Waste heat utilization technology of coke oven flue gas

This paper introduces the waste heat utilization technology of flue gas, such as heat pipe technology, coal humidity control and negative pressure ammonia evaporation. Through the comparative analysis of the waste heat recovery effect, it is pointed out that independent coking enterprises should adopt coke oven gas heating and heat pipe technology to produce steam (or negative pressure ammonia evaporation); Blast furnace gas is used for heating in iron and steel complex, and coal humidity control technology is recommended.

20210821015758 72387 - Waste heat utilization technology of coke oven flue gas

The temperature of coke oven flue gas is 180 ℃ – 300 ℃, and the heat carried out accounts for about 17% of the total output heat of coke oven. At present, most coking plants disperse coke oven flue gas into the atmosphere through chimney, resulting in great energy waste. Under the background of advocating circular economy and sustainable development, the recovery and utilization of waste heat from coke oven flue gas has great economic benefits and significance of energy conservation and emission reduction. At present, the waste heat utilization technologies of coke oven flue gas mainly include heat pipe technology, coal humidity control, negative pressure ammonia evaporation, heating and hot water bathing.

Utilization of waste heat from flue gas

Heat pipe technology

In recent years, the technology of using heat pipe waste heat boiler to recover waste heat from coke oven flue gas to produce steam has developed rapidly because of its low investment and quick effect. Process principle of flue waste heat recovery to produce steam: the heat of hot fluid is transmitted from the heat pipe to the water in the water casing at the heat release end and vaporized. The produced steam water mixture reaches the steam drum through the steam riser, and is output through the steam main control valve after centralized separation. Because the heat pipe continuously inputs heat into the water in the water casing, and completes the basic steam water cycle through the rise and fall of the external steam water pipeline, so as to cool the hot fluid and convert it into steam.
The process flow of steam production from waste heat of coke oven flue gas: a hole is opened in front of the flap valve of coke oven main flue to lead out the waste gas of coke oven main flue, which is introduced into the waste heat recovery system through the regulating butterfly valve. After heat exchange and cooling, the flue gas at about 170 ℃ is pumped through the fan, then discharged into the underground main flue behind the flap valve of main flue through the switching butterfly valve, and finally discharged into the atmosphere through the coke oven chimney. The boiler water is vaporized after being heated, and then incorporated into the steam pipe network after being measured in the steam drum for use by each production workshop. The waste heat recovery system consists of demineralized water treatment unit, deaerator, water tank, deaeration feed pump, boiler feed pump, heat pipe steam generator, soft water preheater drum, riser, downcomer, etc. Its core technology is heat pipe technology to recover sensible heat from flue gas and heat softened water into steam.
The steam production system for waste heat recovery of coke oven flue gas is an energy-saving and emission reduction project. The saturated steam generated can be incorporated into the steam pipe network of coking plant for low-pressure steam users.

Coal humidity control

Coal moisture control is to remove part of the moisture of coking coal before charging, keep the moisture of charging coal stable at about 6%, and then charge the furnace for coking. It is widely used in Japan, Russia and other countries. In China, coal humidity control has become a key technology developed and actively promoted in the coking industry. The use of coke oven flue gas coal humidity control process can not only save energy and reduce the emission of waste gas, waste water and waste heat, but also improve the bulk density of charging coal, the heating rate at the initial stage of coking and shorten the coking time, so as to achieve the purpose of energy saving and consumption reduction.
At present, the heat sources of coal humidity control device mainly include heat transfer oil, steam and coke oven flue gas. In comparison, the coal moisture control process with heat transfer oil and steam as heat sources has the problems of cumbersome equipment and high operation cost; The coal humidification process with coke oven flue gas as heat source can use the waste heat of waste gas to dry the incoming coal, with high thermal efficiency and good energy-saving effect. At present, the coal humidity control processes with coke oven flue gas as heat source mainly include fluidized bed type, pneumatic selective type and fluidized bed type.

Fluidized bed coal humidity control

In October 1996, Japan put into operation a fluidized bed coal humidification device that uses coke oven flue gas to dry coal materials in Muran plant of Beihai iron making company. Its process flow is as follows: the crushed coal materials are sent from the coal bunker to the fluidized bed dryer, the coke oven flue gas entering from the distribution plate is directly in contact with the coal materials to dry the coal materials, and the humidified coarse coal particles are discharged from the dryer into the screw conveyor, The remaining pulverized coal enters the bag filter along with the flue gas of the coke oven, and the recovered pulverized coal is sent to the belt conveyor through the screw conveyor. In order to suppress dust, the humidifier is used to properly humidify the dry pulverized coal, so that the pulverized coal and coarse coal particles are sent to the coke oven coal tower through the belt conveyor.
Process characteristics of fluidized bed coal humidity control: the device is located behind the pulverizer in the coal preparation workshop. In the fluidized bed dryer, the high-speed oblique air flow is ejected by the air distribution nozzle to fluidize the coal and move it to the outlet; Only dry and de powder, not graded.

Pneumatic selection of coal humidity control

In 2007, Jigang put into operation a set of pneumatic selective coal humidity control device. The device is located in front of the pulverizer and has the function of air separation. Firstly, the coal material with qualified particle size less than 3mm is air selected to reduce the load of the pulverizer; The pulverized coal filtered by the bag filter is pressed into briquette coal and fed into the furnace for coking. The process flow is as follows: the coal a enters the air separation and humidity regulator first through the distributor, the coke oven flue gas is blown in the lower part of the air separation and humidity regulator by the blower, the boiling layer is established in the coal seam flowing above the air separation and humidity regulator, the light particles and fine particles are dried and separated into fine particles, and the coal flow B directly enters the coke oven coal tower. The heavy particles and large particles are separated into coarse particle coal flow C and sent to the crusher for crushing. The crushed coal material D passes through the transfer station and re merges with the primary coal flow a of the matching coal before the inlet of the air separation and humidity regulator. After that, the coal material e enters the air separation and humidity regulator for re humidification.
Characteristics of pneumatic selected coal humidity control process: the unit is located in front of the pulverizer in the coal preparation workshop and has the function of air separation. The coal material less than 3mm is directly sent to the coal tower, in which the large particles and heavy particles enter the pulverizer for crushing, and the crushed coal is subjected to air separation to form a closed cycle. There are no large particles and heavy particles in the charged coal, and the moisture is reduced to 6% ~ 7%, It not only realizes the pneumatic selective crushing of the incoming coal, but also realizes the moisture regulation of the incoming coal; The pulverized coal filtered by the bag filter is pressed into the furnace for coking.

Vibrating fluidized bed coal humidity control

The vibrating fluidized bed coal humidification device can be used for coal humidification of incoming coal and can also be used to cooperate with coal grading humidification.
The process flow: the coal material is continuously thrown onto the vibrating fluidized bed staged dryer through the sowing device. Under the action of vibration force and high-speed air flow, all coal materials are in violent movement 3mm large particle coal is sent to the pulverizer; The medium particle coal material rises rapidly with the air flow from the central area of the air distribution plate, falls down along the two side walls or falls down from the low speed. After such circulation for several times, it can reach the discharge end; Small particles rise with the air flow and are carried out with the air flow and collected by the dust collector.
Characteristics of coal moisture control process of vibrating fluidized bed: the coal moisture control device is located in front of the pulverizer, the coal material is on the bed surface of the vibrating fluidized bed staged dryer, and all particles are in violent movement under the action of vibration force and high-speed air flow; Different particles have different motion states; Fine particles are subject to entrained flow bed drying, medium particles are subject to internal circulating fluidized bed drying, and large particles are subject to vibrating fluidized bed drying; High classification efficiency; It is selective for the drying of coal particles with different particle sizes; > 3mm moisture regulating coal is discharged from the dryer and sent to the pulverizer for crushing.

Problems needing attention in coal humidity control technology

  • 1) Since the moisture of coal material decreases after humidity regulation and the dust generated during transportation increases, it is necessary to strengthen the tightness of belt, corridor and other devices and set dust removal facilities.
  • 2) Graphite removal facilities must be set to effectively remove graphite to avoid affecting the operation of coke oven.
  • 3) After humidification, the pulverized coal is mixed into the gas purification system, and the tar quality decreases. An air jet tower must be added in front of the primary cooler to wash the raw gas, and a super centrifuge must be used to deslag and dehydrate the tar to ensure the quality of the tar.

Ammonia evaporation under negative pressure

The negative pressure distillation process technology is based on the principle that the volatilities of each component in the liquid mixture are different and the boiling point of the liquid decreases with the decrease of pressure. After the liquid mixture is preheated to a certain temperature, it is sent to the negative pressure distillation tower for negative pressure distillation, and the tower bottom is heated to separate the components of the liquid mixture. This technology can reduce the operating temperature Achieve the effect of energy saving and consumption reduction.
Negative pressure ammonia evaporation process with coke oven flue gas as heat source: the remaining ammonia is sent to the ammonia distillation tower for distillation after heat exchange through the heat exchanger. After the ammonia steam on the top of the ammonia distillation tower is cooled by the divider and cooler, the cooled ammonia enters the reflux tank, and the non condensable steam on the top of the tank enters the gas suction pipeline after cooling by the cooler under the suction of the vacuum device; The ammonia water at the bottom of the tank is pumped out by a reflux pump, part is sent to the top of the ammonia distillation tower for reflux, and part is sent out as product ammonia. The ammonia evaporation wastewater at the bottom of the ammonia evaporation tower enters the flue gas heat pipe heat exchanger for circulating heating and then returns to the ammonia evaporation tower; The other part of ammonia evaporation wastewater is sent to the wastewater treatment unit after heat exchange and cooling with the residual ammonia of raw materials.
The traditional ammonia steaming method directly uses steam to heat and discharge it together with the tower bottom wastewater after condensation. The ammonia steaming process not only does not form wastewater emission reduction, but also increases the ammonia steaming wastewater and consumes a lot of steam heat energy, resulting in a waste of energy and polluting the environment.
The process principle of negative pressure ammonia evaporation technology is the same as that of steam production by coke oven flue gas heat pipe technology, which uses the waste heat of coke oven flue gas. The difference is that negative pressure ammonia evaporation technology directly uses heat pipe heat exchanger to heat ammonia evaporation wastewater without producing steam. When ammonia is evaporated under negative pressure, the pressure at the top of the ammonia distillation tower is reduced from the original 10KPA to about – 40kpa, and the distillation temperature is reduced from 105 ℃ to about 80 ℃.

Comparative analysis of waste heat recovery benefits

Due to the different types of coke oven heating gas, the temperature of waste heat utilization of coke oven flue gas varies greatly. The heat pipe technology for steam (or negative pressure ammonia evaporation) produced by coke oven flue gas with an annual output of 1.2 million tons of coke is compared with coal humidity control technology.
When the waste heat of coke oven flue gas is used to produce steam (or negative pressure ammonia), the coke ovens of independent coking enterprises are mostly heated with coke oven gas. The inlet temperature of flue gas is high, and the temperature difference of available waste gas is high, so the steam output is high. The coking plants of iron and steel complex mostly use blast furnace gas for heating, the inlet temperature of flue gas is low, and the steam output is small. It can be seen from table 1 that the steam output of flue gas heated by coke oven gas is more than that heated by blast furnace gas.
When the coke oven flue gas is used for coal humidification, the water content in the flue gas is high due to the high hydrogen content in the coke oven gas. When it is used as the heat source of coal humidification, it is not conducive to the evaporation of coal moisture, and generally can only reduce the moisture of the incoming coal by about 2.5%. When the blast furnace gas is used for heating, the blast furnace gas contains less hydrogen, so the moisture content in the flue gas is less. The flue gas can meet the needs of coal humidity control heat. Now, the moisture of the incoming coal is usually reduced from 10% to 6%. The flue gas heated by blast furnace gas can reduce more coking heat consumption than that heated by coke oven gas.
When coke oven gas is used for heating, although the flue gas temperature difference that can be used by coal humidity control technology is greater, it is not conducive to coal humidity control due to the high moisture content in flue gas. Moreover, the coal humidity control technology has long process flow, complex equipment and high relative heat loss. The economic benefit of using heat pipe technology to produce steam from flue gas is better than that of coal humidity control technology.
When blast furnace gas is used for heating, the temperature difference of waste gas that can be used by coal humidity control technology is obviously higher than that of heat pipe technology, and coal humidity control technology has other economic benefits, such as reducing the amount of coking wastewater, improving coke quality and output. Therefore, the economic benefit of coal humidity control technology is obviously better than that of steam production by heat pipe technology.
In conclusion, for independent coking enterprises, due to the use of coke oven gas for heating, it is not recommended to use coal humidity control technology, but rather to use flue gas waste heat heat heat pipe technology to produce steam (or negative pressure ammonia evaporation). For iron and steel complex enterprises, due to the use of blast furnace gas heating, it is recommended to adopt flue gas waste heat coal humidity control technology.

Source: China Gas Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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