Pulverized coal injection into the hearth of blast
KALUGIN JSC proposes the system of pulverized coal injection (PCI) into the hearth of blast furnace, based on the engineering of one of the leading institutes in the People’s Republic of China on iron and steel works designing, with which KALUGIN JSC has set fruitful relations in the development of new technologies in the blast furnace production.
Injection of coal powder remains the most prospective energy-saving arrangement for the blast furnace process. PCI can replace up to 100% of natural gas and up to 40% of coke in the blast furnace (see Fig. 1). In this way, productivity of the furnace increases, and the possibility appears for operational control of hearth temperature condition.
Researches carried out abroad have shown that construction of a PCI unit is 4 times less expensive than construction of a coke battery of a similar capacity. Besides, it is necessary to consider the trend of further increase in the market prices of natural gas and coking coal compared to prices for injection coal.
The main scope of design and supply of KALUGIN JSC and design institute from China includes pulverized coal preparation unit and the unit for pulverized coal injection into the hearth of blast furnace, while the units for coal unloading from railway cars, coal storage facilities and equipment and coal transportation to coal pulverizing unit are usually included in the scope of the customer design and supply. Distribution of the scope and supplies is defined when the contract is finalized.
The coal intended for the complex of pulverized coal injection is stored in a warehouse, where the number of silos or areas required for the storing should be selected taking into account reserve for 7-10 days of operation. Under the silos or bunkers installed in the warehouse storing coal, a system of belt conveyors is provided. With their help, coal is in regular intervals fed to the main belt conveyor supplying raw coal into the bunkers of powder preparation (the further scheme is shown in Fig. 2). To extract iron-containing inclusions, a magnetic metal separator is installed above the coal conveyor belt.
From the bunker of powder preparing unit, using the weight belt feeder, coal enters the roller mill. The weight of coal supplied to the bunker is measured by electronic weighing system, and the volume is controlled by sensors of top and bottom levels. The coal stock in the bunker allows the mill to work in continuous mode for 4-5 hours. Productivity of the mill depends directly on the properties of coal. Size of raw coal pieces is up to 60 mm, its humidity – no more than 10%.
|Fig. 1 – Relation of coke flow rate on the quantity of PCI supplied to blast furnace||
Fig. 2 – Schematic section of powder preparing unit
and unit for pulverized coal injection
Crushed coal by means of hot gas is transported to the separator installed at the top of the mill. Large particles of coal are separated and returned to the platen for re-grinding, while fine ones are entrained with gas from the mill and come through the gas duct into the bag filter. Size of pulverized coal particles is controlled by changing the speed of gas flow throgh the separator and the speed of separator rotation and on the mill outlet is as follows: less than 74 microns – 80%, less than 200 microns – 100%.
To dry the coal powder, waste gases are fed to the mill from the vertical drying gas generator. Drying gas generator has a burner, combustion chamber and mixing chamber. The generator is designed for mixing blast furnace gas and combustion air burner, for burning the combustible mixture in the combustion chamber and producing flue gas, for mixing hot flue gas with the waste gas departing from hot stoves to obtain hot drying gas (its temperature at the inlet to the mill is 200 to 350ºC).
Temperature of the pulverized coal and drying gas mixture after the mill is no more than 100ºC, the oxygen content – not more than 12%. Humidity of pulverized coal after the mill is 1.0%.
The mixture of pulverized coal with drying gases is supplied through gas ducts to the bag filter, where the gases are cleaned from the coal powder deposited in the bunkers of bag filter. Cleaned drying gas is partially returned to the generator, partially released into atmosphere. Powder level controllers are installed in the filter bunkers.
Coal powder from the bunkers of bag filters can move by means of valve-feeders in any of two modular pulverized coal bunkers of injection unit. Volume of the bunker provides continuous injection into the blast furnace during 4-6 hrs. In the bunkers there are systems of electronic weighing and measuring level and temperature of pulverized coal. Drained nitrogen is fed into the bunker for coal powder pseudo-fluidizing that improves loading pulverized coal into the injection unit tank. Nitrogen also provides safety of long-term coal powder storage, which is especially important in the downtime of the injection unit.
The pulverized coal injection unit consists of:
- System loading pulverized coal into the injection unit tank for coal injection into the blast furnace;
- Two injection tanks;
- System supplying nitrogen into the tanks and transport pipelines;
- Overall transport pipeline of pulverized coal to the blast furnace;
- Pulverized coal flow divider;
- Transport pipelines from the divider to the blast furnace tuyeres;
- Systems controlling the pulverized coal flow in the overall pipeline and distributing the coal evenly on the pipelines which deliver it to the furnace tuyeres.
Injection tanks of the unit for pulverized coal injection into the blast furnace are working in turn: one is in coal injecting mode, and the other is loading, setting pressure and expecting the injection. The tanks have electronic weighing system and are equipped with sensors of temperature and pressure. The volume of injection tanks allows injecting coal powder into the blast furnace normally during 30-40 min.
Transportation of pulverized coal in the overall transport pipeline is made with the concentration of 25-40 kg per 1 kg of nitrogen and speed at the beginning of pipeline 3-4 m/sec. As the coal-gas mixture is extremely abrasive, in order to maintain the minimum speed of coal powder transmission, the pipeline diameter increases in steps with the distance from the tank (in process of pressure decrease and density reduction of the flow gas part).
Pulverized coal through the overall pipeline is fed to the powder flow divider. Flow divider is installed directly next to the blast furnace. With the help of divider, the coal powder goes to independent transport pipes DN 25 mm leading to each tuyere of the furnace. To control the uniform coal distribution on pipelines, each line has the powder-gas agent pressure and temperature sensors set with a system of nitrogen feeding to the pipelines. Pulverized coal injection can be carried out both in manual mode from the injection unit control panel, and in automatic mode.
KALUGIN JSC and design institute from China guarantee timely project performance on a high technical level according to Customer’s requirements and the norms of Russian legislation, and also contract supporting at all stages (designing, equipment delivery, construction, supervision, starting-up and adjustment works) till the achievement of guarantee indicators of the project.
Training of Customer’s experts on operation and adjustment of pulverized coal preparation and injection systems is carried out in the People’s Republic of China (a group of 8 persons during two weeks).