What is Pulverized Coal Injection? Why Do You Need It?
Steelmaking industry is a large consumer of energy resources, where 70% of expensive coke and natural gas are used for production of hot metal, the cost of which makes 40-50% of the cost of energy resources consumed.
Therefore, one of the main aims of the blast furnace production optimization is reduction of energy commodities per ton of hot metal.
The analysis of blast furnace production processes shows that the most effective energy-saving technology today is technology of pulverized coal injection (PCI), the efficiency of which is much higher than that of other types of fuels.
Important economic and operational advantages of using pulverized coal in blast furnaces include the following:
✔️ Reduction of specific natural gas consumption to its nearly complete cessation (80-100%);
✔️ Reduction of specific coke consumption (coke substitution coefficient 0.8-1);
✔️ Improvement of blast furnace thermal control;
✔️ Maintaining the required theoretical combustion temperature, reduced by means of pulverized coal injection;
✔️ Increase of reducing ability of gas to ore and thereby improvement of the blast furnace performance, as injected pulverized coal contains more hydrogen than coke;
✔️ A considerable economic effect due to using noncritical and cheaper yolk coals grades;
✔️ Extending the coke furnace lifetime due to decrease of coke for production. It is highly important, as many coke furnaces are coming to an end of their operation life and require significant investment for replacement or maintenance. Capital cost for construction of a new coke battery is three to four times higher than that of a pulverized coal plant of equal capacity;
✔️ Creating conditions for increasing the part of sinter coal in the coke charge and improving the quality of the coke produced.
Currently, blast furnace technology with pulverized coal injection (PCI) has been implemented in more than 30 countries worldwide. More than 50% of the world’s hot metal are produced by means of this technology. Technical and economic indicators of operation of PCI plants confirm effectiveness of replacing a part of coke with pulverized coal when smelting hot metal both from an economic and environmental point of view. Using PCI in the amount of 100-260 kg/t of hot metal made it possible to reduce specific coke consumption by 30-50% and achieve a consumption level of 240-340 kg/t of HM.
Design of a pulverized coal injection plant.
Nowadays, new or modernized blast furnace complexes include construction of PCI plants.
For implementation of pulverized coal preparation and injection technology construction of the following facilities is envisaged:
- Departments for receiving and storage of feed coal.
- Conveyor galleries and coal handling units.
- Coal preparation departments.
- Departments of pulverized coal preparation and injection into the blast furnace.
- Pneumatic transportation line for feeding pulverized coal from the preparation department to a distribution and proportioning unit of the blast furnace.
- Distribution and proportioning unit for dosed feeding and distribution of pulverized coal among tuyeres.
- Power facilities.
Pulverized Coal Injection into a Blast Furnace: Process Overview.
All installations for pulverized coal preparation and injection have the same basic process flow diagram, but may differ from each other in design of:
- pulverized coal preparation facilities;
- means of fuel supply to the distribution and proportioning unit;
- proportioning and distribution facilities.
A typical diagram of a PCI plant is shown in Fig. 1.
Fig 1. Installation for pulverized coal preparation and injection into a blast furnace.
What Equipment is Required to Inject Pulverized Coal into a Blast Furnace?
The most common European scheme of PC injection into a blast furnace consists of the following equipment:
- a PC storage bin (installed on weighing cells);
- two (three) PCI tanks;
- a pneumatic transfer system;
- a distribution station, located near the BF.
From the storage bin, located in the pulverized coal preparation department, the coal dust is alternately fed into an injection vessel, which operates under pressure with the following characteristics:
- max. temperature of pulverized coal fuel – 80°С;
- operation pressure – 1-12 bar;
- design pressure – 16 bar;
For material weighing the vessel is installed on weighing cells with sending information to the line control panel.
The tank is sealed and includes a cylindrical part with a conical bottom and elliptical tank head, a hatch for inspection and possible repair of the internal part, pipe rings for sealing and fluidization.
Before filling the tank with pulverized coal, a pressure of 1 bar is created. After that a signal of the pressure sensor opens the loading valve, located on the pipeline, and pulverized coal fuel comes from the bin into the tank by gravity and fills it to the maximum level. After the maximum level sensor is triggered, the loading valve closes and nitrogen is feed to the tank until a pressure of 12 bar is reached. Nitrogen supply stopps as a signal of a pressure sensor comes and the pulverized coal supply valve opens. Two pipelines go from each injection vessel, which are then combined into two main lines and connected to two static distribution devices located in the blast furnace distribution station. Pulverized coal is transported by nitrogen in a dense layer.
When the pressure in the injection vessel drops to 1 bar, the pressure sensor gives a signal, the PC supply valve closes and the loading valve opens. The injection vessel operation cycle repeats. The excess pressure in the PC bin is discharged through a filter.
Flow meters and control valves are installed on the main PC supply lines, and nitrogen purging of the pipeline takes place, if it is clogged.
A distribution station includes two static distribution devices, connected by pipelines to the tuyeres of the blast furnace. Each tuyer is connected to the distribution device by a separate line. Uneven tuyeres are connected to distribution device No. 1, and even tuyeres are connected to distribution device No. 2.
Uniform flow to each tuyere is achieved by means of static throttle valves, placed in series immediately after the distributor. Thus, the difference in pressure drop caused by the different lengths of pipes between the distributor and tuyeres is compensated.
The scheme, developed and applied by the company’s specialists in Ukraine for the blast furnace of Iron and Steel Works in Donetsk, differs from the above-described installation in means of prepared fuel supply to the distribution and proportioning department, as well as in dosing and distribution devices and consists of the following equipment:
– dust storage bins of the pulverization plant;
– two pneumatic chamber-type pumps TA;
– pneumatic transportation lines for feeding pulverized coal fuel from any chamber of the chamber-type pumps of the pulverized coal preparation plant to the PC storage bin of the distribution and proportioning department;
– shut-off and control valves on pneumatic PC transportation pipelines;
– two stand-alone distribution and proportioning units. Each unit consists of a PC storage bin, intermediate and feeding tanks.
Fig. The scheme, developed and applied by the company’s specialists in Ukraine for the blast furnace of Iron and Steel Works in Donetsk.
Coal dust, accumulated in the stock bin, is pumped into the distribution and proportioning unit bin by a pneumatic chamber-type pump. Purified and dried air, mixed with nitrogen (oxygen content in the mixture no more than 16%), is used as a transporting agent.
There are 4 aeration feeders under each feed bin. Two transport pipelines come out of each feeder for injection of pulverized coal into 8 tuyeres of the blast furnace from one line (the first block supplies the uneven tuyeres, and the second block supplies the even tuyeres).
To determine the total weigh of the PC injected, the intermediate tank is standing on weighing cells and “Silphon”- type compensators are installed between them to decouple the dust bin, intermediate tank and the feed bin.
Material reloading from the storage bin to the intermediate tank and from the intermediate tank to the feed tank is carried out by means of a valve system.
To control the pulverized coal flow through the tuyeres, flow meters are installed.
There is a bag filter with pulse cleaning on the dust storage bin.
Specific features of this installation are individual supply of coal to the blast furnace tuyeres from the feed tank, pneumatic method of dosing, equal hydraulic resistance of the lines, supplying pulverized coal fuel to the tuyeres.
The department of distribution and proportioning is equipped with devices for coal dust weighing in the intermediate tank, and with air and coal dust flow regulators for each tuyere of the blast furnace.
The PC consumption by each tuyere is automatically controled by changing the air flow and pressure drop between the tank and the BF hot blast header.
Special distribution devices – “distributors” – have become widely used.
The PCI plant (distribution station) consists of the following equipment:
- PC storage bin (installed on weighing cells);
- two (three) PC purging tanks;
- pneumatic transportation lines for PC fuel supply;
- distributing facility, located at the furnace top level.
The coal dust is alternately fed from the bunker to the purging bins. The purging bins are operated in a in a continuous mode and supply coal dust from the PC bin to the blast furnace. The amount of coal in the purging bins is controlled by weighing cells. The purging bins operate in the high-pressure chamber-type pump mode with a design pressure of 1.6 MPa.
The purging bins are operated in a cyclic mode as follows:
- Loading fuel from the PC bin at atmospheric pressure;
- Increasing nitrogen pressure to a value, sufficient for injection;
- The purging bin is ready to start purging and is waiting for the purging cycle of the other purging bin to be completed;
- Injection starts when the second purging bin is almost empty;
- Pressure release to atmospheric through the PC bin and pressure relief filter.
Weighing cells in the suspension supports of the purging bins provide information to the control system for determining and monitoring the level of PC fuel in the feed tanks both during their loading and in the operating mode of PC supply to the distributor. The accuracy of the weighing system of the purge bins: is 0.05%.
Pulverized coal fuel is fed by a pneumatic transport system through dust pipelines from the purging bins of the PC preparation department to the distribution station.
Distributors are static devices, which ensure uniform distribution of coal dust along injection pipelines. The distributor body is a cylinder with a conical bottom. Coal dust enters the conical part of the distributor from below. In the upper part of the cylinder there are bends, evenly placed around the perimeter, to which injection pipelines are connected for coal dust injection into the tuyeres. One of the two “distributors” distributes it to even, and the other to uneven tuyeres of the blast furnace.
A preset differential pressure is maintained between the hot blast pressure and the pressure in the distributor. It makes possible to control the coal feed rate into the blast furnace. This requires a signal about the blast pressure value to the pulverized coal injection system. The pressure difference will be adjusted depending on the pulverized coal injection intensity.
Uniform distribution of pulverized coal among the tuyeres of the blast furnace is achieved due to the principle of equal resistance in all injection pipelines. Injection pipelines have a constant internal diameter and the same hydraulic length, ensuring equal pressure drop and flow rate in different pipelines.
The flow rate of the PC fuel supplied is measured and controlled by the loss in weight of the purging bins as they are emptied.
Each injection pipeline is equipped with an automatic valve, located at the distributor level, which allows a specific pipeline to be closed and disconnected from the blast furnace, if necessary.
The Most Common Methods of Pulverized Coal Injection into Blast Furnaces.
Pulverized coal injection plants with the European scheme and a distributing station are successfully operated in more than 120 blast furnaces worldwide. Fluctuations of pulverized coal supply to tuyeres are within ±5-10%. The choice of one or another type of injection unit is determined by specific conditions of the blast furnace shop, in which the construction of a pulverized coal injection complex is planned and also depends on the technology and equipment supplier.
How Can PCI Improve Your Operational Efficiency?
Pulverized coal injection into a blast furnace can enhance operating efficiency by improving combustion and increasing furnace temperatures. It makes possible to:
✔️ Increase combustion heat: Pulverized coal fuel burns efficiently, which contributes to higher temperatures in the reaction zone.
✔️ Reduce coke consumption: a part of energy and carbon can be obtained from pulverized coal, reducing the need for coking.
✔️ Improve gas flow: Pulverized coal fuel helps distribute heat evenly, resulting in a more efficient hot metal melting.
✔️ Reduce emissions: more complete combustion of pulverized coal reduces emissions of carbon dioxide and other pollutants.
As a result, it can lead to resource savings and improves blast furnace performance.
Features of Pulverized Coal Injection (PCI).
To ensure the most favorable conditions for combustion of coal dust in the tuyere zones of a blast furnace, it is necessary:
- to use mixes of coals with different compositions, combine several types of coals;
- to use coals with ash content of 5…12% and sulfur content of less than 1.0% for PCI;
- to ensure stability of quality indicators of coals, used for PCI;
- to grind coal fuel to the required fraction – 80% no more than 80 microns;
- to inject coal dust through a coaxial, adjustable injection lance;
- to increase blast temperature to 1200…1250 °С;
- to assure a uniform feed of pulverized coal into the blast furnace tuyeres (nonuniformity ±5…10%).
To increase efficiency and reliability of the PCI technology it is necessary to apply the following compensating measures:
- to improve the coke quality in terms of hot strength CSR ≥ 60%, reactivity CRI≤28%;
- to prepare coke for blast-furnace smelting in terms of fractional composition: to increase the content of the most effective fractions of 40-80 mm in skip coke to 90% and higher;
- to increase the oxygen content in the blast to 28…33%;
- to increase the blast temperature to 1150-1200 o C and higher;
- to increase the oxygen content in the blast to 25-30% and higher;
- to reduce slag output to 300-350 kg/t of hot metal;
- to reduce the fines content of 5-0 mm in agglomerate to 5% and less.
Preparation of Pulverized Coal for Injection.
The process of coal preparation for injection into a blast furnace is determined by a number of physical and mechanical properties and the initial size of the coal grains, requirements for fractional composition and moisture content.
The coal that enters the pulverization plant usually has pieces of 150-200 mm and a moisture content of no more than 10%.
To convert large pieces of coal into dry fine dust, suitable for injection into a blast furnace, it is necessary to carry out coal preparation, grinding and drying.
According to our experience it is recommended to use a mix of different coal grades for preparation of pulverized coal based on technological and economic feasibility. A mix of coals in a certain ratio with a specified composition is fed to the coal preparation unit.
Coal preparation consists of removing metal objects and other foreign inclusions by means of a separator, installed on the coal feed path, followed by screening and crushing of the oversize product in a hammer or roll crusher. Coal crushing is performed in coal grinding equipment. The size of the input coal pieces should not exceed 50-80 mm. Prepared coal is fed by a conveyor into a raw coal bin for the following stages of dust preparation: grinding and drying.
To obtain fine-dispersed fuel with a grain size of 50-100 microns ball pulverizer mill or roller mills are mainly used.
To improve coal grinding and storage of pulverized coal in a stock bin the grinding process in mills is combined with coal drying to a minimum humidity to privide explosion safety and to prevent from spontaneous dust combustion. Besides, drying of pulverized coal dust reduces the possibility of pipeline clogging and provides high accuracy of pulverized coal distribution in the tuyeres by means of special dividers. Dried coal dust ignites more easily and the combustion process proceeds better.
For coal drying flue gases from hot blast stoves with an average temperature of 2000C are used. To increase the temperature of the drying agent flue gases from combustion of blast furnace gas in a gas generator are used. The temperature of the drying agent at the mill inlet is +250-350◦C, after the mill – no more than +100◦C. Natural gas is used only to warm up the mill after its long shutdown.
When feeding drying agent into the mill the dried and grinded coal is carried out through a separator, in which large particles are separated and returned to the grinding table for re-grinding, and the mix of small (suitable for combustion) pulverized coal particles and drying gases enter a bag filter through a gas duct. By controlling parameters and separator operation, as well as by changing the flow rate of the drying agent through the separator it is possible to obtain pulverized coal fuel with different grain size.
In the bag filter gas-dust mix is separated from the drying agent and enters the dust bin through a screen. The screen filters out oversize PC fuel particles.
The dust preparation system is operated with underpressure. For creating underpressure appropriate draft equipment is installed on the grinding line.
The purified drier agent partially returns to the gas generator and partially is emitted into the atmosphere with a dust content of no more than 20 mg/Nm3.
From the pulverized coal bin the coal comes to injection tanks, from which it is transported in a dense layer through pipelines to the stationary distributors of blast furnaces by means of nitrogen.
At the present stage of the blast furnace technology the operating requirements to pulverized coal, coming to the blast furnace tuyeres, are as follows:
– grinding fineness of pulverized coal within 80% – up to 0.09 mm (90 µm), 100% <1 mm;
– humidity – not over 2%.
Basic flowcharts, used in industrial pulverization plants, depending on the type of the drying agent and coal grinding equipment installed, are shown below as an example.
Fig. 2. Scheme of the pulverized coal preparation plant with a ball pulverizer mill.
- Belt conveyor for feeding prepared coal;
- Raw coal storage bin;
- Sealed feeder;
- 4. Ball betcher;
- Firebox (preparation of heat carrier for drying coal concentrate. The fuel for the firebox is natural gas burned in burners);
- Downflow fuel drying device;
- Ball pulverizer mill;
- Dust separator;
- Dust cyclone;
- Bag filter;
- Aerodynamic dust and gas cleaning device;
- Pulverized coal bin;
- Fan;
- Smoke exhauster;
- Chamber-type pneumatic pump.
Grinding of coal in the ball pulverizer mill (7) is carried out mainly by the impact principle and partially by squashing and abrading pieces in the layer. When the drum rotates, the balls rise to a certain height, then fall and break pieces of coal. As the balls wear out, they are added to the mill through a special pipeline.
The scheme of coal grinding and drying in drum-type ball pulverizer mills is successfully used at the Donetsk Metallurgical Plant. It is the first industrial complex in Europe for preparation and injection of pulverized coal fuel into blast furnaces, which was built in 1980 and subsequently reconstructed in order to increase productivity, modernize equipment, ensure explosion safety and the ability to work with a mix of 2-3 coal grades.
In world practice roller mills of different design configurations and sizes are used for grinding and drying coals for PCI. Their main differences are in the number and geometric shape of grinding elements of the mill. However, the process of grinding coals in any type of roller mills is carried out in the same way: by crushing and partial abrading material between a plate and rollers, pressed against each other by the force of springs or hydraulic devices.
Medium-speed vertical roller mills with conical rollers are also widely used for grinding coal to a powdery state.
Fig. 3. Classic flowchart of a modern pulverized coal fuel preparation plant with a roller-plate mill.
- Conveyor for feeding prepared raw coal;
- Raw coal storage bin;
- Drag chain conveyor;
- Roller-plate mill with built-in static separator;
- Process gas filter;
- Circulation air fan;
- Screen for screening off-grade fuel;
- PC storage bin;
- Injection tank;
- Hot gas generator with mixing chamber;
- Container for off-grade fuel.
Analysis of operation of various pulverization systems showed that the most efficient are pulverization schemes, using roller mills (Fig. 3). This type of mills is very flexible in operation, it is possible to control operating modes with low inertia when changing productivity and they can be quickly adjusted from grinding one coal grade to another, the parameters of grindability (Klo) and humidity of which greatly fluctuate. In addition, the roller mills have a number of other advantages over the Drum-type ball pulverizer mills:
– a roller mill combines drying, grinding and separation processes in one unit, which simplifies the line configuration;
– there is no downflow fuel drying unit;
– in the dust preparation scheme with a roller mill less equipment is installed, if compared to a drum-type ball pulverizer mill. It minimizes the harmful suction of cold air;
– more efficient in terms of performance indicators (installed capacity, specific energy consumption, capital investments);
– specific metal consumption by the wear of grinding elements in a roller mill is lower than in a drum-type ball pulverizer mill;
– metal consumption for production of the mill itself is also a significant factor: for fabrication of a roller mill it is almost 2 times less than for fabrication of a drum-type ball pulverizer mill;
– high smoldering or ignition safety, as there is no accumulation of coal and the grinding chamber can be easily emptied;
– the roller mill works with low noise levels, so it can be installed outdoors, which significantly reduces the cost of construction works and improves the working environment.
In consideration of the foregoing, a vertical roller mill should be the first choice equipment as the most promising type, when constructing new pulverized coal dust preparation and injection complexes.
Influence of Coal Grade and Particle Size on Operating Speed.
For complete combustion of pulverized coal in a blast furnace the choice of injection coal is very important. Incomplete combustion of pulverized coal causes a decrease in the coke replacement coefficient and an increase in carbon content in smelting products. Besides, the smelting intensity decreases due to deterioration of gas-dynamic characteristics of the burden layer and, as a consequence, it hinfluences the furnace productivity.
The higher is the carbon content, the more heat is released and the higher is the coke substitution coefficient. In the case of low-volatile coal injection, 10% less coal burns in the tuyere zone, if compared to high-volatile coal. However, high-carbon coals burn worse than high-volatile coals. The best performance is achieved when injecting coal mix, containing up to 60% flammable coal, and the rest being high-calorific coals. In the case of poor-quality raw materials the ratio should be 80 to 20. The first coals ensure early ignition of the mix and its heating, the latter provide high thermal capacity and a high coke substitution coefficient.
Technical requirements to PC raw materials for blast furnace production:
– ash-contents ≤ 12 %;
– sulphur – 0,5-1,0 %;
– humidity – 10-12 %;
– volatile matter – 8-38 %, optimal 10-20%;
– coal grinding fineness (content of 80 µm fraction not less than 80%, and 100 µm fraction – 100%);
– stable chemical composition;- rated grinding property – 45-60 (HGI);
M HEAVY TECHNOLOGY know-how for Optimizing Pulverized Coal Injection.
M HEAVY TECHNOLOGY experts have necessary experience and knowledge for implementation in construction of new and reconstruction of existing PCI plants.
Our team took part in designing the following facilities:
- PCI plant at CJSC «Donetskstal-MZ» was reconstructed in 2002-2008. The aim of revamping was increasing its productivity, modernization of equipment and ensuring explosion safety.
- OJSC “Alchevsk Iron and Steel Works”. Designing PCI plants for blast furnaces No. 1, 3, 4, 5, based on basic engineering of «Kuettner», 2006-2010.
- PJSC “ArcelorMittal Kryvyi Rih”, Blast Furnace Shop No. 2. New construction of a complex for the preparation and injection of pulverized coal fuel into BF 9. Coal receiving and storage department, 2014-2015.
- Construction of a PCI system for blast furnaces of PJSC “Azovstal”.
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