Is wood gas a biofuel? Is it renewable?

It sure is, because it is made from renewable feedstock – wood or any other biomass. If it’s made from some other biomass like grass or rice husks, it’d probably be called producer gas though.

Is wood gas a clean fuel?

In most cases, burning wood gas produces almost none harmful substances.

Wood gas combustion produces carbon dioxide, the most important greenhouse gas. But since the wood gas was made from renewable feedstock (biomass), the carbon dioxide cycle closes. It is a commonly accepted rule that combustion of biomass does not produce extra carbon dioxide, because the carbon bound within biomass was taken from the atmosphere by the plants.

As for the sulphur oxides, their emissions negligible compared with the case of burning such as coal. Wood contains a lot less sulphur than coal, so it’s combustion produces a lot less sulphur oxides.

Dust? Also, it can be assumed that emissions of particulate matter is negligibly small, because of two factors. First of all, dust particles are removed from wood gas before it’s fed to the engine. Secondly – from the engine exhaust pipe is located on a small height, so that even if the exhaust gases contain some dust, they fall to the ground soon.

Nitrogen oxides are also emitted in smaller quantities than for petrol or diesel fuel. Wood gas has smaller calorific value (heating value) than petrol / diesel, so peak temperature in combustion chamber is lower than for those two fuels. This leads to lower content of thermal nitrogen oxides in the exhaust gases.

The only not safe component present in wood gas exhaust gases is carbon monoxide. It is an essential component of the wood gas itself, so using this fuel instead of petrol / diesel / LPG will lead to its increased presence in exhaust gases. When wood gas is burned in some burner, for example in central heating boiler, this is not a concern, since there’s enough time for the carbon monoxide to be burnt fully to carbon dioxide.

Other issues

Producer gas can be produced from many different fuels. It can also be made through gasification of waste that can’t be used for any other purpose. I had the opportunity to visid a plant that makes producer gas out of tannery waste, conaminated with chromium (used in processing of hides). The only left waste from this gasification is ash, that can also be used! It containes the chromium so can be used to make some paints. If not gasified, this waste can be only stored at a specially prepared landfill.

Compared to many so called biofuels, wood gas is quite environment-friendly. It does not require any energy to be used to make this fuel, except for the energy used to make the wood chips fed to the gasifier. You don’t need to use natural gas or coal to distill it, like ethanol, or heat it, like biodiesel.

If you use wood gas in your internal combustion engine, designed for gasoline or diesel fuel, you’ll notice some power loss. In this article I’ll describe why such power loss does occur and how to overcome this effect.

First of all, it needs to be stated, that wood gas has very low heating value. For wood gas that consists of 17-22% (by volume) carbon monoxide, 12-20% of hydrogen and 2-3% of methane, the heating value is only 5,000-5,900 kJ/m³. When you mix wood gas with air in ratio close to 1:1,1 (stoichiometric mixture), its heating value is around 2,500 kJ/m³. Stoichiometric mixture of gasoline with air has heating value of 3,800 kJ/m³ [1]. Because of that, less energy is introduced to the engine, and less power is generated by the engine.

In most cases, this power loss can be up to 30-40%. But hey, if you want to have your car fueled by wood gas, you probably found some other good reason to build your gasifier. Nevertheless, you’ll probably interested in overcoming this power loss.

We’ll use the great advantage of wood gas – the high octane rating. This rating describes how good does fuel work in spark ignition engines, i.e. when does it start to knock (it’s a measure of anti-detonation of fuel). If a fuel has high octane rating, it can be used in engines with high compression ratio. And guess what? Wood gas has octane rating greater than gasoline!

What does it mean? While you can’t use standard gasoline in engines with compression ratio higher than something around 10:1, because you’ll have knocking (detonation combustion). And the higher the CR, the higher the engine efficiency (the more power it gives from the same amount of fuel). So if you plan on using your engine exclusively on wood gas, you might consider increasing the CR, for example by installing thinner cylinder head gasket. Increasing CR will help you overcome the wood gas power loss.

If you don’t want to increase engine CR, you can give it additional power by installing supercharger or turbocharger. This latter device increases the amount of fuel / air mixture using the kinetic energy of hot exhaust gasses from your engine.

Diesel engines don’t use stoichiometric fuel / air mixtures, but lean mixtures. The cylinder is full of air and some fuel is introduced. So total energy introduced to the engine per unit volume is closer to case when you filled the cylinder with wood gas / air stoichiometric mixture. Because of that, in diesel engines the power loss is much smaller than for gasoline engines!

[1] Wood gas as engine fuel, FAO Forestry Department, Forest Industries Division, Mechanical Wood Products Branch

Wood gas is an interesting fuel for internal combustion engines. It can be used in almost all four-stroke engines, both compression ignition (diesels) and spark ignition engines. In this article you’ll read how to use wood gas in engines.

Spark ignition (gasoline) engines

In gasoline engines the fuel is ignited by an electric spark made by spark plug. If you substitute the gasoline / petrol with some other fuel, like LPG for example, the engine runs fine. So why not fuel your engine with wood gas?

For engines with carburettor you simply turn the gasoline flow back to the fuel tank. You add wood gas / air mixture (of proper composition!) in the same place where the air was originally introduced. The mixture is compressed inside the engine and then ignited by spark plug.

For engines with fuel injection you need to buy some electronic device that’ll “cheat” the injection controller. Such a device is a part of LPG or CNG conversion sets. When you stop the gasoline flow in the engine, the injection controller will probably stop your engine or do something even more stupid.

Compression ignition (diesel) engines

Fuel in diesel engines is ignited by itself, because it is injected to the engine when air is under high pressure and hot. Wood gas will be introduced as in gasoline engines, in a form of wood gas / air mixture. Compressing it will not lead to ignition, so you need to leave a small dose of diesel fuel to be injected to the engine. This dose is called pilot charge. It can be petrodiesel, biodiesel or even vegetable oil. It will be ignited and ignite the fuel mixture. This way you can save up to 80% of diesel fuel or even 95%, if you substitute the injection nozzles with smaller ones.

Oh yeah, there’s also another way to ignite woog gas in diesel engines. You can substitute the whole injection system with ignition system (ignition coil, spark plugs instead of glow plugs). But if you decide to try this, you wouldn’t be able to use diesel fuel in this engine anymore.

In this article we’ll answer the question: which gaseous biofuel is better: biogas or wood gas? Both of those fuels may be used as energy source for your engine or boiler. They differ in composition and heating value, but they’re both combustible and that’s important. If you need to know anything more about those two fuels, check the articles I posted (biogas and wood gas). For now it has to be enough.

Feedstock

Wood gas can be made from wood. Or straw. Or rice husks. The feedstock needs to be dry. The less ash the better. The less volatile fractions (tar), the better.

Biogas can be made from almost any biomass. It is produced in landfills, marshes, swamps and such. So if you have wet grass, leaves or some other biomass residues, you can make biogas, but not wood gas.

Production

If you want to make wood gas, you start the gasifier. The gas is produced when it is necessary, so sometimes it is called producer gas. The wood gas production is not very difficult to start or stop on demand. You just need some electricity to power the fan used for startup.

Making biogas is not easy and cannot be regulated. Biogas is produced by some bacteria. You can’t tell them “start making biogas now” or something like that. When you input some biomass to the digester, some gas will come out later. When exactly? You can’t determine it precisely.

Storage

If you really need to store energy in biogas or wood gas, biogas is your choice. It has greater heating value per unit volume, so you can store more energy in a tank of given volume.

Powering your car

Biogas can’t be used directly in your car because of two main reasons. First is the inability to control the biogas production. Second is the inability to carry the digester around on your car. It is far too big and heavy, it is a stationary device… So you make some biogass and compress it and put it into your car’s gas tank. I guess it’d be suitable to use CNG gas tank. For every liter of the tank’s volume you’ll be able to store energy of 1/8 liter of gasoline!

If you want to use wood gas, you make it while driving your car. As I mentioned above, storing this fuel under pressure is not energy efficient. You’ll waste a lot of energy to compress the gas to required level and you’ll have to carry really large tank with you! So you carry the gasifier. And fuel for it.

Conclusions

If you have large amounts of different biomass to be disposed, make biogas out of it! If you get some tank to store biogas for some time, you can use it for making electricity, heating or cooking in your home. Without the tank it’d be difficult, since you wouldn’t be able to get as much biogas as you would need. You can also use this fuel for your car, but it’s not very wise…

If you need this fuel to drive your car, use wood gas. It’s much easier to produce than biogas.

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Wood gas

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Woodgas flame from a vehicle gasifier unit

Wood gas is a syngas fuel which can be used as a fuel for furnaces, stoves and vehicles in place of petrol, diesel or other fuels. During the production process biomass or other carbon-containing materials is 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.

Contents 1 History 2 Usage 2.1 Internal combustion engine 2.2 Stoves, cooking and furnaces 3 Production 4 See also 5 References 6 External links

History

The first wood gasifier was apparently built by Bischof in 1839. The first vehicle powered by wood gas was built by Thomas Hugh Parker in 1901.^[1] 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.^[2]

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

A wood gas truck in North Korea.

Usage

Internal combustion engine

Wood gasifier on a Ford truck converted into a tractor

Wood gasifier system

Wood gasifiers can power either spark ignition engines, where 100% of the normal petrol 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 WWII 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 km drive 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 (not including the energy needed to extract, transport and refine the oil from which petrol is derived, and not including the energy to harvest, process, and transport the wood to feed the gasifier). This means that 1000 kg of wood combustible matter has been found to substitute 365 litres of petrol during real transportation in similar driving conditions and with the same otherwise unmodified vehicle.^[3] 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 kg wood combustible matter / kWh electricity.^[4]

Gasifiers have been built for remote Asian communities using rice husk, which in many cases has no other use. One installation in Burma uses an 80 kW modified diesel for about 500 people who are otherwise without power.^[5] The ash can be used as Biochar fertilizer, so this can be considered a renewable fuel.

Against general belief, exhaust gas emission from an internal combustion engine is significantly lower on wood gas than on petrol.^[6] Especially the HC emissions are low on wood gas.^[7] 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.^[8]

Stoves, cooking and furnaces

Certain stove designs are in effect a gasifier working on the updraft principle—the air passes up through the fuel, which can be a column of rice husks, 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.

Coaxial downdraft gasification stove

An alternative stove based on the downdraft 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 LDO (light density fuel oil) in industrial furnaces.^[9]

Production

Fluidized bed gasifier in Güssing, Austria, operated on wood chips

A wood gasifier takes wood chips, sawdust, charcoal, coal, rubber or similar materials as fuel and burns these incompletely in a fire box, producing solid ashes and soot (which have to be removed periodically from the gasifier) and wood gas. The wood gas can then be filtered for tars and soot/ash particles, cooled and directed to an engine or fuel cell.^[10] Most of these engines have severe 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. to “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 build-up 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 WWII 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 (more capacity per 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 U.S. meaning wood gas produced for use in a combustion engine) is rather low compared to other fuels. Taylor ^[11] reports that “producer gas” has a lower heating value of 5.7 MJ/kg versus 55.9 MJ/kg for natural gas and 44.1 MJ/kg for gasoline. The heating value 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

• 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.^[10]

See also

Energy portal

• Rocket stove
• Water gas
• Gasification

References

1. ^ “Thomas Hugh Parker”. http://www.localhistory.scit.wlv.ac.uk/genealogy/Parker/ThomasHughParker.htm. 
2. ^ Wood gas vehicles: firewood in the fuel tank Low-tech Magazine, January 18 2010
3. ^ Mikkonen, Vesa (2010). Wood Gas for Mobile Applications. Published by the author, available at www.ekomobiili.fi. pp. 31. 
4. ^ Mikkonen, Vesa (2010). Wood Gas for Mobile Applications. Published by the author, available at www.ekomobiili.fi. pp. 142. 
5. ^ Burmese village rice husk gasifier
6. ^ Mikkonen, Vesa (2010). Wood Gas for Mobile Applications. Published by the author, available at www.ekomobiili.fi. pp. 3. 
7. ^ Mikkonen, Vesa (2010). Wood Gas for Mobile Applications. Published by the author, available at www.ekomobiili.fi. pp. 4. 
8. ^ Mikkonen, Vesa (2010). Wood Gas for Mobile Applications. Published by the author, available at www.ekomobiili.fi. pp. 70. 
9. ^ Rajeev Jorapur and Anil K. Rajvanshi, Sugarcane leaf-bagasse gasifiers for industrial heating applications Nimbkar Agricultural Research Institute, 11 April 1997, doi:10.1016/S0961-9534(97)00014-7
10. ^ ^{a b} Electricity from wood through the combination of gasification and solid oxide fuel cells, Ph.D. Thesis by Florian Nagel, Swiss Federal Institutte of Technology Zurich, 2008
11. ^ Taylor, Charles Fayette (1985). Internal-Combustion Engine in Theory and Practice – Vol.1. Cambridge: The MIT Press. pp. 46–47. ISBN 0-262-70027-1. 

External links

Wikimedia Commons has media related to: Wood gas

Wikimedia Commons has media related to: Gasification

• Food and Agriculture Organization of the United Nations: Wood Gas as Engine Fuel, 1986

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