Recovery of waste heat

At present, there are three main forms of industrial waste heat utilization. The first one is waste heat boiler to recover waste heat steam for process use of saturated steam or for power generation; the second is to use heat pump or lithium bromide absorption unit to recover waste heat and make hot water or Steam, used in process, air conditioning and domestic heating; the third is to use the screw expander to recover waste heat, directly drive the generator to generate electricity or drive pumps, fans and compressors. In addition to recovering waste heat, the screw expander is also used in the recovery of residual pressure.

1. Middle and high temperature waste heat utilization

The waste heat boiler produces steam steam heat boiler to recover steam by recovering waste heat from the production process.

At present, waste heat boilers are mainly used to recover gaseous residual heat such as high temperature flue gas and combustible exhaust gas. The steam produced has two directions of use, one can be directly used for production and domestic steam, and the other is steam can be used for steam turbine generators to generate electricity.

2. Low-temperature waste heat utilization

The heat pump unit and the lithium bromide absorption refrigeration unit comprise two types of units. First, the lithium bromide absorption refrigeration unit utilizes industrial waste heat to provide cold water or air conditioning refrigeration for the industry. Second, heat pump units (such as lithium bromide absorption heat pumps) use hot water or steam to absorb heat from low-grade heat sources for industrial or urban heating. The driving heat source of the lithium bromide absorption heat pump is steam, high temperature flue gas, heat generated by direct combustion fuel (gas, fuel), waste hot water, waste heat steam, and the like.

Compared with waste heat boilers, lithium bromide absorption heat pump units are commonly used for low temperature waste heat recovery, while waste heat boilers are more used for medium and high temperature waste heat recovery. Secondly, the application fields are different. The hot water and steam supplied by the lithium bromide absorption heat pump are used for process and air conditioning refrigeration, and the steam provided by the waste heat boiler can be used for steam turbine power generation. Third, in the current waste heat recovery project, the waste heat boiler is mostly used to recover gaseous waste heat, and the lithium bromide heat pump unit can recover waste heat (such as waste hot water and fuel) after recovering gaseous residual heat.

3. The use of residual pressure

Screw expander screw expander can use steam, high temperature hot water, vapor-liquid two-phase fluid and other media as the power, convert thermal energy into mechanical energy to drive generators to generate electricity or directly drive pumps, fans, compressors, etc. . At present, there are two main methods for the application of the screw expander: one is to recover the residual pressure of the steam, and the other is to use the organic refrigerant Langken cycle system to recover the waste heat.

(1) Recover steam pressure difference

Case: If the steam generated from the boiler is supplied to the industrial steam after being depressurized, the screw expander can be used to recover the residual pressure for power generation.

Unit composition: screw expansion power machine + generator (circulation pump, compressor, fan, etc.) + condensing unit (optional)

Supplement: It is especially suitable for saturated steam differential power generation system with steam pressure ≥0.3MPa, temperature ≤300°C, and inlet and outlet pressure difference between 0.2~1.0MPa. When the pressure difference is greater than 1.0 Mp, multiple screw expanders can be stepped down in series. In the waste heat recovery, the steam pressure generated by the waste heat boiler is usually larger than that of the industrial steam, and the screw expander can be combined with the screw expander to realize the pressure reduction and power generation.

(2) Organic working fluid circulation system to recover industrial waste heat

Case: If there is industrial hot water (above 60 °C), organic working fluid can be exchanged with hot water for heat exchange. The organic working fluid becomes vapor-liquid two-phase and then enters the screw expander. The screw expander drives the generator through work. The power is generated, and the steam-liquid mixture after the work enters the condenser to be condensed, and then returned to the heater through the working fluid pump, so that the work is cycled.

Unit composition: heat exchanger + screw expansion power machine + generator + high efficiency condensing unit + auxiliary equipment
Supplement: screw expander recovery The main difference between waste heat power generation and waste heat boiler combined with steam turbine power generation is that the screw expander replaces the steam turbine. Compared with the steam turbine, the advantage is that the working fluid entering the screw expander can be hot water (above 60 ° C), steam, gas and liquid, and the steam turbine can only be steam; secondly, the screw expander can drive the generator. Power generation, can also directly drive fans, pumps, compressors; third, steam in the screw expander flow rate is low, low noise, internal wear and tear.