Cereals: an alternative energy source
by G Lund, CADDET Danish National Team

The use of cereal crops as a source of energy is an important part of Denmark's long-term energy strategy. A four-year research programme is investigating the technical challenges that energy crops present.

Loading of energy crop bales for transport to the Grenaa CHP plant.

Introduction

One of the goals of the Danish energy plan is a 20% reduction in CO2 emissions by the year 2005, compared to the 1988 level. The utilisation of biomass resources, such as straw, liquid manure, wood-chips and wood, as well as organic waste from domestic households and industry, is important in achieving this target. Biomass consumption is expected to increase from 7% of Denmark's total gross energy consumption in 1996 to 11% in 2005 and 21% in 2030. The Danish policy is to exploit the available biomass residues first, before initiating major exploitation of energy crops.

Implementation of energy crops is not planned until 2005; after that, their input is expected to increase gradually to 45 PJ/year from 200,000­300,000 ha in 2030. In order to meet this target, a four-year research and demonstration programme on energy crops, financed by the Danish Energy Agency and the Ministry of Food, Agriculture and Fisheries, started in 1997.

Cereals as an energy source

The term "energy crops" is used here to describe biomass grown for energy production, as opposed to biomass that is a by-product of food production (such as straw) and timber production. Examples of annual energy crops may be cereal grain or rape, or cereal grain/rape plus straw (whole crops) harvested together. Burning of cereal grain in boiler plants larger than 250 kW is prohibited in Denmark.

One of the current research activities is to investigate if cereals and whole-crop triticale (a cross between wheat and rye) could be an alternative energy source to fossil fuels. Part of this programme is focusing on:

  • selection of high yield types of grain with improved combustion characteristics;
  • comprehensive fuel and ash analyses;
  • combustion tests in laboratories.

Annual crops such as wheat, rye or triticale, cultivated as an alternative energy source, do not require high investment by the farmer and are easy to rotate in the crop cycle. However, the identification of cereal species and varieties with high biomass yield, high combustibility, low ash content and low potential for boiler corrosion, is a priority. Cultivation, fertilisation and harvesting techniques are essential to ensure optimal use of resources.

Full-scale tests

Generally, food crops grown in larger quantities have not been considered desirable as a fuel in the Danish energy supply system. However, the set-aside land scheme has changed this view. According to the EU set-aside scheme, farmers are obliged to take some of their land out of food production each year. This has sparked interest in growing energy crops on fallow fields. In 1994, the Danish Ministry of Energy and Environment granted exemptions allowing some farmers to grow wheat, rye and triticale as energy crops on 500 ha at five different locations in Denmark. In 1995, the total harvest amounted to 5,100 tonnes.

Fuel handling

The potential for biomass production of eight varieties of three annual cereals ­ winterw heat, winter rye, and triticale ­ has been tested in field trials. The results show that rye and triticale can be recommended as energy crops, giving high yields (10.3 tonnes/ha of dry matter in the two years of test harvesting) with the minimum use of pesticides and fertilisers.

Methods of handling large straw bales are well known in Denmark and have been used for about
20 years. For large energy crop bales, the same handling, storage and transport procedures can be followed as for large straw bales. A survey in 1995 revealed some technical difficulties in the handling of whole crops to produce bales of sufficiently high quality with regard to shape and stability, because of stiff stalks. Baling quality can be improved by the use of swath treatment; however, this can also increase the field losses of grain in wet years, and carries a risk that energy crops left in swaths will sprout.

The heating plants that tested the combustion of the energy crops had problems in lifting heavy whole crop bales (600­700 kg). Well-shaped, compact bales of a suitable length and weight are necessary for the crane to operate satisfactorily.

 

Emissions from Holeby.

Combustion tests

Eight energy plants participated in the combustion tests, including four district heating plants (Holeby, Tullebølle, Lohals and Haunstrup) where tests were carried out 24 hours a day for one week. In order to make a comparison with straw burning, a one-day test was carried out, during which efficiency and emissions were measured.

Like straw, whole crops can be used as fuel in utility plants and co-combusted with coal in circulating fluidised bed (CFB) plants. Only marginal deviations in the combustion properties were found but, in general, whole crops can be considered as a fuel with similar qualities to straw. Rye and triticale are better than wheat because they have lower grain losses and require less fertilisers, pesticides and insecticides.

Emissions and efficiencies were more or less the same for energy crops as for wheat straw.
However, a marked rise in the NOx emission (measured as NO) could be seen at a few of the plants, since the protein content of the grain releases nitrogen during burning.

Other factors that need to be considered are the high alkali content of the fuel, which can lead to corrosion in the boilers, and the high content of heavy metals in the ashes, which mean they cannot be spread on farmland. When co-combusted with coal, the ashes cannot be used in the cement industry because of their high potassium content.

Combustion tests in 1995

 Plant

 MW heat

 MW electricity

 Plant type

 Fuel

 Quantity in tonnes

 Holeby

 3.1

 0

 District heating
Grate-fired

 Wheat and triticale in large bales

 394

 Tullebølle

 1.6

 0

 District heating
Whole bales

Triticale in large bales

 169

 Lohals

 1.4

 0

 District heating
Sliced bales

 Triticale in large bales

 51

 Haunstrup

 0.5

 0

 District heating
Stoker

 Rye grains

 222

 Rudkøbing

 7.0

 2.3

 Steam
Grate-fired

 Triticale in large bales

 380

 Studstrup 1
power station

 0

 150.0

 Steam
Powder burner

  Triticale in large bales

 1,100

 Grenaa CFB-boiler

 60.0

 18.6

 Steam
Fluidised bed boiler

  Triticale in large bales

 2,000

 Østkraft, Bornholm

 35.0

 16.0

 Steam
Spreader stoker

 Wheat grains

 781

Economics and the environment

Considerable interest has been shown by the Danish Farmers' Association, but when it comes to hard economic realities, there is a huge gap between the farmers' need for profitable production and the market prices anticipated in the energy strategies.

Producing and using biomass requires other inputs such as diesel oil, nitrogen fertiliser and pesticides which, depending on the type, dosage and utilisation, have a negative impact on the environment compared to that of fallow fields.

However, estimates of Danish biomass resources in the next 30 years show that energy crops have the highest potential, so they form a major element in the long-term Danish energy strategy. The challenge for the research programme is to find the most cost-effective and least damaging way of using that resource.

For more information contact the CADDET Danish National Team in Tølløse.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.