Boiler flue gas in Weiqiang Mine and the industrial site of the coal preparation plant
EPC of dust removal, desulfurization and denitration system reconstruction project
1. Introduction
Coal-fired boiler is a facility for mine production and domestic heat supply, and the flue gas containing sulfur dioxide, nitrogen oxides and soot produced by combustion must be treated and discharged into the atmosphere. There are two sets of 20t/h chain steam boilers in the boiler houses of Weiqiang Mine and the industrial site of the coal preparation plant. The flue gas was originally treated by multi-pipe dust removal and magnesium oxide desulfurization without denitration facilities, and the emission concentrations of soot, sulfur dioxide and nitrogen oxides in the treated flue gas exceeded the standard. The project mainly reformed the dust removal and desulfurization system and added denitration system and on-line monitoring facilities to ensure that the treated flue gas met the requirements of national pollutant emission standards. The main difficulties to be solved in this project are as follows: (1) The project site is small. According to the requirements of the mine, dust removal and desulfurization need to be reformed in the existing dust removal and desulfurization room, which is difficult to reform; (2) The flue gas outlet temperature of the boiler is higher than that of the conventional chain furnace, and the temperature is as high as 300ºC under the full load operation condition of the boiler; (3) Whether the position of spray gun in SNCR denitration process is reasonable or not directly determines denitration efficiency; (4) The construction period is tight, and the total contract period is 105 calendar days. During this period, the construction drawing design, equipment procurement, site construction and personnel training will be completed, otherwise the heating of the whole mine will be affected.
2. Treatment solution
Through scheme comparison, taking full account of site conditions and project economy, the process of the project was finally determined as follows: raw boiler flue gas → flue gas cooling system → bag filter → induced draft fan → desulfurization tower → chimney, and SNCR furnace denitrification was adopted for denitrification. The specific scheme was as follows: (1) Bag filter technology was adopted for flue gas dust removal, multi-pipe dust collector was removed, induced draft fan was replaced, and bag filter was built at the position of multi-pipe dust collector. The size parameters of dust remover were redesigned to meet the site conditions. In order to reduce the project investment, the dust collector adopted low-temperature filter bag, and a flue gas cooling system was set in front of the bag filter. The flue gas cooling system adopted the mode of "economizer+cooling tower" for the first time, which has the advantages of less investment, small land occupation, flexible layout and good cooling effect, and reduces the flue gas temperature below 160ºC to prevent damage to the filter bag. Because the height of dust collector exceeded that of the roof of dust removal and desulfurization room, partial roofs and beams were removed and reinforcement measures were taken at the same time. (2) Magnesium oxide method is still used for flue gas desulfurization. In order to improve desulfurization efficiency, the existing desulfurization tower increased, anticorrosion was carried out again, four layers of spray were arranged, demister was replaced, Venturi pipe was set before the entrance of desulfurization tower to reduce sulfur dioxide content, so that desulfurization tower has both desulfurization function and dust removal function. The desulfurization efficiency is ≥90%, and the collected particulate matter is ≥ 5μ m. Re-arrange the circulating water pipeline system, add sludge pump and filter press in the circulating pool room, and filter presses the sediment in the circulating pool to facilitate outward transportation. (3) SNCR process was adopted for flue gas denitration, and urea is adopted as denitrifying agent. A new denitration room was built, which is equipped with a drug dissolving system, a urea solution storage and dilution system, a batching and conveying system and an air compressor room. The urea solution is delivered to the two-fluid spray gun by the batching delivery system. Combined with compressed air, the urea solution is fully atomized when sprayed through the nozzle and then sprayed into the furnace at a certain angle. The setting of spray gun, through monitoring the temperature in the furnace and experience, arranges the nozzle in the boiler temperature range of 850~1100ºC to ensure the denitration efficiency ≥40%.
There were many links in EPC general contracting project of dust removal, desulfurization and denitration system reconstruction project. Our Institute gave full play to its professional advantages. The project department was composed of design team, procurement team, construction team and commissioning training team. Under the organization of the project manager, each team was coordinated organically and in the way of design while construction. At the same time, the project management platform was adopted, which effectively shortened the construction period.
3. Construction situation
The scope of this project covers the transformation of boiler flue gas dust removal and desulfurization system, the addition of denitration system and online monitoring system, and the replacement of the original induced draft fan in boiler room. Including the design, procurement, construction and transformation of the above-mentioned items, the system has passed the environmental protection department's standard acceptance, operation license handling, technical service, personnel training and so on. During on-site construction, 4 sets of large equipment were dismantled, with 32 meters of flue, 379 sets of equipment were installed, more than 6,000 meters of cables were laid, and a new denitration room was built.
After careful organization, the preparatory work before commencement was completed within 15 calendar days after signing the contract, and the approval of the mining party and the supervision party was obtained for the commencement report, and the construction workers entered the site at the same time. In the process of construction, the project department strictly followed the construction organization design and special scheme, overcame the difficulties such as the space limitation of dust removal and denitration room and the inability to use large-scale construction machinery in the demolition project, realized safe production, and successfully completed all contents in the general contract.
Demolition and hoisting engineering operation
Construction of the denitration room
Installation of dust remover and urea solution tank
Education and training
4. Achieved accomplishment
After treatment in this project, the concentration of dust particles is ≤50mg/Nm3, sulfur dioxide ≤300mg/Nm3 and nitrogen oxides ≤200mg/Nm3, meeting the relevant national emission standards. Reasonable flue gas cooling system can effectively control the flue gas temperature entering the system within 160ºC.
The reconstruction of the project makes the best use of the existing one, which saves the investment of the mining side and reduces the emission of flue gas pollutants, and has great economic and social environmental benefits. The flue gas venturi pretreatment scheme can provide reference for other coal mines and has certain application value.
Bag filter, desulfurization tower
Some equipment of denitration system
5. FAQ
Q1. How is the flue gas dedusting, desulfurization and denitration treatment process selected in this project?
A: The treatment effect, construction and operation costs, advantages and disadvantages were compared through different schemes, and finally determined by referring to similar coal mine boiler flue gas treatment schemes.
Q2. Spray solution in desulfurization system is easy to cause secondary pollution. What treatment measures have been taken?
A: The sewage treated by the desulfurization tower is discharged into the desulfurization circulating pool, the sludge in the pool is regularly lifted to the filter press for treatment, and the circulating liquid enters the desulfurization system for reuse.
Q3. What is the pretreatment effect of Venturi tube?
A: The content of sulfur dioxide in flue gas is high, and full contact in venturi can reduce the content of sulfur dioxide and the temperature of flue gas entering desulfurization tower, which can achieve obvious effect and increase the removal rate by 5%-10%.
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