Ferrosilicon Furnace Waste Heat Power Generation Technology

Ferrosilicon Furnace Waste Heat Power Generation Technology

Ferrosilicon furnace waste heat power generation technology is a technology that uses the waste heat of ferrosilicon smelting submerged arc furnace flue gas to generate electricity. As an important part of the steel industry, the ferroalloy industry consumes a lot of energy every year. Its products are high-energy carriers, and the high-temperature flue gas and equipment heat dissipation generated during the smelting process take away a lot of energy. Ferrosilicon, one of the three major ferroalloy series products, consumes almost 50% of the electricity consumed by the entire ferroalloy industry.

Ferroalloys are very important raw materials in the steel industry. They are used as deoxidizers, desulfurizers, alloy element additives, etc. in the steelmaking process. According to the main elements, they can be divided into: silicon-based ferroalloys, manganese-based ferroalloys, chromium-based ferroalloys and special ferroalloys.

Ferrosilicon alloys are alloys composed of iron and silicon. They are mainly used as steelmaking deoxidizers and alloy raw materials for silicon-based alloy steels. The main equipment for producing ferrosilicon is the electric furnace, which uses electricity as the heat source, silica, steel scraps and other iron materials as raw materials, and coke as the reducing agent. It reacts chemically at high temperature (1700~1900℃), and the silicon dioxide in the silica is reduced to silicon through carbon, and ferrosilicon is formed with molten steel scraps and other iron materials.

Energy Consumption Status

The production of ferroalloys is a high energy (electricity) consumption industry, so the production of ferroalloys is affected by the cost of energy and electricity supply. With the consumption and shortage of energy worldwide, the impact of electricity costs on the ferroalloy industry will become greater and greater.

Taking ferrosilicon as an example, 60% to 75% of the production cost of ferrosilicon is electricity cost. Adequate and stable power supply is the basic guarantee for ferrosilicon production. Therefore, the adjustment of electricity prices will significantly affect the operating costs of ferrosilicon enterprises. The electricity consumed to produce one ton of ferrosilicon is 8400-9000 kWh. The current electricity price in Ningxia is 0.42-0.43 yuan/kWh. Based on this, the electricity cost per ton of ferrosilicon is 3740 yuan.

Input and output of ferrosilicon smelting energy:

Input: During the ferrosilicon smelting process, 45% of the energy input to the electric furnace is electric energy, and most of the remaining energy comes from smelting chemical reactions.

Output: The energy input to ferrosilicon smelting is mostly transferred to the flue gas and the molten finished product (ferrosilicon alloy) during smelting, and the sensible heat carried by the flue gas can be used.

The large amount of heat energy in the flue gas of the ferrosilicon electric furnace is equivalent to about 50% of the total input energy. The current utilization rate is quite low, and most of the flue gas is carried away and discharged into the atmosphere.

Environmental Protection Needs

The waste heat power generation technology of ferrosilicon submerged arc furnace flue gas has the advantages of dust reduction, waste heat utilization, and reduction of various emissions compared with coal-fired power plants. According to the above conditions and the current national pollution control, especially the frequent pollution control policies, the waste heat power generation technology of ferrosilicon submerged arc furnace flue gas will be fully promoted and developed.

Technical Introduction

Ferrosilicon waste heat power generation technology recovers the waste heat of flue gas originally discharged into the air through the air cooler through the waste heat boiler for power generation, which not only recovers heat energy and reduces environmental pollution, but also promotes more stable operation of submerged arc furnace.

Ferrosilicon alloy production adopts the smelting of submerged arc furnace with huge power consumption. The flue gas generated by smelting carries a lot of heat. The waste heat boiler is used to recover this part of the heat for power generation, which can greatly save energy for ferrosilicon smelting. Ferrosilicon waste heat power generation technology solves the key problem that the flue gas with high dust content is easy to adhere to the pipe wall and affect the boiler performance. The technology is relatively mature.

Similar to the production of ferrosilicon alloys are ferrochrome alloys, silicon-manganese alloys, calcium carbide, industrial silicon, etc. They all use high-power ore furnaces for smelting, and at the same time, the amount of flue gas emitted is large, the heat energy carried is large, and the smoke dust has high adhesion. Ferrosilicon waste heat power generation technology is also suitable for waste heat recovery in these fields.

Technical Difficulties

The core equipment of ferrosilicon flue gas waste heat power generation is the waste heat boiler. The difficulty of the waste heat boiler is that the SiO₂ in the flue gas of ferrosilicon electric furnace is extremely fine, and its tiny particles are called silicon micropowder. The average particle size of silicon micropowder is 200-400 nm, the surface area of the dust wall is 20m²g (very easy to generate static electricity, very easy to adsorb and has very good thermal insulation performance), the content of silicon micropowder in the flue gas is 5-7g/Nm³ (high concentration), and the dust accumulation density is 0.18t/m³ (very light); it has extremely strong adhesion and very high heat resistance and thermal insulation. If the silicon powder is not cleaned up, it is difficult for the boiler to operate normally.

Advantages and Disadvantages of the Main Cleaning Methods

Mechanical Brush Cleaning

Simple structure, good cleaning effect, direct cleaning with mechanical brush, has a good effect on cleaning dust accumulation with strong adsorption and fine particles in the ore furnace. However, there are the following disadvantages:

1) There are dead points in the cleaning, and the steel brush cleaning can mainly clean the surface directly in contact with the steel ball;

2) Due to the modular assembly of the horizontal boiler, an ash hopper needs to be installed under each module, and the air leakage is relatively large;

3) The mechanical failure rate is relatively high, the steel brush is replaced frequently, and the wear and maintenance workload of the equipment is large;

4) The waste heat boiler can only be arranged horizontally, occupying a large area.

Shot Blasting (Steel Ball) Ash Removal

A steel ball sowing device is arranged on the top of the furnace, and a steel ball separation and collection device is arranged under the ash hopper at the bottom of the furnace. A circulating steel ball mechanical transport device is arranged between the steel ball sowing device and the steel ball separation and collection device. The ash removal effect is good, but the disadvantage is that the cost is high and the whole device consumes a lot of steel.

Gas Pulse (Shock Wave) Ash Removal

Use acetylene, natural gas or liquefied gas and air to mix, and ignite at high frequency to generate high-temperature and high-pressure explosive gas. Through instantaneous release, it rushes into the furnace in the form of impact kinetic energy, heat energy and sound energy, and acts on the heating surface of the boiler to make the ash fall off. This ash removal method has a good effect and no operating moving parts. The main disadvantage is that it consumes fuel and increases operating costs.

Main Technical Indicators

1) Flue gas temperature of ferrosilicon furnace: 300～700℃

2) Main steam parameters of waste heat power station: 1.25 MPa, 350℃

3) Waste heat boiler efficiency: about 50%

4) Power consumption per ton of ferrosilicon: 8300～9000 KW (75 silicon)

5) Power generation per ton of ferrosilicon: 800～900 KW (75 silicon)

6) Unit kilowatt investment: 7000 yuan (domestic technology)

7) Power generation cost: about 0.15 yuan (of which depreciation accounts for 37%~39%, operation costs account for 25%~28%, and other costs account for 18%~22%).

8) Factory power consumption rate: about 7%

Technical Requirements for Power Generation from Waste Heat of Ferrosilicon Furnace Flue Gas

(1) The design should be reasonable, especially the flue gas pipeline from ferrosilicon furnace to boiler; first, it should be internally insulated, and second, the distance should be as short as possible (arranged close to the ferrosilicon furnace, and the horizontal boiler should be elevated). Too long flue will lead to large pressure loss and temperature loss of flue gas, causing the actual operating parameters of the project to be lower than the design rated parameters.

(2) The insulation and sealing of each pipeline including the boiler should be done well to prevent heat dissipation, air leakage and freezing. Lax supervision of waste heat boilers can easily lead to imperfect sealing, insulation and bypass sealing of boilers, and large air leakage rate and heat dissipation rate.

(3) The boiler cleaning method should be reasonable. There are currently three cleaning methods, mechanical wiping, shock wave cleaning and shot blasting. It is recommended to use mechanical wiping.

(4) The distance from the steam engine room to the boiler should be short to reduce the main steam pressure and temperature drop.

(5) The ferrosilicon production and boiler power generation system should be closely coordinated.

The owner arbitrarily modified the submerged arc furnace, resulting in unstable furnace conditions, which easily affected the steam production of the boiler.

(6) In some project cases, the exhaust temperature of the waste heat boiler reached 200℃, but it can actually be reduced to 160℃, and the waste heat of the flue gas was not fully utilized.

(7) The operations of charging, ramming and discharging of the ore-heating furnace have a great impact on the efficiency of waste heat power generation. Improving the coordination between the production of ore-heating furnace and waste heat power generation can effectively increase the power generation capacity. The waste heat power generation system invested and constructed by the ferrosilicon production enterprises is closely coordinated with the production of ore-heating furnaces. In the waste heat power generation projects constructed by the investors, the production of ore-heating furnaces and the waste heat power generation system are not well coordinated.

(8) The procedures for connecting waste heat power generation to the grid should be taken seriously and prospective.