Biomass is a form of renewable energy which creates heat energy through the burning of wood, typically as wood pellets. It is a type of renewable energy which is considered to be carbon neutral. The carbon dioxide released during burning is only that which was absorbed by the plant during its life. Biomass heating has many benefits, not least for the environment. Biomass boilers and stoves can generate heat very efficiently if they are installed correctly and serviced regularly. A highly efficient boiler can provide a great return on investment through the government’s renewable heat incentive scheme (RHI). However, without regular servicing and maintenance, a biomass boiler may become less efficient. Our senior biomass engineer provides some valuable information on biomass boiler servicing.
What indicates that a biomass boiler needs servicing?
There are a variety of signals which may mean that your boiler needs servicing. The boiler may have a few or all of these indicators:
- Excessive visual levels of unburnt fuel within the ash deposit chamber
- Excessive visual levels of soot within the combustion chamber
- Excessive levels of clinker within the combustion chamber (clinker is solidified ash which has been taken past its melting point and allowed to cool)
- Excessive black smoke production during combustion
- Large yellow, wispy flames within the combustion chamber
- Excessive flue gas temperatures at exit point of boiler (indicates heat exchanger lining is coated with ash/debris and is acting as insulator, preventing correct heat exchange of flue gasses to water jacket)
- Adverse fluctuations in the O2 content of the flue gasses causing incomplete/inefficient combustion
- An engineer’s analysis of the flue gases may show incorrect combustion reflected in adverse readings of CO, CO2, NOx, SO2 and the CO-CO2 ratio results
Quality of biomass fuel
The quality of the fuel used can have a significant impact on the reliability and efficiency of a biomass boiler. The use of contaminated fuel material can lead to adverse flue gas emissions and reduce boiler efficiency, as well as causing long term damage to the internal boiler components. Earth, stones and other inert matter can seriously damage a wood fuel discharge system, particularly those with an automatic feed. Factors such as the method of wood fuel harvesting and the care of handling of the product will determine the proportion of contaminants present in the wood fuel.
Ash is the by-product of burning wood fuel and it can act as an insulator and so reduce the efficiency of the boiler. Typically the ash content for wood chips is between 0.7% and 10% of the oven dry weight of the fuel, depending on the classification of the fuel (i.e. A0.7, A1.5, A3.0, A6.0, and A10.0). De-barked wood for wood chip production reduces the ash content of the fuel.
The ash produced from burning woodchip fuel in a modern efficient system is typically between 0.7% and 3% of the oven dry weight of the wood fuel.
How ash affects biomass boilers
The main parts of the boiler that are affected by ash deposits are the combustion chamber, the heat exchanger and connected flue/chimney system surfaces.
Virtually all automated biomass boilers have an automatic heat exchanger cleaning mechanism. These mechanisms periodically clean a reasonable percentage of the overall surface of the heat exchanger to assist with the efficiency of the boiler. However, these components gather ash over time which restricts the flow of the flue gasses. This in turn reduces boiler efficiency and output.
Large commercial biomass boilers flue systems are fitted with cyclonic dirt scrubbers to remove fly ash and debris from the flue gases as they pass through the system. These scrubbers require cleaning to ensure efficient removal of the materials.
How often should a biomass boiler be serviced?
Different manufacturers give different recommendations. Typically there should be a minimum of one main service a year. Two services would be more effective and these do not necessarily need to be 6 months apart. It is more appropriate to service the plant proportionally with the actual operational hours of the boiler.
Large biomass boilers have a greater number of safety systems, motors, fans, cleaning components etc. and these items generally have a quarterly servicing regime.
Over and above the main services the boiler should be cleaned, lubricated and visually inspected on a regular basis.
What is the efficiency level of a biomass boiler before and after a service?
This depends on the level of boiler cleaning automation, customer servicing ability and fuel type and quality. Typically, the efficiency could be improved with servicing by between 7% and 15%, depending on the condition of the boiler.
Between services maintenance
The most important thing is continue to monitor fuel quality and consumption. The higher the quality of the fuel, the more efficiently the boiler operates. Burning the fuel at maximum efficiency results in less maintenance and cleaning. This leads to reduced maintenance and running costs and ensures the boiler maximises its operational lifetime.
Having a well serviced and maintained biomass boiler plant and associated wet heat distribution system will maximise RHI income. It makes sense to get a biomass boiler under a maintenance contract to be able to budget effectively.