Global agriculture produces millions of tons of residues every year, mainly in the form of straw. DP is the world leader in straw-firing, having built over 40 straw power plants in Europe and China. The straw fired boiler consists of a complete range of specially designed components including the water cooled vibrating grate which is defined to handle different types of straw like wheat, corn and rice straw.
Straw is an agricultural crop by-product, made up of the dry stalks of cereals and legumes after removal of the grain and chaff. Typical straw crops are wheat, oats, barley, rice, rye, millet and other varieties of grain. Normally straw residue is about half of the crop yield, making straw residue one of the most abundant agricultural residues available. Straw can be more of the more woody variety if the definition of straw is extended to include other plant residues. Irrespective of straw type, DP is a pioneer and acknowledged straw firing expert, with experience in combustion of all straw types and over 60 different types of fuel.
| Factor | Yellow straw | Grey straw |
|---|---|---|
| Water content | 10-20% | 10-20% |
| Ash | 4% | 3% |
| Carbon | 42% | 43% |
| Hydrogen | 5% | 5% |
| Oxygen | 37% | 38% |
| Chloride | 0.75% | 0.20% |
| Nitrogen | 0.35% | 0.41% |
| Sulphur | 0.16% | 0.13% |
| Heating value | 14.4 MJ/kg | 15.0 MJ/kg |
Whilst most moisture is vapourised on combustion, a moisture level of over 20% can increase the risk of condensation and corrosion. The dry matter contains about 50% Carbon, 6% Hydrogen, 42% Oxygen as well as Nitrogen, Sulphur, Silicium, Alkali, Chloride. Chlorine and Alkali in the flue gas can lead to the development of Sodium Chloride and Potassium Chloride, which are highly corrosive for boilers and pipes. Ash content can vary between 2-10%, although the average is 4%. High levels of ash content can cause slagging and fouling of equipment, increasing maintenance and replacement costs. The importance of understanding the chemical composition and the combustion characterstics is critical for the design and operation of a straw biomass power plant.
