Universal boiler for burning (not only) whole bales of straw

Biomass-fired boilers with outputs ranging from 100 to 1,000 kW are now mostly designed for either burning straw or waste wood.

A straw-burning boiler is designed to burn straw pellets and straw that is cut or sorted from bales or possibly split slices in form of square bales. Making pellets from straw is expensive with no benefits for such output levels. Burning cut straw bales or splitting bales in slices that are transported into the boiler and burned is somehow more cost-effective, however, burning whole bales of straw is the most economical method.

A boiler fired on waste wood can be designed for burning pellets, with the same disadvantage as for straw pellets, or to burn wood chips, which is much more cost-effective.

Concerning the range of output given above, the current energy situation in our country will demand a universal environmental friendly biomass-fired boiler that could burn whole bales of straw and, with minimum adaptation, even waste wood or possibly low-sulphur coal.

Step TRUTNOV Company, a boiler manufacturer, has designed and produced a pilot boiler meeting the requirement above, which is now operated for heating Company’s own manufacturing premises. A unique construction is used here allowing for burning whole bales without any necessary previous sorting or splitting. Burning unsorted whole bales of straw with stoking demanding electric input 6.2 kW for motors will cut capital and running costs and decrease installed wattage, thus saving some 37 kW required for a sorter, air-powered transporter of the sorted straw including worm conveyors, and rotary turnstile. Compared to splitting, the high capital costs of the entire cutting and feeding device is not necessary, and installed wattage equals to some 7.5 kW.

The 100 – 600 kW series hot water boilers with new design consist of a pre-chamber, in which a whole bale of straw is placed vertically, a combustion chamber as such, and an exhaust gas channel discharged into a vertical fire tube exchanger. A major part of both chambers is cooled with water. An overfeed stoker on the bottom of both chambers feeds the straw from the lower part of the bale. As the straw burns down, ashes in the end of the stoker is unloaded out by a worm conveyor. The stoker design allows for burning wood chips and bark in the same way.

The combustion chamber has openings for inlet of secondary air to facilitate full combustion with low CO content.

Horizontally placed straw bales sized 80 x 120 x 220 cm are transported by a chain slide onto a tilted and water-cooled gate of the pre-chamber. The gate is positioned vertically by means of a hydraulic system. By closing the gate, the bale is inserted into the pre-chamber, which is also closed.

The output is controlled by a number of motion cycles of a special piston separating a part of the straw from the lower part of the bale, feeding the straw onto the overfeed stoker. Air necessary for combustion is controlled by revolutions of an exhaust fan, i.e. by alternating negative pressure of flue gases in the combustion chamber, and an air fan supplying air to jets located in two points of the combustion chamber.

As soon as the boiler indicates a burnt out bale, another bale is inserted in the pre-chamber. A lower output required result in a time delay in operation of the special piston and thus inserting another bale. Analysing and testing the pilot boiler with designed output at Step TRUTNOV has proved overload capacity of the heat output by some 50 %. Any weaknesses that appeared during three-month operation period are resolved step by step. They include variation of CO content in the area of minimum output, which will be resolved by maintaining low negative pressure of flue gases in the combustion chamber, and not fully tested burning of wood chips and coal.

The industrial use lies within burning whole bales of straw without any necessary treating as well as defect-free burning of straw containing stones, soil, etc. Furthermore, starting burning of wood chips and pellets or even low sulphur coal will take only five minor steps of adjustment. The equipment is not susceptible to fluctuating humidity of straw bales as with any other existing boiler.

The above type of boiler enables all gas boiler plant operators in any industry independence on gas when supplying heat. Heat production costs are extremely low, and at the same time, the boiler technical design allows for environmentally friendly firing on other types of biomass such as chaff, rape straw, reeds, hemp, etc.