What is it?
It’s turning biomass (often wood chips and offcuts) into usable gas. The flames we see in an open fire are actually the burning of gas which comes off wood – about 70 to 80% of its total mass. If wood is heated but then starved of oxygen, so that it doesn’t burn with a flame, gas is released and char is left behind (this is the ancient method of charcoal production). This wood gas (called ‘producer gas’) has a high calorific value that can then be transformed into kinetic energy by combustion in an engine cylinder, and into grid-independent energy if the engine is attached to a generator. This heating without oxygen is called pyrolysis – which is just a stage in, and not synonymous with, the gasification process.
Gasification is not a new technology. Coke and coal gasification in the 1850s was followed in the early part of the 20th century by systems that could accept wood scraps and agricultural residues, reaching a technological peak during the 1930s and 40s. It changes a bulky fuel into a gas that is easier to transport, burns cleaner, and burns hotter. Internal combustion engines (both spark and compression types) can be converted to run on producer gas, and during the Second World War, when there were shortages of petroleum and diesel, wood-fuelled vehicles became relatively common. System use later declined when oil became cheap, but some countries – particularly in Scandinavia – continued to use the technology. Interest has returned because of global warming and future fuel insecurity.
Small-scale gasifiers also have the potential to turn any carbon-based waste (old shoes etc) into energy. They’re not at this stage yet, but expect to see many more of these systems in future. Large-scale gasification and pyrolysis as a way of turning municipal solid waste into energy is subject to increasing investment due to the bad press that incineration has received, and government subsidies for novel waste-to-energy systems. So far, the track record of large-scale systems is poor, due to the variable nature of the waste, with numerous failed projects and system shut-downs for exceeding emission limits or for poor operational performance.
What are the benefits?
Biomass gasifiers have an advantage over wind and solar renewable technologies because they provide power and heat on demand rather than being limited by seasonal or diurnal variations in supply. There is no need to store energy. The systems operate like a car with a fuel tank of wood chips. Make the engine run faster and the wood will be used up more quickly; run it more slowly, or switch it off completely, and the wood will last as long as needed. They can be more efficient than a biomass boiler as they have no water-heating requirements for energy transfer.
Gasifiers produce lower concentrations of NOx and SOx pollutants than combustors. The wood char-ash which gasifiers produce as a by-product can also be used as a soil fertiliser/conditioner.
Biomass is only an ethical and sustainable energy source if plant regrowth equals removal, and if no associated fossil-fuel energy is used. Due to the large volumes required, demands on suppliers to be reactive to market requirements, and the extra costs involved in locking in large areas of space, biomass supplied to power stations is usually force dried in large fan-assisted ovens, pelletised, and transported internationally. Small-scale use of bio-waste does not have these associated carbon inputs, and it doesn’t necessitate the use of quality agricultural land.
Small-scale gasifiers will be cost effective where there is a waste wood resource for free. When land is cleared or timber harvested, anything other than heartwood is chipped anyway, and either left in situ or landfilled. It has zero or negative value since landfill disposal is costly. This has possible community enterprise potential, making use of local authority waste. In the developing world, gasifiers can provide something that the grid cannot – reliable electricity for industry.
What can I do?
Biomass gasification reactors are simply steel vessels filled with wood chips. With one or more small openings to permit some air to enter, they are connected directly to an engine, and the engine suction pulls the gas through. They need no external water, electricity or gas, as they self-sustain their internal temperature. Reactor designs have changed little since the 1920s, and are based around three main types: updraft, downdraft, and cross-draft. There are also fixed and fluidised bed gasifiers (air or steam gasification), but these are industrial-scale systems. Downdraft, fixed-bed gasifiers are the most widely used, as they produce high-purity gas with relatively low quantities of tar.
For anyone with basic fabrication skills, small biomass gasifiers can be self-built, and full details of sizings and components can be found online. There are a number of companies offering commercial systems – such as GEK, Volter and Ankur. These are not at present, ‘push-button’, low-user interaction, kitchen-type appliances, but they’re getting there. Made from sturdy basic components, gasifiers are at present much like a classic car, which needs routine maintenance to ensure reliability, but which could last a lifetime. Specialist biomass gasification consultants are rare.
Gasifiers are proven to work with wood chips, but poor-quality wood chips can lead to problems if the set-up is not right. In particular, before buying a system, look for the small print about what the feedstock requirements are, since some (but not all) systems will stipulate wood chip moisture levels below that which can be achieved naturally. Fines content can also be a problem. Outside of design specification feedstock range, temperature in the reactor varies and tar comes through in the gas, which can clog engine pipework. Some systems use water scrubbing to clean tar from the gas, which can result in high disposal costs throughout the lifetime of the system. Others use basic wood chip filters, which work effectively.
Wood gas comprises about 20% carbon monoxide, which is fatal if inhaled. However, considering that oil and petrol are highly flammable, carcinogenic liquids, producer gas just poses a different hazard. Operating temperatures inside the reactor are between 800-950°C for optimum clean gas. The gasifier reactor is silent when operating, but the engine will be noisy. It’s perfectly legal to own and operate a wood gas vehicle, and as they only produce smoke at start up and shut down, they can be used in smokeless zones.
Wood gas generators are stand-alone, but can be grid-tied. In the UK, they are presently not eligible for feed-in tariffs, but system owners can get credits from ROCs and income from the Renewable Heat Incentive, although there are caveats.
And finally – remember Wacky Races? The Arkansas Chuggabug with Lazy Luke and Blubber Bear? Pretty sure that’s a gasifier at the back there.
Thanks to Dr. Andrew Rollinson of Blushful Earth for information.
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Whittaker, C., The nature of the wood pellet supply to the UK, 2014. In: Torrefaction Workshop, University of Leeds, 2-3 April 2014.
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