Straw as a Fuel


The most important argument for using straw for energy purposes is that this fuel is CO2 neutral and therefore does not contribute to increasing the CO2 content of the atmosphere, thereby resulting in an aggravation of the greenhouse effect.
Straw used for fuel purposes usually contains 14-20% water that vaporises during burning. The dry ...

 

 


...matter left 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.

Combustion takes places in 4 phases.
During phase 1, the free water vaporises.
In phase 2, the pyrolysis (gasification) occurs, during which combustible gases are produced depending on the temperature. There will always be a certain content of carbon monoxide (CO), hydrogen (H2), methane (CH4), and other hydrocarbons.
Phase 3 is the combustion of gases. If sufficient oxygen is supplied, a complete combustion occurs where the residual products are carbon dioxide
(CO2) and water. Where the oxygen supply is insufficient, carbon monoxide, soot (finely divided carbon), tar, and unburnt hydrocarbons are produced. During phase 4, the charcoal burns.
By complete combustion, carbon dioxide is produced. By reduced supply of oxygen, monoxide is produced. Finally, there is only ash that consists of incombustible inorganic matter.

By incomplete combustion, the ash may also contain unburnt straw residues.
The air that is supplied in excess of that theoretically required for complete combustion is called excess air.
A certain amount of excess air is necessary in order to secure sufficient air all over the area where the gases are to burn although the gas/air mixture is
never quite uniform. The ratio between the air supplied and that theoretically required is called the excess air ratio (lambda).

 

 


Through boiler walls and fire tubes, the major proportion of the combustion heat is absorbed by the water in the boiler, while the remainder disappears
through the chimney as a mixture of carbon dioxide, vapour, and small amounts of carbon monoxide and other gases, e.g., tar and compounds of chlorine. In addition, the flue gas contains small particles of ash and alkaline salts.
The presence of chlorine and alkali in the flue gas is problematic, since these matters undergo chemical reactions into sodium chloride and potassium chloride that are extremely corrosive in respect of the steel of boiler and tubes, particularly at high temperatures.

The ash is not without problems either, since its softening temperature is relatively low in proportion to other fuels, beginning at 800-850°C. How ever, it has even been demonstrated that the ash may become viscid already at 600°C /ref. 31/. This is of importance, in particular, at power plants
where a high steam temperature is desired in order to achieve a great efficiency.
This requires a high superheater temperature, thereby risking extensive deposits on the superheater tubes.

Where a combination of straw and coal is used as a fuel, the presence of alkaline matter in the ash indicates that - contrary to pure carbon ash - it cannot be used as a filler in building materials, but must be dumped at controlled disposal site.

 

 


Straw Pellets

Experiments have been carried out on the use of straw pellets, i.e., comminuted straw that has been pressed into pellets of a diameter of 8 or 10
mm/ref. 13/. The experiments showed that straw pellets can be used as a fuel in large boilers, whereas ash and particularly slagging problems make
straw pellets less suitable for use in small boiler plants. Straw pellets can be pressed with molasses as a binding agent thereby admixing an antislagging
agent, e.g., kaolin, in order to make them more stable during transport and in order to counteract the tendency of the ash to become viscid and form clinker. The calorific value of the pellets is 16.3 MJ/kg at 8% water, and the volume weight is 4 times larger than that of straw baled into big bales, i.e., approx. 550 kg/m3.

 

 


Washing of Straw
It has been known for a long time that straw that has been lying in the field and has been exposed to rain has a reduced content of the corrosive matter,
chlorine and potassium. Contrary to “yellow” straw, this “grey” straw is more lenient to the boiler, since part of the matter that corrodes boiler wall
and tubes has been removed.
Grey straw also has a somewhat higher calorific value than yellow straw.
In order to reduce the corrosive effect of straw upon the boiler plant, the ELSAM - Electricity Utility Group of Jutland-Funen implemented in the
spring of 1996 experiments on the removal of the unwanted components by boiling the straw at 160°C.
Later it was demonstrated that chlorine and potassium can also be washed out at lower temperatures. At present, it is considered most economical to wash the straw at 50-60°C. So far, straw washing has only been tested at small plants. According to ELSAM, a plant that can treat 125-150,000 tonnes of
straw per annum will most probably cost about DKK 200 million/ref. 26/.
The energy losses caused by washing, drying and the leaching of organic matter make out approx. 8% of the calorific value of the straw. This
cost is offset though by the prolonged life of the boilers, because corrosion problems are avoided.
Washing of straw is also expected to give advantages in respect of the subsequent application of the fly ash, since straw ash that does not contain alkaline
salts and other impurities can be used as a filler in building materials.

 


Straw Price
The market price of straw for energy purposes is still being intensely negotiated among suppliers and purchas-ers. With demands by the large electrical
power producers steadily increasing over the recent year due to the Biomass Agreement, the market has been characterised by a certain
increase in prices.
Thus the cost price is not only an expression of the cost of producing the straw and the profit, but it is also part of the parties’ strategy in respect of performing the Biomass Agreement.
The producer’s straw price includes in addition to return on investment and depreciation on the machinery used in connection with the gathering
in of straw and a range of other elements, e.g., wages and cost of fuel in connection with turning, baling, gathering in, and costs of storage and
transport to heating plant. In addition to that, lost fertiliser value, insurance, and the producer’s risk covering, i.e., due to shrinkage caused by bad
weather conditions during the period of harvest and shrinkage during storage.
A term that is now and then used is the “socio-economic straw price”.
That is a price that is cleaned for direct and indirect taxes so that it reflects the actual cost of producing it.
The socio-economic price is used, e.g., for a comparison between prices of various domestic and imported fuels and is more a planning tool than a
price calculation for use in the day-todaytrading.

 


Transport Energy

Admittedly, the great number of trucks transporting straw to plants or transport over great distances emit CO2 to the atmosphere caused by the engines
of the trucks.
A truck travels 2-3 km on one litre of diesel oil, thereby emitting 2.7 kg CO2.
Therefore, the CO2 emission can be estimated at approx. 1 kg per km travelled.
A truck load of straw with a calorific value of 14.5 GJ/tonne weighs 11-12 tonnes and represents an amount of energy of approx.
170GJ.
Since the CO2 emission from coal is approx. 100 kg/GJ, then the straw corresponds to a CO2 emission of approx.
17 tonnes subject to coal being burnt instead of straw.
That means that the truck should travel 17,000 km with a load of straw in order to emit the same amount of CO2 that is saved by using the truck
load of straw as a fuel instead of coal.
It can also be expressed by saying that the CO2 saving is reduced by approx. 0.6% per 100 km transport distance
travelled.

 

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