Design for Pollutant Emission Control in Blast Furnace
Introduction
Iron production using blast furnace is a major source of air pollutants causing negative effects to human health and environment. The most hazardous emissions are dust, sulfur dioxide (SO2), nitrogen oxides (NOx), and carbon monoxide (CO). The pollutants emitted from blast furnace can be minimized through various techniques such as primary, secondary and tertiary control measures. This paper provides an overview of design considerations for pollutant emissions control of blast furnace.
Primary Pollutant Control Measures
Primary control measures are those implemented during the design stage of the furnace and during initial construction. This includes the design of process heat, raw material ingredients and composition, quality of fuel, operating temperature and pressure, and condition of equipment. These measures should be considered to minimize pollution from iron production.
The selection of raw materials for use depends upon the degree of emission reduction. High iron content is preferred for reducing the emission of SO2 and CO, which reduces the need for use of auxiliary fuel. The addition of coke and limestone to the ore further reduces emission and should be taken into account during the selection of the raw materials.
The fuel quality used for the furnace should also be carefully chosen. The use of low-sulfur and low-volatile coal reduces the emission of SO2, CO and particulate matter. The optimum air/fuel ratio should be maintained to reduce the emission of all pollutants. The rate of combustion and operating temperature is also an important factor in reducing emissions.
Secondary Pollutant Elimination Techniques
Secondary control measures are applied after the blast furnace has been operating for some time, such as installing exhaust gas cleaning units for pollution control. One of the more popular secondary methods is electrostatic precipitators and bag filters, which are used to capture dust from the furnace and keep it from entering the atmosphere.
Electrostatic precipitators work by creating a duct, where an electrical current is applied to particles suspended in the air. Due to the charge, the particles collide with walls of the duct and is collected, reducing overall emissions. This method is designed to provide high collection efficiency and is a cost effective solution.
Bag filters are also an effective method, whereby a porous fabric bag filter is installed within the process stream. The bag acts as filter media, where dust particles are trapped as they pass through an air stream. Bag filters are efficient in collecting fly ash and other particulate matter. This method is relatively low-cost, but requires frequent maintenance, as the accumulated dust needs to be removed from the bags.
Tertiary Pollutant Control Measures
Tertiary control measures are the last resort for pollution control and should be considered after the primary and secondary measures. These include the use of wet scrubbers, which use strong acidic/basic solutions to remove pollutants from the flue gas streams of the blast furnace. The wastewater from the scrubbers then requires further treatment prior to disposal into the environment.
Another method for reducing stack emissions is the use of desulfurized substances. This involves adding substances like magnesium oxide and calcium oxide to the gas stream that reacts chemo-catalytically with sulfur dioxide to reduce its concentration. This method is the most expensive, but best suited for controlling emissions of SO2.
Conclusion
Controlling emissions from blast furnace involves the consideration of multiple aspects such as selection of raw material, fuel quality, control of air/fuel ratio, exhaust gas cleaning devices and desulfurized substances. The design of a blast furnace should consider this to ensure that the pollution caused by the blast furnace is minimized.
