Present Situation and Prospect of EAF Gas Waste Heat Utilization Technology

2.1 Scrap preheating

In the 1970s, the outbreak of the oil crisis promoted the development of Japan’s steel industry energy saving technology, and preheating scrap by using EAF waste gas was developed. Preheating scrap technology can be greatly recycling physical sensible heat of high temperature gas to reduce power consumption with elevating the temperature of the scrap. In theory, the scrap preheating temperature raises 100℃, and power consumption save 20 KWH/t steel. If considering the energy effective utilization, the power consumption saving is 15 KWH/t steel for the increase of per 100℃.

Scrap preheating process not only saves energy but also shortens the time of smelting. Details are as follows [8]:

1. The consumption of electrode and refractory material is reduced to prolong the service life of the furnace with smelting cycle shorten.

2. Water and organic waste in scrap are fully removed by preheating, which reduce the air content in the molten steel to create favorable conditions for subsequent refining.

3. As the smelting time shortens, dust amount is reduced to reduce the dust removal equipment running load, which has good social environmental benefits.

Now, scrap preheating technology with successful industrial application can be roughly divided into four kinds: comelt furnace, twin shell furnace, shaft furnace, and consteel furnace [9].

However, there are some new environmental problems for scrap preheating. Scrap preheating will lead to dioxins concentration in EAF gas increased significantly, which is because the gas cooling rate decrease will further increase the dioxins concentration in gas. To meet the requirements of dioxin emission reductions in steel plant, these technologies (separation of scrap steel, gas temperature control after preheating and spray lime) need to be considered when making scrap preheating system design. When necessary, dioxins removal devices also need to be set [10].

2.2 Steam production

Preheating scrap is the use of high temperature gas energy into the steelmaking process timely and directly, while steam production will be more flexible, mainly displays in:

1. Steam can be used as process steam for heating and can also be used to drive the compressor and power;

2. Similar to the exhaust gas temperature, steam temperature range is wide;

3. Steam is convenient for transportation;

4. Water is cheap and is nontoxic medium.

The research of Xiao-qian Ma [11] shows that recycling waste heat by steam production is about 2.5 times than that of preheating scrap. Steam production is more reasonable on the energy recovery efficiency.

However the steam production technology mainly affected by the three factors:

1. The gas volume and temperature are affected by hot metal ratio affects. Volume and energy of gas are relatively small by full scrap smelting. In China, hot metal ratio can reach 70 %. This difference has a great influence on the design and application for steam production technology.

2. EAF smelting periodic is against for application of steam production technology;

3. The amount of dust that has the characteristic of thick and tiny in gas is high, which is against for application of steam production technology;

Therefore, waste heat utilization device needs high requirements for the characterization of EAF gas. It not only considers the gas temperature fluctuations but also considers the effects of high dust concentration on the equipment. In addition, the efficiency of waste heat utilization equipment is also crucial for the system running. Higher recovery efficiency leads to greater the amount of steam and lower the gas temperature at the outlet. Regenerator’s role is to balance the amount of steam in waste heat utilization device and to stabilize flow rate and pressure of steam in system export.

In China, the Lai steel (50 t EAF) and Xingcheng special steel (100 t EAF) are successful in EAF gas waste heat utilization. Both enterprise directly used steam for vacuum furnace, and develop the user of steam, which achieved good economic benefits and stable running.

A lot of sensible heat in gas cannot be used by water cooling pattern, which wasted energy, increased the consumption of cooling water, consumed large amounts of electricity with industrial water cycle. In order to solve this problem, Xingcheng special steel (100 t EAF) changed water cooling to evaporation cooling, and recycled waste heat in gas by using radiation heat exchanger and heat pipe exchanger (Figure 1) [12].

Figure 1:

Xingcheng special steel EAF gas waste heat utilization system.

Lai steel (China) successfully run a set of new type of EAF dust removal and waste heat utilization system (Figure 2). High efficient waste heat utilization system has been completely replaced the oil burning boiler for VD furnace, which greatly reduces the cost and has very significant economic benefits [13].

Figure 2:

Flow chart of the dust abatement and residual heat recovery system.

In the steel plant, the gas produced in EAF process can be used as a heat source to produce steam. And VD furnace needs stability of steam to provide to jet pump for vacuum pumping (Figure 3). At present, most of the steel plants have to be equipped with special boiler for VD furnace, which needs to consume large amounts of energy. The way that EAF gas surplus heat is provided to VD furnace will improve the energy efficiency in EAF steelmaking process.

Figure 3:

Germany GMH steel EAF waste heat utilization system (waste heat boiler).

ECS system exhaust pipe of GMH steel plant in Germany by the method of radiant heat transfer can effectively reduce the exhaust temperature to around 600℃. Under this temperature can effectively improve the thermal efficiency through convection heat transfer. Therefore, gas waste heat that the temperature is 180℃–600℃ can be recycled by waste heat boiler, which also needs to consider the problem of high dust content of the EAF exhaust gases.

2.3 Power generation

Under the condition of mature technology for steam production, the way that converted high temperature steam to electricity will further improve the EAF gas utilization value. However, as the high volatility of the flow rate and temperature and high dust content (>12 g/m³) for EAF gas, the gas waste energy is difficult to use in power generation.

Li-bo He [14] use the gas pulse soot blowing, steam accumulator, and power generation with saturated steam technology to solve the above problems. The technology system is shown in Figure 4.

Figure 4:

EAF saturated steam waste heat power generation system.

2.4 Secondary combustion

With the greatly increase of physical heat and chemical heat supply by the increase of hot metal charging in EAF steelmaking process, electricity demand decreased, smelting rate increased, and waste energy of gas increased.

In EAF steelmaking process, scrap preheating and steam production (power generation) are simply the recycling of the high temperature gas thermal physics. As the CO content of gas increased with the high hot metal ratio, a large number of chemical heats carried in gas have not been used. Secondary combustion technology (Figure 5) is to provide oxygen for secondary combustion, which leads CO fully burnt into CO₂. The secondary combustion heat will transfer to the molten pool by the way of diffuse heat transfer and radiation heat transfer. Secondary combustion can obviously shorten smelting time, lower unit power consumption, increase productivity, and reduce the heat load of the furnace. Meanwhile, poisonous gas (CO and NO_X) in gas reduced with secondary combustion. With excess oxygen consumption for secondary combustion, the relationship between the energy recovery efficiency and the oxygen consumption needs to be balanced [15].

Figure 5:

The secondary combustion system.

2.5 Coal gas production

The secondary combustion is a way for the waste heat utilization that releases chemical latent heat of furnace gas. As the material conditions (hot metal ratio) of EAF are similar to BOF, and coal gas production of BOF is mature, coal gas production for direct recycling furnace gas chemical latent heat in EAF steelmaking process will be a more efficient way of waste heat utilization. Coal gas production fully recycles the chemical latent heat of gas, which is higher efficiency than other ways.

For hot metal ratio is 50 %, the physical heat of the high temperature gas is 140 KWH/t, and chemical heat of that is 134 KWH/t. Therefore, by the use of the physics heat of high temperature gas, recycling chemical latent heat of that is also to be great value [5, 16].

In the study of recycle of BOF gas, there are some problems: BOF gas content changing with blowing time; certain requirements for gas content.

There are much more influence factors for BOF coal gas recycling including: BOF equipment condition, raw material, the carbon content of molten steel, air intake, coal gas recovery conditions, the oxygen supply intensity, etc.

If BOF coal gas recovery conditions changed fromφ(CO)≥30 %,φ(O₂)<1.5 % toφ(CO)≥18 %,φ(O₂)<1.5 %, and the recovery time increased by 0.5 min, gas per ton steel for recycling will increase nearly 3 m³/t.

In EAF steelmaking process, a large amount of air (O₂, N₂) is involved into the high temperature furnace gas for the EAF sealing. In this case, carbon monoxide content in furnace gas is reduced to less than 15 %, leading to that coal gas calorific value is reduced greatly. Meanwhile, the oxygen content is increased to more than 3 %, leading to carbon monoxide burning explosion. So coal gas is difficult to recover as the high volatility of in EAF steelmaking process. In order to realize coal gas production in EAF steelmaking, EAF sealing technology, oxygen removal technology, and gas purification technology need to be studied for coal gas production in EAF steelmaking process [17].