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Recovery of waste heat from flue gas

Time:2019-5-28 22:07:12From:SagreonConcern:153


The high temperature flue gas discharged from various metallurgical furnaces often takes away 20-50% of the heating capacity of the furnaces. The flow direction of flue gas and preheated medium in metallurgical furnace is opposite. The waste heat of flue gas can be fully utilized to reach a higher preheating temperature. The wall of the furnace needs to be made of heat-resistant materials.

In the past ten years, due to energy shortage, energy conservation work has been further carried out. Various new and advanced energy-saving furnaces are becoming more and more perfect, and the use of new refractory fibers and other high-quality insulation materials makes the heat loss of furnaces and kilns significantly reduced. Advanced combustion devices are used to intensify combustion, reduce incomplete combustion, and the air-fuel ratio tends to be reasonable. However, the technology of reducing heat loss of flue gas and recovering waste heat of flue gas is still not progressing rapidly. In order to further improve the thermal efficiency of kilns and achieve the purpose of energy saving and consumption reduction, the recovery of waste heat from flue gas is also an important way of energy saving.

Flue gas is the main way to waste energy for general energy-consuming equipment, such as boiler exhaust, which consumes about 15%, while other equipment such as printing and dyeing industry stereotypes, dryers, kilns and other major energy consumption is through flue gas emissions. The waste heat recovery of flue gas mainly converts the heat carried by flue gas into the available heat by some heat transfer method.

There are two ways to recover waste heat from flue gas: one is to preheat the workpiece; the other is to preheat the air for combustion. Flue gas preheating workpieces need to occupy a larger volume for heat exchange, which is often limited by the working site (intermittent kilns can not use this method). Preheated air combustion-supporting is a better method. It is usually disposed on the heating furnace, and it can also enhance combustion, accelerate the heating speed of the furnace and improve the thermal performance of the furnace. This not only meets the requirements of the process, but also achieves remarkable comprehensive energy-saving effect.

In addition, preheaters of preheated air have been used in industrial kilns since the 1950s in China. The main forms of preheaters are tube heat exchangers, cylinder radiation heat exchangers and cast iron block heat exchangers, but the exchange efficiency is low. In the 1980s, domestic heat exchangers, such as jet heat exchangers, jet radiation heat exchangers and double-table heat exchangers, were developed successively, mainly to solve the waste heat recovery at low and medium temperatures. Significant results have been achieved in waste heat recovery of flue gas below 100 degrees, and heat transfer efficiency has been improved. However, at high temperatures, the material limitations of heat exchangers, low service life, heavy maintenance workload or high cost of fixing affect the promotion of use.

Ceramic heat exchangers were developed in China at the beginning of the 21st century. Its production process is basically the same as that of kiln furniture. Thermal conductivity and oxidation resistance are the main application properties of materials. Its principle is to place the ceramic heat exchanger near the flue outlet and at a higher temperature, without mixing cold air and high temperature protection. When the kiln temperature is 1250-1450 C, the outlet temperature of the flue should be 1000-1300 C. The waste heat recovered by the ceramic heat exchanger can reach 450-750 C. The recovered hot air is fed into the kiln and the gas shape. Combustion with mixed gas can save energy by 35%-55%, which directly reduces production costs and increases economic benefits.

Ceramic heat exchanger has been well developed under the limitation of metal heat exchanger, because it solves the problems of corrosion resistance and high temperature resistance, and becomes the best heat exchanger to recover high-temperature waste heat. After many years of production practice, the results show that the ceramic heat exchanger is very effective. Its main advantages are: good thermal conductivity, high temperature strength, oxidation resistance and thermal shock resistance. Long life, small maintenance, reliable and stable performance, simple operation. At present, it is the best device for recovering waste heat from high temperature flue gas. At present, ceramic heat exchangers can be used in main thermal kilns in metallurgical, non-ferrous, refractory, chemical and building materials industries.


1. Waste heat reclaimer (gas-water)

Heat pipe waste heat reclaimer is a special equipment for coal-fired, oil-fired and gas-fired boilers. It is installed in the flue gas of boilers to reclaim waste heat from flue gas to heat domestic water or make-up water of boilers. The structure is shown in the figure: the lower part is a flue, the upper part is a water tank, and the middle has a partition. There are safety valve, pressure gauge and thermometer interface at the top, and water tank has water inlet and outlet and sewage outlet. When working, the flue gas washes down the lower end of the heat pipe through the flue of the heat pipe waste heat reclaimer. After the heat pipe is absorbed, the heat is transferred to the upper end, and the water is heated by the heat pipe's upper end. In order to prevent ash plugging and corrosion, the flue gas temperature at the outlet of waste heat recoverer is generally controlled above dew point, i.e. the exhaust gas temperature of oil-fired and coal-fired boilers is 130 C, and that of gas-fired boilers is 100 C, saving fuel by 4-18%.


2. Waste heat reclaimer (gas-gas)

Heat pipe waste heat recovery device is a special equipment for oil, coal and gas fired boilers. It is installed in the flue or flue of boilers. The waste heat of flue gas is recovered and heated to air. Hot air can be used as combustion aids and drying materials of boilers. Its structure is shown as follows: the tube box around, the middle partition separates the two sides of the passage, the heat pipe is a full finned tube, and a single heat pipe can be replaced. When working, the high-temperature flue gas flows upward from the left channel to scour the heat pipe. At this time, the heat pipe absorbs heat and the flue gas exothermic temperature decreases. The heat pipe will absorb the heat, leading to the right end, cold air from the right channel downward reverse scour the heat pipe, at this time the heat pipe exothermic, air endothermic temperature rise. The outlet flue gas temperature of waste heat recovery unit is not lower than dew point.


3. Waste heat ammonia water absorption refrigeration

With ammonia as refrigerant and water as absorbent to realize solution cycle, the absorption refrigeration unit is ammonia water absorption refrigeration unit. Because ammonia is used as refrigerant, the refrigeration temperature range ranges from - 30 to 5 degrees, which has a wide range of applications. Waste heat recovery refrigeration can be used as air conditioning or industrial cooling source. After nearly 10 years of research, Taishan Group has made some achievements in this area.


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