The present invention concerns a method of and a roller grate for burning garbage. A layer of garbage is burned on a grate while continuously conveyed along it. Primary air is supplied to the garbage through the grate from below.
SU 1 756 741 A1 describes a roller grate wherein every intermediate component comprises a system of stationary bars with gaps between them. Accommodated below the intermediate components are air boxes. Air is supplied to the boxes by a fan that is independent of the primary-air system, The particular object is to improve burn-up.
As far as is known, the approach suggested in this reference has never been applied to garbage incinerators. Specifically, supplemental air is not supplied to the nips or spaces between the rollers in the roller-grate combustion systems currently known and being built. All of the primary air supplied to the fuel bed arrives through the surfaces of the rollers.
The layer of fuel in the vicinity of the nip or space is considerably thicker than in the areas where it rests against the rollers. The fuel also has a tendency to become denser in the vicinity of the gaps, where there the material is finer. A lot of air is accordingly needed in these areas. The increased impedance offered by the garbage resting there on the other hand forces the primary air out of the nips or spaces and into the regions when the layer is thin and loose, at the edges of the grate for example.
To improve burn-up in the incineration of garbage, the combustion air in the migrating-grate furnace specified in U.S. Pat. No. 3,403,643 is enriched with 25 to 50% oxygen. It is doubtful, however, whether such enrichment is possible in practice. Even a slight increase in the percentage of oxygen will increase the temperature beyond the materials-dictated thresholds. Furthermore, increasing the oxygen to the aforesaid extent would make the process unacceptably expensive.
EP 0 496 325 B1 describes, with reference to a roller-grate furnace by way of example, a method whereby the combustion temperature and a volume of steam are supposed to be maintained essentially constant. The flow of garbage is controlled in accordance with a measured volume of steam. An extinguishing liquid consisting of water, sewage, or sludge is sprayed into the furnace. Primary and secondary air are enriched with oxygen in accordance with the liquid's rate of flow. From one of the examples it is evident that enrichment to approximately 50% results in a lot of oxygen being consumed. A garbage-combustion rate of 800 tonnes a day requires approximately 600 tonnes a day of oxygen. Even a relatively low enrichment of less than 24% requires more than 100 tonnes a day of oxygen to burn just 500 tonnes a day of garbage.