The Good, the bad and the ugly
Using biogas, which is generally a waste or by-product, to generate electricity can make sound economic sense for an organisation, even before considering the green arguments. There are however some buts. Most of the major issues are technical and it essential that you choose the right partner for such an enterprise with care.
Biogas has contaminants and these will be different on each site. Their effects will vary and will have an impact on the generator sets, the installation design, the maintenance regime needed and ultimately whether the project is economically viable. There is no standard solution and it is best to talk to experts.
Such contaminants might include: acids if chlorine, fluorine and hydrogen sulphide is present with water; siloxanes a gaseous form of silicon; particulates such as dust; condensates; and other waste stream contaminants. The challenge is to design a complete system that can either remove the contaminants and/or reduce their effect on the generator.
So before starting on any project the first task is to analyse the fuel gas for contaminants. Samples are collected on site and sent to a laboratory for analysis and from here it is possible to design a system that will cope with the measured gas quality.
Two of the more common contaminants are inorganic acids and siloxanes.
If there is a problem with acids then the trick is to convert them into a vapour and prevent them condensing on any cold spots.
Gas manifolds will heat up the fuel gas before it enters the engine and the temperatures of the cooling water jacket, lubrication oil and ambient air are all kept high. The jacket water heaters are kept on when the engine is turned off to prevent condensation.
Engine design also plays its part. Components are hardened against the effects of corrosive materials. Such an engine would often have positive crankhouse ventilation to evacuate blowby gases quickly in order to minimise contact with its interior surfaces. A large capacity sump means there is more lubrication oil to dilute any contaminants.
Organic silicon, present in the form of siloxanes is another common problem. These can enter the engine with the fuel mixture or get into the lubrication oil by blowby. They cause problems by forming deposits on the internal surfaces of the engine’s combustion chamber as silicates, often to a depth of several millimetres. Unfortunately high efficiency engines, which operate with a higher exhaust oxygen content, only make the problem worse.
If siloxanes are present then a bespoke maintenance contract will be needed. Other solutions may be to divert waste high in siloxanes away from the site, especially if it is mostly from one source. Another solution is to inject a fine water vapour into the engine to help prevent deposits building up. Note the latter should only be considered if the fuel gas is low in acids.
Whatever the contaminants their concentrations can vary over time. It is therefore best to test the lubrication oil weekly, after the engine has been running, to show both contaminant levels and also engine wear. For example the presence of chrome in the oil may indicate top piston ring wear.
Biogas always varies from site to site and will change over time so there are no standard answers. In addition to modifying the generator set, the gas can be cleaned or conditioned, the design of the installation can help minimise the problem and the maintenance regime needs considering.
Taking these one by one. First there are several ways to condition or clean the gas. A cyclone, or centrifuge and filters can help remove particulates and liquid from the gas. Coalescing filters or chillers can remove moisture in the vapour phase whilst activated carbon filters will remove some acids and siloxanes. It is also wise to consider absorptive filters for the oil.
All of the above will affect your capital expenditure and in our experience none of these equipment manufacturers will give you a commercial guarantee. It is wise to do a cost benefit analysis and consider all of these measures as part of a complete solution, not as a solution on its own.
Common sense engineering when designing the plant can also make a big difference. Consider the gas pipework. Solenoid operated drain valves will remove water collected in the bottom of the pipework, whilst taking the fuel gas from the top of the pipe will tend to prevent particulates and liquids moving downstream. Also, attempting to keep the gas above its dew point will help to avoid condensation.
Preventative maintenance with planned inspections and condition monitoring is even more critical for Biogas fuelled engines than other machines operating on conventional fuels. The actual maintenance schedule will depend on what contaminants are present, in what concentrations and how these vary over time.
Information is vital, so robust data collection and recording how the engine performs over time is critical. This can now be monitored remotely 24/7 with alerts ensuring that maintenance occurs before breakdown.
The main generator set is designed to be durable and, as already discussed, measures can be taken to minimise the affects of contaminants. Another option, particularly if the gas is very acidic, is to mix the biogas with natural gas from the mains to dilute the contaminants. This can be controlled by engine management systems such as our LIMA system.
Finally, for a smooth operation it is important to select the correct lubrication oil based on the results of the condition monitoring. It should have the lowest possible Total Base Number (TBN) that gives the best possible oil and component life. If siloxanes are a problem then the gas is less acidic and the engine needs a lower TBN oil, if acids are more of a problem then a higher TBN oil will be needed and when the TBN drops to a certain point the oil will need changing. The exact lubrication oil needed may well change over the lifetime of the installation so ongoing condition monitoring is important.
An efficient fallacy
Most people’s natural inclination is to specify the most efficient engine possible. For Biogas fuelled generator sets this is not necessarily the best option.
A more efficient engine has a higher Break Mean Effective Pressure (BMEP), which makes it run better, but makes it less tolerant of combustion chamber deposits. Higher compression ratios, cooler running temperatures and a leaner fuel mixture with higher free oxygen will also increase deposits. This puts more pressure on both the spark plugs and the oil.
Remember Biogas is not the same as natural gas.
Towards an answer
At the beginning of this article I outlined why burning biogas to generate electricity makes a lot of sense. It can save you money, help move your organisation towards carbon neutrality and even generate income by selling excess electricity to the grid.
But before going any further there is much to consider. Contamination of the fuel gas is a major issue. Is such a solution commercially viable for your organisation and what are the risks? Will your service partner provide commercial guarantees and will the generator set be able to cope?
Unfortunately there is no set answer. Making the most out of Biogas electricity generation is a continual balancing act. Your best approach is to seek professional advice from a supplier with relevant experience, a track record and long-term financial security.