Technical research on the utilization of waste heat and waste gas from small power generation equipment

　　As fossil energy is exhausted, the energy supply that supports the rapid development of the world economy is becoming increasingly tight, and the environmental problems caused by irrational use of energy are becoming more serious. Energy crisis and environmental pollution have become the fuse that restricts global economic development and troubles human life. my country is a major energy producer and consumer, and economic development depends on energy. Under the new situation of increasingly severe energy problems, resource conservation and environmental protection have put forward higher requirements for the efficient use of energy. Therefore, changing traditional energy utilization technology and improving energy utilization efficiency are of great significance to my country's economic development and environmental protection.

　　Low energy utilization efficiency has become one of the important reasons for the tight energy supply and demand in my country. The output efficiency of each ton of standard coal in my country is only equivalent to 10.3% of Japan and 28.6% of the United States. In industrial energy consumption, nearly 60-65% of the energy is converted into waste heat resources. If it is used reasonably, the energy utilization efficiency can be greatly improved. At present, the United States is the country with the largest waste heat utilization, with a utilization rate of 60%, Europe's utilization rate is 50%, and my country's is only 30%. Therefore, there is still a lot of room for development in the field of waste heat and waste energy utilization in my country. The utilization of waste heat and waste energy in my country is mainly based on the steam turbine power generation system composed of waste heat boilers and steam turbines. Although this technology has been applied in industries such as steel and metallurgy that contain medium and high temperature waste heat steam, it is not widely used in the field of medium and low temperature waste heat and waste gas resources. The waste heat and waste gas utilization system based on two key equipment, reciprocating internal combustion engines and screw expanders, has gradually been recognized by the market for its wide range of applications, strong flexibility, and high cost performance. It has become an important way to utilize low-temperature waste heat and waste gas.

　　1. Reciprocating internal combustion engine exhaust gas utilization technology

　　Among industrial waste gases, due to the constraints of gas source quality, available scale, equipment cost and other factors, there are waste heat and waste energy power generation technologies such as coalbed methane (coalbed methane mined from ground drilling, gas extracted from coal mines), coke oven gas, smelting tail gas (blast furnace gas for ironmaking and converter gas for steelmaking), low-concentration gas, recovery and utilization of associated gas from oil fields, blast furnace and converter gas recovery power generation, and low-temperature waste heat power generation. It can be seen that the vigorous development of waste heat and waste energy power generation technology during the 12th Five-Year Plan period will be of great benefit to solving the increasingly severe energy conservation and emission reduction goals. Therefore, it is necessary to carry out research on low-temperature waste heat waste gas utilization technology, conduct extensive research on application cases, and accumulate rich practical experience to lay the foundation for further improving equipment reliability, technical stability and promotion and application.

　　Keywords: Analysis of waste heat and exhaust gas utilization technology of small power generation equipment Combustible waste gases such as biogas, straw gas, carbon black gas, refining tail gas, shale gas and associated petroleum gas that cannot be exported are not suitable for gas turbines and steam turbines. The reciprocating internal combustion unit power generation system can make full use of them and turn waste into treasure.

　　1.1 Technical Principle

　　The reciprocating internal combustion engine power generation system mainly uses the reciprocating internal combustion engine to drive the generator to generate electricity, and matches the waste heat boiler to form a gas power plant to achieve the waste gas utilization technology for heating and cooling. Generally speaking, it can be divided into the gas power plant heating waste gas utilization system and the gas power plant cooling waste gas utilization system.

　　(1) Gas power plant heating waste gas utilization system

　　The combustible waste gas is passed into a reciprocating internal combustion engine (gas engine), and the combustion work drives the generator to generate electricity. The generated flue gas (about 600℃) enters the waste heat boiler, and at the same time, the cooling water (about 90℃) flowing through the gas engine body transfers the heat to the waste heat pipeline through the heat exchanger. The water is heated by the heat exchanger and then enters the waste heat boiler to generate high-temperature hot water or steam. Steam can be used for industrial production, and hot water can be used for heating and domestic hot water. This technology realizes cogeneration of heat and power, which can generate electricity and provide heat. (See Figure 1)

　　Figure 1 Working principle diagram of a typical gas power plant heating system

　　(2) Gas power plant refrigeration waste gas utilization system

　　The combustible waste gas is passed into the gas engine, and the combustion work drives the generator to generate electricity. The generated flue gas (about 600℃) enters the waste heat boiler. The water passes through the engine body heat exchanger and then enters the waste heat lithium bromide refrigeration unit to produce low-temperature water and higher temperature water. Low-temperature water can be used for air conditioning refrigeration, and higher temperature water can be used for domestic hot water. This technology realizes the cogeneration of heat, electricity and cooling, which can generate electricity, and also provide heating and cooling. (See Figure 2)

　　Figure 2 Working principle diagram of a typical gas power plant refrigeration system

　　1.2 Technical Features

　　The power generation efficiency of a traditional gas turbine is generally around 35%. The current common method to improve efficiency is to recover heat energy through a waste heat boiler and convert it into steam, drive a steam turbine to generate electricity again, and form a gas turbine and steam turbine combined cycle power generation, thereby improving efficiency. However, this combined cycle system has high requirements for water resources, the system is relatively complex, the construction investment is large, and relocation is relatively difficult.

　　Compared with the traditional gas turbine power generation system with large investment, long construction period and large land area, the reciprocating internal combustion engine unit is used as the power equipment of the gas power station. The power generation efficiency of the unit is usually between 30% and 40%, and the more common models can generally reach 35%. Its most prominent advantage is the relatively high power generation efficiency, followed by high equipment integration, quick installation, low requirements for dust in the gas, basically no water, and the unit kilowatt cost of the equipment is relatively low. The supporting waste heat utilization equipment required for the construction of gas power stations is also relatively flexible, and small waste heat equipment can be well matched. In general, the gas power station built on the basis of reciprocating internal combustion units has all the thermal power functions of the gas turbine combined cycle power plant in the field of waste heat utilization technology, and has the advantages of low requirements for gas supply quality, fast commissioning, low investment, and a wide range of applications. At present, the reciprocating internal combustion engine units produced by domestic enterprises can achieve at least 30% power generation efficiency when applied to power stations. With waste heat boilers or refrigeration units, they can achieve a heat absorption rate of about 30%, and absorb about 20% of the heat through heat exchangers, thereby achieving a comprehensive energy utilization rate of more than 75% in power stations. Although my country's reciprocating internal combustion engine technology is relatively mature, the construction of gas-fired power stations based on it still faces problems such as insufficient engineering design experience, small scale, low compatibility of power station equipment, and weak intelligent and information management, which leads to challenges in its large-scale development and application.

　　As relevant national policies and regulations are gradually implemented, measures such as establishing waste heat power generation gas power plant demonstration projects, formulating a unified development plan and standard system, and strengthening technical cooperation and technical exchanges among supporting enterprises will be conducive to promoting the resource integration and optimal allocation of waste heat utilization auxiliary equipment such as reciprocating internal combustion engines, effectively alleviating the problem of uncoordinated and asynchronous development of gas power plant construction, and ultimately achieving the synchronous development of power generation projects and waste heat utilization projects.

　　2. Waste heat and waste energy utilization technology of screw expander units

　　2.1 Technical Principle

　　The screw expander power generation system is mainly a waste heat and waste energy utilization technology that uses the screw expander to recover waste heat steam, hot water, hot liquid and flue gas waste heat to drive the generator to generate electricity. According to whether the low calorific value heat source directly drives the screw expander to do work, the screw expander power generation system can be divided into a conventional cycle system and an organic Rankine cycle system.

　　(1) Conventional circulation system

　　Conventional circulation system, also known as single circulation system, is to introduce heat-containing fluid directly into the screw expander unit, which drives the main engine to expand and work. This type of system is relatively simple and suitable for energy recovery of high-temperature and high-pressure fluids. Limited by the expansion capacity, the application range of heat sources for directly driving screw expanders is steam with a temperature of 0.15~3.0MPa below 300℃ or hot water with a pressure of more than 0.8MPa and higher than 170℃.

　　(2) Organic Rankine cycle system

　　The organic Rankine cycle system, also known as the dual-cycle system, is a system that introduces the gaseous working medium into the main unit of the screw expander after heat exchange between the working medium and the heat-containing fluid, pushing the main unit to expand and do work. For steam less than 0.1MPa or hot water with a pressure below 0.8MPa and above 85°C, a secondary circulation organic working medium screw expander system can be used to recover waste heat. For flue gas above 200°C, a screw expander unit equipped with a waste heat boiler can be used for waste heat recovery.

　　In engineering applications, the organic Rankine cycle screw expander power station system pressurizes the low-pressure liquid organic working fluid through a working fluid pump, and uses the evaporator to absorb the heat of the working fluid and convert it into high-temperature and high-pressure steam. The high-temperature and high-pressure steam then drives the screw expander to do work and generate energy output. The low-pressure steam at the outlet of the expander enters the condenser, releases heat to the low-temperature heat source and condenses into liquid, and the cycle repeats.

　　Figure 3 Schematic diagram of the organic Rankine cycle screw expansion power station system

　　2.2 Technical Features

　　As the main equipment of the screw expander power generation system, the screw expander has fewer main parts and very low operation and maintenance costs. It does not need to be cranked or warmed up during operation, and it will not run away. It can be directly started by flushing. It is simple to operate and can be unattended, making it very suitable for use in industrial and mining enterprises. In addition, the screw expander is also suitable for high-salinity strong alkaline fluids. The body itself can descale and clean itself, so it does not require high quality of waste heat fluids, further expanding its application range.

　　Compared with steam turbines, which can only be used for heat sources such as superheated steam and clean steam with relatively stable steam flow and parameters, the screw expander power generation system is suitable for different types of industrial waste heat such as superheated steam, saturated steam, steam-liquid two-phase mixture, flue gas, polluted hot water, hot liquid, etc. When the waste heat source parameters (pressure, flow, etc.) change significantly, the unit efficiency can still remain relatively stable. In practical applications, screw expander power generation systems are generally used for heat sources below 300°C and occasions with small waste heat scale. When the heat source temperature is around 200°C, its heat-to-work conversion efficiency (the ratio of the system's external output mechanical energy to the heat energy contained in the low-temperature heat source) can reach about 15%. If the quality is lower, the heat-to-work conversion efficiency can reach 8%~13%.

　　At present, the screw expander power generation system has gained a certain market space by virtue of its technical advantages such as being not picky, not complicated, occupying less space, requiring less construction, and having high cost performance. It is widely used in the fields of petroleum, chemical industry, metallurgy, steel, cement, papermaking, printing and dyeing, etc. Although there is a broad market space for the use of screw expander units to recover waste heat for power generation, the actual development situation is not optimistic. The main reason is that due to the limitation of expansion capacity, the power of a single unit is generally not high, and users cannot comprehensively measure the energy-saving effect. In addition, the low technical threshold for industry access, fierce competition among industry enterprises, and the lack of unified development plans and standards have also had a certain impact on its development.

　　In order to realize the country's long-term plan of energy conservation and emission reduction, the screw expander waste heat and waste energy utilization technology can become another important waste heat and waste energy utilization method after the micro gas power generation system and reciprocating internal combustion engine power generation system by finding the right market positioning, establishing a standardized industry order, and formulating unified standards.

　　3. Conclusion

　　In summary, the full utilization of waste heat and waste energy provides us with a new development idea for solving the energy crisis. However, the development of the waste heat and waste energy industry still faces many problems such as imperfect market mechanism, low equipment reliability, lack of industry standards, etc. If these problems are not solved in time, the utilization of waste heat and waste energy may remain in the stage of paper talk, and even restrict the smooth realization of the national energy conservation and emission reduction goals. From a long-term perspective, taking measures such as overall planning by the state, strict supervision by relevant functional agencies, and enterprises conscientiously carrying out technical research, innovation and cooperation, and formulating unified technical standards will be conducive to promoting technological progress in the field of waste heat and waste energy utilization, ensuring the good and rapid development of the industry, and making greater contributions to my country's energy conservation and emission reduction cause.