The waste heat includes two parts of heat that are available and unavailable. The recovery rate of waste heat energy refers to the percentage of recovered waste heat resources to the total waste heat. Waste heat utilization technology is the study of recoverable waste heat and its recycling methods, technologies and economic benefits.

Waste Heat Resources Classification

Traditionally, people often divide waste heat resources based on the temperature of the waste heat resources, which is convenient and intuitive, and can reflect the size of the waste heat. Generally, it can be divided into:

1) High temperature residual heat, greater than 500°C.

2) Middle temperature residual heat, 300~500℃.

3) Low temperature residual heat, less than 300℃.

Industrial boilers have a wide range of applications and a large number, most of which are in low-temperature waste heat. Recovering a large amount of low-temperature waste heat is of great significance to energy conservation and emission reduction.

Steam Condensate Recycling

In the process of steam utilization, energy saving and emission reduction can be carried out. Three energy-saving links must be grasped, namely, steam production, transmission and terminal use.

How to recycle steam condensate? To save energy and reduce emissions, it is necessary not only to recover clean water resources, but also to recover the latent heat of condensate, so as to promote the efficient use of steam-using equipment.

At present, we have manufactured various types of traps and studied various recycling methods. Due to the high steam leakage rate and short life of most steam traps, it cannot recover all the drain and heat energy to the boiler room. Among them, the introduced SG nozzle type steam trap and multi-aperture rotary disc nozzle type steam trap can form a condensate recovery system and can achieve satisfactory energy-saving and emission-reduction effects. The condensate recovery rate is 100%, the condensate heat recovery rate is more than 85%, the energy saving of the steam boiler is more than 20%, and the output of steam equipment and product quality can be improved.

Wastewater Recycling

1. Sewage Heat Energy Utilization

Boiler blowdown water is saturated water under operating pressure and contains physical sensible heat at the corresponding temperature. For every 1% increase in the blowdown rate of general industrial boilers, the fuel consumption will increase by more than 0.2%, and at the same time, the cost of water supply treatment will increase and the environment will be polluted.

Many boiler manufacturers have adopted methods to recycle the thermal energy of sewage, and have achieved good energy-saving effects, which can save fuel by 2%-5%.

2. Sewage Resources Utilization

Ways to Recycle Sewage Resources:

1) For water film dust collector to improve flue gas desulfurization efficiency.

The sewage is the high-alkalinity boiler water discharged from the boiler. After the heat energy is recovered, it can also be used for water film dust removal, SO₂ removal from the flue gas, or neutralization. Generally, the desulfurization efficiency can reach 30%~50%.

2) It is used to neutralize the waste acid to achieve discharge standards.

After sewage treatment, the alkalinity of the boiler sewage is very high, with a pH value of about 12, which can be used to neutralize acidic wastewater and add necessary chemicals to meet the discharge standards and save water treatment costs.

3. Recovery and Utilization of Alkali in Wastewater

Boiler sewage contains a certain amount of alkaline substances such as NaOH and Na₂CO₃. The alkali content of the boiler sewage is different under different pressures.

The content of NaOH, Na2CO3, etc. in boiler sewage is relatively high. These useful substances should be recycled and purified. This is not difficult in the chemical industry.

Waste heat recovery boiler

4.Apply reverse osmosis water treatment technology to solve the sewage problem

In order to ensure the boiler safe and economical operation, sewage discharge must be implemented, which wastes a lot of water and heat and causes environmental pollution. Reverse osmosis water treatment technology can produce pure water, which is more suitable for boiler water quality standards, and the pollutant discharge rate can be reduced to about 1%, realizing scale-free operation.

Waste Heat Recovery Heat Energy Conversion Device

Waste Heat Boiler

Waste heat boiler is also called waste heat boiler. It is a heat exchange device that utilizes various waste heat resources discharged by industrial and mining enterprises in the production process, especially high-temperature waste heat sources, and recycles these waste heat sources to produce steam or hot water.

Waste Heat Boiler Characteristics

The biggest feature of waste heat boiler is that the produced high-temperature, high-pressure steam can not only generate electricity, but also provide heat. The implementation of combined heat and power has the most economic benefits and plays an important role in energy saving and emission reduction.

Waste Heat Boiler Types

(1) High temperature flue gas waste heat

It is characterized by large output, strong continuity, and easy recycling. It is the most common form of waste heat. This kind of waste heat is most suitable to use a waste heat boiler to recover heat, used to produce steam, preferably a combined heat and power, and the capacity is generally relatively large.

(2) Residual heat of chemical reaction

In the production process, there is a lot of waste heat generated by chemical reactions, such as sulfuric acid, phosphoric acid, fertilizer, chemical fiber, ceramics, metallurgy and other industries.

(3) Residual heat of combustible exhaust gas and waste liquid

Such as blast furnace gas, converter gas, coke oven gas, catalytic cracking regeneration waste gas from refineries, exhaust gas from carbon black plants, black liquor from paper mills, etc.

(4) High temperature slag waste heat

Such as blast furnace slag, converter slag and electric furnace slag. The slag temperature is above 1000℃, and the heat content per kilogram of slag is 1250-7150kJ.

(5) Residual heat of high-temperature products

Such as cement burned mature material, coke oven coke, steel ingot billet, high temperature forging, etc. Generally, the temperature is very high and contains a lot of waste heat.

(6) Cooling medium, residual heat of condensate water

A large amount of cooling water and low-temperature steam discharged from various cooling devices, steam used in the production or life process of industrial boilers, becomes water after cooling, and all contain a large amount of waste heat.

Waste Heat Boiler Classification

According to the structure of the waste heat boiler, it can be divided into shell-and-tube waste heat boiler and flue type waste heat boiler.

(1) Shell and Tube Waste Heat Boiler

Shell and Tube Waste Heat Boiler Structure:

The flue gas of a shell-and-tube (fire tube) waste heat boiler generally runs through the tube side, and the shell side is hot water or a mixture of steam and water.

Shell and Tube Waste Heat Boiler Features :

①The airtightness is good, and it can be operated under positive pressure;

②The heated tube is washed longitudinally, and it is not easy to accumulate dust. The heated tube can be internally threaded to enhance the heat transfer effect;

③Quick start and fast steam production;

④Compact structure, small size, light weight, small footprint and low metal consumption;

⑤The layout is flexible and the installation is convenient;

⑥Reliable work, safe operation and easy operation;

⑦Low operating cost, less maintenance, long life, and significant benefits.

Shell and Tube Waste Heat Boiler Application :

Shell-and-tube waste pyrolysis households are mostly used for the recovery of flue gas with complex components, corrosive media or waste heat with a high amount of flue gas. Such as the sulfuric acid industry, phosphoric acid industry, glass fiber, carbon black industry and other industries, but the general capacity is relatively small.

(2) Flue Type Waste Heat Boiler

Practice has proved that since the 1970s, the flue-type waste heat boiler is relatively reliable after long-term operation.

Flue-Type Waste Heat Boiler Structure

As a kind of waste heat boiler, flue-type waste heat boiler is mainly used for waste heat recovery with high smoke content, low smoke melting point, high smoke temperature, easy adhesion, easy to cause ash clogging and abrasion, and is a common furnace type at home and abroad.

Flue-type Waste Heat Boiler Features

① Achieve the due dust collection effect. The waste heat boiler should have a large enough cooling space so that the dust particles in the flue gas have a fixed settling time.

②Prevent the dust accumulation and the removal of dust accumulation. Considering the design of boiler structure to prevent dust accumulation as much as possible, such as setting a sufficiently large radiation cooling chamber to cool the flue gas below the bonding temperature of the flue dust.

③ Prevent corrosion. For low-temperature corrosion, make the pipe wall temperature higher than the dew point temperature of sulfuric acid (increase the operating pressure of the boiler to increase the saturated water temperature).

④Reduce wear. Reduce the flue gas flow rate, and at the same time, because the flue gas longitudinal scouring wear is much lighter than the horizontal scouring wear, so most of the heating surface should be longitudinal scouring.

(3) Straight-Through Waste Heat Boiler

As a kind of waste heat boiler, the straight-through waste heat boiler is mainly used in the chemical industry, fertilizer industry, ethylene industry, methanol industry, coke industry, building materials industry, etc. This type of industry produces a large amount of flue gas, and the flue gas has a high dust content, so it can be designed into a large-capacity, high-parameter waste heat boiler.

Straight-Through Waste Heat Boiler Type

There are many types of straight-through waste heat boilers, the common ones are double-boiler tube-bundle waste heat boilers and tunnel-type waste heat boilers.

Straight-Through Waste Heat Boiler Features

①The boiler adopts a single-drum vertical or horizontal cloth, the flue gas has no turning in the flue, and the circulation resistance is small:

②The flue gas washes the heated surface horizontally, the entire heated surface is washed intact, the heating is even, and dust accumulation and corrosion are reduced;

③Compact structure, micro-negative pressure operation, outsourcing adopts sealing and welding structure, good sealing performance, no environmental pollution, high waste heat utilization, convenient operation, safe and reliable;

④The boiler has low manufacturing cost, convenient construction, short installation period, low cost, and low capital investment;

⑤There are multiple ash outlets in the lower part of the boiler. In order to facilitate the ash removal of the boiler, the entire boiler system is installed on a foundation plane 1.5~3m above the ground;

Tunnel-type Waste Heat Boiler Characteristics:

1. The drum is placed outside the flue, not heated, and can expand freely. All connections of the boiler are welded, which improves the safety performance of the boiler;

2. The heating surface of the boiler is all manufactured in the factory, the quality is easily guaranteed, and the on-site installation workload is small;

3. The boiler is manufactured and installed in sub-components, which is convenient for replacement and maintenance.

Precautions for designing straight-through waste heat boiler

① Corrosion problem. The problem of low-temperature corrosion can be solved by increasing the operating pressure of the boiler and the use of special materials for the air preheater; measures such as controlling the temperature in the furnace and keeping the heating surface clean to prevent high-temperature corrosion of the boiler.

②Dust accumulation problem. A large cavity radiation cooling chamber can be used to reduce the flow rate of the flue gas, so that the large particles in the flue gas can be fully settled. In the convection section, the flue gas scours the tube bundle laterally to reduce fouling and corrosion. For occasions where smoke and dust are easy to stick, try not to install a superheater to prevent high-temperature corrosion on the one hand, and prevent dust accumulation on the superheater on the other hand.

③Abrasion problem. Try to ensure that the flue gas flow in the waste heat boiler is stable and uniform, and control the flue gas flow rate according to the characteristics of the flue gas in the flue gas, which can effectively reduce the wear of the flue gas on the heating surface.

Conclusion:

There are many ways to recover waste heat, but there are three main ways: direct use of waste heat, waste heat power generation, and comprehensive utilization of waste heat. As explained above, there are three perspectives for the recovery and utilization of industrial boiler waste heat resources: the recovery and utilization of steam condensate, the recovery and utilization of sewage, and the waste heat recovery heat energy conversion device.

In the daily operation of industrial boilers, if we pay attention to energy recovery and utilization in these aspects, this will save fuel, improve economic efficiency, and play an important role in energy saving and emission reduction.