Straw for Energy

Straw for Energy (PDF)

Introduction

It is accepted that straw is not a prime energy source that will solve the world’s energy problems. However, given that straw is a by-product integrated with food production, its potential for use as a sustainable, renewable, alternative fuel source is noteworthy. In addition, increasing straw utilisation will have knock-on benefits to the economy, rural employment and farm income levels. Ireland’s area under cereals amounts to almost 270,000 hectares and yields approximately one million tonnes of straw. The traditional markets for straw include animal bedding, animal feed and chopping and plough back to increase soil carbon content.

Straw has been used in other EU countries for decades as a combustion fuel for both heat and electricity production. The Irish government and the EU have made clear their wish to promote the use of increasing amounts of biomass for the production of energy. The value of straw like any resource depends on demand and availability. Traditionally straw prices have been very volatile, but fixed price contracts are likely to be more attractive to growers now than they have been in the past.

Straw use

Straw can enter any of the following markets:

• animal feed (barley);
• mushroom compost (wheat); animal bedding (oaten, barley, wheaten); and,
• chopping to return to soil (mainly on winter crops).

Straw for energy

For the use of straw as fuel, its use in boilers might possibly lead to corrosion problems on heat exchange surfaces. There are a number of boiler manufacturers who will apply their boiler warranty to straw. Straw is a CO₂ neutral fuel and that is the reason why it should be promoted in the energy supply chain. Straw has been used as an energy fuel in Denmark since the early seventies.

Boilers

There are a number of boilers developed which can handle the more complex chemistry of straw for combustion. Batchfired boilers are available equipped with combustion air fans to control the air supply. Boiler plants for straw can be used for on-farm heat generation and for district heating. Straw can be presented in chaffed, pellet or whole-bale form depending on the boiler technology.

Competing uses

Of the one million tonnes of straw produced in Ireland annually, approximately 60% of this theoretical yield is used on farms as bedding and feed. This figure would have been higher in the past but has reduced due to improvements in farm waste management practices. The next largest user of straw in Ireland is the mushroom composting industry, which uses wheaten straw to produce new mushroom compost. Mushroom compost is manufactured from wheaten straw and poultry manure, with the addition of water and gypsum (calcium sulphate). Based on the mushroom composition percentages and the amount of compost used in 2011, it is estimated that the mushroom compost industry uses around 85,000 tonnes of wheaten straw produced in
Ireland annually (8.5% of the total theoretical straw resource). Given that the present day combined estimate of straw used for animal husbandry and mushroom composting is around 70% of the theoretical resource of one million tonnes, it is considered reasonable that 25-
30% of the theoretical resource (250,000-300,000 tonnes) would be available for energy purposes.

Straw harvesting

Winter crops of barley, wheat, oats and oilseed rape become available from July to August. The spring-sown crops become available in late August to end of September. Straw is normally baled in round or square bales and transported from the field to storage or directly off farm. Straw may also be recycled to the soil to improve soil structure and increase soil organic matter content.

The 8x4x4 (square) 450kg straw bales used for fuel purposes usually contain 14-20% moisture that vaporises during burning. The remaining dry matter consists of less than 50% carbon, 6% hydrogen, 42% oxygen, and small amounts of nitrogen, sulphur, silicon and other minerals, e.g., alkali (sodium and potassium) and chloride.

This fact sheet was produced by Barry Caslin, Teagasc, Rural Economy Development Programme.