Farm Diversification

When considering wood fuel there are two distinct scales of operation; producing wood fuel for sale or providing wood fuel heating for the farm and associated businesses such as holiday cottage. The main market driver has been the rising cost of heating oil which has made alternative fuels such as wood more attractive with considerable cost savings possible. Unlike renewable electricity there are no direct subsidies for production with government support mainly focused on capital grants for timber processing equipment and heating systems.

How much energy in a tonne of biomass?
Various types of biomass have been considered as potential fuels, to replace fossil fuels for the production of heat and power.  These include woodchips, wood pellets, straw and grain.  To date there are few, if any options for producing power from any of these materials but heat production is becoming widespread.  The approximate energy content of biomass is around 17 – 18 MJ per kg of dry matter.

The problem is often that the moisture content of the material is high so energy is used to evaporate the water, rather than emitted for useful purposes.  Some examples of typical moisture contents, energy contents and the equivalent amounts of heating oil are shown below:

The values in table 1 refer to the energy in the fuel there will be some inefficiency resulting in waste heat during burning, depending on the type of burner: modern condensing oil boilers are about 90% efficient, wood burners around 60 – 80%.

Wood logs for heating
Log burners are increasingly popular especially in rural domestic properties.  They are much more efficient at providing heat than an open fire and have the benefit that they provide direct radiant heat to those nearby.  Stoves are ideal in conjunction with other renewable heating options, which provide background heating of the home.  Connecting log burning stoves into a central heating system, to work in with an existing gas or oil fired boiler, is possible but not entirely straight-forward. 

The main downside to log burners is that most are completely manually fed.  Some more sophisticated systems will automatically feed logs into the boiler but these are most suited to sites that have a constant heating demand.  For easier automation woodchips, or pellets, are preferable.

Woodchips
What is the normal moisture content of woodchips?
The moisture content of freshly felled timber can be around 50% - 80% moisture content.  If the timber is left to dry before chipping it will reduce, getting down to about 30 – 35% moisture.  At this level it is best to chip it, as leaving it to dry more will mean that extra power will be needed by the chipper.  Reducing the moisture content of the chips further is desirable as the fuel will have more energy when burned and the chips will store better, creating fewer hazardous moulds and fungal spores.  However artificial drying will be expensive so the market must be able to withstand the extra cost for the higher quality product. 
Remember, as woodchips loose water from say 30% to 15%, so their price per wet tonne must increase, before any drying costs are added.

What is the current cost of woodchips?
The current cost of woodchips is around £60 per tonne but this is highly variable, depending on the location and source of the wood.  At this price woodchip fuel costs about 1.94 p/kWh of energy contained in the woodchips.

What are the options for drying woodchips?
Proprietary driers do exist that are modified continuous flow grain driers.  These dry at high temperature using fossil fuels.  As there is much more water to remove from chips than grain this is an expensive operation that will only be justified in markets requiring the highest quality.  High temperature barrel driers are also used in Europe but only suit large installations.
More normally the chips may be aerated in storage to gradually dry, but even with no added heat the cost of the power for the ventilation fans needs to be monitored to ensure that the process is cost effective.

How much storage space will be required for woodchip fuel?
Woodchips are a low density material.  Storage will require about three to four m3 per tonne.

What chippers are available?
Compared with equivalent small-scale wood log producing machinery, wood chippers may be more than three times the capital cost. Therefore, unless the intention is to produce large quantities or to supply a large woodchip burner, purchase may not be recommended.  The alternative is to get a contractor to chip a batch of timber periodically.

What systems are needed to burn chips and are they viable?
Woodchips would normally be tipped into a store that is equipped with an automated loading system that feeds chips to the burner.  As chips do not flow well they need to be forcibly conveyed using powered augers, to ensure the correct flow rate is maintained.   Clearly precautions need to be taken to prevent the possibility of fire passing back along the feed system to the store.  The burner will typically take more space than an equivalent oil boiler and often needs extra space around it for access and servicing.
In situations where continuous heating is required a backup system will be needed to cover periods of servicing or breakdown of the burner.  This will increase the cost of the setup compared to oil or gas fired systems.  In some less critical situation for example some domestic heating set-ups this duplication may not be required.
Wood fuel burners need special attention to the chimney which will need to be longer than an equivalent conventional boiler and well insulated.  Most burners can cope with chips at 30% moisture content but some need drier fuel as the condensate causes them problems.
Fuel hopper size or adjacent fuel storage capacity is a point of consideration. This will also be influenced by the likely frequency of fuel deliveries, and haulage on-costs for small deliveries.  As a rough guide a small domestic (20 kW) system could use about 0.6 m3 of chips per day in the heating season.

Why are pellets preferable to wood chips?
Pellets are made using a process similar to the production of animal feed pellets.  Nothing is added to bind the material the wood is just compressed into a pellet of about 25 mm length and about 5 – 6 mm diameter.  Wood pellets are often made from compressed sawdust from dried wood so are considerably drier than woodchips.  Energy may still be needed to dry the material down to 10% moisture content and to compress the chips.  This means that they will be more expensive but the product is easier to store and handle.  The bulk density is higher so the storage space is reduced.  The energy output is increased and as the pellets flow better the product is much easier to transfer from store to burner. In some cases the pellets are blown into store via a long pipe from a tanker, meaning the store can be sited almost anywhere.  For these reasons pellets are normally preferred for domestic situations however in cost terms may not prove any cheaper as a fuel source than oil with added boiler costs.

Woodchippers

One well set-up chipper machine can supply a large number of small-scale woodchip heating installations at a capital cost of between £14,000 and £20,000 (source Ecodyfi). Costs of wood chip production range from £1.70 to £3.00/m3 depending on machinery output and technology. Because bulk fuel storage facilities will probably be limited at small-scale woodchip fuel installations, a centralised bulk storage facility will be necessary if the chipper machinery is to be fully utilised.  For efficiency and competitive supply costs, the above considerations should be combined at one, or a limited number of locations.

Chippers that are too small and not powerful enough make chipping very labour intensive and may affect the long term viability of the process.

Woodchip boilers
Estimated costs for wood chip boilers range from £3,700 for a 20kW system to £22,000 for a 300kW system (source Ecodyfi). Most small-scale woodchip appliances are of foreign manufacture and prices have tended to fluctuate, particularly with exchange rates.
Hot water storage, (accumulators), may cost from £300 to £700 dependant on size. A rough guide to the required size, in litres, is about 40 to 60 times the boiler heat output in kW.
Recent quotes for wood chip heating schemes for domestic properties have been in the range £20,000- £25,000.  Grant assistance will help with this to some extent but it is clear that compared to an equivalent fossil fuel system wood fuel system for domestic applications can be considerably more expensive. The viability of wood fuel heating is also dependent on a ready supply of wood fuel at a considerably lower cost than the equivalent heating oil.

What is the cost of heating with pellets?
The cost of a tonne of pellets is about £140 - £160 depending on whether it is supplied in bulk or bags.  At £140/tonne this makes pellets about 3 p/kWh of energy contained in the pellets.

Can electricity be produced from biomass, or just heat?
In most small scale situations it is not practical to produce electrical power from biomass.  In most cases the wood is burnt and the heat used to create steam which drives a turbine connected to a generator.  The problem is that these systems are complex and expensive.  For this reason generating power is only worth considering when there is a large demand for both heat and power.  As twice as much heat will be produced than power a high heating requirement is most important.  There are quite a large number of systems already installed, mostly in schools and municipal buildings, but there are still some questions with reliability and cost effectiveness.  One of the providers of this technology is Talbott Ltd, but there are some others.  Talbott’s have a system installed at Harper Adams agricultural college.  At the last report the heat was not being used from this system.  An example of this technology is shown in Figure 1, generating 100 kW of power and producing 200 kW of heat.  The cost of the unit is around £435 000 but the installation and connection costs are additional.These costs should be taken as approximate only and would best be used for comparative purposes. Most small-scale woodchip appliances are of foreign manufacture and prices have tended to fluctuate, particularly with exchange rates.

Compared with equivalent small-scale woodlog producing machinery, wood chippers may be more than three times the capital cost. Therefore, unless the intention is to produce in bulk or to supply a large woodchip burner, purchase may not be recommended.

Machine design falls into three types. In all types wood is cut across the grain. Two types chip the wood by the action of a set of knives, set round the surface of either a disc or a drum.The third has a continuous knife formed in the shape of a spiral cone, (ie. a"screw" chipper). All cut the wood by trapping it between the knife,and an anvil, set to give a small working clearance for knife rotation. Cut chips are ejected forcibly out of a directable spout. Wood feed is either by hand or by a hydraulic loader. Drum and disc chippers have hydraulic powered feeding rollers but screw chippers usually do not, depending on the coarseness of screw thread, ('pitch') for feeding rate, and therefore chip size. Different screw pitches are available. The rotation rate of any hydraulically powered feed rollers is usually adjustable. Given a constant chipper drum/disc speed, feed speed adjustment alters the average size of the chip produced.

One well set-up chipper machine can supply a large number of small-scale woodchip heating installations. Table 3 gives an idea of the outputs and costs involved. Because bulk fuel storage facilities will probably be limited at small-scale woodchip fuel installations, a centralised bulk storage facility will be necessary if the chipper machinery is to be fully utilised. For efficiency and competitive supply costs, the above considerations should be combined at one, or a limited number of locations.

Some typical tested woodchipmachines, outputs, and costs

Generally the 'cost payback' term will depend on the initial capital cost of installation and the amount of annual usage the system gets. To give some idea of chip fuel economy a Swedish appliance manufacturer states that 12 m3 of (fresh)chips can give similar warmth as 1m3of oil fuel, (1,000 litres). Approximately 2.5m3 of woodchip may be produced from 1m3 of round wood, (Approximately equivalent to 1tonne of Greenwood). Calculations show that wood fuelled systems can break even with alternative systems in approx. six to nine years, at about half expected equipment life.

Source: http://www.ecodyfi.org.uk/energy/DyfiEcoWdFfinl.htm

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The Scottish Rural Development Programme (SRDP) offers potential funding for farmers investing in renewable energy production including grants on a proportion of capital costs for wind turbines, hydro schemes, anaerobic digesters, biomass heating schemes etc. Rural Development Contracts - Rural Priorities (RDC-RP) include a theme on adaptations to mitigate climate change and may provide opportunities for further support.Through the Scottish Rural Development Programme (SRDP) 2007-2013 funding is now available for rural businesses throughout Scotland for diversification and renewable energy projects.

For further information about what specific support is available see the section on Rural Priorities - Renewable Energy from Agriculture and Rural Priorities - Renewable Energy from Forestry and Rural Priorities - Renewable Energy - Non Land Based

The launch of the Renewable Heat Incentive means that grants are not available for the installation for the installation of wood-fired heating systems.  It is not possible to receive a grant and claim the RHI.  However, grants are available for investments needed to establish woodfuel supply operations through the Scotland Rural Development Programme.