermochemical gasification of biomass factsheet

By Katie Fletcher | July 28, 2015

In June, World Bioenergy Association launched a factsheet on thermochemical gasification biomass—the eighth in a series of publications. The factsheet overviews the technology, basics of gasification, feedstock and current global use.

WBA President Heinz Kopetz states in the factsheet that thermochemical gasification of biomass is an important technology for converting agriculture and forestry products and residues into various forms of energy. “This technology has immense potential in industrial and developing countries for providing easily accessible and affordable electricity and heat,” Kopetz said. “It is important to publicize this technology and present science based facts for increasing its uptake and this factsheet aims to do just that.”

Thermochemical gasification of biomass is a high temperature process—700 to 1,600 degrees Celsius (1,292 to 2,912 degrees Fahrenheit) in the presence of a gasification medium—that converts biomass into a clean fuel gas called producer gas or synthesis gas (syngas). Biomass feedstock typically includes cellulosic biomass, such as wood chips, pellets or wood powder, or agricultural byproducts like straw or husks. The process involves air, oxygen, steam or a mixture of these media. Gasifiers used range from just a few kilo-watts (kW) up to a couple of hundred MW.

The process involves several steps including pretreatment of the feedstock, the gasification itself, the gas cleaning and the utilization of the gas in a gas engine or any other device depending on its end use.

The syngas can be used for many purposes such as for heat and electricity generation, high temperature heat, transportation fuel, as raw material for chemical production and, when cleaned and upgraded to near pure methane, it can be injected into gas grids. The factsheet states two other advantages are that the technology has a higher electrical efficiency than other conversion processes and the produced gas is well suited for cogeneration units, especially small-scale gasifiers.

The technology has higher efficiency with its gasification process because it can run at a higher temperature than combustion. Around 70 to 80 percent of the energy contained in the feedstock is converted to the energy content of the producer gas, the rest being heat and losses.

The factsheet describes three main types of gasification technologies: fixed bed, fluidized bed and entrained flow, providing advantages and disadvantages of each, as well as capacities and typical particle sizes for each.

Various properties of feedstock were discussed in the factsheet, as well. According to the document, the physical and thermochemical properties of the feedstock influence the quality of the producer gas, and therefore play a decisive role on the selection of the gasification technology. For input in a gasifier, feedstock should be relatively dry—less than 30 percent moisture content—to give a reasonable gas heating value, but preferably even drier feed—down to 10 percent—for pellets.

The main limitation with feedstock is significantly oversized materials or excessive fines that should be screened out, according to WBA. Also, with biomass feedstock there are differences between woody biomass and agricultural biomass like straw, rice husk, miscanthus, etc. The factsheet states, due to its lower ash melting temperatures, herbaceous biomass is better suited for larger gasification units.

The report rounded off with commentary on the global use of biomass gasification. According to the factsheet, the annual biomass gasification capacity can be estimated to be around 15 units of production (PJ). In Asia and Africa, biomass gasification in small units is of particular interested as a way to improve upon electricity supply in rural areas. In China, biomass gasification power plants of various types and scales are being used. China’s government has also supported the technology and has a current target of 30 gigawatts (GW) of bioenergy supply capacity by 2020. In addition, in India and other Asian countries, the gasification of residues of agriculture and forestry is spreading mainly to supply electricity to rural regions. According to data from the Indian government, around 150 MW of biomass gasifiers were in use in 2011. On the European front, development of the technology is mostly in electricity. In Germany, more than 100 power plants based on biogasification units are in operation.

Overall, WBA considers the gasification of biomass, especially in the form of residues from agriculture and forestry, as an important technology with a significant future in industrialized countries as well as in developing countries.

The complete factsheet can be downloaded here.