What Is a Gas Cleaning System for Blast Furnaces?

Blast furnace or top gas – is a by-product of the hot metal production. It contains a high concentration of combustible components (around 30-35%), mainly carbon monoxide (CO), and has a relatively high calorific value (3.5-4 MJ/m3), making it suitable for being used as fuel.      However, this gas contains dust, resulting from charge materials grinding, when it exits the furnace. Dust composition and quantity depend directly on the grade of hot metal being smelted, charged materials composition and smelting mode conditions. 

The blast furnace gas cleaning process includes several stages and is performed by several types of cleaning equipment:

  • Rough cleaning is performed in inertial dust catchers, where about 50% of large dust particles are separated; 
  • Semi-fine cleaning is performed in blast furnace scrubbers. The process is accompanied by coagulation of particles on droplets, after which residual dust content in gases is no more than 0.5-1 g/m3;
  • Fine cleaning to a residual dust content of 5-10 mg/m3 can be done in a throttle unit, a wet-type electrostatic precipitator, or a Venturi scrubber with a 12-15 kPas pressure drop.

Blast furnace gas cleaning systems are traditionally divided into “wet” and “dry.” 

“Wet” -type systems consist of:

  1. An inertia-type dust precipitator where large dust particles are separated.
  2. A scrubber and Venturi tubes (adjustable or non-adjustable) where fine cleaning of tiny dust particles to a concentration of no more than 5 mg/nm3 takes place.
  3. A demister where condensed moisture is removed from the gas stream. 

“Dry”-type systems consist of:

  1. An inertia-type dust precipitator where about 50% of large dust particles are separated. 
  2. Bag filters where fine cleaning from small dust particles takes place to the residual concentration in the BFG of no more than 5 mg/Nm3.

Benefits of an Efficient Blast Furnace Gas Cleaning System

When designing blast furnace gas cleaning systems, M HEAVY TECHNOLOGY specialists keep balance between blast furnace construction and reconstruction costs, operation costs, and key project indicators.  

A modern blast furnace gas cleaning system contemplates:

  1. Increasing dedusting efficiency at the first gas cleaning stage in an inertial dust catcher. To solve this problem, M HEAVY TECHNOLOGY specialists developed an upgraded blast furnace dust catcher design. The use of this dedusting equipment increases dust-catching efficiency to 70% – 75%. 
  2. Using bag filters as fine cleaning devices ensures residual dust content is less than 4 mg/nm3, increasing blast furnace gas temperature to 130°C – 170°C. 
  3. Using top-pressure recovery turbines (TRT) generates electricity and maintains top pressure. 
  4. A high level of automation to control smelting processes and monitor blast furnace gas parameters at the furnace exit.

Fig.1 – A modernized blast furnace dust catcher. 1. Bearing metalwork. 2. Modular multi-cyclone element. 3. Outlet tubes. 4. Dust-collecting bin. 5. Dividing barrel.

Blast Furnace Gas Cleaning System Types

“Wet” – type gas cleaning system

Traditionally, a “wet”- type cleaning system was used to clean blast furnace top gas in the former USSR countries and Europe. It means sequentially using an inertial dust catcher – a scrubber – Venturi pipes – a throttle group, and a demister. This BFG cleaning technology makes it possible to achieve a residual dust content of no more than 5 mg/nm3 and cool gas to a temperature of 40°C. This provides a gas calorific value of 900-1000 kcal/m3, making it suitable for fuel use. 

When blast furnaces are operated with a top pressure of 0.2 – 0.25 MPa, top-pressure recovery turbines (TRT) are commonly installed to generate electricity using energy from increased blast furnace gas pressure. The use of TRTs made it possible to reduce hot metal production costs.

Disadvantages of the “Wet”-Type Blast Furnace Gas Cleaning Technology:

  1. Low efficiency of the first stage cleaning – no more than 50%. The dry dust from the gas cleaning plant is fully used when returned to the sintering and blast furnace production.
  2. Large amounts of sludge with high zinc content complicate their further processing. As a rule, such waste is accumulated in sludge dumps and causes soil and groundwater pollution. 
  3. Water consumption in the “wet”- type gas cleaning system is 3.0 – 4.0 l/Nm3, which makes:
  • 0,53 m3/h for a 1513 m3 furnace (BF №2 Zaporizhstal)
  • 1,28 m3/h for a 2000 m3 furnace (BF №3 Severstal)
  • 2,55 m3/h for a 5500 m3 furnace  (BF №5 Severstal)

“Dry” – type gas cleaning system

In China and Japan, blast furnace gas cleaning is performed by the “dry” method using electrostatic precipitators and fabric filters. This blast furnace gas cleaning technology solution prevents us from constructing a recycling water supply system with pumping stations, water treatment, and cooling systems and excludes sludge formation. “Dry”-type gas cleaning facilities have lower hydraulic resistance than Venturi tubes, where pressure drop can be 0.1 MPa. It is advisable to use this pressure drop at TRTs to generate electricity. The “dry”- type BFG cleaning technology makes it possible to obtain cleaned blast furnace gas with a temperature of 120°C – 170°C, which also increases the power of TRT.

Fig. 2 – Blast furnace gas cleaning system using bag filters.

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What Does a Gas Cleaning System Modernization Look Like?

Main Goals of Gas Cleaning Systems

The main goal of modernizing a traditional “wet”-type blast furnace gas cleaning plant is to increase the efficiency of dry flue dust catching. This solution allows us to solve some issues:

  1. Recycling and returning dust, collected in the first stage and having a low Zn content, back into production. 
  2. Reduction of water consumption for the “wet”-type gas cleaning system by reducing the input dust concentration.
  3. Reduction of liquid sludge from a “wet” gas cleaning plant.

At the same time, it is pretty standard to modernize a “wet”-type blast furnace gas cleaning system by using an adjustable Venturi tube. Thus, the position of the Venturi tube control element maintains a preset gas top pressure and controls gas extraction from the blast furnace.

Best Ways to Modernize GCS?

According to M HEAVY TECHNOLOGY, the most promising direction is a complete replacement of the “wet”-type gas cleaning system with a system that uses bag filters and reconstruction of the first stage by installing a modernized blast furnace dust catcher. 

This solution makes it possible to:

  1. Preserve bearing steel structures, the body of the dust catcher, and the dust removal system when reconstructing a blast furnace dust catcher and minimize reconstruction costs;
  2. Completely get rid of the sludge facilities of the “wet”-type gas cleaning plant, a recycling water supply and cooling system, groups of pumping stations, etc.;
  3. Avoid costs for sludge processing. Disposal of liquid sludge is 5-7 times more expensive than the disposal cost of dry blast furnace dust.

Why To Choose a Gas Cleaning System of M Heavy Technology?

M HEAVY TECHNOLOGY has experts of various specializations in its team:

  • Blast furnace and sinter production.
  • Gas cleaning system.
  • Steelmaking technologists.
  • Highly qualified specialists in auxiliary production (handling of ore materials and their processing into a marketable product). 

When choosing a General Contractor for your project, consider a significant advantage of M HEAVY TECHNOLOGY – availability of engineers of all disciplines (technology, piping, mechanical, electrical, instrumentation & control, civil, architecture,  construction). 

The absence of contractors allows us to manage risks effectively and to design blast furnace gas cleaning systems with subsequent stages of project implementation.

Author of the article: Dmytro Semenov