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Biomass continues to play a pivotal role in increasing the UK’s use of
renewable energy sources. Andy Owens, biomass technical sales
manager with Hoval, explains why it pays to look at all the angles
As the Carbon Trust increases its efforts to accelerate the uptake of biomass
as a renewable heat source, we are set to see a significant boost to what is
already a fast-growing sector of the heating industry. To that end, it’s
important that specifiers and installers of such systems ‘do their bit’ to ensure
that such projects are successful.
In this respect, there are several key factors to consider and these range from
the most appropriate choice of biomass fuel to the design of the fuel storage,
access for delivery vehicles and the duties of maintenance staff. Experience
shows that addressing all of these issues and liaising closely with the end
user is vital in achieving that success.
When problems occur in biomass projects, it may be because the specifier or
installer had little experience of biomass, or the end user may not have fully
understood the key differences between biomass boilers and the more
familiar gas and oil boilers.
In this respect, biomass projects differ from most other building services
installations because the specifier or installer needs to look beyond the basic
engineering design and consider the day-to-day practicalities of living with a
biomass system.
Nevertheless, the engineering is still important and the first stage is to
understand the range of heat loads through the year and to design as much
as possible for continual load with minimum cycling. For example, when there
is only a demand for domestic hot water (DHW), will it be worth firing the
biomass boiler at all during these periods or would it be more cost-effective to
include a small gas-fired boiler – or perhaps solar thermal – in the project?
And would a suitably sized buffer vessel help the operation of the boiler?
It’s also important to understand how different biomass boilers deal with
variable loads. Some may go into standby mode without going out, offering a
rapid response to a call for heat. Others may go out completely and use autoignition
to re-fire when required, offering a slightly slower response time.
In terms of fuel, the choice in the UK usually comes down to wood chips or
wood pellets, both of which have their place but offer different characteristics.
Wood chips, for instance, undergo less processing than pellets and tend to
show greater variation in combustion efficiency and physical size. Crucially,
the calorific value and the bulk density of chips is lower than pellets so will
require higher volumes of storage and/or more frequent deliveries than pellets
to deliver the same heating capacity. In addition, the variable moisture content
and size of chips may necessitate more complex, and therefore more
expensive, controls to maintain even combustion.
The irregular shape and size of chips means they need more robust handling
equipment with larger augers and motors, compared to pellets. In addition, the
distance from storage to boiler should be as short as possible to minimise
damage to the fuel during mechanical handling.
As the choice of fuel influences storage volumes and frequency of deliveries,
it also influences the type of delivery vehicle – the size of which will dictate the
required dimensions of access routes and turning circles.
Generally, chips would be delivered by a tipper lorry, necessitating belowground
storage and a mechanism (‘walking floor’) for moving the chips out of
the way as more flow into the space. A cyclone can be used to decelerate and
spread the pellets but will occupy some headroom and possibly reduce the
storage volume.
Pellets are usually delivered by a smaller ‘blower’ vehicle with less demanding
access requirements and the best results are achieved by using a vertical
storage silo as this maximises fuel recovery. The silo may be manufactured
from GRP (glass reinforced polyester) for outdoor use, or a fabric silo can be
assembled in-situ indoors. Because of their height, outdoor silos may require
planning permission.
All of these factors need to be balanced against the fact that chips cost less
than pellets so life cycle costing will also play a role in deciding on the most
appropriate choice.
Another contributor to life cycle costs is maintenance – for example, cheaper
wood chips tend to produce more ash than pellets or higher quality chips, so
this will increase maintenance requirements. This is important in terms of
explaining everything to the end client; the cheaper fuel may seem attractive
but they need to fully understand the implications.
In addition, the people carrying out the maintenance should be made aware of
how the maintenance of biomass boilers differs from that of other boilers.
Once the installation is up and running the acceptance of the people who are
dealing with the boilers will be critical to the end user’s perception of the
system. In our experience, early engagement with key staff at the client end,
combined with regular contact even after handover is plays a major role in a
successful project.
Clearly, then, there is no such thing as a standard biomass project and each
needs to be carefully evaluated in the light of the factors that have been
discussed here. Equally, for those who are less experienced in biomass
installations, it makes sense to call on specialist expertise. |
