Straw as biofuel

Straw can be used for bedding, fodder, and for the production of bioheat. The use of straw as bioenergy has been increasing in recent decades. Many farmers have invested in straw heating plants to heat livestock rooms and residential buildings on the farm. Many people grow grain in combination with chicken and/or pig production. These productions, and especially chicken production, have a large heat requirement, and many have solved this by investing in straw heating plants.

Climate benefit

Straw is a renewable resource and CO₂ since it has a short turnover time from decomposition in the soil and incorporation into a new plant during next year's photosynthesis. Working it into the soil can help increase or at least maintain the carbon content in the field soil and it is not recommended to remove the straw every year.

It has been common to set the climate impact to zero when we burn biofuels. It is still debatable whether this is the case. Some (fossil) fuel is used for packaging and transport and this is estimated at 7 g CO₂ per kWh. A straw boiler will also be produced and this has a climate footprint. Another way to calculate the climate benefit is to look at which heat source is replaced. If it is propane with emissions of 230 g CO₂ per kWh it provides good climate benefits to replace it with bioenergy no matter how we calculate.

Energy

The energy potential for using straw for bioenergy is estimated at about 4.5 TWh. 1 TWh corresponds to 1 billion kilowatt hours, which is equivalent to the energy needs of 50,000 homes.

Straw to be used in a heating system must be dry when harvested and stored dry. At 15% moisture, the calorific value is 4 kWh/kg straw. Damp straw gives a lower calorific value and more smoke from the straw system. Smoke from the straw system can be a problem for the surroundings. The strongest smoke will be just after new straw bales are placed, before the heating has started.

The amount of straw varies depending on the type of grain grown and the stubble height at harvest. The use of straw shortening agents will also reduce the amount. The harvestable amount of straw will be between 60 -70 % of the grain crop.

In a straw system, the water stored above the boiler is heated when the straw burns. There are systems for loading several straw bales into the boiler at the same time. There are also systems with automatic straw feeding.

Other dry wood can also be burned in the plant, e.g. wood, cardboard, etc.

A high potassium content in straw leads to a certain amount of slag deposition in the boiler room. This must be removed over time to maintain good heat capacity.

If the straw is removed from the soil every year and used as fuel, it may eventually lead to a certain decrease in the content of organic matter in the soil. Organic matter is important for maintaining soil life and favorable soil structure. Therefore, it is recommended to plow down the straw every 3 years.

Task:

A farmer has a grain area of 250 acres, and has achieved an average yield of 475 kg of grain per acre. All the straw will be used in the straw facility built on the farm.

a) How dry should the straw be before it is put into storage?

b) How much energy can be expected to be obtained if all the straw is burned in the straw plant?