Discussion on waste heat power generation process of lime rotary kiln

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Discussion on waste heat power generation process of lime rotary kiln

As a raw material for metallurgy, chemical industry, papermaking and other industries, active lime is in great demand, Application of rotary kiln in active lime production A large amount of waste gas and waste heat are discharged from the preheater outlet of rotary kiln, which is directly discharged without utilization, resulting in waste heat resources. Therefore, how to recover this part of waste heat and improve the economic benefits of enterprises is the common desire of the majority of owners, which also meets the requirements of energy conservation, emission reduction and environmental protection. One of the main ways of waste heat utilization is waste heat power generation. Due to the low temperature of rotary waste gas from lime kiln, which is generally around 250 ℃, the efficiency of Langken cycle power generation using water as medium is low, so it is not economically feasible. In order to improve the thermal efficiency of waste heat power generation system, ORC can be used. When the waste heat temperature is constant, the thermal efficiency of ORC system depends on the selected working fluid. How to improve the thermal efficiency of ORC system by selecting suitable circulating working fluid Active lime kiln The key problem of waste heat power generation.

 Lime rotary kiln

1、 Available waste heat conditions

Taking the 1000 t / D lime rotary kiln as an example, referring to the actual operation of this type of lime kiln in China, the high temperature flue gas discharged from the shaft preheater at the kiln tail has no other use and can be used as waste heat.

1. Determination principle of available waste heat conditions

Waste heat power generation process is a process that combines the main process with the power generation process. The design of waste heat power generation follows the principle of "determining power by heat", which includes the following four aspects

(1) The power generation system is attached to the main line process and does not affect the distribution of energy, temperature and materials in the production process of the main line;

(2) Based on the comprehensive investigation of the available waste heat generated in the production process of the main line, the sustainable and stable heat source is determined;

(3) The design of power generation system and equipment should be "customized" according to the waste heat conditions. For example, according to the dust content of flue gas and the cohesiveness of fly ash, the methods of dust collection and boiler ash cleaning are formulated; according to the content of SO2 in the flue gas, the reasonable exhaust gas temperature of the boiler is set to prevent acid corrosion;

(4) According to the available waste heat conditions, the economic installation scheme is determined.

2、 Process scheme of thermal power generation

According to the above waste heat conditions, flue type heat exchanger is adopted in the project. Waste gas is used to heat water, and then hot water is used to heat organic working medium to obtain high-pressure steam, and then the expander is driven to drive the generator to generate electric energy.

Compared with steam, organic working fluid has lower sound velocity, and can obtain favorable air kinetic energy coordination at low blade speed, and can produce higher isentropic efficiency at 50 Hz. Therefore, the isentropic efficiency of steam turbine in the model is 0.8.

The results show that the ideal cycle efficiency can be achieved when the dry working medium steam enters into the turbine expansion work in saturated state, and the superheating process has no significant effect on improving the cycle efficiency, which is greatly different from that of steam. The saturated steam can be slightly superheated at 5 ℃. When there is enough temperature difference to calculate the heat transfer process, the general calculation is to calculate the heat transfer through the given temperature difference of relevant nodes, or to check the temperature difference of each node after given the heat transfer.

In conclusion, several organic working fluids have the following characteristics:

(1) When the power generation system reaches 150 ℃, the butane evaporation power reaches 150 ℃. At 140 ℃, the power generation power of R245fa system is higher than that of other working fluids at the same evaporation temperature, but lower than that of R123 system under high temperature evaporation condition;

(2) It is not suitable to use R113, n-pentane and isopentane working fluids under the condition of relying on engineering heat source;

(3) The use of R245fa, n-butane and isobutane may cause the problems of large evaporator volume and high equipment investment;

(4) From the point of view of system performance, R21, R123 and r245ca are suitable working fluids, but R21 is a wet working medium, which is generally not used to ensure the safety of the turbine; r245ca is a new type of environmental protection working fluid, which is expensive.

Although the use of alkanes can obtain higher power generation power and lower working fluid cost, it is not recommended because of the potential safety hazard.

R123 has high thermal efficiency under high-temperature evaporation conditions, and there is no hidden danger of alkanes. At the same time, the price of R123 is much lower than that of r245ca and R245fa.

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