Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 214

Biomass gasification is the complete conversion of biomass to gaseous products consisting of carbon monoxide (CO), hydrogen (H₂), and methane (CH₄), with traces of other gases depending on the character of the biomass feedstock.

This gaseous product is usually a low-to-medium Btu gas (producer gas) which can be used to run internal combustion engines (both compression and spark ignition), can be used as substitute for furnace oil in direct heat applications, and can be used to produce, in an economically viable way, methanol – an extremely attractive chemical which is useful both as fuel for heat engines as well as chemical feedstock for industry.

Since any biomass material can undergo gasification, this process is much more attractive than ethanol production or biogas where only selected biomass materials can produce the fuel.

The preliminary gasification of biomass in (preferably) pressurized circulating fluidized bed reactor with secondary processing of the obtained product gas in a slagging entrained flow gasifier is another option. Usually, little or no pretreatment of the biomass is needed (particle size up to 5 cm is acceptable) and that a wide range of biomass feedstock can be processed.

Similar to pyrolysis, the availability of some bed material in the product gas, which is fed into the entrained flow gasifier, is not a concern either, as it improves the properties of molten slag.

Maintaining the stability of the feed flow necessary for the safe operation of entrained flow gasifiers could be a problem, because circulating fluidized bed gasifiers are characterized by some variations in the product gas flow. In order to keep the amount of nitrogen in the product gas under control, the first step circulating fluidized bed gasification is performed with steam, not with air.

After being pyrolyzed in a low-temperature gasifier, biomass pyrolysis gas and char are fed to an entrained flow gasifier and a tar-free gas with high content of carbon monoxide and hydrogen is obtained. The clean gas is cooled down to approximately 200°C (390°F) in a heat exchanger, increasing thereby the overall energy efficiency of the process by producing high-quality steam for power and/or heat generation. Next, the gas is cleaned from dust particles (in a de-duster) and from components, other than carbon monoxide and hydrogen (in a washer). At the end, clean synthesis gas, consisting of carbon monoxide, is obtained. Sufficient gas cleaning represents a key point in syngas and liquids production.

The catalysts for the synthesis of liquid fuels and chemicals are easily poisoned even by small amounts of alkali metals, halides, sulfur compounds, and carbon dioxide which therefore have to be removed to ppm and even ppb levels.

Besides torrefaction, pyrolysis, and pre-gasification, there is a fourth option to convert biomass into a semi-finished material. This is the hydrothermal upgrading process (HTU process), which, under certain conditions, could be considered also as a pre-treatment alternative for biomass conversion.

See also: Biomass – Gasification, Carbo-V Process, Hydrothermal Upgrading Process, Torrefaction.