Utilization of the universal boiler for combustion of whole bales of hay

Recently, the attractiveness of energy crops for burning has increased namely in the field of agriculture. This mainly involves burning hay harvested from permanent grass stands, especially meadows and pastures. The interest in this type of alternative fuel has been raised namely by the escalating prices of natural gas, heat oil, or also electrical power.

Key reasons for the lack of using hay from grass stands for burning have involved the high costs of harvesting quality hay with maximum water content of 20% as well as the fact that hay and straw have a lower ash melting point than wood. Thus, burning the type of fuel above will require corresponding boilers avoiding fouling and sticking. Certain grass species and varieties can achieve a very good production potential, and are thus perspective for use in energy production.

A boiler meeting the above requirements has been produced and is now operated by Step TRUTNOV plc, the boiler manufacturing company, for heating the Company’s own manufacturing premises. A unique construction used here allows for burning whole bales without any necessary previous sorting or splitting. The boiler has now been operated for one heating season, which has tested the boiler design appropriately. This experience indicates that the principle opted for burning whole bales of straw have proved good. Of course, the comprehensive operating tests involved burning grass hay as well. The combustion tests performed demonstrated that when burning hay using the boiler, emission limits can be even met. However, to keep the CO emission limits low, burnt out flue gases and secondary combustion air will have to be mixed more carefully in hay combustion compared to straw.

Bent grass and fescue grass are useful species. For combustion purposes, these grasses should be harvested as late as possible following their harvest maturity for seed. Meadow grasses for energetic use can be harvested in June and July, which is the same period as for the fodder crop agricultural practice, where hay of these grasses must always have water content below 20%.

Testing the possibility of burning grasses in the types of energetic equipment designed for burning whole bales of cereal or rapeseed straw will help to resolve the given issue, making a difference in use of surplus hay in this field. This issue concerns making use of fallow land as well as a social aspect.

Boiler description

The 100 to 1,000 kW series warm-water boilers that feature the design indicated above consist of a pre-chamber, in which a whole bale of straw is placed vertically, a combustion chamber as such, and a flue 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 are unloaded by means of a worm conveyor. The combustion chamber has openings for inlet of secondary air to facilitate complete 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 the speed 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 at 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 results in a time delay in operation of the special piston and thus inserting another bale.