Dr. Michael Lemperle
New perspectives for the gas-fired cupola furnace
State of the art
Gas-fired cupola have a water-cooled grate. On the grate are balls of refractory material, which serve to superheat iron and slag. The gas burners are installed below the grate. Iron and slag are extracted from the same taphole and separated from each other on the outside.
The liquid iron with temperatures in the range of 1370 ° C - 1400 ° C must be superheated to about 1500 ° C in a downstream induction channel furnace. The carburization of the iron takes place on the iron runner by adding carbon carriers such as petroleum coke.
The natural gas burns completely with a small excess of air in the furnace hearth. The furnace top gas has a very low content of combustible substances, which can form from the organic components, for example, oil impurities, which are often located on the surface of the feedstock and gasified in the upper part of the shaft.
The thermal efficiency of the furnaces is very good and the flue gas volume is low. The flue gas, which is hot during the melting-down phase, can be cooled by spraying water behind the shaft. The cooled gas is cleaned in a filter and emitted through a silencer and a stack.
Cupola operated with natural gas have an additional advantage when melting ductile iron, because there is no absorption of sulfur in these furnaces. Of course, particularly favorable conditions for the use of these furnaces arise when natural gas is available in large quantities and at low cost. The use of steel scrap has so far been limited to 30 to 40%.
A few gas-fired furnaces were in operation for many years. The process is mature with melting rates in the range between 5t/h and 18t/h. In the beginning problems were caused by the setting of a sufficiently flowable slag at low temperatures and its tendency to foam.
Cupola conversion from coke to gas operation
The shaft size of coke and gas fired cupola are very similar. The majority of infrastructure can be used and the melt shop can remain in its position without changing the in plant iron logistics. Therefore, the modification of existing plants is low in capital expenditures. With the available experience also larger Cupola of up to 40 t/h can be realized.
Innovation for better overheating and increasing the use of steel scrap
The superheating of the iron and steel droplets depends on the residence time of these droplets in the zone through which they pass after melting. In the gas-fired furnace, this zone is essentially the layer of refractory balls on the grate. The residence time is directly proportional to the passing height and the amount of iron droplets per volume. Attempts to increase the passage height by increasing the quantity of balls resulted in problems regarding the permeability of iron, slag and gas. The consumption of the cost-intensive refractory balls also increased too much. An alternative is to increase the residence time by increasing the iron droplets per volume. For this purpose, the spherical shape of the refractory material must be changed to an irregular shape like coke e.g. by using low cost refractory waste material.
The problems of foaming slags are almost always due to the reaction of the iron oxides dissolved in the slags with the silicon carbide used for silicification. To this end, investigations must show whether the use of ferrosilicon can solve the problem.
Thermodynamic calculations show that the high coke bed temperatures achieved with coke cannot be achieved with gas firing. The flame temperatures calculated on the basis of the calorific values alone are not meaningful because they do not take into account the temperature reduction, for example, by the dissociation of the water. In addition, the more precise calculations also provide information about the contents of hydrogen, nitrogen oxides and other relevant species.
Dual fuel perspective / safety aspects
If so-called green hydrogen is available at acceptable prices in the future, natural gas can be replaced both in part and completely by hydrogen. However, the burners must be adapted to the increased gas volume when hydrogen is used. The furnaces running on hydrogen or natural gas can be operated in such a way that no critical hydrogen content occurs in the exhaust gas.
With natural gas-operated furnaces, CO2 emissions drop from around 330 kg CO2 t iron (in coke operation) to around 1/3, namely 110 kg CO2 / t iron (in natural gas operation). By adding hydrogen, the CO2 emissions can be further reduced, even down to zero in operating with pure hydrogen.