01 Existing problems

Two clinker production lines of a company (line #1 designed clinker production capacity 700t/d, line #2 designed clinker production capacity 2500t/d) are equipped with a 7.5MW pure low-temperature waste heat power generation project. The single-pressure system is adopted. The steam turbine is low-parameter pure condensate steam turbine, with rated power of 7500kW, rated inlet pressure of 1.0MPa, rated inlet temperature of 340℃, exhaust pressure of 0.008MPa, internal efficiency of 81%, and speed of 3000r/min. Supporting the form of four furnaces and one machine, #1 line cannot generate electricity alone, #2 line can generate electricity alone.

At the beginning of operation, both lines were produced, and the power generation per ton of clinker was about 33kWh. However, since 2014, the power generation decreased year by year. In 2015, with the shutdown of #1 production line, the power generation was less than 30kWm. Main problems: steam turbine exhaust temperature, water temperature of water injection tank, condensation water temperature, vacuum degree, temperature difference of circulating water inlet and outlet water, end difference value are seriously beyond the reasonable value; The heat exchange effect of SP boiler and AQC boiler is not good, and the inlet temperature and pressure of AQC boiler fluctuate greatly. The total gas production is low, the steam temperature and pressure are also low, resulting in tons of clinker power generation capacity cannot reach the design value for a long time.

02 Cause Analysis

(1) After the shutdown of the inspection, it was found that there were more solid sediments in the circulating pool, the jet tank and the condenser of the waste heat power generation system, and there were harder fouling in the 20mm stainless steel pipe junction inside the condenser. About 230 pipes were blocked, leading to abnormal operation of the condenser, and the parameters could not be controlled within the normal range.

(2) The temperature of fume air at kiln head of Line #2 is unstable. The temperature of fume air at kiln head fluctuates with the fluctuation of cement kiln. Most of the time, the temperature of kiln head is lower than 380℃, and the lowest temperature is only 260℃.

(3) It was found during shutdown maintenance that the evaporator exhaust pipe of Line 2 kiln head boiler was partially deformed, about 20% of the surface had ash accumulation, the area of smoke air flow was reduced, and the thickness of clinker scale on the outer surface was about 0.5mm.

(4) There is a lot of ash accumulated between the tubes of the boiler at the end of the kiln of #2 lines, which affects the heat dissipation. The original vibration hammer of the boiler at the end of the kiln often breaks, which cannot guarantee full operation, leading to poor ash removal effect.

(5) After calibration, the air leakage of the by-pass valve at the end of the kiln is large.

03 Solutions

3.1 Waste heat power generation system

(1) The successful application of real-time online cleaning and enhanced heat exchange system (RCCS) in each heat exchange tube of the condenser. The device includes: screw button plate, special steel support, ceramic bearing and other components. Without external power, the circulating water is driven by its own velocity. Under the action of spiral torque, the circulating water rotates rapidly with the flow, changing the flow form of circulating water from laminar flow to turbulent flow, so as to prevent particles from staying and gathering in the pipe and prevent the formation of hard scale. After 7 months of use, it was found that there was slight mud and no hard scale on the surface of the screw button of the RCCS by checking the condenser at the kiln stoppage.

(2) 20 rows of the AQC boiler super heater were replaced by maintenance, and the clinker scale of the other rows was thoroughly cleaned.

(3) All the vibration hammers of the SP boiler ash cleaning system were transformed, and the original bolt type was transformed into an integrated one, which significantly reduced the failure rate and effectively improved the vibration effect.

(4) Improve the utilization rate of waste heat power generation circulating water pump. The original operating procedures stipulate that the circulating water pump is equipped with two for one, and only runs two for a long time. The pressure has been found to be on the low side. In order to support the normal operation of the RCCS system, the operation procedure was changed to three pumps running simultaneously for 4h every day and running once every 12h. The water pressure difference between the inlet and outlet of the condenser was increased from the original 0.1MPa to 0.16MPa, which effectively ensured the water flow rate.

3.2 Cement kiln head part

(1) The smoke air duct was modified. The original air intake point was at the front end of the second section of the grate cooler, and the same pipe runoff outlet was added in the first section of the grate cooler (the second chamber), as shown in Figure 1. Two pipes are installed with a high temperature butterfly valve to adjust and control, effectively improve the smoke air temperature, the smoke air temperature from the original 360℃ to more than 430℃.

Fig. 1 New heating air duct diagram

(2) the grate cooler is reformed, the fixed grate plate from 3 rows increased to 7, the grate plate structure is optimized, the characteristics of the fixed grate bed is for the way the wind is in accordance with the order of grate plate “column” order for the wind, all will be a grate plate replacement for control flow aeration grate plate and resistance grate plate, effectively improve the efficiency of the clinker cooling, at the same time improve the kiln head smoke wind temperature and secondary air temperature.

(3) After air volume balance calculation, the rated parameters of the original four high-pressure cooling fans supporting the grate cooler did not meet the design requirements, so the four fans 3302, 3304, 3305 and 3306 were transformed, and the parameters before and after the transformation were shown in Table 1. After the modification, the outlet pressure of the fan in the fixed bed was increased from the original 7500Pa to 11000Pa, and the lower pressure of the grate was increased from the original 6500Pa to 7500Pa, and the clinker temperature of the grate cooler was significantly reduced.

Table1 Technical parameters before and after modification

3.3 Cement kiln tail

(1) Effectively seal of SP boiler bypass shutter valve, the gate when the gate valve opened over the 10 # steel channel of invert welding on the gate when the gate valve opened under the bearing surface of welding on the surface of channel steel, channel steel welding to ensure channel steel and deduction to come into contact with the center of the channel, each other, form a labyrinth seal, the valve plate is not affected when open, reliable sealing.

(2) All the flanges of the high temperature fan are made into a box body with thin steel plate for sealing, so as to avoid the air leakage of the high temperature fan.

3.4 Ingredients and operation adjustment

(1) The original three component ingredients of limestone, clay and mining waste rock were adjusted to four component ingredients, and 1.2% coal gangue was added to the raw material ingredients to increase the C1 temperature by 15℃ on the basis of original temperature and increase the gas production of the kiln tail boiler.

(2) Strictly control the three rate value of stable clinker. The clinker rate value will not only affect the calcination and clinker quality of cement kiln, but also have a great influence on the waste heat power generation. When the clinker KH is too low, the heat needed for clinker firing is less, and the heat brought out by the clinker cooling is also less. When the clinker KH is too high, the clinker granulation is not good, there are more flying sand, the grate cooler bed is not easy to control, and the power generation will also be reduced. According to the comprehensive efficiency of cement kiln and waste heat power generation, the three-rate value of the company is controlled as KH =0.890, SM=2.7 and IM=1.6.

(3) The operation concept was changed from “just focus on the kiln without taking into account waste heat power generation” to “taking into account both cement kiln and waste heat power generation”, and the power generation capacity of tons of clinker was included in the assessment index of cement kiln operators. The inlet air temperature and air volume of AQC boiler are strictly required, and shall not be adjusted substantially without special circumstances. Under the premise of ensuring grate cooler clinker material layer, air volume of grate cooler fan in each chamber, and air volume of kiln tail and kiln head, the clinker cooling efficiency is significantly improved, and the temperature and air volume of secondary air and waste heat AQC smoke air for power generation are stable.

04 Effects

Through the implementation of the above transformation measures and the adjustment and coordination in operation and batching, the power generation of the waste heat power generation system has been steadily improved, and the cement kiln system has been running stably. Comparison of data of waste heat power generation system in recent 3 years is shown in Table 2.

parameter	In 2015	In 2016	From March to July 2017
AQC inlet temperature	351	400	450
AQC inlet negative pressure	-630	-620	-780
AQC outlet temperature	114	120	105
SP inlet temperature	305	335	330
SP inlet temperature	-6525	-6600	-6910
SP outlet temperature	180	187	185
Main steam temperature	322	340	360
Main steam pressure	0.71	0.75	0.78
Main steam quantity	22	23	27
Vacuum degree	-0.076	-0.078	-0.081
terminal temperature difference	14	5.6	4.5
Bypass valve opening	0	0	0
Generating power	3100	3800	4500
Tons of clinker power generation	30	31	34.5

Table 2 Comparison of data of waste heat power generation system in recent years

It can be seen from table 2, along with the grate cooler and the AQC boiler at draught forward and double valve adjusting control, import AQC boiler smoke wind is above 50 ℃ higher than the original, the AQC boiler imported 150 pa, negative pressure increased and AQC boiler outlet temperature is not rising, AQC boiler efficiency improved obviously, the AQC boiler steam pressure and temperature will increase; SP boiler inlet temperature than the original 25℃ or so, under the condition of the same high temperature fan speed, import negative pressure increased by 300Pa, the boiler ventilation volume significantly improved, bypass leakage volume significantly reduced, SP boiler efficiency to improve; The main steam temperature and pressure increased year by year, the main steam volume increased more than 4t/h than the original, providing more and better steam for power generation; The condenser works more stably and efficiently, avoiding the problem of efficiency decline caused by high exhaust steam temperature. The end difference temperature can be controlled below 5℃ for a long time, and the true kiln degree can be stabilized at about -0.08MPa. The power generation of clinker increased by 3.5kWh/t.

-END-