Green Coal

• What is it?

It’s miscanthus grass (elephant grass) chopped up and compressed into coal sized briquettes, packed into 10kg plastic bags and ready for burning in a traditional wood stove, open fire or a chiminea.

• What is miscanthus grass?

A bit like bamboo.  It is grown in the UK for fuel or animal bedding and other uses.  It can be grown on poor quality land, requires very little treatment with fertilisers, doesn’t need planting as it is a perennial and can be harvested with normal equipment.  It is supplied locally by New Energy Farms¹ and the image below is a picture from their brochure:

• How do I store it and use it?

It needs to be dry as it absorbs moisture.  Keep it in its bags in a rainproof area or preferably inside a hut or garage.  Its bulk density is 500kg per cu. meter so it doesn’t take up a large amount space.   Use it a bit like logs or barbecue charcoal.  We have tested it on ordinary wood stoves and it lights easily and burns cleanly.  You control the rate of burning like logs.  It burnt a little faster than wood but seemed to give out plenty of heat.  The following morning there was a large pile of ash.  The stove reached a surface temperature of 210⁰C.

The thumbnail images below show the sequence of tests carried out.

Click on any of the images to start a slide show of the full set of images:

• How much does it cost and where do I get it from?

The retail price is expected to be £5 per 10kg bag and will obtainable from garden centres and DIY stores.  50 bag pallets are expected to be £150, delivered price, equivalent to £3 per 10kg bag.  These have to be ordered from, and delivered by fuel suppliers.

• How does it compare with logs?

The difficulty in comparing the two is that wood is bought by volume and is quite variable in type and quality so a lot will depend on the assumptions made.  The Forestry Commission and others² suggest that 1 cu. m of  loosely stacked logs without excess bark contains about 0.5 cu. m of solid wood.  Beech and spruce have very similar calorific value, approximately 4 kWh/kg.  Miscanthus has 5.1 kWh/kg.  Comparing what you would typically get in a bulk delivery of wood this works out at about 4 p/kWh for wood and 5.9 p/kWh for miscanthus.  However, the 4 p/kWh for logs has quite a variation depending on the weight and quality your log supplier delivers.  For one local supplier this figure is as high as 5.2 p/kWh.

• Any drawbacks?

One or two.

1. Miscanthus absorbs moisture, which is why it is used for animal bedding.  It will have to be kept in its plastic bags, or if kept loose, in a dry environment.
2. Green coal is not as tough as fossil coal so will have to be handled more carefully.  It was noticed that the bottom of the bag had already accumulated quite a bit of loose miscanthus.
3. The laboratory analysis of green coal shows that it produces 2.9% ash after burning.  Wood typically produces about 0.3% ash.  After the trial the wood stove had accumulated as much ash as 3 or 4 days of continuous log burning.  The ash, like wood, is suitable for spreading on gardens.

• So is it worth having?

It is a local green fuel and works well in a log stove.  It is clean to handle and easy to store.  It is ‘green’ in that the carbon is taken from the atmosphere and returned to it after burning.  Bought in small quantities in economic terms it is similar to buying wood logs, straw logs or kindling wood.  In bulk quantities for domestic use it is more costly than logs and probably won’t replace them.

• What of its future?

It is cheap to grow.  Current farm gate prices for dried miscanthus according to DEFRA are around £50 - 60 per tonne.  This would give about 1 p/kWh which is highly competitive.  Larger heating installations for schools, offices and warehouses using specially designed boilers with bulk handling equipment would likely save considerable sums on their heating bills.  Miscanthus fuel is also covered by the Renewable Heat Incentive³ where a tariff is paid for every kWh produced.  Coupled with a long term growing contract with a local farmer and, providing it is not grown on high grade farming land, miscanthus looks like it should have an excellent future for larger scale heating applications.

1. New Energy Farms

2. Wood Fuels Handbook published by AIEL, Italien Agriforestry Energy Commission:

http://nuke.biomasstradecentres.eu/Portals/0/D2.1.1 - WOOD FUELS HANDBOOK_BTC_EN.pdf

3. For the RHI expected deal:

 http://www.rhincentive.co.uk/eligible/levels

4. For comparative energy prices:

http://www.biomassenergycentre.org.uk/portal/page?_pageid=75,59188&_dad=portal