The use of biofuels in power generation presents a number of challenges for manufacturers and plant operaters. But such fuels can also bring opportunities and environmental benefits.
In 1900, Rudolf Diesel presented an engine which ran on peanut oil and wisely stated in 1912 that, “The use of vegetable oils for engine fuel may seem insignificant today. But such oils may become in the course of time as important as petroleum and the tar products of the present time”.
The Finnish engine manufacturer, Wärtsilä, has been developing a modern day engine technology that can economically utilize vegetable oils or bio-oils as an efficient fuel oil for its engines. The company has already been awarded a contract to deliver gensets for a power plant which will run on bio-oil. The Italgreen Energy power plant is located in Italy and will use vegetable oil as fuel.
The Italgreen Energy power plant, known as Pentesilea, is currently being built and will start commercial operation in the spring of 2004. The plant will supply electricity to the Italian public grid and will produce steam for industrial processes. The plant will be commissioned using bio-oils (vegetable oils) from the very start-up of power generation.
Wärtsilä has been carrying out research and testing work on using bio-oils as diesel engine fuel since the mid 1990s. The development of bio-oils such as rape seed and palm oils as fuel oils has been a gradual process. Most bio-oils being tested are used oils from restaurants, or rejected oils from the food industry which have failed to meet the industry’s quality demands on food safety.
The utilization of bio-oils as an effective fuel source is the next step in a long line of alternative fuel oils that have been introduced by Wärtsilä in recent years. The fact that engines can now run efficiently using poor quality heavy fuel oil (HFO) and crude oils is taken for granted these days. Other fuels now in common use for engines now include: natural gas and liquefied petroleum gas as well as refinery burn off gases and even orimulsion.
There are a number of ways in which bio-oils differ from normal diesel fuel oils. For example, bio-oils have higher pour points and they feature large variations in ash content and acid numbers.
In operation, the pre-heating of bio-oils is normally required and methods must be adopted to avoid their high ash and acidity causing deposits and corrosion within the engine. To cater for the bio-fuels low heating value, the engine’s fuel delivery system must be adapted for the higher flow rates needed.
Two Wärtsillä 18V32 engines will be used at the Pentesilea power plant in Italy. The plant is currently under construction and will be commissioned on bio-fuel in early 2004
The first evaluations for bio-oils as fuel for diesel engines were made in 1995; this resulted in the approval of refined rape seed oil as a diesel fuel. This work has continued with the evaluation of other bio-oils such as palm oils and olive oil.
The development of bio-oil as a fuel depends on comprehensive studies of the physical and chemical properties of the fuels, combined with a good design of the fuel handling and fuel injection systems. Naturally, the only way to verify their functionality is through extensive testing.
A high proportion of the operating costs of a diesel power plant normally comes from the fuel. The higher the fuel price, the less profitable the power plant is likely to be in operation. It is important to assess whether fuel costs can be reduced by using other fuels sources like bio-oils – such as palm oil, coconut oil and rape seed oil. Unfortunately, producing these oils directly for power production purposes has one major disadvantage: the price is prohibitive.
However, to avoid this cost it is possible to consider vegetable oils that have already been used in restaurants or are a by-product of the food industry, plus oils that have been rejected by the food processing industry as they fall below standards can be considered as well. These oils are invariably considered to be a waste product.
Assuming these oils can be made to burn efficiently in diesel engines, they can be considered as a fuel oil as they have all the ingredients for a successful power plant solution:
- Low fuel price because vegetable oils are a waste product
- A sustainable solution that does not deplete fossil fuel reserves
- An environmentally friendly solution
- Emission trading and tax advantages.
When evaluating a new type of fuel the first step is to ask: ‘How does this differ from other known and approved fuel oil types? A comparison of three bio-oils under development today – refined palm oil, palm stearine and olive oil – and typical HFO and LFO (light fuel oil) grades shows that bio-oils have:
- High pour points
- Large variations in ash content
- Large variations in acid number
- Low LHV (heating value), and
- Extremely low sulphur levels.
Stearine oil is actually solid at room temperature. The properties of bio-oils have several consequences for engine design:
- The fuel-related systems must be heated/preheated
- High ash content and acidic fuels causing deposits and corrosion must be avoided
- The fuel delivery system must be checked for higher flow rates (low heating value)
- No sulphur emissions will be experienced.
The bio-oil processing industries offer several by-products that could be used to fuel diesel engine power plants. However, the end product properties of these options vary depending on the refining process, the origin of the raw material for oil production, and even the way the plants are operated. This also applies to used vegetable oil from the food processing industry and for this reason it is essential to analyse the full range of fuel properties in each case.
The chemical composition of vegetable oils and animal fats consists of triglyceides (glycerol bound to three fatty acid molecules) in different proportions. The most important difference in chemical composition between vegetable and fossil oils is the oxygen content.
Vegetable oil transported from Italy is used for testing at the Wärtsilä factory in Finland. Bio-oils have high pour points, low heating value and very low sulphur levels compared with conventional engine diesel fuels
Vegetable oils contain 10-12 per cent oxygen while fossil fuels contain insignificant amounts. Due to their oxygen content, vegetable oils normally have a higher cetane number, or ignition quality, and lower specific energy than conventional diesel oils. An important factor is that vegetable oils do not contain sulphur.
The only way to verify the suitability of new fuels is to perform engine tests with them. The goal of the test is to gain adequate performance data – specific fuel oil consumption and exhaust gas emissions – and to establish the functionality of the engine and fuel system with them.
For example, Wärtsilä has undertaken bio-oil testing for its 6L32 engine. With properly preheated fuel there was no difficulty getting the Wärtsilä 6L32 engine to start and run on refined palm oil, palm stearine and olive oil. The fuel consumption figures, appropriately corrected for heating value variations, were largely the same as in HFO operation.
The exhaust gas emissions from bio-oils are in most respects superior to conventional fuel operation. The NOx emissions may in some cases exceed the corresponding values for HFO operation but can be reduced using SCR technology.
The CO emissions are at roughly the same level as in LFO and HFO operations. The total hydrocarbon emissions are at clearly lower levels than with conventional fuels. Particulate emissions are in most cases lower than with conventional fuels except in the case of high-ash olive oil.
However, the real benefit of using bio-oils is the extremely low SOx emissions, which are close to zero. Carbon dioxide emissions are regarded as non-existent because they are part of the global carbon cycle.
During 2002, the EU commission proposed that there would be a 20 per cent use of substitute fuels in road transport by the year 2020. The short term targets are to reach two per cent by 2005 and 5.75 per cent by 2010.
The commission proposed that alcohol (ethanol) will be blended into petrol and that diesel oil will be partly replaced by vegetable oil derivatives. There are two approaches towards the solution: the use of pure vegetable oils; and biodiesel (transesteified vegetable oil or animal fat).
Following Wärtsilä’s series of tests on bio-oils functionality and performance based on short-term results seem to be good. Long-term experience will be needed to achieve a consistent and reliable view of the viability of using bio-oils. Fuels with high ash contents and exhibiting high acidity numbers, however, should be avoided as they will cause corrosion and deposit formation.
From an environmental point of view, using bio-fuels offers several advantages. These include the combustion of waste while simultaneously generating power and heat, the negative consumption of fossil fuel resources, no sulphur emissions, and no greenhouse effect are certainly advantages. These factors, when combined with emission trading, may give a superior power solution.