Substitution of natural gas by converter exhaust gas for hot blast stove heating using a control system with a fast responding gas analyser

摘要

The gasification of coke with hot blast produces the heat and the reducing conditions for iron making in blast furnaces. The preheating of the air for blast furnaces in ironworks is accomplished in large ceramic regenerative heat exchangers (hot blast stoves or cowpers) in alternating firing and blast operation. To achieve constant hot blast flow and temperature typical arrangements consist of 3 or 4 cowpers with each 20-60 MW. Hot blast stoves are basically heated by the combustion of the blast furnace top gas, which has a low calorific value. To achieve constant hot blast temperatures above 1000 deg C heat exchanger peak temperatures in the range of 1300 deg C are required, which is not possible to achieve with exclusive blast furnace top gas combustion. One commonly applied method is the addition of up to 5 percent natural gas to achieve higher calorific values of the fuel gas, resulting in increased operation costs. The major objectives of the project were the substitution of natural gas by converter exhaust gas to reduce operation costs of a blast furnace by reduction of the requested amounts of natural gas for the hot blast stove heating in parallel with a reduction of the exhaust gas emissions of the hot blast stoves. Two major modifications were carried out therefore at the hot blast-stove system, a) the supply of steel converter exhaust gas for the substitution of natural gas and b) the development and installation of a new control system to achieve constant blended fuel gas quality and combustion conditions. Steel converter exhaust gas offers sufficient high energy content for upgrading the weak blast furnace top gas, but quality and available amounts vary strongly due to the batch operation of the converters. The optimisation of the combustion was realised with a gas blending system for the mixing of the three gas types using a control system with a fast responding gas analyser. The installation of the converter gas addition system resulted in a reduction of the continuous natural gas consumption of approx. 8 MW with a corresponding reduction of the operation costs. In addition, by the application of the system the CO emissions are reduced compared to an equal hot blast stove system which was in operation without the control system and without the utilisation of converter gas at the site.