homebrew power : run your engine on wood !
Posted 11 July 2009 - 05:43 PM
So what is gasification?
Gasification is the high temperature conversion of combustible solids (e.g. wood, coal, coke, charcoal, municipal waste etc) into a gaseous fuel mixture containing amongst others, hydrogen (H2) and carbon monoxide (CO). Depending on the production context, the gas mixture generated may be known variously as “wood gas”, “synthesis gas”, “producer gas” or “coal gas”
Is gasification a new technology?
Far from it, it has been around for over 100 years.
Why do we need biomass gasification for renewable energy, isn’t wind and solar enough?
Wind and solar power are key components to the future electricity generation mix because they are low maintenance and greenhouse neutral. Solar generation has the advantage of peaking at the same time as peak air-conditioning loads, and wind energy when networked sufficiently widely with appropriate interconnection can provide base-load generation. However, neither can provide on-demand generation for load-matching. Bio energy based generation such as gasification can provide this, and if supplied from sustainably-harvested fuel, can be a truly greenhouse-neutral on-demand generation system to complement wind and solar.
What about coal gasification? Isn’t that supposed to be good for greenhouse gas emission reduction too?
Yes, coal gasification can be done. (Most town gas used to be produced from coal gasification processes.) Whilst coal gasification in an integrated gasification combined cycle (IGCC) system can offer improved efficiency in a centralized power station over combustion alone, it represents only a transitional step in greenhouse gas emission reduction, and is not a long-term option without a viable and cost-effective carbon-sequestration technology.
But isn’t wood gasification just an excuse to chop down even more trees?
We hope not. We think it is a practical means for deriving energy (and an income) from the sustainable management of native re-vegetation schemes, off-setting fossil fuel electricity production. We think it is a good reason to do more than just plant the trees, but sustain and maintain them and generate renewable power along the way.
What are the waste products from the wood gasification process?
A high-performance power gasifier is optimized to generate the maximum gas and little else. By-products of the process are a small amount of low-grade activated carbon, some ash and water. Ideally, these by-products should be combined and returned to the soil as a soil conditioner. Waste heat is also produced by the process and can be used for water or space-heating, or for pre-drying the fuel for the gasifier.
What’s the advantage of biomass gasification over production of bio fuels like ethanol or biodiesel?
A well-designed gasifier can accept a range of feedstocks and thus the technology supports a bio diverse resource base rather than a monoculture energy crop. However the energy density of the gas is far lower than that of liquid bio-fuels, making it more suitable for stationary applications than transport. Although amateurs are now powering a diverse range of vehicles on woodgas.
What is in the gas that comes out of the gasifier?
Whilst the exact gas composition will be particular to the feedstock and operating conditions, typical gas composition from an air-blown gasifier is around 20% carbon monoxide (CO), 18% hydrogen (H2), 10% carbon dioxide (CO2), 1% methane (CH4) and 51% nitrogen (N2). It is a very high octane but low specific energy gas mixture that has a relatively low flame temperature and combustion flame speed due to the large amount of dilution gases (CO2 and N2) present.
What can the gas produced in the gasifier be used for?
The gas mixture produced by the gasifier can be used for process heating applications by operating the gasifier in a forced draft (positive pressure) mode and passing the gas directly to a burner. In induced-draft mode it can be inducted into internal combustion engines as a fossil-fuel replacement for developing motive power, driving generators etc.
What sort of engines can you run on woodgas?
Both spark-ignition engines (petrol or gas), and compression ignition engines (diesel) can run on wood gas. 100% petrol or gas replacement can be easily achieved on spark-ignition engines; compression ignition engines still require around 10 to 20% diesel for ignition of the fuel charge as the gas will not compression-ignite in a diesel engine. Sometimes spark-plugs are retrofitted to diesel engines for 100% fuel replacement. In large applications, the gas can also be use to drive gas turbines.
What about exhaust emissions from engines running on wood gas?
Exhaust emissions from an engine running on wood gas are typically more benign than petrol or diesel, ie lower in nitrogen oxides (NOx), sulphur oxides (SOX) unburned hydrocarbons and particulates. The major components of the exhaust gas from the engine are nitrogen, carbon dioxide and water vapour.
Isn’t the carbon monoxide dangerous?
Yes! Carbon monoxide is an odourless, but highly poisonous gas and care should be taken not to inhale it. In the gasification process, the carbon monoxide is an important component of the fuel gas mix but exists only for a short time in the process between gas production in the gasifier and combustion in the engine. With the engine running properly, carbon monoxide is not present in the exhaust gases. Our particular gasifier operates at sub-atmospheric pressure to minimize the possibility of a gas leak. But, just like a petrol-powered car, a gasifier should never be operated in an enclosed space in order to further reduce the risk of a carbon monoxide build-up.
Can an engine be damaged by running on wood gas?
The single biggest risk to an engine running on wood-gas is the risk of tar formation in the gas. This only happens if the gasifier is either badly designed or operated improperly. The tar will cause engine parts (e.g. valve stems, pistons) to stick and this will lead to disasters like bent and broken pushrods, rocker-arms, con-rods etc. An engine should never be operated if there is tar present in the gas stream.
Why not run micro turbines on the gas rather than piston engines?
We get asked this a lot for some reason. Piston engines are cheap and easily available. Micro turbines are not. Old car engines will run happily for years on wood gas and using second-hand engines reduces eco-footprint considerably over buying either new engines or micro turbines. Micro turbines are far less tolerant of contaminants in the gas stream (e.g. particulates and moisture) than piston engines. Also, when a car engine breaks down in many cases you can fix it yourself or worst case it can be taken to the local mechanic. But try fronting up to your local garage with a busted micro turbine and see how far you get.
Can I retrofit a gasifier to my little portable petrol or diesel generator?
Sadly, in most cases the answer to this is “No”. The reason is that the small generators are typically of the 2-pole type and operate at 3000 rpm for 50 Hz (3600 rpm for 60 Hz). This reduces the weight of both the engine and the generator, improving portability. Wood gas simply doesn’t burn fast enough to run an engine much over 1800 rpm and hence is only suitable for the larger-scale 4-pole generator systems operating at 1500 rpm for 50 Hz (1800 rpm for 60 Hz).
Posted 11 July 2009 - 06:10 PM
Posted 11 July 2009 - 07:55 PM
It certainly is a viable option. They had trucks that were road-ready that ran on wood gas, stationary power plants are a great match-up.
Some woods not great for home heating can work quite nicely when gasified.
Methane generation from animal/household waste is also good for home electricity generation.