Potential of straw as biofuel
According to calculations Ukraine annually obtains from grain crops, soya and rape almost 40 million tons of straw .
According to data up to 20-25% of total amount of straw is used for stockbreeding needs. Almost 70-75% of straw is milled and spread across the fields during harvesting of grain at that half of this amount is simply burned on the fields to avoid expenses related to its mixing with soil (plowing) and impossibility of putting nitrogen fertilizers after harvesting. 5% of straw is used for other needs of which only 2-3% go for energy needs – production of fuel (pellets, briquettes) and direct combustion for heat generation.
Despite the trend of increasing use of straw for energy needs, due to the constant rise in prices for traditional energy resources, monopoly of suppliers of energy resources, economic policies of developed countries to increase use of biomass, including straw as fuel, this agricultural resource of restorative local fuel, which can significantly improve energy efficiency of district heating in all regions of Ukraine, is not used properly. From each "grain" hectare 3 tons of straw can be used - that has calorific value equivalent to 1000 m3 natural gas. According to experts and based on experience of some Western countries, notably Denmark, for energy needs annually Ukraine can use up to 20% of obtained straw without any significant effects on the fertility of the soil, which is almost 8 million. Tons of straw for the local energy use.
Now Ukraine uses direct combustion of straw (in bales or rolls) to produce hot water for heating and to produce hot air for drying crops. The total amount of heat generators up to 1 MW in Ukraine, which is used for boiler straw as fuel, totals 120-130 pc. which use to 100 thousand tons of straw per year, which do not corresponds neither to possible capacity nor national interests of Ukraine.
In Denmark - the leader of combustion of straw - 14% of the total harvesting - today operates 10 thousand farmer boilers up to 1 MW and about 70 large boilers that burn straw in the cities for heating systems. Furthermore, Denmark operates 7 straw-based power plants.
At realization of potential of use of straw at 8 million tons the country can save up to 3 billion m3 of natural gas. Thus spread of use of straw for heat generation plays a significant role in the import substitution of natural gas.
Specification of straw as biofuel
The straw used as fuel, is capable of providing high energy requirements of enterprises for generating thermal energy (hot water, air) to obtain highly technological steam that can be used for electricity production. The calorific value of wheat straw is 17-18 MJ / kg, rape straw - about 16-17 MJ / kg. For comparison, the calorific value of wood averages 17,5-19 MJ / kg. Natural gas 52.2 MJ / m3. Thus, 2.9 tons of straw replaces 1000 m3 natural gas.
The use of straw as an energy carrier for direct combustion has some difficulties that need to be taken into account.
Temperature of softening and melting of ash of straw is relatively low due to high content of alkaline metals. As consequence slag formation may appear on low-temperature surfaces. High chlorine content, which is observed in the straw oats, barley, and oilseed rape, can lead to increased corrosion of the boiler components.
At combustion of straw two main characteristics are very important: moisture content of straw and degree of its fading. Humidity range optimal for straw combustion is 10-25%. (WARNING Excessive moisture by 1% leads to a drop of heat generator capacity to 1%). The degree of decay indicates how long straw remained on the field after harvest, and what was the amount of rainfall during this period. The higher the degree of fading, the greater reduction in concentration of alkali metals and chlorine compounds in the straw. 5-7 days are enough for leaching of chlorides from straw. Thus risk of corrosion of surfaces of equipment components and appearance on them of slag formation is reduced.
According to various calculations and estimations costs for straw as an energy are distributed as follows: 25% - for collection, 55% - for logistics costs, 5% - for storage of straw and 15% - for retention of nutrients in the soil (additional fertilization depending on plants used for production of straw, this value may vary).
It should be noted that use of straw as a fuel is fully mechanized process, with little use of manual labor.
Procurement and storage of straw
The efficiency of transport and logistics operations is a key factor in profitability of harvesting straw. It should be noted that for long distances it is more advisable to transport pellets and briquettes than the straw in bales or rolls, because density of bales, bales of straw is 0,12-0,36 t/m3, while density of the pellets from straw bulk is 1-1.2 t/m3, density of briquettes is 1 t/m3. Attention! When using straw as fuel with a transportation lever up to 30 km, as practice shows, the most advantageous (2-3 times) is direct combustion of straw in bales or rolls.
In the matter of logistics of straw one should consider separately the processes of collecting straw (roll forming), transportation from the field and storage at warehouse.
Forming of straw roll.
After harvesting grain in field straw remains. It must be formed into a roll for transportation and its subsequent use as a fuel or raw material for production of pellets, briquettes. Composition of units that can be used to form rolls of straw in rolls is as follows: tractor in set with the baler.
Transportation of straw.
Transportation of straw in rolls from the field is divided into the following processes: selection, loading, transport, unloading in warehouse. Composition of units required for this is roughly as follows: tractor in set with trailer-bale transporter and front loader or tractor with attachments for front loading operations (front loader), car with a platform for transportations of rolls (bales).
During the loading and unloading telescopic handlers are the most effective.
Storage of straw.
Straw can be stored in a closed area (for example, in hangar), under roof on racks (Dutch storage) or in open area. When stored in the open area humidity of straw increases, therefore straw bale density at preparation plays an important role. At dense compaction of rolls, bales, straw is stored in open ventilated area without danger of its getting wet.
Area required to store 1 ton of rolled bales of straw (2 bales) in a single layer of 4.2 m2, with the following bale dimensions: length - 1,4 m, diameter - 1.5 m. If stored in 2 layers, respectively area required for storage will be reduced by 2 times.
As a rule, bales of straw are stacked outdoors in four layers, bales of straw - depending on the size.