The trolley type forging heating furnace is a key energy consuming equipment in the forging process, and the main fuel used is coke oven gas, which accounts for 70% of the company's total gas consumption. In order to reduce the energy consumption of heating furnaces, multiple energy-saving renovations have been carried out in the past two years: firstly, combined with the overhaul of the furnace, full fiber lining and thermal storage renovation have been carried out; Secondly, adopting computer control technology to improve control accuracy; Again, regarding the recovery and utilization of waste heat from high-temperature flue gas, explore air preheaters suitable for high-temperature flue gas. Through the aforementioned improvements, significant energy-saving effects have been achieved. Due to the high cost of full fiber lining or thermal storage renovation, and the large amount of renovation required for the furnace type, it can only be combined with major and medium repairs; For some furnaces that are not yet in the overhaul period, energy-saving retrofitting of air preheaters has significant economic benefits.

1. Existing issues

The forging heating furnace has a furnace temperature of up to 1300 ℃, and the exhaust gas temperature is above 900 ℃. The air preheater material used in the past was stainless steel. Stainless steel material will oxidize in a high-temperature environment for a long time in an oxidizing atmosphere, and will peel off under the erosion of flue gas, causing the metal tube to become thinner, resulting in burning, bending and deformation, and a short service life. Therefore, in practical use, it is necessary to add a cold air device before the heat exchanger to cool the high-temperature flue gas and then pass it through the air preheater, resulting in a significant loss of heat from the high-temperature flue gas. The air can only be preheated to below 300 ℃, which cannot fully recover and utilize the waste heat of the forging heating furnace, resulting in low thermal efficiency of the forging heating furnace.

2. Analysis of renovation plan

2.1 Comparison of Waste Heat Recovery Methods

There are usually three methods for recovering waste heat from flue gas: first, preheating the workpiece; The second is to preheat the combustion air; The third is to preheat the gas. Preheating the workpiece with flue gas requires a large space for heat exchange, which is often limited by the working site (this method cannot be used for intermittent production of trolley type furnaces). Preheating gas does not require the use of such high temperature flue gas, and for safety reasons, it will not be implemented temporarily. Preheating combustion air is a good method, which is usually installed on heating furnaces to increase the theoretical combustion temperature of fuel, improve combustion conditions, and increase the speed of combustion gases, thereby achieving energy-saving goals.

2.2 Selection of Air Preheaters

After extensive market research and technical exchanges with multiple preheater manufacturers, we ultimately chose a silicon carbide ceramic heat exchanger for the selection of air preheaters. Compared with metal heat exchangers, silicon carbide ceramic heat exchangers have the characteristics of corrosion resistance and high temperature resistance. They have been well developed in the limited use of metal heat exchangers and have become a new type of heat exchanger for recovering high-temperature waste heat. After more than two years of renovation, the silicon carbide ceramic heat exchanger has achieved good performance, with the main advantages of good thermal conductivity, oxidation resistance, thermal shock resistance, high strength at high temperatures, long service life, low maintenance, stable performance, and easy operation. Especially solving the problem of high exhaust temperature and ineffective utilization of waste heat in various high-temperature industrial kilns. Performance comparison between silicon carbide ceramic heat exchanger and metal heat exchanger:

(1) High temperature resistance. The operating temperature of silicon carbide ceramic heat exchangers is between 1350 ℃ and 1450 ℃; The operating temperature of the metal heat exchanger is 700 ℃.

(2) Use environment. Silicon carbide ceramic heat exchangers can be used for a long time at 1350 ℃ without the need for high temperature protection; When the flue gas temperature is above 700 ℃, the metal heat exchanger must be mixed or blown with cold air for high temperature protection.

(3) Service life. Due to the high temperature resistance, corrosion resistance, and oxidation resistance of silicon carbide ceramic heat exchangers, their service life is several times that of metal heat exchangers under the same usage conditions.

(4) Disadvantages. Poor seismic resistance, due to the high brittleness of silicon carbide material, the preheater cannot withstand significant vibrations; The maintainability is also poor. If there is damage, it needs to be replaced as a whole and cannot be repaired locally.

2.3 Renovation Plan

(1) To maximize the heat recovery efficiency, the silicon carbide ceramic heat exchanger is placed near the outlet of the forging furnace flue at a higher temperature, and the original double stroke is changed to four stroke to increase the heat transfer area. When the temperature of the kiln is 1250-1450 ℃, the temperature of the flue gas at the inlet of the flue is 1000-1300 ℃, and the preheating temperature of the air can reach 450-750 ℃. The hot air is sent into the kiln as combustion air and mixed with the gas for combustion, greatly reducing the heat absorbed by the combustion air. Considering the safe use of pipelines and valves, as well as the design capability of the air supply system and the original burner, the air preheating temperature is controlled below 500 ℃ without extensively modifying the forging heating furnace by replacing only the air preheater.

(2) Partial modification was carried out on the original flue to ensure a tight fit between the silicon carbide ceramic heat exchanger and the inner wall of the flue, effectively preventing air leakage and improving heat recovery rate.

(3) Due to the increase in heat exchange area, the length of the silicon carbide ceramic heat exchanger has been extended by 700mm compared to the original stainless steel heat exchanger, and corresponding modifications have been made to the air duct inlet and outlet of the heat exchanger.

(4) Considering that the increase in length of the heat exchanger will lead to an increase in smoke exhaust resistance in the flue, resulting in an increase in furnace pressure, a 250mm wide backup auxiliary flue has been added to the foundation side of the renovated flue to regulate furnace pressure; In addition, when the air preheater is damaged, it can be used as a backup flue without affecting the normal production of the heating furnace.

3. Energy saving effect

3.1 Calculation Data

According to the relationship chart between air preheating temperature and fuel savings in the industrial furnace design manual, it can be found that increasing the air preheating temperature from 300 ℃ to 500 ℃ can increase the fuel savings rate from 16% to 25%, and the savings rate increases by 56.25%.

3.2 Actual measurement data

After six months of use, the preheated air temperature is between 460 and 490 ℃. The comparison data of gas consumption before and after the transformation is as follows:

(1) The average gas consumption in the 800 ℃ insulation section decreased by about 60m3/h, a decrease of 15.7%;

(2) In the heating stage of 1230 ℃ (taking 4h to reach the required temperature as an example), the average gas consumption decreases by about 75m3/h, a decrease of 9.16%;

(3) At 1230 ℃ insulation section, the average gas consumption decreased by about 130m3/h, a decrease of 35.12%.

3.3 Benefit Analysis

According to the usage statistics after the renovation, a heating furnace produced a total of 57 furnaces, saving a total of 428700 m3 of gas compared to before the renovation. Based on normal production volume, the annual gas cost can be saved by 764800 yuan. The investment for the renovation of an air preheater is about 150000 yuan, and the investment can be recovered in six months.

4. Conclusion

(1) The use of silicon carbide ceramic heat exchangers for energy-saving transformation of high-temperature flue gas waste heat utilization is a cost-effective and effective energy-saving transformation method that does not require major equipment modifications.

(2) Silicon carbide ceramic heat exchangers have advantages such as high temperature resistance, corrosion resistance, and long service life, but they also have disadvantages such as poor seismic resistance and difficulty in maintenance, making them unsuitable for applications with high vibration.

(3) Due to the special nature of its material, silicon carbide ceramic heat exchangers require on-site construction and have long downtime for equipment maintenance.