Waste heat recovery from coke oven flue gas

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The coke oven flue gas waste heat boiler is now officially introduced: This boiler enables the recovery of 80 Kg of steam per ton of coke, allowing the entire investment to be recouped within one year. Typical applications of this waste heat boiler include: Shanxi Taihua – 900,000 tons per year, 1 unit; Tangshan Dafeng Steel – 700,000 tons per year for the first phase, and 1,100,000 tons per year for the second phase, 1 unit each; Tangshan Dongfang Coking – 1,200,000 tons per year, 1 unit; Jiangxi Heimao Carbon Black – 600,000 tons per year, 1 unit; Jingdezhen Coking – 600,000 tons per year, 1 unit. I. Overview of the coking process: The coal blend delivered from the coal handling area is stored in coal towers. Coal trucks retrieve coal from these towers according to a predetermined schedule, and after measurement, it is fed into the carbonization chamber. In the carbonization chamber, the coal undergoes high-temperature dry distillation over a coking cycle to produce coke and raw coal gas. Once the coke in the carbonization chamber is mature, it is pushed out using a coke pusher; it is then guided into the coke quenching car by a coke stopper car, and the coke quenching car is pulled to the coke quenching tower by an electric locomotive for water quenching. After coking is stopped, the coke is unloaded onto a cooling bed; after cooling for a certain period of time, it is sent to the coke screening section, where it is sorted by grade and stored for shipment. The raw gas generated during the carbonization process of coal in the carbonization chamber gathers in the upper space of the chamber, and then enters the gas collection pipe via the rising pipe and bridge pipe. The raw gas at around 700°C is cooled to about 90°C by spraying it with ammonia water inside the bridge tube. Tar and other substances in the waste gas are also condensed at the same time. Gas and the condensed tar, together with ammonia water, are sent to the gas purification workshop via gas suction pipes. Coke oven gas for heating the coke ovens is introduced via external pipelines overhead. After preheating, the coke oven gas is sent to the basement of the coke oven, where it is fed into the bottom of the combustion chamber’s flame channel through lower nozzles, where it burns in combination with the air that enters via the exhaust gas exchange valve. The exhaust gases after combustion enter the downward-flowing flue through the holes at the top of the vertical flue, then pass through the heat storage chamber; where the grid bricks recover some of the sensible heat contained in these gases. After that, they proceed through the small flue, the exhaust gas exchange valve, the branch flues, the main flue, and finally are discharged into the atmosphere via the chimney. PDF files were created using the trial version of “pdfFactory Pro”. 3www.fineprint.com.cn 1 There is still significant potential for heat recovery in relation to the hot flue gases that enter the chimney through the main flue. This solution is designed to recover the waste heat from this portion of flue gas. II. Process flow diagram for waste heat recovery 1. Flue gas flow: An opening is made in front of the flap valve in the underground main flue, allowing the hot flue gas from the main flue to be directed out of it. After passing through the waste heat recovery system where it loses heat and its temperature is reduced to around 160°C, the gas is sent back into the underground flue behind the flap valve via the boiler exhaust fan, and finally discharged through the chimney. 2. Composition of the waste heat recovery system: This system consists of a water softening unit, a deaerator, water tanks, a deaerated feedwater pump, a boiler feedwater pump, a medium-temperature heat pipe steam generator, a soft water preheater, a low-temperature heat pipe steam generator, a drum, rising pipes, descending pipes, external connection pipelines and control instruments, as well as a boiler exhaust fan; these components operate independently of one another. PDF files are created using the trial version of “pdfFactory Pro” at www.fineprint.com.cn 2 3. Steam generation process: Industrial softened water is pumped by a soft water pump into a thermal deaerator for deoxygenation. Part of this deoxygenated water is fed by a feed pump into a heat-tube soft water preheater, where it is preheated before entering the steam drum. The water then flows through downcomers into a medium-temperature heat-tube steam generator, where it absorbs heat and becomes saturated water. This saturated water subsequently enters the steam drum through upcomers, where water and steam are separated to produce saturated steam at 0.6 MPa, which is then sent to the main steam pipeline or to end-users ； Another portion of the deoxygenated water is fed by the feed pump into the low-temperature heat pipe evaporator; after being heated, it enters the low-pressure drum where steam and water are separated to produce saturated steam at 0.3 MPa, which is then sent to the deaerator for further deoxygenation or to the end users. 4. Main principles of waste heat recovery: 1) The principle of the steam generator is as follows: The heat from the hot fluid is transferred through heat pipes to the water inside the water jacket at the heat-exchanging end (water enters via the downcomer), causing it to vaporize. The resulting mixture of steam and water rises to the steam drum through the steam upcomer; after separation, it is then discharged through the main steam control valve. In this way, as the heat pipe continuously transfers heat to the water inside the water jacket, and the basic steam-water cycle is carried out through the rise and fall of the external steam-water pipes, it is possible to cool the hot fluid and convert it into steam. 2) The working principle of the economizer is as follows: The heat from the hot fluid is transferred to the water inside the water jacket at the heat-dissipating end through the finned heat pipes. The water absorbs this heat, which causes the hot fluid to cool down; as a result, the water in the jacket moves from an undersaturated state to a saturated state at the corresponding pressure, before entering the steam drum to participate in the natural circulation process. 3) Working principle of the low-temperature steam generator: The heat from the hot fluid is transferred by heat pipes to the water inside the water jacket (the water enters via the downcomer), causing it to vaporize. The resulting mixture of steam and water rises through the steam upcomer to the low-pressure steam drum; after centralized separation, it is then discharged through the main steam control valve. In this way, the heat pipe continuously transfers heat to the water inside the water jacket, and the basic steam-water cycle is accomplished through the rise and fall of the external steam-water pipes, thereby achieving the purpose of cooling the hot fluid and converting it into steam. Its low-pressure saturated steam is mainly used for thermal deoxidation in systems. 5. Main features of waste heat recovery equipment: (1) High heat transfer coefficient. The heat exchange between the exhaust gas and water as well as water vapor takes place on the outer surface of the heat pipe. Additionally, fins are present on the outside of the exhaust gas heat pipe, which increases the heat exchange area and enhances heat transfer, thereby resulting in a significant increase in the heat transfer coefficient ； (2) It has strong resistance to dust accumulation, clogging, and corrosion. By adjusting the ratio of the heated surfaces in the hot and cold sections of the heat pipe, it is possible to raise the wall temperature above the dew point temperature of the flue gases or the area subject to the most severe corrosion. The PDF file can be created using the trial version of “pdfFactory Pro” at www.fineprint.com.cn. (3) The hot and cold fluids are completely separated, effectively preventing leaks in the water vapor system. During operation, due to the intense scouring by exhaust gases, even if the tubes suffer some damage, it prevents steam and water from the cold side from leaking into the hot side, thus ensuring the safe operation of the system. This is the main feature that distinguishes this device from conventional waste heat recovery devices used in flues. (4) It has low resistance loss, making it suitable for the retrofitting of old units. Under normal circumstances, with the addition of waste heat recovery equipment, the resistance of the hot exhaust gas increases by about 800 Pa. (5) The damage to one or multiple heat pipes does not affect the overall operation of the equipment. III. Technical and Economic Indicators
Coking waste heat recovery: 1.3 million yuan per year per ton (as an example)

1. Original process parameters:
- Flue gas volume: 250,000 Nm3/h (provisional)
- Flue gas temperature: 260°C (provisional)
- Number of working days per year: 330

2. Parameters of the waste heat recovery system:
- Flue gas temperature at the outlet of the heat pipe exchanger: ≤150°C
- Pressure drop across the heat pipe exchanger: ~0.8 kPa
- Operating pressure of the deaerator: 0.15 MPa
- Operating pressure of the steam drum: 0.7 MPa
- Steam flow rate: >13.5 t/h
- Electrical power required for the system: ~400 kW

3. Estimated investment cost for the system
Summary table of estimated costs for waste heat recovery project
| Serial No. | Item | Formula/Source | Cost (10,000 yuan) |
|-------------|------|----------------|--------------------|
| 1 | Thermal equipment | – | 520 |
| 2 | Electrical and instrumentation equipment | – | 55 |
| 3 | Thermal construction costs | – | 55 |
| 4 | Electrical and instrumentation installation costs | – | 15 |
| 5 | Boiler frame | – | 40 |
| 6 | Steel flues | – | 50 |
| 7 | Insulation | 1,400 square meters × 30 | 42,000 |
| 8 | Infrastructure costs | – | 50 |
| 9 | Design fees | – | 20 |
| 10 | General contracting management fees | – | 20 |
| 11 | Commissioning costs | – | 5 |
| 12 | Contingency fund | – | 30 |
| 13 | Total cost | – | 890 |

4. Summary table of main process equipment
| Serial No. | Equipment name, model, technical specifications, performance unit | Quantity | Remarks |
|-------------|----------------------------------------------------------------------------------|----------|---------|
| 1 | Boiler body | 250,000 Nm3/h; maximum inlet flue gas temperature: 250–300°C; maximum outlet flue gas temperature: 160°C | 1 set | |
| 2 | Exhaust silencer | P=0.6 MPa; exhaust flow rate: 20 t/h; noise reduction level: 40 dB(A) | 1 set | |
| 3 | Deaerator feed pump | IS65-50-160 | 2 units | 5.5×2 KW |
| 4 | Boiler feed pump | DG25-30X5 | 2 units | 22×2 KW |
| 5 | Deaerator | RLCY-20 | 1 set | |
| 6 | Induction fan | Y4-73 NO.25D | 1 unit | 400 KW |

6. Economic benefit analysis
Energy consumption after the renovation:
Soft water: 14.5 t/h, at a cost of 6.5 yuan per ton. Power consumption: 400 kW, at 0.7 yuan per kilowatt-hour. Energy output after renovation: Steam: 0.7 MPa, 14 t/h, at 140 yuan per ton. Energy efficiency benefits after the renovation: 14×140 – (14.5×6.5 + 400×0.7) = 1585.75 yuan/hour. Labor costs: 120,000 yuan/year. Equipment maintenance costs: 120,000 yuan/year. The PDF file was created using the trial version of “pdfFactory Pro” at www.fineprint.com.cn. Total annual operating hours of the system: 360×24 = 8640 hours. * Annual economic benefits after the renovation: 8640×1585.75/10,000 – 12 – 12 = 13,460,880 yuan. 7. Bonus for energy conservation and emission reduction that can be claimed: 1) Annual savings in standard coal: 14×24×360×0.16 = 19,353.6 tons/year ； 2) According to the **standard, a reward of 300 yuan is given for each ton of standard coal saved: 19353.6×300/10000 = 5,806,080 yuan. 8. Conclusion: Based on a total project investment of 8.9 million, the entire investment can be recovered within approximately 150 days. Payback period for investment: (890 – 580.608) / 1346.088 × 12 × 30 = 82 days. (Note: Without applying for energy-saving subsidies, the payback period is generally around 230 days.) For more information on heat pipe waste heat boilers and their application in waste heat boilers in other industries, please visit the company’s website. Tianjin Huaneeng Energy Equipment Co., Ltd. Website address: http://www.tjhuaneng.cn Phone/Fax: 022-29110031 Contact person: Chen Fujun, 13752373530 Email: 13752373530@163.com Photo 1 PDF document