Your choices so far:
1 Residual oils/fats etc.
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| 1 Residual oils/fats etc | Local heating (ind. house) |
Today, there are commercial, bio-based diesel substitutes available on the commercial market, one of the more well-known being RME or Rape-Methyl-Ester though the more general name is FAME or Fatty Acid Methyl Esters.
These fuels are produced using a low-temperature chemical conversion based on fatty acids. The raw material can be excess vegetable oil from agricultural production, rapeseed oil, soybean oil and alike but also residual cooking oils from – for example – restaurants or from food processing.
The advantages with the upgrading process are mainly three: First it serves to homogenise the properties of the fuel so that the very inhomogeneous feedstock becomes a standardised fuel within narrow quality limits. Second it serves to make the fuel storable. The raw oil qualities will slowly oxidise and solidify unless contaminants and unsaturated fatty acids are removed or neutralised and this is achieved by the upgrading. Third the fuel produced attains sufficient quality for the immediate use in the transport sector. Not only is this one of the few processes that can yield a diesel oil substitute from waste fractions but the transport sector is also one where the willingness to pay is the highest, so this is a sector where the upgrading process may be paid for by the added value.
The feedstock needs to be filtered and free from solid impurities and water prior to the process, so the collection and handling needs be such as to provide a reasonably clean feed. Basically, the process is to add OH-groups to the fatty acid and thus transform it into an ester. This is achieved by the addition of an alcohol, typically ethanol or methanol in the presence of a catalyst, typically at low process temperatures. As compared to the raw vegetable oils, esters have favourable properties with respect to storage (they are more stable).
The resulting fuel quality, as measured by the cetane number, is strongly depending on the combination of feedstock and alcohol but some combinations – like coconut oil and ethanol – will typically yield cetane numbers > 70. Such quality fuel, provided it is not contaminated, can serve as a diesel substitute without any need for modifications of the engine while other fuels, such as RME produced from rapeseed oil and methanol (cetane number ≈ 50) may call for engine modifications.
It is also important to remember that the final product must be purified so as to be free from residual fatty acids and from water since they are very corrosive. The process involves common bulk chemicals like ethanol or methanol, it runs at low temperatures and the total conversion may be almost 100%. The main by-product is glycerol, which requires separate handling.
The total energy balance is strongly depending on the addition of alcohol and typically the total energy provided from the alcohol amounts to about 10 – 20% of the energy contained in the final product. The main by-product is glycerol, which requires separate handling.
FAME – fatty acid methyl esters – as is the common acronym for this group of fuels, can be used as diesel substitutes as well as diesel additives or can be used to replace light fuel oil in any process.
It is also possible to co-combust the raw oils together with a primary fuel, preferably a fossil oil or gas, directly in boilers and so replace part of the fossil fuel with biofuel.
Oil and fat residues lend themselves well for anaerobic digestion and in case the production of FAME is not favoured they may well be co-digested with other substrates to produce methanol.
In case the feedstock is sufficiently homogeneous – for example as a side product from a food processing industry –the use of the raw oil in a robust, low-rpm, adapted engine for internal electricity production may be feasible. The efficiency from fuel energy to mechanical energy (i.e. to electricity) in such engines is well below 15% and this is the maximum amount of electricity that can be produced from the oil residues in case this route is used.
Theoretically there will be another route, and that would be to burn or to co-combust the oil residues with oil, gas or some other fuel in a steam boiler and then to use a steam cycle to produce electricity.
Even so, the cost will still be high, and only with very favourable subsidies or tax reductions – or with a high cost to dispose of the residues – will electricity production from FAME become a feasible alternative.
Due to the competition about fertile land, FAME production should only be based on residual oil fractions, or at least on non-edible biomass, and it must also be considered that oil-rich residues like press cakes should better be used for fodder than for energy purposes.