The process of discharging medium and low temperature waste heat exists in many industries, such as building materials, metallurgy, chemical industry and so on.

Taking a cement clinker production line as an example, the ideas and methods of low and medium temperature waste heat power generation are introduced.

In the more advanced cement production process, the burning process of cement clinker adopts kiln tail pre-decomposition and rotary kiln firing process, in which raw material absorbs the heat released by pulverized coal combustion and is forged into cement clinker II, which then enters the next process of the kiln head castor cooler after air quenching. The above-mentioned production process will produce two parts of waste heat. The first part is the flue gas discharged from the kiln tail decomposition furnace. It is generally used as the drying heat source of raw materials in the process. However, after flowing through the four-stage cyclone separator, the temperature of the above-mentioned production process will remain at about 400 degrees C, which is much higher than the technological requirements of raw materials drying at the temperature of the heat source (generally not less than 16 degrees C). It's 0 C. The heat between these temperature differences can be regarded as waste heat.

The second part is the heat carried by the air flowing through the kiln head as the clinker cooling medium. The temperature of this part of the hot air is close to 400 C, which is no longer used in the production process. It is usually discharged into the atmosphere after dust removal, thus becoming the waste heat completely.

In view of the different characteristics of two parts of waste heat in cement production, different heat exchangers are needed to recover it. The heat exchanger used to recover waste heat of flue gas at the kiln end is called SP furnace. The heat exchanger used to recover the heat energy of hot air at the kiln head is called AQC furnace. The lower limit of temperature for boiler recovery and utilization should be determined by the ratio of steel to coal. In this case, the outlet flue gas temperature of SP furnace is 180 C, which is determined by the drying process requirements of cement raw materials. Because the heating medium in the furnace is hot air rather than the combustion product of fuel, it does not contain. With gas, there is no acid corrosion problem. On the basis of considering the ratio of steel to coal, the temperature of hot air can be lowered to a lower level in order to maximize the recovery of waste heat from the air discharged from the kiln head.

It should also be noted that due to various factors, the heat energy carried by the hot air at the kiln head has great fluctuation. When the AQC furnace with installed energy acts as a heat source of a turbogenerator unit alone, the instability of the temperature or flow rate of the steam will lead to frequent start-up and shutdown of the unit. For this reason, SP furnace is installed at the kiln head and kiln tail at the same time. With AQC furnace, the steam from the two furnaces is mixed into the human steam turbine for power generation. The relative stability of the furnace can make up for the relative stability of the furnace and ensure the stability of the generating unit.

2. Setting up multi-stage flash evaporator and adopting mixed-steam power generation system

In order to maximize the recovery of waste heat, it is necessary to reduce the discharge temperature of heat medium as much as possible. For the furnace, the outlet temperature of heat medium is not only restricted by factors such as the ratio of steel to coal, acid dew point of flue gas and so on, but also by the temperature requirement of the drying process of cement itself. For furnaces, the heating medium is hot air discharged into the atmosphere, which is mainly restricted by technical and economic comparison.

The effective way to reduce hot air at the outlet of AQC furnace is to use multi-stage flash evaporator. The medium of the multistage flash is from the preheating section of the AQC furnace. This increases the heat recovery in the preheating section of the AQC furnace and effectively reduces the outlet temperature of the thermal medium of the AQC furnace. Because the steam parameters of flash evaporator are low, and when flash evaporator is in series, its steam parameters will be reduced step by step. In order to make full use of these steam, mixed-steam power generation system can be used.

The so-called mixed-steam power generation system is to adopt multiple-intake steam turbines, that is, on the basis of the normal entry of main steam into the steam turbine, to introduce low-parameter steam into the intermediate stage of the steam turbine in order to form the mode of multiple intake and graded work, so as to improve the effect of waste heat power recovery.