Optimized application of exhaust heat in paper mill

The hot and humid air after drying the paper contains a lot of heat energy. Although most of them are recycled by a heat recovery device and then discharged to the external environment, the discharge temperature is still high. People try their best to recover the waste heat from the waste water and waste gas of the paper mill. However, due to the low taste of energy, the low economic efficiency of recovery, and the lack of suitable energy use places in the paper mill, a large part of the waste heat of the paper mill has been wasted.

Typical waste heat recovery methods in paper mills

1. Waste heat recovery of closed hood exhaust of paper machine drying section

Generally, it is a two-stage recovery, as shown in the figure, the air-air heat exchanger preheates the air inlet of the hood, and the steam water heat exchanger heats the water in the paper mill.

2. Waste heat recovery of hot air / infrared dryer of coater

After the paper is dried by hot air dryer of waste heat recovery coater, the exhaust temperature of hot air dryer is about 325 ℃, and the humidity is about 300-350g / kg dry air. After the air / air heat exchanger is preheated, the temperature is still as high as 200-250 ℃, which contains a lot of heat energy. As shown in the figure.

3. Waste heat recovery of hot air dryer in Yankee dryer of  tissue machine

After the fresh air is preheated by the heat exchanger, the temperature of the exhaust air is often over 230 ℃, the humidity is about 300-500g / kg dry air, and the heat waste is medium. The recovered heat usually provides heat for the ventilation of the plant, but only a small part of the heat can be used in this way, and most of the energy can be directly discharged. As shown in the figure.

Analysis of existing hot air recovery

All of the above schemes are to use the exhaust waste heat to pass through the air / air heat exchanger and air / water heat exchanger to preheat the process air and process water. In short, the exhaust waste heat is used as the heat source to heat the medium.

For example, the waste heat from the hot air dryer of the coater, the waste heat from the hot air dryer of the Yankee vat of the tissue paper machine, etc. Although the quantity is massive, the waste is not very high. At present, the demand has not been found in the paper factory, so it has to be distributed to the environment.    

Technology of optimization scheme

Cooling capacity is generally through compressor refrigeration, from the conversion of electrical energy. In terms of cooling capacity, it can also absorb refrigeration and transform from heat energy. In terms of the efficiency of energy utilization, the conversion from thermal energy to electric energy is more efficient than that from electric energy. Moreover, there are a large number of medium and low quality waste heat in the paper factory, which has not yet been found for proper use. This part of waste heat can be used to convert into cooling capacity through lithium bromide refrigeration technology, so as to save high-grade electric energy.

There are two main types of cold places in the paper factory: air conditioning system, process low temperature water system (mainly used for sizing or coating workstation and other equipment requiring cold.

1. Air conditioning system

Almost all the modern paper mills of the waste heat recovery equipment manufacturers need to use air conditioners to ensure the safe operation of electrical equipment and meet the improvement of people's working environment.

2. Low temperature water system

In the paper factory, the use of low-temperature water is very small. Generally speaking, paper machines equipped with sizing or coating need to use low-temperature chilled water. It mainly includes: coating preparation workshop chilled water system, coating workstation box sandwich chilled water supply system (working station feed tank), coating head sandwich chilled water supply system, as shown in Figure 6.

Because the coating preparation system and coater are two different systems, which are provided by different suppliers, the general paper mill has more than two sets of compression refrigeration system, which not only consumes a lot of power but also invests a lot.

The method of waste heat recovery in this scheme: first, the hot air is transformed into hot water of a certain temperature (generally above 85 ℃) through heat exchange equipment, such as heat pipe, and then the hot water is introduced into the lithium bromide refrigeration equipment to recover the waste heat that can be used in the hot water, partially or completely replace the current compression refrigeration device, to produce low-temperature water for air conditioning and process use, so as to save a lot of electricity power.

Economic benefit analysis

1. Calculation of the cooling capacity of the exhaust heat of the hot air dryer of the coater

It can be seen from the calculation that in the exhaust heat of a hot air dryer of a coater, the heat that can be recovered per hour is 7994836.9kj/h. If the heat is used for refrigeration, the refrigerating capacity is 1776.6kw (6395869.5kj / h).

2. Calculation of the cooling capacity of the exhaust waste heat of the hot air dryer of the tissue machine

It can be seen from the calculation that in the exhaust heat of the hot air hood of the yankee cylinder of the tissue machine, the heat that can be recovered per hour is 10734377.5kj/h. If these heat is used for refrigeration, the refrigerating capacity is 2385.4kw (8587502kj / h).

This kind of waste heat recovery method can be used in combination with the existing refrigeration system for the paper plant that has been running, greatly reducing the power consumption; for the new paper plant, it can also save the investment of electric refrigeration equipment, and for the factory with living facilities (such as canteen, dormitory, hotel), it can also provide the cold source, saving the investment and operation cost of air conditioning system. At the same time, in winter, when the cooling demand is reduced, the hot water produced by the system can also be used as a heat source to heat pulp, process water, domestic water, ventilation and heating of the plant, and even make-up water for the boiler of the power plant and preheat the air entering the furnace.