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Natural gas, biogas, landfill gas and coal mine gas are common gases that can be used in gas-fired internal combustion engines. In addition, some industrial waste gases contain combustible components, such as hydrogen and carbon monoxide. The exhaust gas from the submerged arc furnaces is one of the industrial waste gases that can enter the gas internal combustion engine for power generation. Although this waste gas has many impurities and low calorific value, it can enter the gas internal combustion engine for power generation after purification, so as to turn waste into treasure, save energy and reduce emissions, and create considerable economic benefits.
1. Source of Tail Gas from Submerged Arc Furnaces
A submerged arc furnace is also called an electric arc furnace or resistance furnace. It is mainly used for reducing smelting ore, carbonaceous reductant, solvent, and other raw materials, and mainly produces ferrosilicon, ferromanganese, ferrochromium, ferrotungsten, silicon manganese alloy, and other ferroalloys. The submerged arc furnace is mainly divided into two types: semi-closed furnace and closed furnace. The tail gas produced by the closed furnace contains more combustible components and can be used for gas internal combustion engine power generation.
The off-gas (fof) produced in the process of ferroalloy production in submerged arc furnaces is mainly composed of CO and a small amount of H2, CO2, etc. Carbon monoxide (CO) gas is formed due to the presence of carbon in the reducing agent. If the reducing agent contains volatile hydrocarbons, hydrogen (H2) will be formed. In the past, the tail gas of submerged arc furnaces was usually discharged after direct combustion, but now it can be generated by the efficient gas-fired internal combustion engine, and the heat energy of high-temperature flue gas can be used to make efficient and full use of energy.
2. Combustion Characteristics and Utilization Difficulties of 2 Submerged Arc Furnace Tail Gas
The calorific value of fof is usually only about 1/3 of that of natural gas, that is, compared with natural gas models with the same output power, the air intake of fof generator sets needs to be increased by more than 3 times. The antiknock property of fof is also significantly different from that of natural gas. Its methane number (MN) is only about 60% of that of natural gas, which means that this gas is more prone to knock in the cylinder of the internal combustion engine. Therefore, the gas generator set needs to monitor the occurrence of knock-through sensors to adjust the operating state of the unit.
On the other hand, the laminar flame speed (LFS) of fof combustion is much faster than that of natural gas, which is related to the proportion of hydrogen content in the gas. The laminar flame velocity is the velocity at which the oxidation reaction takes place at the laminar front, that is, the relative velocity between the unburned mixture and the flame front. According to the research data of MIT, when the equivalence ratio is 2, the LFS of the mixture composed of 5% H2 and 95% CO is about 50cm/s, while the LFS of the mixture composed of 50% H2 and 50% CO can reach 160cm/s. In view of the special combustion characteristics of fof, the intake system and control parameters of gas-fired internal combustion engines need a special design to adapt to its characteristics, and must also meet the emission requirements of NOx.
The Yanbach advanced dia The ne xt4 system automatically adjusts the settings when the gas quality fluctuates, so it can adapt to complex operating conditions. Meanwhile, yanbach’s lianox lean combustion control technology can control the emission of NOx in real-time to ensure that the strict environmental protection requirements are always met, so that the NOx at the unit’s exhaust outlet reaches 250-500mg/nm3, and there is no need for complex tail gas post-treatment devices such as SCR. With the latest yanbach asset management system, my plant can monitor, control and predict the operation status of the unit in real-time, and provide forward-looking maintenance suggestions.
3. Requirements for Impurity Content of Gas in Gas-fired Internal Combustion Engines
The composition and gas volume of tail gas are related to the operating parameters of the furnace and are affected by factors such as raw material quality, furnace control, and operating status, so they usually change continuously. It is very important for the gas internal combustion engines to use gas holders to reduce the fluctuation of gas calorific value and pressure. When the gas supply to the furnace is interrupted, the gas holder can also play a buffer role.
The fof before entering the internal combustion engine needs to be pretreated to meet the gas quality requirements. It is usually necessary to remove dust, excess moisture, tar, sulfur, and other harmful impurities. The content requirements of sulfur, ammonia, and other components are related to the calorific value of fuel gas, and the upper limit value must be determined according to the actual calorific value.
4. Typical Case of 4 Submerged Arc Furnace Tail Gas Power Generation
1) South Africa Ilmenite Smelter Submerged Arc Furnace Tail Gas Power Generation Project
In 2012, the largest ilmenite smelting plant in South Africa adopted 8 yanbach j620 gas-fired generator sets, each with a power of 1698 kW and a total capacity of 13.6 MW. The generated power is incorporated into the company’s 11 kV internal network to meet part of the power demand of the plant.
There are two closed DC electric arc furnaces in the plant. The main components of tail gas are 73% Co, 14.5% H2 and a small amount of CO2. It is estimated that the project can reduce 84432 tons of carbon dioxide emissions on average every year. It is a CDM registration project of the United Nations.
2) Norway Ferromanganese Submerged Arc Furnace Tail Gas Power Generation Project
The largest ferromanganese alloy manufacturer in northern Europe has adopted a yanbach j620 gas-fired generator set to generate electricity from the tail gas of a submerged arc furnace, with an output power of 1.5MW. The main components of tail gas are 65% Co, 5% H2, 8% CO2, etc., and the low calorific value (LHV) is about 2.8 kWh/nm3. The project will be commissioned and put into commercial operation in 2020.
By recycling the tail gas from the submerged arc furnace for the combined heat and power supply system (CHP), the power demand of the plant from the power grid is reduced and the power cost is saved. The high-temperature flue gas generated by the system is used to dry the ore before smelting. The goal of the plant is to increase its energy efficiency of the plant by 40% through the use of CHP. At the same time, the project has also received strong support from the Norwegian government for its outstanding contributions to environmental protection and energy conservation and emission reduction.
5. Application Prospect
The power generation application of the tail gas from the submerged arc furnace makes full use of the waste gas and converts it into electric energy and heat energy. It is an example of energy conservation, emission reduction and circular economy in the smelting industry. Through the utilization of waste gas, we can create new value while improving environmental emissions, and saving the operating cost of the enterprise. Yanbach has rich experience in this field. Its generator unit products have high power, high power generation efficiency, short commissioning cycle, and good reliability, which can effectively help users obtain considerable benefits and significant carbon emission reduction effects. With the continuous improvement of environmental protection requirements for the metallurgical industry, the efficient utilization of the tail gas from submerged arc furnaces will have a broad development prospect.