Wood Gasification

Wood gasification is the process by which wood is burned at very high temperatures, to create a cleaner, more efficient fuel than traditional wood burning can produce. (Many naturally-occurring fuel sources (or feedstock) can be gasified, including coal, petroleum and some biomass, the common of which is wood.) Wood gasification can provide fuel to power vehicles, greenhouses, homes and farms. It produces less greenhouse gases than traditional wood burning, and its by products can often be used for fertilizer. Since wood is a renewable resource, and gasification produces more energy from smaller amounts of wood, wise use of wood gasification can result in a sustainable global alternate energy source.

History

Interest in gasification dates back to the seventeenth century, when the first experiments were conducted in pyrolysis, the process of superheating coal and biomass, such as wood. The first patent for gasification was obtained in 1788 by Robert Gardner.

Just four years later, in 1792, Scottish engineer William Murdoch first realized the commercial potential of heating coal in the absence of air to produce gas. Murdoch used this gas to light his home while continuing to modify and refine the method of gas production. He collaborated Matthew Boulton and famous steam engine manufacturer James Watt to provide widespread gas lighting throughout England by 1798. In 1807, “town gas” began to be used for street lighting, and by 1816 most of London was using the gas.

In 1861, Siemens created the first successful commercial wood gasifier. In the decades to follow, manufacturing plants throughout the U.S. and Europe produced manufactured gas (also known as wood gas, syngas, or town gas) for widespread use as a gaseous fuel.

In 1901, Thomas Hugh Parker built the first motor vehicle powered by wood gas. By then, wood gas produced from coal was being used in many cities to power and heat residences. The Fishcer-Tropsch process, of sorting and reassembling manufactured gas into liquid fuel, was used by Germany during World War II and by South Africa during the apartheid years.

After 1945 gasoline and diesel became more readily accessible and usable as a cheap, plentiful fuel source for powering engines and machines. As a result, gasification became a "forgotten" technology. It took nearly 30 years before renewed interest in gasification brought about ongoing development in the technology for small-scale power generation.

A detailed and complete history of coal-gas and gasification is available at www.reference.com.

The Basics of Pyrolysis and Gasification

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"Pyrolysis" refers to any "chemical change brought about by the action of heat." Pyrolysis is the process at the heart of gasification. Gasification occurs when wood, coal, or any other feedstock containing both carbon and hydrogen is treated at high temperatures and pressure (i.e., pyrolysis), causing changes in the original substance at the molecular level.

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The processed feedstock is broken down into different gases, including hydrogen and carbon monoxide. The gases are separated and cleaned of impurities by scrubbers and distillers; the purified gases can then be used to create various kinds of fuel, including "ethanol, methanol, butanol and other alcohols as well as methane gas, synthetic petroleum, dimethyl ether, kerosene, diesel and other gases and liquids. Even the “waste” gases can be used to form marketable products such as fertilizers and chemical feedstocks."

Specifics of Wood Gasification

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Wood gasification has four distinct steps:

1. Drying: In order to have effective pyrolysis, the biomass feedstock must be dried, that is, the moisture inherent in the feedstock must be removed or vaporized. Where and how the moisture is removed in the process can affect the quality of the fuel: water must be removed from the feedstock before the temperature reaches 100 degrees celcius to produce clean gas.

1. Pyrolysis: Once the biomass is dried, the biomass is subjected to extremely high temperatures and pressure, in the absence of air. This causes the biomass to break down rapidly into volatile gases and liquids (known as "tar") and solids (known as "charcoal").

1. Combustion: The tar gasses or char produced during pyrolysis are the fuel sources that are burned or broken in the combustion stage, in order to create the heat and gases needed for the final stage of gasification. Combustion involves combining hydrocarbons with oxygen to produce heat, carbon dioxide (CO₂) and water vapor (H₂O), both needed for reduction, while eliminating the tar from the outgoing gas.

1. Reduction: The reduction reaction is the final stage in gasification. Reduction, the opposite process to combustion, involves stripping off the oxygen molecules from hydrocarbons as those hydrocarbons are passed over red hot char. The combustion gases become fuel products again, and can be piped away to perform work elsewhere.

The process of gasification is more completely described at the ALL Power Labs website.

Current Gasification Technology and Types of Gasifiers

With renewed interest in gasification in the last several decades have come significant developments in the efficiency and output of the modern gasifier, including the development of different types of gasifiers for commercial and private consumer use. This article will discuss the primary types of private consumer gasifiers; for an explanation of the types of gasifiers commonly available for commercial use, see the Energy Independence website.

Although the gasifier still operates on the same general principles, today's consumer wood gasifiers come in several different forms, each with its own benefits and challenges: the updraft gasifier, the downdraft gasifier, and the crossdraft gasifier.

1. Updraft Gasifier: The updraft gasifier uses the flow of air from bottom to top. Fuel enters the gasifier unit from the top and air enters from the bottom. The manufactured fuel gas flows out from the top in a relatively simple, uniform process. Updraft gasifiers tend to be more robust and simple to use, able to withstand feedstocks with higher moisture content than downdraft gasifiers. Feedstock can also vary more in size and shape, making the updraft gasifier more versatile. However, the manufactured fuel gas typically contains a high amount (10-20%) of oil and tar, because the products of the process exit along with the manufactured gas.

1. Downdraft Gasifier: The downdraft gasifier operates in the opposite direction of flow from the updraft gasifier. The biomass feedstock and the air enter from the open top (in the open model) or the middle (in the throated variety), the biomass is dried and pyrolized before being partially combusted, and manufactured gas is drawn off through a grate at the bottom of the unit. This results in more uniform burn and more complete combustion. As a result, the downdraft gasifiers have the advantage of producing cleaner fuel gas (with less tar) than updraft gasifiers. Downdraft gasifiers require low-ash, uniform feedstock, however, limiting their use with a variety of biomass feedstock.

1. Cross-draft Gasifier: In the crossdraft gasifier, air enters from one side of the gasifier, and manufactured fuel is released from the opposite side. Crossdraft gasifiers are the simplest to use, but require low-tar fuels like charcoal and uniformity of size of the biomass feedstock. Because other types of gasifiers are more efficient across fuel types than the cross-draft, this type of gasifier is less commonly used.

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