Plant & Works Engineering

Heating in harmony

Published:  29 September, 2007

It has now become clear that concerns about the relationship between energy consumption and climate change are not going to be some passing fad; they are here to stay and need to be taken seriously. It's equally clear that those organisations that use the most energy will come under the most pressure -though they also have the most to gain because of the significant reductions in overheads that can be achieved.

Much of this energy consumption will result from the manufacturing process and there may not be a great deal that can be done to reduce that. However, one of the key areas to look at for making these savings is the heating plant. In any building, whatever its use, heating for hot water, steam or space heating will be making a significant contribution to the overall energy bills and carbon emissions.

There"s quite a lot that can be done by ensuring the heating plant is being maintained properly to optimise the performance that it can deliver. Its also important to ensure that the control strategy delivers the maximum energy savings. But there will come a time when new plant is the only answer not just for efficiency but also for the ongoing reliability of the heating systems.

At this stage there will be a number of factors that may influence the decisions about new plant, including the possibility of introducing renewable energy technologies such as biomass fuel and solar heating. In many cases, where renewables are selected, they may be combined with conventional technologies as well, such as high efficiency gas or oil condensing boilers.

In all cases, one factor that will undoubtedly exert an influence is the return on investment, so it will be important to select the mix of heating technologies that offers an acceptable payback.


Wood as a carbon-neutral fuel

One option that is proving increasingly popular is the use of wood burning boilers, which are now available from 10kW up to 1MW. Wood fuel is classified as carbon-neutral because the carbon dioxide emitted from burning them is equivalent to the CO2 that was taken from the atmosphere for photosynthesis when the plants were growing.

Contrary to what might be expected, they are also very easy to use. The latest wood pellet boilers, for example, include fully automated operation, function control and fuel dosage, using automatic pellet feeding systems. They are also able to switch on and off automatically or modulate to match the operation of the boilers to the heating demand.

Unlike early wood boilers, which had efficiencies of 50% or less and carbon monoxide emissions of around 20,000 mg/m3, modern wood boilers offer efficiencies of over 90% with carbon monoxide emissions down to 100 mg/m3.

These efficiencies, though, are very dependent on the type of wood fuel used and there are essentially three to choose from logs, wood chips and wood pellets.

While being a traditional form of wood fuel, logs have the disadvantage of being bulky to handle and requiring a great deal of storage space. They also vary greatly in combustion characteristics depending on the species of the tree and the water content of the wood. However, modern log-burning boilers that combine gasification with high temperature combustion can provide efficiencies of more than 90% with properly seasoned wood.

Water content is a very important consideration because the higher it is the more water has to be boiled off before the wood burns.

Wood chips, which are produced by chipping or shredding various sources of wood, can vary greatly in water content typically ranging from 25% to 55%. This lack of homogeneity means that consistent combustion with wood chips can only be achieved through the use of sophisticated boiler controls to handle the varying fuel properties; thus increasing the capital, installation and commissioning costs for the installation.

For these reasons, wood pellets have been selected as the optimum fuel for many biomass projects. They are manufactured from sawdust, shavings and finely reduced wood waste, some of which comes from further processing of wood chips. Compressing these materials into pellets releases lignin from the wood, binding the pellets firmly with no additional binding agents.

Pellets from accredited sources are also a consistent size as specified in international and British standards [DD CEN/TS 14961:2005]. Good fuel specification and quality control will result in a homogenous fuel that burns predictably with a consistent heat output and requires lower boiler attendance.

Where wood boilers are selected its also important to consider fuel delivery and storage. If coal or oil boilers are being replaced, then clearly the appropriate access for delivery vehicles will already be in place. If coal storage is available this can be readily converted for use with wood pellets, and oil tanks can be replaced with wood pellet silos. As a rule of thumb, each kilogram of wood pellets will provide 4.9kWh of heating and a cubic metre of storage space will accommodate 600 to 650kg of wood pellets.

As noted earlier, there may be potential for combining biomass boilers with solar heating. When solar heating is used for hot water it will, on sunny days, heat all of the water to the required temperature and when less solar energy is available it may be used to pre-heat mains cold water before it is brought up to temperature by another heat source. It is important to note that any pre-heated potable water storage volumes should be kept to a minimum, as these may have to be regularly pasteurised as part of the anti-Legionella regime.

In these circumstances it is beneficial to store the solar energy in a thermal storage vessel and pre-heat the cold feed water through a suitable heat exchanger. This also has the benefit of allowing the solar energy to be safely stored at higher temperatures without the risk of scalding while also saving on the energy that would be needed for pasteurising large volumes of potable water.

In other situations, a combination of these renewables with high efficiency condensing boilers may be an even better solution. Where there are high demands for hot water, for instance, the solar heating and biomass boilers may be used for hot water while the condensing boilers supply the space heating and take full advantage of the low return temperatures to achieve maximum efficiency.

The key thing is that every facility has different requirements for heat so the system needs to be designed and configured to meet those specific needs. It also needs to incorporate a control strategy that will make the optimum use of each of the technologies available whether its just condensing boilers or a mixture of fossil and renewable fuels.

Getting this balance right requires specialist expertise and experience, so it makes sense to team up with companies that have experience of all of these technologies and can work with you to find the best solution for your needs.

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