A guide to Biomass
Boilers for Educational
Constant Energy Ltd
Office: +44 (0)161 813-1760
HOW DO MODERN BIOMASS BOILERS WORK?
WHY INSTALL A BIOMASS BOILER?
WHICH FUELS SHOULD BE USED?
FUEL DELIVERIES AND ACCESS
BIOMASS CABIN SCHEMATICS
Biomass is a renewable energy source that is obtained from living or once living material including
wood, grass, wheat or suitable waste materials. Biomass is mostly used in the production of heat and
the generation of electricity. When burnt, biomass sources release CO2, equivalent to the amount that
a plant would release when it dies and decomposes.
Hence, the carbon released has a limited effect on the environment as it is a part of the natural
carbon cycle. This is unlike fossil fuels which continually increase the carbon levels in the environment
when burnt. The history of biomass heating is mainly based on basic biomass log stoves which were
very high maintenance and not very energy efficient for the amount of warmth that they generated.
Technological advancements have seen major improvements to these biomass devices which are now
capable of operating at efficiency levels of over 95% which is comparable to modern oil-fired and gasfired boilers.
If oil is currently used to fire a central heating boiler, biomass is the closest alternative renewable
technology as the appliances operate in a very similar way but with lower CO2 emissions and residues.
Biomass for heating can take many forms but the main ones are likely to be wood chip or wood pellet.
People have been producing energy from biomass for centuries, and in many parts of the world it is
still the principle source of heat. However modern technologies are far more efficient and cleaner
than the traditional open fire and there are an increasing number of fuels are being used namely logs,
wood chips, wood pellets and wood briquettes.
Energy from biomass refers to energy produced from organic matter of recent origin. This excludes
fossil fuels which have taken millions of years to evolve. Biomass is also referred to as ‘Bio-energy’ or
‘bio fuels’ (in terms of renewable energy).
‘The Definition of Biomass’
“Any solid, liquid or gaseous fuels produced from organic materials either directly
from plants or indirectly from industrial, commercial, domestic or agricultural wastes”
However, we are only concerned with woody biomass. Producing energy from wood brings
environmental and economic advantages both nationally and locally and has considerable potential
within the UK. Biomass heating systems, unlike other renewable energy sources, do emit carbon
dioxide. However, it is the carbon dioxide (CO2) taken from the atmosphere by trees for
photosynthesis that is released during burning. This closed CO2 cycle means that biomass heating is
considered a renewable energy source, and therefore eligible for the Renewable Heat Incentive (RHI).
For sustainably managed woodland, or energy crops, the process is similar. Wood is never removed
faster than it is added by new growth, therefore the CO2 released when the wood fuel is burned is
never more than the CO2 absorbed by new tree growth. Biomass heating systems in reality create
small net emissions of CO2 to the atmosphere through operations including harvesting, transport,
processing and the construction and commissioning of the boiler. Wood fuel emits 25 grams of CO2
per kilowatt-hour (g/kWh); Wind Energy emits 8g/kWh; gas emits 194g/kWh; oil emits 265g/kWh and
coal emits 291g/kWh.
THE CARBON CYCLE
When grown sustainably, wood pellets are a carbon neutral energy source .
When trees grow, they absorb atmospheric carbon dioxide (CO2). During the burning of wood, carbon
dioxide which was initially absorbed, is release back in to the atmosphere. In essence, this provides no
overall increase in the level of atmospheric CO2. If the tree was to die naturally, it would still break
down and thus eventually release the same amount of CO2 back into the atmosphere. The burning of
wood pellets just speeds up this process. This process is often referred to as the Carbon Cycle.
HOW DO MODERN BIOMASS BOILERS WORK?
In the case of wood chip and wood pellet boilers, the fuel is fed automatically into the boiler from the
fuel store using an auger screw system (see below). The fuel then burns in the combustion chamber,
where a regulated flow of oxygen ensures a clean and efficient combustion process. The resulting hot
gases then heat water in a heat exchanger which feeds the hot water storage tank, or for smaller
stoves a back boiler, and ultimately the heating circuit (radiators in most cases).
Biomass boilers are as controllable as modern gas condensing systems; heating controls allow the
user to adjust all personal parameters for the central heating and domestic hot water. Some controls
even allow the user to switch on the boiler remotely via text messaging.
Basic operation of a wood pellet and wood chip boiler
HOW DO MODERN BIOMASS BOILERS WORK?
It is important to realise that biomass boilers are very different from their fossil fuel counterparts.
They prefer to run at their maximum capacity, primarily as a result of limited ability to react to spikes
in demand. Most installations are optimised to provide around 50-60% of peak load in order to
maximise plant running hours, minimise the capital outlay and increase both the efficiency of the
boiler and carbon reductions. This would then be teamed with either an accumulator tank or a back
up supplied from fossil fuel sources. Hence, a school currently heated with a 300kW oil boiler will
probably opt for a biomass boiler in the range of 150 -180kW; the actual size will also depend on the
age and efficiency of the current boiler.
A Biomass Boiler housed in an Energy Cabin
Many biomass boiler systems are now specified with an accumulator tank (a large hot water storage
tank) which is integrated into the heating system. It collects and stores heat energy from the system
to allow its flexible use at all times and to smooth out daily demand profiles. The boiler works flat out
to heat the water contained in the tank until it reaches 75-80OC. Once this temperature is achieved
the boiler reverts to slumber mode and only starts up again when the water in the accumulator falls
below a certain threshold (perhaps 70OC). In this way, repeated starts and shut-downs which can lead
to inefficiencies and running problems are substantially reduced, increasing the boiler’s lifetime and
minimising emissions. Tanks are normally sized in litres corresponding to 20-30 times the boiler rated
WHY INSTALL A BIOMASS BOILER?
Many schools, colleges, universities and other educational organisations could make savings on their
heating fuel costs by replacing their current fossil fuelled system with a biomass boiler fuelled with
wood. As mentioned, as long as wood is obtained from sustainable sources it is extremely ‘carbon
lean’ leading to large savings in carbon dioxide emissions. In addition, housing a boiler at an
educational establishment provides the opportunity for wider curriculum benefits giving a practical
demonstration of a low carbon technology.
It is also important to think about the age of the current fossil fuel boiler and whether it is due for
replacement. It is easier to make the financial case for replacing a 15 year old boiler than a 5 year old
boiler. If older boilers are still in good condition they can often be retained to back up a new biomass
boiler to meet peak loads.
Where schools have swimming pools or are located adjacent to leisure centres the cost efficiency
can be improved due to the significant demand for heating and hot water provision throughout the
year. For instance, a school without a swimming pool might have a capacity factor of 16% (i.e. 1,400
full load hours) whilst one with a swimming pool might have a capacity factor of 30% (2,600 hours).
In recent years many schools have been designed to be ‘learning centres’, with the intention that
they are used by the wider community in the evenings and at weekends for adult education and
other purposes. This increase in operational time makes Biomass Boilers an ideal solution for power
& heat production.
Whilst biomass requires more maintenance than fossil fuelled heating systems the activities are not
particularly onerous. It is important that each school has a caretaker or other member of staff who
can take on the regular inspections, routine maintenance checks and activities such as de-ashing.
With highly automated systems the day to day activities required should be minimal. For instance
some machines have in built ash bins on wheels. The efficiency is such that they only need to be deashed every four weeks.
The boiler room should, where possible, be accessible for educational purposes as it can provide an
interesting demonstration of renewable energy at work as well providing a real life installation on
which students could base mathematics, sustainability and climate change projects. The Queen
Elizabeth School in Wimborne, Dorset is building a sustainability centre to showcase its biomass
boiler and other on site renewable energy technologies. The boiler installation will include a perspex
‘see through’ to allow students to see the woodchip moving through the auger.
WHICH FUEL SHOULD BE USED?
Wood pellets are consistent in size and have low moisture content whereas woodchip can come in a
range of specifications. Most modern biomass boilers that are specified for woodchip can also burn
wood pellets, although the reverse is not true. In an ideal world most large schools should use wood
chip as it is by far the cheapest fuel and there are likely to be local suppliers. However, it is up to three
times as bulky as pellets and therefore requires either a large fuel storage facility or more frequent
deliveries. Our advice is to use wood pellets for practical storage reasons.
A Typical pellet fuel delivery
Fuel Deliveries & Access
For fuel deliveries, good access to the fuel store is required and the impact of any noise or other
disturbance on the surrounding area should be considered. There are a number of storage methods
available and the option chosen depends largely on the quantity of fuel required, the space available
and the delivery vehicle used by your fuel supplier. Where space is an issue subterranean fuel stores
may be specified. This involves using standard tipping vehicles to deliver woodchip into pits excavated
under ground level. This kind of storage provides more flexibility but will cost more to install unless
there is an existing height difference to work with.
Above ground fuel stores include permanent containerised fuel store or pre-filled hook-bin
deliveries. Using a containerised fuel store would require fuel to be blown in using
specialised equipment. Pellets flow more easily than chips and are frequently delivered by
the blown method - as above.
Biomass boilers have a significantly larger footprint than their fossil fuel equivalents and allowance
should therefore be factored in. Firstly, the size of a biomass boiler itself is about double that of a gas
or oil system.
Secondly, wood fuel has a much lower energy density than fossil fuels so much higher volumes of
storage are needed. However, these problems have been addressed with the construction of
containerised 20ft or 40ft Energy Cabins. The Energy Cabins house both the biomass boiler and the
fuel store all in one complete unit. They are constructed off-site and installed with the minimum
interruption to the educational establishment, as any grounds works are completed prior to the
delivery, installation and commissioning of the biomass boiler housed in the Energy Cabin.
A 40ft Energy Cabin with fuel store
Of course a school currently with mains gas heating will have no need to store fuel in the first place so
this would also need to be factored in. Also, in most situations, the existing fossil fuel boiler will be
retained as back up. Even if there is room, installing the system into the existing boiler house could be
problematic as the boilers are not always modular and can rarely be taken apart.
ENERGY CABIN SCHEMATICS
A single 20ft Energy Cabin
OUR INSTALLATION PARTNER
Our installation partner are plumbing and heating engineers that have been established for over 60 years and
have been chosen due to their longstanding experience in the heating sector. They provide heating services for
home and industrial purposes with a strong emphasis on wood burning heating systems.
‘Our partner is Gas Safe, Oftec and Hetas registered, so all modifications and improvements to the existing
system can be done internally.’
‘Our partner consists of building services engineers which provide full biomass system packages including
installation and servicing.’
Our installation partner has undertaken a number of biomass projects recently including;
• Q Hotels (Norton Park – 200kW – Biomass Energy Cabin),
• Cosmopolitan housing (Liverpool – Twin 80kw Pellet Boilers to heat a social housing project),
• Mersey Forest Trust (50kW log boiler and full central heating system) ,
• Peak Oak (Onecote – 60kW log boiler to heat farm buildings and showroom),
• Galliford Try (Warrington – 50kW pellet boiler for a block of flats).
Constant Energy Limited provides a complete and expert turnkey service in the commissioning, design,
finance, installation, management and maintenance of biomass boilers, to the public and private
sectors. We bring together commercial experience, expertise and a unique funding model to biomass
boiler installations, a proven, clean technology fuel, which has been used successfully in the Nordic
countries for many years.
Constant Energy Limitedʼs project management includes the full design of your biomass boiler system,
supply and installation of the boiler(s), buffer vessel, fuel store, controls and heat meters, flue and
connection to your existing hot water flow return system. Constant Energy Limitedʼs also includes
ongoing monitoring and maintenance of the system, and fuel supply under our long term fuel supply
We also finance packages that are tailored to our clients circumstances, be it 100% financing or our
truly Free to Fit model, backed by our investment partners. This enables us to deliver to our clients a
structured and immediate cash flow saving on the biomass boiler system and clients will also benefit
from reduced fuel costs.
Our service takes care of every aspect of biomass boilers from initial consultation through design,
installation, commissioning, ongoing operation and fuelling. Our state of the art systems will monitor
the day to day operation of the system, and will even tell us when fuel levels need replenishing. Our
aim is to make saving money with biomass very straightforward.
Constant Energy Limited only works with experienced market leading partners and sub-contractors
who have completed many biomass boiler installations throughout the UK, including;
Q Hotels - Norton Park Hotel. 200kW Biomass energy cabin.
Cosmopolitan Housing - Liverpool. Twin 80kW pellet boilers to heat a social housing project.
Mersey Forest Trust -‐ 50kW Log boiler and full central heating system.
Constant Energy Ltd, Magnetic House, Media Village, Salford Quays, Manchester, M50 3XW
Office: +44 (0)161 813 1760