Dahlman OLGA

Biomass & waste gasification; a brief introduction

Nowadays, renewable energy is a hot topic. The stock of the fossil fuels is not inexhaustible and therefore a lot of other energy sources (biomass, sun, water etc.) are under investigation at this moment. An energy source, with a large potential is biomass. Biomass is renewable by definition. During growth of plants, crops and trees CO₂ is withdrawn from the atmosphere. The release of this CO₂ when biomass is used for energy consumption just closes the CO₂ cycle. Although biomass is the oldest energy source known to mankind, its current contribution to the primary energy consumption in the Western world is small. Biomass is replaced by cheap fossil fuels like coal, crude oil and natural gas when these were found in huge amounts.

To achieve the Kyoto objectives, the share of renewable energy in the total consumption must be increased significantly. Biomass is generally considered as one of the most important renewable energy sources. Biomass is the only renewable carbon source! Gasification of biomass opens the full potential of biomass. Nowadays, combustion is still more applied, but gasification has more potential. It is more efficient for electricity production and opens up the possibility to produce products like SNG, Hydrogen or Fisher Tropsch Diesel.
A biomass gasification system can roughly be split up into four steps:

 

 

Biomass conditioning & transport

Depending on the biomass feedstock and the type of gasifier, the biomass must be conditioned (sized, dried etc.) and transported into the gasifier. Achieving a good continuous and reliable biomass conditioning is often one of the most important points in operation of a gasification plant.

 

Gasification reactor

There are several different types, all with their own characteristics:

• Fluidised bed gasifiers (BFB of CFB)
• Fixed bed gasifiers (updraft or down draft)
• Entrained flow gasifiers
• Indirect gasifiers (separate combustion & gasification)

 

When discussing biomass gasification, it is important to understand that there are two main gasification routes each resulting in a different product:

The first route is high temperature gasification (> 1200 °C), which produces biosyngas. Biomass or waste is completely converted into H₂, CO, CO₂ & H₂O. Biosyngas is chemically similar to syngas derived from fossil fuels and can be used for the same applications. Applications for biosyngas are:

• Fisher Tropsch diesel production
• Methanol / DME production
• Ammonia production
• Hydrogen source etc.

The advantages of entrained flow gasification are that there is a lot of experience with large scale plants (coal & refinery waste gasification) and that syngas is free of tars. For 100% biomass this technology is not applied. Feeding “raw” biomass into such a plant is often not possible and the low energy density of biomass favours smaller scale plants for which this technology is not economical.

The second route is ‘low’ temperature gasification (< 1000 °C), which produces product gas. Biomass or waste is converted into: H₂, CO, CH₄, C_xH_y, aliphatic hydrocarbons, BTX, tars, CO₂ and H₂O The syngas components H₂ and CO typically contain approximately 50% of the energy in the gas. Typical applications for product gas are:

• Electricity production
• Synthetic Natural Gas (SNG)

The advantages of low temperature gasification are:

• Lower investment costs. Particularly for air blown or indirect (steam blown) gasification as an oxygen production plant is not necessary.
• Higher energetic efficiency. Lower gasification temperature & no energy needed for air separation.
• More suitable for smaller scale plants. Transport costs for biomass feedstock are high, which gives a considerable market for smaller scale plants.

Both gasses need additional gas cleaning and conditioning, before they can be used for their final application. Although we focus ourselves on the OLGA tar removal technology, we can also offer other equipment for solids removal and removal of inorganic impurities and also we have good experience in syngas purification.

Our partner ECN has a lot of experience in gasification of many different feedstock’s. Composition data  summarised in the “Phyllis” database is available on the internet.