We generally associate biodiesel with transport, but recent research shows that it could have significantly more potential as a domestic heating fuel when it comes to reducing UK carbon emissions. Andrew Robertson explains why recent field trials provide fuel for thought
More and more motorists are opting for the green fuel biodiesel. According to figures from the Department of Transport, bio-diesel consumption shot up from just over two million litres a month in 2003 to an estimated 12 million litres a month by the end of 2005.
However, while biodiesel offers a straightforward way to cut carbon emissions, the only blend that is currently universally accepted by the automotive industry is B5 - or 5% bio-diesel, 95% mineral diesel - which results in just modest CO2 savings overall.
While we associate biodiesel with road transport, it could end up contributing significantly more to cutting CO2 emissions in another way altogether. Researchers at The Clean Energy Consultancy in Norfolk have been exploring the possibility of using biodiesel as a replacement for kerosene in domestic oil-fired heating plant.
Kerosene is the main fuel used in the domestic heating market and offers the biggest opportunity for cutting emissions. It also has similar properties to biodiesel (see Table 1, opposite) - which means that the heating appliances currently installed in houses will be compatible with the alternative fuel, therefore eliminating any prohibitive capital costs needed to modify plant.
Biodiesel combustion
The Clean Energy Consultancy began by laboratory testing three types of biodiesel: B25, B50 and B100. The names refer to the concentrations of biodiesel mixed with kerosene, ie B25 is 25% biodiesel with 75% kerosene and so on.
This initial combustion data was used to determine the specifications for the (oil-fired boiler) pressure jet burners for use with biodiesel prior to the field-testing. It demonstrated that while biodiesel can combust effectively with a given nozzle and burner combustion head with correct fan pressure, the B100 was unable to provide effective combustion.
When the ambient temperature of the test laboratory fell from 17ºC to 13ºC, the test boiler began to experience combustion failure with the B25 and B50 fuels. The boiler failed to ignite despite burner adjustments. The problem was eventually overcome by using a fuel preheat burner to raise the temperature of the fuel within the burner to around 70ºC. This reduces the fuel viscosity to a level that enables it to atomise and combust effectively.
Interestingly, using the preheat burner, the B100 also demonstrated good combustion with the correct nozzle, fuel pressure and combustion head combination.
This shows that pressure jet burners on existing oil-fired boilers will need to be upgraded with a fuel preheat facility. While some boiler models will simply require a preheat kit to be fitted to the existing burner unit, others will require a replacement biodiesel specification pressure jet burner. New boilers specified for biodiesel could be to the same boiler specification as oil boilers and incorporate a fuel pre-heat burner.
The biodiesels were then tested in a series of field trials over a six-week period between December 2005 and February 2006 in a family dwelling, using the household oil-fired boiler to determine the potential boiler reliability. B20 was used in preference to B25, as it demonstrated improved fuel stability characteristics.
The results of these trials are detailed overleaf (see box: The field trial results). However it was discovered that the effective combustion of biodiesel in an oil-fired boiler is dependent on two critical factors: correct burner/boiler matching and fuel stability.
Although the requirements of biodiesel burner/boiler matching are similar to those for kerosene and gas oil, one significant difference is the need of a preheat burner when using biodiesel. Failure to use a preheat burner will result in burner failure at internal ambient temperatures below 15ºC.
Burner fan static was also found to be an important consideration. The combustion flame must be kept off the combustion head, otherwise black gum deposits build up on it, eventually affecting burner reliability.
For this reason, high-pressure jet burner fan pressure (fan static) is an important requirement for biodiesel pressure jet burners. The higher fan pressure essentially pushes the combustion flame off the combustion head and further into the combustion chamber of the boiler.
Fuel stability
The winter field trials reveal just how critical fuel stability becomes in terms of boiler reliability. The B100 fuel must be stored above 25ºC in order to remain stable. Once the B100 fuel temperature falls below this level, the fuel becomes opaque and gradually, over several hours, or a few days depending on the size of temperature drop, combustion failure occurs due to the partial blocking of the nozzle filter with separated fatty waxy material.
The same problem is true of the B50 fuel, although the required temperature for this to stay clear was lower, at around 15-20ºC.
B20 is more stable than the other biodiesel fuels tested, giving better combustion results. Even at fuel temperatures below zero, it remains stable. During the boiler tests and field trials, the internal condition of the test boilers were examined. The boiler heat exchangers were found to be in clean condition, as were the heat exchanger baffles - suggesting that biodiesel is compatible with oil-fired boilers.
In terms of fuel storage, both B100 and B50 will require thermal insulation and heating to maintain their stability. As a precaution, all the boiler's internal fuel pipes and external fittings need to be insulated, which could be a prohibitive capital cost for some consumers.
B20, on the other hand, is not affected by severe winter weather conditions and can be used with existing domestic fuel storage systems without incurring additional costs. It is possible that any fuel seals made from natural rubber may need to be replaced with synthetic rubber seals, but in practical terms, all consumers who wished to switch from kerosene to B20 biodiesel would need to do is order their initial biofuel delivery when their fuel tank was nearly empty. When B20 was added to the existing kerosene in the tank, the two fuels would mix without a problem.
The big question, though, is whether a switch to biodiesel for domestic heating could deliver significant CO2 reductions. During 2003, CO2 emissions for oil-fired space heating and domestic hot water were around 7.5 million tonnes. This is only 6.4% of the emissions from the road transport sector. However, the objective of the UK government's "Green Fuel Obligation" is to reduce road transport CO2 emissions by one million tonnes per annum by 2010.
If all kerosene heating oil can be substituted with biodiesel, potential CO2 savings could be as high as 7.5 million tonnes per annum.
The worst-case scenario in terms of reducing boiler CO2 emissions would be to use the B20 bio-diesel. This fuel blend will save 1.5 million tonnes of CO2 per annum and could be produced from oilseed rape crops grown on around 330,000 ha of land.
This represents greater CO2 savings than the road transport CO2 savings proposed by the UK government's 2010 road transport bio-fuel obligation and incurs a significantly lower impact on agriculture.
Furthermore, if the UK heating market consumes biodiesel fuel produced in the UK, kerosene heating oil imports could be reduced - and this would offer both environmental and fuel security benefits as it would reduce our reliance on Middle East kerosene.
All this certainly provides fuel for thought: are you ready to make the switch?
What is biodiesel?
There are two main types of biodiesel. The UK’s main source is used vegetable oil (UVO) for cooking, which has been cleaned and filtered. This is usually described as Fatty Acid Methyl Ester (FAME).
The other source is harvested rapeseed, known as Rapeseed Methyl Ester (RME).
Vegetable oils have a high viscosity and as such are not suitable as a direct diesel replacement; instead, their viscosity must be reduced by transesterification. This involves reacting the vegetable oil with an alcohol in the presence of an alkaline catalyst, which produces a methyl ester of the vegetable oil and glycerine as a byproduct. The act of separating the glycerine from the vegetable oil molecule reduces the viscosity of the vegetable oil. Both types of fuel must conform to the biodiesel European quality fuel standard, BSEN 14214:2003.
Notes for Table 2
When it comes to using biodiesel in domestic boilers, the problems associated with fuel viscosity are a concern with regard to maintaining boiler reliability during the winter. In the field trials carried out, the B100 and B50 fuels were unstable at low temperatures.
Looking at the viscosity trend for B20 FAME biodiesel, it is similar to kerosene and gas oil. However, the B50 and B100 viscosity trend increases significantly over kerosene and gas oil, suggesting B100 and B50 will result in fuel stability problems during colder weather – which could cause winter boiler reliability problems.
Data for RME biodiesel suggests the viscosity is lower than that of the FAME biodiesel at similar fuel temperatures. This indicates RME biodiesel may be a more stable heating oil source during the colder winter months.
The field trial results
B100
Initially, the boiler operated without any problems. However, after a few days, the boiler began to lock out. Inspection revealed that the combustion head was blackened with gum and the fuel was opaque and cloudy in appearance. The fuel was stored at an ambient temperature of around 13ºC. It was noted the combustion settings and data had changed from the original settings. The burner had to be adjusted significantly to maintain the original combustion emissions. Further examination revealed the burner nozzle filter was partly blocked with a fatty, waxy material. The fuel was found to be separating inside the fuel container, with the suction end of the burner fuel pipe immersed in the layer of separated material.
The nozzle filter was replaced and the fuel stored at a warmer temperature of around 25ºC that kept the fuel clear in appearance. Once the fuel had separated, by heating it to around 25ºC, the separated fuel mixed back into the fuel and eventually it became completely clear in appearance.
The boiler operated without further incident using the heated fuel until day 14 of the trials, when it again locked out. The fuel had become cloudy in appearance and the fuel temperature had dropped to around 12ºC. At the end of the B100 trial, the burner components were examined. Despite the problems of the boiler locking out, the nozzle and combustion head were in very clean condition and had no traces of soot or carbon build-up, suggesting the fuel combustion was good.
B50
The fuel was replaced with B50 biodiesel (50% biodiesel and 50% kerosene).
There were a few problems initially caused by a partially blocked nozzle fuel filter due to cooler fuel temperatures and cloudy fuel. The fuel was then heated to around 20ºC and the boiler operated without any further problems. At the end of the trial, the burner combustion head and nozzle assembly were found to be in good clean condition, suggesting clean combustion.
B20
The fuel was replaced with B20 biodiesel (20% bio diesel and 80% kerosene).
The nozzle filter was replaced with a new filter as a precaution. Test samples of B20 stored at temperatures of -5ºC indicated the fuel remained stable and clear, so for the purposes of the field trial, the fuel was not heated prior to use. All the fuel was stored outside at ambient temperatures of between -6ºC and 4ºC.
There were no combustion problems: the combustion head and nozzle assembly remained clean and there were no traces of soot or any other debris on the combustion head or burner nozzle assembly components.
Source
Building Sustainable Design
Postscript
Andrew Robertson is an environmental, energy and architecture consultant for the Clean Energy Consultancy
No comments yet