Automatically Fired Boilers
Automatically Fired Boilers
The first boilers with automatic feeding device were developed in order to primarily ease the work of firing with small bales, since the originally simple
batch-fired boilers could often only contain 2-4 small bales.
At automatic firing, a bale conveyor of, e.g., 10-20 m length is filled with straw bales once every 24 hours.
From there, the bales are slowly conveyed towards the boiler. Before the firing, the straw is being disintegrated by means of a rotating shredder/cutter.
The transport system between the shredder and the boiler may be either a worm conveyor or a blower system.
The most widely used system is the blower system, since it offers the greatest flexibility by positioning of the shredder in relation to the boiler, and
in addition, it gives a good security against backfire/burn-back between the boiler and the shredder. However, the blower system is more energy
consuming than is the worm conveyor system. The straw is fed into the boiler by a worm conveyor, often called the screw stoker.
Continuous firing results in a more stable combustion in the boiler and thereby a higher efficiency and reduced smoke nuisance compared to batch-fired boilers. The boiler heat transmission is adjusted by on/off operation, controlled by a thermostat that reacts on the boiler water temperature.
Control of Firing
In order to achieve a good combustion, the amount of straw fired should be adapted to the amount of combustion air that is introduced by a fan. In
order to secure a constant amount of straw being fed, the newest automatic boilers are therefore equipped with oxygen-controlled screw stoker which means that the amount of straw fired is automatically adjusted to the oxygen content in the flue gas from the boiler. The oxygen content in the flue gas is recorded by means of the same type of oxygen probe that is described under batch fired boilers, and the amount of straw is thus adjusted by starting and stopping the screw stoker at short intervals.
Traditionally, the aim is an oxygen content of the flue gas of approx. 7%. If the oxygen content exceeds this level considerably, the screw stoker operates uninterruptedly until the oxygen content drops to about 7%.
Then the automatic step function will try to keep this oxygen percentage by stopping the screw stoker at short intervals. If the oxygen content drops
to a level far below 7%, the screw stoker stops completely, until the oxygen content starts rising again.
By using oxygen control at an automatic boiler, the efficiency is improved by 5-10 percentage point on a rough estimate, because the conditions
for the combustion are better. Simultaneously, the CO content of the flue gas is reduced, and the smoke nuisance from the boiler is reduced.
Straw firing systems 200 - 1500 kW
The system consists of a straw shredder with 1, 2 or 3 shredder drums, depending on the size of the system.
On the shredder a straw conveyor is coupled up in modules of each 3 metres.
Shredder with conveyers are mounted directly in the straw store.
Shredder and conveyor are availabe in versions suitable for Mini-, Medi-, Round- og Bigbales.
The shredded straw is transported in a closed pipesystem directly to the cyclone in the stoke room.
From the cyclone the straw is transported directly to the boiler where it is efficiently burned in a hearth.
From the boiler the heat is lead on via a network to house heating, grain drying etc.
A smoke suction efficiently moves the smoke from the boiler to the chimney possibly via a smoke cyclone
thereby obtaining a cleaner and more environmentally compatible smoke quality.
The ashes is periodically pushed from the combustion hearth to an ash worm.
The worm then leads the ashes on to an ash container. The system is fully automatically controlled via a
PLC-control unit, which at the same time monitors whether the system is constantly performing its best.
Straw, district heating plants - output up to 10 MW
Product development at LI-N-KA has resulted in the development of the fully automatic straw firing system for Big Balls.
The system is supplied with fuel by means of an automatically controlled crane or conveyor, which can be delivered in one or to storeys. When the fireproof stokeroom has received a bigbale from the feed channel the drawer will be closed and subsequently be in stoking position immediately in front of the hearth door.
When the automatic controls have controlled all of the safety precautions, a signal is given to stoke. The hydraulically operated firedoor opens, and in a matter of seconds the bale is in the pre-combustion chamber, the blower starts and combustion is in action.
The stoked bale is in the gasification- and combustion fase in the pre-combustion chamber.
During this the combustionair is applied as respectively primary and secondary air.
The pre-combustion chamber and boiler are constructed as one unit.
From the boiler the flue gases pass through a smoke filter, smoke suction and canal system to the chimney.
The boiler features automatic ash removal with outlet to an ash container.
The combined main-, monitor- and controlpanel is built with the most up to date PLC technology
to ensure maximum reliability, ease of operation and maximum fuel utilization.S
traw is transported by means of a two storeys conveyor.he crane solution for straw transport