Boiler Plants for farms

The present use of straw firing in agriculture expanded heavily due to the energy crisis in the 1970s with the following subsidy schemes
and easy terms concerning depreciation on straw-fired boilers. In the middle of the 1980s, approx. 14,000
straw-fired boilers had been installed.
In 1997, it is estimated that there are approx. 10,000 straw-fired boilers in agriculture. The reason for the drop in numbers is that the
early installed boilers were small, primitive boilers designed for batch firing that have not all of them been substituted by a straw-fired boiler.



There are two types of boilers, the batch-fired and the automatically fed boilers. The batch-fired boilers are always installed in combination with a storage tank that can absorb the heat energy from one firing (1-4 bales).
Thereby, the energy content of the straw is utilised better, because the boiler can operate at full boiler load.
Automatically fed boilers are fed by a conveyor that, e.g., is loaded with straw approx. once every 24 hours.
The conveyor feeds bales into the boiler automatically, concurrently with the consumption of heat.
Over the recent 10-15 years, great technological advances have been made in respect of these boilers with a view to achieving a greater efficiency and reducing the smoke nuisance. Greatest technological advances have been in the field of batch-fired boilers, where the efficiency has increased from 35-40% in 1980 to 77-82% in 1997. This can be ascribed, in particular, to a better control of the air supply required for combustion.
The smoke nuisance is reduced considerably with increased combustion quality.


Batch-fired Boilers

Batch-fired Boilers

Whereas the small straw bales earlier dominated the market, most batchfired boilers are to-day designed for
big bales (round bales, medium-sized bales, or big bales). The boilers are often built together with a storage tank as an all-in-one unit for outdoor location.
The outdoor location highly reduces the danger of fire.
The batch-fired boilers are produced in a wide range of sizes, containing from one medium-sized bale to three big bales in the combustion chamber. The most widely used boiler size is a boiler for one medium-sized bale or alternatively 8-10 small bales.
When disregarding firing with small bales, firing and the removal of ash are usually performed by a tractor with front-end loader.


Batch-fired boiler for round bales Batch-fired boiler for round bales or big bales located in a separate housing, thereby eliminating the danger of fire in respect of the farm buildings.


Control of Air Supply for Combustion

Control of Air Supply for Combustion

Today, all batch-fired boilers are equipped with combustion air fans, where the amount of air and the distribution of air between primary and
secondary air is controlled by an electronic control unit. The flue gas temperature and oxygen content are used as a control parameter. In addition,
the boiler has refractory linings of firebricks round the walls of the combustion chamber in the upper part in order to secure a high combustion temperature.
Measuring of the flue gas temperature secures that the boiler output is kept within certain determined limits,as, e.g., a high flue gas temperature
is an expression of the boiler being overloaded, i.e., the combustion produces more heat than can be absorbed by the water in the boiler. Similarly a too low flue gas temperature is an expression of a too low boiler output.
Measuring of the oxygen in the flue gas is used for adjusting the combustion excess air by opening and closing the primary and secondary air.
The ideal target is an oxygen percentage in the flue gas of 6-7% which is equal to an excess air, lambda, of approx. 1.5.
The oxygen content is measured continuously by an oxygen probe of almost the same type as that used for controlling the petrol injection in modern
car engines.

The electronic control unit converts the signals from the probe into air-inlet-open- and shut-off impulses to the motor-driven air valve.
If the oxygen percentage is too high, a small amount of primary air is allowed to enter, thereby shutting off a bit the secondary air inlet. Similarly, the primary
air inlet shuts off a bit, and the inlet of secondary air opens a bit if the oxygen content decreases too much.
It is important that the electronic control unit is capable of keeping the oxygen percentage constant, since fluctuations in the oxygen percentage result in too high CO values and too low boiler efficiency. Therefore ongoing developments aim at improving the straw-fired boilers by developing a very accurate and reliable control system with oxygen probe. In addition, it is also important that the air nozzles are designed and positioned so that the proper turbulence is created in the combustion zone.
In order to attain a good combustion with a low CO content in the flue gas, it is also of decisive importance that the straw that is used is of a good quality. That means first and foremost that the straw should be dry before baling and be stored in a dry place.
But also, the straw should be well field cured (i.e. it has been left in the field exposed to rain and has turned grey) before baling, since the too early gathered “yellow” straw normally has poor combustible properties (see Section
In order for the boiler to keep a stable rate of combustion at maximum boiler load without interruptions throughout all combustion stages, all
batch-fired boilers designed for straw are equipped with a storage tank. The storage tank will usually contain 60-80 litres of water per kg of straw contained
in the combustion chamber.
This is equal to the temperature in the storage tank increasing 30-40°C at the time of firing if not simultaneously heat is drawn from the tank. The storage
tank is typically a separate tank that is located on top of the boiler, but the boiler may also be built into the storage tank. Figure 15 illustrates the principle of separate tank.



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