Grate kiln system

Work to test the feasibility of the grate-kiln system for the hardening of iron ore pellets began at the Allis-Chalmers pilot plant at Carrollville near Milwaukee, Wisconsin, USA, in the 1950s. The first commercial plant started-up in the mid 1960s at the Humboldt Mining Co., in Michigan, USA [Section 13.3, Ref 16]. 

The exhaust gases are generally discharged into dust and fume knockdown equipment to avoid contamination of the atmosphere. Gas-cleaning equipment includes cyclones, settling chambers, scrubbing towers, and electrical precipitators. Heat-recovery devices are utilized both within and outside the kiln. These result in an increase in kiln capacity or a decrease in fuel consumption. Waste-heat boilers, grates, coil systems, and chains are used for this purpose. 

Fig. 6.8-16 sketch of the grate-kiln hardening systems for iron ore... 

Fig. 6.8-17 I sometric drawings of the three major iron ore pelletizing processes [B.48, B.97], a) Balling drum circuits with shaft furnaces, b) Balling drum circuits with straight travelling grate machine, c) Balling drum circuit(s) or balling pan(s) with grate-kiln hardening system...

In multiple-chamber incinerators, automatic ash removal systems usually feature mechanical grates or stokers. In rotary kiln systems, ash removal is accomphshed via the kiln rotation. However, automatic, continuous ash removal has historically been diflicult to achieve in controlled air systems which have conventionally featured stationary, or fixed, hearths. 

With regard to adaptation to the kiln system the grate cooler is more versatile than the other systems of clinker cooler. More particularly, it offers the best conditions for optimal extraction of hot air for precalcining kiln systems with separate tertiary air supply. 

For kiln systems with grate-type preheaters, in which the exit gases cannot be utilized for material drying purposes, there is practically no economical alternative to the grate cooler, because in such cases the exhaust air from the cooler can always be utilized. 

The next two diagrams (Figs. 5 and 6) show the various loss proportions - varying with the clinker exit temperature - for grate coolers and for planetary or rotary coolers, on the assumption that these operate with a heat-saving kiln system and that these coolers can at best attain about 66% efficiency. 

A further lowering of the heat consumption of the rotary kiln can - under appropriate operating conditions - be achieved by utilization of the heat contained in the exhaust air from theclinker cooler (grate cooler) insofar asthisair cannot be supplied as secondary or tertiary air to the kiln system. An additional heat recovery of about 260 kJ per kg of clinker can thus be gained from the exhaust air of the cooler. 

When making up the heat balance of the kiln system, the cooler loss is determined. This loss is the sum of the heat in clinker, excess air and surface loss. The normal values are 130 kcal/kg clinker for a grate cooler and approx. 120 kcal/kg chnker for a Unax cooler (planetary cooler). 

The two leading methods of generating energy from the incineration of municipal solid waste (MSW) are the mass-bum system and the refuse derived fuel (RDF) system. The mass-burn system incinerates unprocessed MSW to recover energy and the RDF system processes unprocessed MSW into a usable fuel prior to incineration. Both methods use either starved-air modular, stoker grates, rotary kiln, or fluidized-bed units for incineration. While the mass-burn system is currently more widely utilized, both systems may be used for large waste capacities. 

V0lund has designed a system in which a rotary kiln is located downstream of a forward-acting grate. Gas-phase conversions have been implemented in furnaces with parallel flows of the primary combustion products and secondary air flows, as well as with counterflows and mixed flows of these gases. 

Sandstorm Kiln. A system of coal-firing for kilns for earthenware tiles, in which an alternating arrangement of grates and secondary air flues was claimed to provide a more even distribution of hot gases over the fired ware. (G.H.Downing and H.R.Holding Br. Pat 391 175,1933 and 464 128,1937). 

The heat lost from the grate cooler in the exhaust air can in part be recovered. With exhaust air utilization the most favourable conditions are obtained if the largest possible thermal gradient is available for the kiln by the different types of coolers. The best hot gas utilization ratings for raw material drying are obtained with grate coolers equipped with air circulation systems. 

From its Slite works, Cementa AB has been supplying a district heating scheme for a neighbouring village since 1984, generating aimual heat sales of some 15 GWh from the waste heat of its No 8 kiln, a 1.4 mta capacity line, utilising waste heat from the grate cooler. Total investment costs to develop this system are put at US 2.0 million and by 1990 the net profit of the project are Forecast at about US 0.5 million per... 

---