Wood gas

Wood gas is a syngas fuel which can be used as a fuel for furnaces, stoves and vehicles in place of gasoline, diesel or other fuels. During the production process biomass or other carbon-containing materials are gasified within the oxygen-limited environment of a wood gas generator to produce hydrogen and carbon monoxide. These gases can then be burnt as a fuel within an oxygen rich environment to produce carbon dioxide, water and heat. In some gasifiers this process is preceded by pyrolysis, where the biomass or coal is first converted to char, releasing methane and tar rich in polycyclic aromatic hydrocarbons.

The first wood gasifier was apparently built by Gustav Bischof in 1839. The first vehicle powered by wood gas was built by Thomas Hugh Parker in 1901. Around 1900, many cities delivered syngas (centrally produced, typically from coal) to residences. Natural gas began to be used only in 1930.

Wood gas vehicles were used during World War II as a consequence of the rationing of fossil fuels. In Germany alone, around 500,000 “producer gas” vehicles were in use at the end of the war. Trucks, buses, tractors, motorcycles, ships and trains were equipped with a wood gasification unit. In 1942, when wood gas had not yet reached the height of its popularity, there were about 73,000 wood gas vehicles in Sweden, 65,000 in France, 10,000 in Denmark, and almost 8,000 in Switzerland. In 1944, Finland had 43,000 “woodmobiles”, of which 30,000 were buses and trucks, 7,000 private vehicles, 4,000 tractors and 600 boats.

Wood gas was used, among other things, to drive internal combustion engines of motor vehicles. The generators were built outside the body or carried as a trailer. The technical system, the wood gasifier, was filled with firewood and worked as a fixed bed gasifier. By heating, the flammable gas mixture (wood gas) escaped from the wood. Until the early 1950s, a number of small trucks were in use in Germany with a special driver’s license, for which only certified and approved beech logs were usedcould be used. It was about one liter of gasoline can be replaced by the amount of gas obtained from 3 kg of wood. The wood, which was specially dried for wood gasification and shredded to the right size, was called tank wood and produced and stored in so-called tankwood factories.

At the end of the Second World War, there were about 500,000 generator gas cars or wood gas cars in Germany. Its supply was provided by the Ministry of Power Generator Company for fuelwood and other generator fuels with their associated filling stations.

In the Soviet Union, wood-carbureted lorries were mass-produced. Particularly noteworthy are the models ZIS-21 (based on the ZIS-5 ) and the GAZ-42, of which almost 35,000 copies were produced between 1939 and 1946. The reason was that, especially in the far north of the Soviet Union, the fuel supply in the 1930s and 1940s was not yet secured.

In Schaanwald in Liechtenstein there is a private museum with around 70 wooden gas vehicles from the motorbike to the tractor. The vintage cars are roadworthy and are moved from time to time, that is operated with waste of a furniture factory.

Wood gasifiers are still manufactured in China and Russia for automobiles and as power generators for industrial applications. Trucks retrofitted with wood gasifiers are used in North Korea in rural areas, particularly on the roads of the east coast.

As part of the discussion on the increasing use of renewable raw materials at the end of the 20th and the beginning of the 21st century, wood gasification and the gasification of other organic substances, especially of organic residues, for the recovery of gaseous fuels for heat and power generation were taken up again and implemented in individual demonstration plants. Based on this purely energetic use, the use of the product gas as a raw material for the chemical synthesis of biofuels and products of the chemical industry was also targeted and will be realized in the near future, especially for BtL fuels, dimethyl ether and methanol. By a subsequent methanationand treatment, it can be fed into the natural gas grid as Substitute Natural Gas (SNG). High-quality product gases containing more than 50 percent hydrogen are also referred to as so-called biohydrogen.

Wood gas consists of burning constituents, mainly of carbon monoxide 34% and methane 13%, as well as minor proportions of ethylene 2% and hydrogen 2%, as well as non-combustible components such as nitrogen 1%, carbon dioxide 49% and water vapor. Wood gas is about 1.5 kg / m 3 heavier than air under normal conditions. The calorific value of wood gas is about 8.5 MJ / m 3 in conventional autothermal gasification and over 12 MJ / m 3 in allothermal gasification.

According to the production, the composition of the wood gas may vary widely. When using air (21 vol.% Oxygen, 78 vol.% Nitrogen), the product gas contains a very high proportion of nitrogen, which does not contribute to the calorific value of the gas and reduces the hydrogen yield. In contrast, the product gases contain no nitrogen when using oxygen and water vapor and accordingly have a higher calorific value and a high hydrogen yield.


Internal combustion engine
Wood gasifiers can power either spark ignition engines, where all of the normal fuel can be replaced with little change to the carburation, or in a Diesel engine, feeding the gas into the air inlet that is modified to have a throttle valve, if it didn’t have it already. On Diesel engines the Diesel fuel is still needed to ignite the gas mixture, so a mechanically regulated Diesel engine’s “stop” linkage and probably “throttle” linkage must be modified to always give the engine a little bit of injected fuel, often under the standard idle per-injection volume. Wood can be used to power cars with ordinary internal combustion engines if a wood gasifier is attached. This was quite popular during World War II in several European, African and Asian countries, because the war prevented easy and cost-effective access to oil. In more recent times, wood gas has been suggested as a clean and efficient method to heat and cook in developing countries, or even to produce electricity when combined with an internal combustion engine. Compared to World War II technology, gasifiers have become less dependent on constant attention due to the use of sophisticated electronic control systems, but it remains difficult to get clean gas from them. Purification of the gas and feeding it into natural gas pipelines is one variant to link it to the existing refueling infrastructure. Liquefaction by the Fischer–Tropsch process is another possibility.

Efficiency of the gasifier system is relatively high. The gasification stage converts about 75% of fuel energy content into a combustible gas that can be used as a fuel for internal combustion engines. Based on long-term practical experiments and over 100,000 kilometres (62,000 mi) driven with a wood gas-powered car, the energy consumption has been 1.54 times higher compared to the energy demand of the same car on petrol, excluding the energy needed to extract, transport and refine the oil from which petrol is derived, and excluding the energy to harvest, process, and transport the wood to feed the gasifier. This means that 1,000 kilograms (2,200 lb) of wood combustible matter has been found to be equivalent to 365 litres (96 US gal) of petrol during real transportation in similar driving conditions and with the same, otherwise unmodified, vehicle. This can be considered to be a good result, because no other refining of the fuel is required. This study also considers all possible losses of the wood gas system, like preheating of the system and carrying of the extra weight of the gas-generating system. In power generation, reported demand of fuel is 1.1 kilograms (2.4 lb) wood combustible matter per kilowatt-hour of electricity.

Gasifiers have been built for remote Asian communities using rice hulls, which in many cases have no other use. One installation in Burma uses an 80 kW modified Diesel-powered electric generator for about 500 people who are otherwise without power. The ash can be used as biochar fertilizer, so this can be considered a renewable fuel.

Exhaust gas emission from an internal combustion engine is significantly lower on wood gas than on petrol. Especially the hydrocarbon emissions are low on wood gas. A normal catalytic converter works well with wood gas, but even without it, emission levels less than 20 ppm HC and 0.2% CO can be easily achieved by most automobile engines. Combustion of wood gas generates no particulates, and the gas renders thus very little carbon black amongst motor oil.

Stoves, cooking and furnaces
Certain stove designs are, in effect, gasifiers working on the updraft principle: The air passes up through the fuel, which can be a column of rice hulls, and is combusted, then reduced to carbon monoxide by the residual char on the surface. The resulting gas is then burnt by heated secondary air coming up a concentric tube. Such a device behaves very much like a gas stove. This arrangement is also known as a Chinese burner.

An alternative stove based on the down-draft principle and typically built with nested cylinders also provides high efficiency. Combustion from the top creates a gasification zone, with the gas escaping downwards through ports located at the base of the burner chamber. The gas mixes with additional incoming air to provide a secondary burn. Most of the CO produced by gasification is oxidized to CO2 in the secondary combustion cycle; therefore, gasification stoves carry lower health risks than conventional cooking fires.

Another application is the use of producer gas to displace light density fuel oil (LDO) in industrial furnaces.

Gas usage
The gas produced in the biomass gasification can be used both energetically and materially.

Energetic use by combustion
The currently common use for the gas mixture of the biomass gasification is the motor use (according to the gasoline or diesel principle) or the combustion in corresponding incinerators for the production of heat (steam) and electric power, using a force-heat Coupling a very high energy conversion efficiency is achieved. The wood gas condensate produced during gas cooling must be properly treated in these plants before it can be sent to a receiving water, as it requires a high level of biochemical oxygenHas. Alternatively, the gas mixture of the biomass gasification in solid oxide fuel cells can be converted directly to electricity. The active principle was already proven in experiments in 2004.

Use as synthesis gas
In addition, a product gas of carbon monoxide and hydrogen for the chemical synthesis of various products can be used as synthesis gas. The material use of synthesis gas from biomass gasification is still in the development, such plants are currently only in laboratory and demonstration scale. The large-scale production and use of CO / H 2 -Synthesegas accordingly takes place exclusively on the basis of natural gas and other fossil fuels such as coal and naphtha.

The chemical-technical utilization options are primarily the production of hydrogen and the resulting production of ammonia using the Haber-Bosch process, methanol synthesis, various oxo syntheses and the production of biofuels (BtL fuels) and other products via the fishermen -Tropsch synthesis:

in ammonia synthesis according to the Haber-Bosch process

in the synthesis of methanol

in the oxo synthesis

in the Fischer-Tropsch synthesis

In addition to these chemical-technical applications, syngas can also be used biotechnologically via synthesis gas fermentation. Products of this option can be, for example, alcohols such as ethanol, butanol, acetone, organic acids and biopolymers. This use is currently still in the development stage and is not used accordingly on a large scale.

In all these types of use, it should be noted that the water condenses as part of the process chain with a cooling of the gas and to varying degrees as wood gas condensate is variously contaminated with organic matter; the proper disposal of this waste water (about 0.5 liters per kg of wood) is listed here in the BtL scheme as “by-products”, but it is an integral part of such systems.

A wood gasifier takes wood chips, sawdust, charcoal, coal, rubber or similar materials as fuel and burns these incompletely in a fire box, producing wood gas, solid ash and soot, the latter of which have to be removed periodically from the gasifier. The wood gas can then be filtered for tars and soot/ash particles, cooled and directed to an engine or fuel cell. Most of these engines have strict purity requirements of the wood gas, so the gas often has to pass through extensive gas cleaning in order to remove or convert, i.e., “crack”, tars and particles. The removal of tar is often accomplished by using a water scrubber. Running wood gas in an unmodified gasoline-burning internal combustion engine may lead to problematic accumulation of unburned compounds.

The quality of the gas from different gasifiers varies a great deal. Staged gasifiers, where pyrolysis and gasification occur separately, instead of in the same reaction zone as was the case in, e.g., the World War II gasifiers, can be engineered to produce essentially tar-free gas (less than 1 mg/m³), while single-reactor fluid-bed gasifiers may exceed 50,000 mg/m³ tar. The fluid bed reactors have the advantage of being much more compact, with more capacity per unit volume and price. Depending on the intended use of the gas, tar can be beneficial, as well by increasing the heating value of the gas.

The heat of combustion of “producer gas” — a term used in the United States meaning wood gas produced for use in a combustion engine — is rather low compared to other fuels. Taylor reports that producer gas has a lower heat of combustion of 5.7 MJ/kg versus 55.9 MJ/kg for natural gas and 44.1 MJ/kg for gasoline. The heat of combustion of wood is typically 15-18 MJ/kg. Presumably, these values can vary somewhat from sample to sample. The same source reports the following chemical composition by volume which most likely is also variable:

A charcoal gas producer at the Nambassa alternative festival in New Zealand in 1981
During the production of charcoal for blackpowder, the volatile wood gas is vented. Extremely-high-surface-area carbon results, suitable for use as a fuel in black powder.

Nitrogen N2: 50.9%
Carbon monoxide CO: 27.0%
Hydrogen H2: 14.0%
Carbon dioxide CO2: 4.5%
Methane CH4: 3.0%
Oxygen O2: 0.6%.

It is pointed out that the gas composition is strongly dependent on the gasification process, the gasification medium (air, oxygen or steam) and the fuel moisture. Steam-gasification processes typically yield high hydrogen contents, downdraft fixed bed gasifiers yield high nitrogen concentrations and low tar loads, while updraft fixed bed gasifiers yield high tar loads.

Also in the production of biofuels, the gas produced in the gasification product gas is used as synthesis gas in the synthesis processes already described. The focus is on gaseous fuels such as biohydrogen, natural gas substitutes (methane, SNG) and dimethyl ether, as well as liquid fuels such as methanol and BtL fuels. [8th]

Biohydrogen is extracted from the synthesis gas by steam reforming, methane can be produced by methanation of the gas. For the preparation of methanol and dimethyl ether, the synthesis of methanol is used. BtL fuels are produced via the Fischer-Tropsch synthesis, whereby both gasoline and diesel fractions can be produced based on the process parameters.

Source from Wikipedia