1365 Gasification

The fact that biomass is a solid fuel limits its direct use in advanced power generation systems. Biomass can readily be burned to provide heat for conventional steam-cycle generation, but that is inefficient at a small scale. To use biomass as a fuel for advanced power generation systems such as gas turbines, microturbines, or fuel cells, it is necessary to convert the biomass into an intermediate gaseous or liquid fuel. Biomass thermal gasification produces a low- or medium-energy fuel gas (5 to 15 MJ/Nm3) that can be cleaned and used as a substitute for natural gas. The product gas contains mixtures of combustible gases including CO, H2, CH4, and others, as well as CO2 and sometimes N2. The gas cleaning process must remove unwanted ash and tars from the product. Biomass gasifiers are commercially available on a small scale (~10 MWt output or less) and have recently become available on a larger scale (~60 MWth). Gas cleanup technologies are available to clean the product gas for high-demand applications such as gas turbines. The gasification process allows biomass to be used in advanced-technology power generation technologies with little or no modification. The cost of gasifying the biomass will vary depending on many factors, but the overall cost of high-efficiency gasification/power generation systems is projected to be less than lower efficiency, conventional combustion systems at equivalent scales. For the very small scale required for microturbines (~100 kWe), the biomass preparation/gasification/cleanup steps are roughly estimated to require capital costs of about $800/kWh of installed generating capacity.

Biological gasification processes are more appropriate for converting wet biomass and wastes to fuel gases. Biological conversion changes the carbohydrate portion of the biomass to a medium gas consisting primarily of CH4

and CO2. The lignin portions of the feedstock (10 to 40%, depending on species) cannot be converted biologically, and must be disposed of as a sludge. Biological conversion of low-sulfur feedstocks produces a readily usable product that can directly replace natural gas in most applications. Wet wastes such as animal manures and MSW usually contain sulfur that is partially converted to H2S in the digestion reaction. In some cases, equipment must be included to remove the H2S prior to use in electrical generation systems.

Anaerobic digestion systems using manures are commercially mature but are seldom used in the U.S. because they are not economically competitive. The collection and cleaning of the gas costs more than natural gas in the U.S., although some systems are used in situations that mandate environmental cleanup. Some digestors are also used in developing countries. Landfill gas collection sites are a specialized instance of biological gas generation. The landfills provide a localized source of biomass, and environmental/safety regulations may require methane recovery. Commercial power generation from landfill gases is widely practiced in Europe, and in North America to a lesser extent.

Biomass can also be converted to a liquid product using pyrolysis technology. The liquid is a highly oxygenated "biocrude" with a heating value of about 60% of diesel fuel. The technology is compatible with power generation systems in the 0.5 to 10 MWe size range. The compatibility of this product with advanced power generation systems has not yet been shown, although the technology is used on a small scale for the production of liquid smoke, a food additive. The oil contains some particulates and alkali salts that may impact advanced generation technologies. Research on these products is ongoing.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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