Application of Circulating Water Wastewater as Desulfurization Process Water
The flue gas desulfurization adopts the limestone/gypsum wet desulfurization process, and the annual desulfurization water consumption is about 2.3 million m3.
If the circulating water sewage can be used for desulfurization, it can not only save water but also reduce waste water discharge.
The power plant personnel successfully solved this problem by using the circulating water sewage for desulfurization production through experiments.
Desulfurization water usage
The limestone/gypsum wet flue gas desulfurization (FGD) process is to grind limestone into powder, then prepare a slurry, and spray it into the desulfurization absorption tower to react with the sulfur dioxide in the flue gas, and finally generate gypsum.
The process has the advantages of stable operation and high desulfurization efficiency, but the water consumption is large.
The water used in the wet desulfurization system can be roughly divided into four categories: limestone pulping water, equipment flushing water, operating equipment cooling water and wastewater treatment system water.
The desulfurization water consumption of each unit in this power plant is 50-60t/h, with an average of 55t/h.
1.1 Water used in limestone pulping system
The power plant uses a wet ball mill to self-grind limestone powder, and the limestone powder is made into a slurry with a mass fraction of 25% to 35%. The pulping water consumption accounts for 20% to 35% of the total desulfurization water consumption.
1.2 Equipment flushing water
The equipment flushing water is mainly the demister flushing water and the vacuum belt conveyor flushing water, and there are also temporary flushing water, such as the flushing water of the slurry conveying equipment and the pipeline.
1.2.1 Mist eliminator flushing water
The desulfurization mist eliminator is installed at the top of the absorption tower. Its function is to remove the dust mist and droplets after desulfurization. The retained dust, gypsum particles and salt will stick to the mist eliminator. In order to ensure the long-term and efficient operation of the mist eliminator, it must be timely Rinse it.
The flushing water will fall into the absorption tower, and at the same time play the role of adjusting the density of the slurry and the water level of the absorption tower. This part of the water consumption is more, accounting for 55% to 65% of the total desulfurization water.
1.2.2 Washing water of vacuum belt conveyor
In order to keep the gypsum filter cloth clean and reduce the gypsum fine ash impurities and soluble salt content, the filter cake and filter cloth need to be washed continuously during the gypsum dehydration process.
The amount of flushing water accounts for 10% to 15% of the total desulfurization water, but most of it will be returned to the absorption tower for reuse, and the supplementary water accounts for about 1% of the total desulfurization water.
1.3 Cooling water for rotating equipment
The booster fan, slurry circulating pump and other pumps will generate heat during operation and need to be cooled. This part of the water is less, accounting for 2% to 4% of the desulfurization water consumption. This part of the water flows into the absorption tower with almost no loss.
1.4 Water used in wastewater treatment system
Waste water will be generated during the operation of the desulfurization system, and chemicals need to be added to treat the waste water. Part of the water is consumed when preparing the agent, and this part of the water accounts for less than 1% of the desulfurization water consumption.
Water quality requirements for desulfurization water
There are no special provisions for desulfurization water in the national standard and industry standard, and each type of water is explained separately.
2.1 Water for pulping
Because the slurry will eventually enter the desulfurization absorption tower, the quality of the pulping water should comprehensively consider the requirements of sulfur dioxide absorption, gypsum oxidation reaction, and equipment anticorrosion.
2.1.1 Chloride
Excessive chloride ion mass concentration will cause pitting corrosion of metal parts. Therefore, it is generally required that the chloride ion mass concentration in the absorption tower be controlled within 20,000 mg/L.
For pulping water, the mass concentration of chloride ions should be controlled as much as possible to avoid being forced to increase the discharge of desulfurization wastewater due to the increase in the mass concentration of chloride ions.
2.1.2 Organic matter (COD)
When the organic content is too high, foaming will occur in the upper part of the absorption tower during operation.
For example, when the COD of the process water of a power plant reaches 40mg/L, a large number of bubbles are generated in the absorption tower, and the generation of bubbles has to be suppressed by adding a defoamer. In addition, experiments have also confirmed that most organic compounds have an inhibitory effect on the oxidation of calcium sulfite.
Comprehensive consideration, it is necessary to control the COD of pulping water below 30mg/L.
2.1.3 Suspended solids
Suspended solids will cover the limestone surface, affecting the absorption reaction of sulfur dioxide and reducing the desulfurization efficiency.
Therefore, it is necessary to control the mass concentration of suspended solids in the process water within a certain range, but this control value is not seen in the desulfurization standards and technical specifications.
Years of operation experience of the power plant show that the concentration of suspended solids is controlled within 200mg/L, which has no obvious adverse effect on desulfurization operation.
2.1.4 Oils
Compared with the oxidation inhibition effect of general organic substances on calcium sulfite, the oxidation inhibition effect of oil substances on calcium sulfite is much faster, and the oil substances entering the absorption tower will rapidly deteriorate the quality of the slurry.
Therefore, the mass concentration of oil substances in the process water must be controlled within a certain range. Years of operating experience of the plant show that the mass concentration of oil substances is controlled within 5mg/L, which has no adverse effect on desulfurization operation.
2.1.5 Organophosphines
The organic phosphine water treatment agent also has a certain degree of inhibition on the oxidation of calcium sulfite, but when the mass concentration of the organic phosphine is controlled below 5 mg/L, its effect on the oxidation of calcium sulfite is not obvious.
2.2 Mist eliminator flushing water
In the wet desulfurization system, for the water quality of the demister flushing water, it is necessary to prevent the nozzle of the demister flushing water from being blocked due to the high content of suspended matter impurities in the process water, and also to prevent the nozzle from being caused by the high content of hardness ions. Scaling phenomenon.
2.3 Cooling water for rotating equipment
In order to slow down the scaling, corrosion and blockage of the cooling system of rotating equipment as much as possible, relatively high requirements are put forward for the water quality of cooling water for rotating equipment.
The quality of cooling water for rotating equipment should meet the following requirements: pH value, 6.5~9.5; hardness (calculated as CaCO3), ≤250mg/L; mass concentration of total suspended solids, <50mg/L (units above 300MW), <100mg/L (other units).
Water quality of circulating water and sewage
The circulating water of the power plant adopts the treatment process of adding sulfuric acid and adding scale inhibitor, and the surface water is used as supplementary water, and the concentration ratio is controlled at 4-5.
See Table 1 for the quality of circulating water and sewage. The Langelier saturation index (L.S.I) is often used to judge the scaling of water quality: L.S.I>0, scaling; L.S.I=0, no scaling and no corrosion; L.S.I<0, corrosion.
After calculation, the L.S.I of the sewage is 1.83-2.18, which belongs to the scaling type water.
The Langelier index does not take into account the induction of crystallization by suspended impurities and corrosion products in water, nor the hindering and dispersing effects of scale inhibitors on crystallization growth.
Because the changes of water quality itself and external conditions often lead to the drift of the equilibrium and stable point, the application of this index has limitations and can only be used as a reference for judging water quality.
Circulating water drainage is used as desulfurization process water
After analysis, the power plant personnel believed that the water quality of circulating water and sewage can basically meet the water quality requirements of pulping water and wastewater treatment water;
Although there is a risk of scaling when used as flushing water and cooling water, it is not easy to scale due to the addition of corrosion and scale inhibitors to the circulating water and in the flowing state.
First, a unit was tested, and after 3 months of operation, it was found that there was no adverse effect, and then it was popularized and applied in the whole plant, and the effect was good.
When the circulating water sewage is used as the desulfurization process water, the carbonate hardness in the water will participate in the sulfur dioxide absorption reaction, which can save the amount of limestone while saving water;
However, there is a risk of scaling when the circulating water discharge water is used as the rinsing water of the mist eliminator and the cooling water of the rotating equipment, and the equipment needs to be checked when the unit is out of operation.
Economic analysis
The use of circulating water drainage as desulfurization process water requires additional pipelines, valves and construction costs, but can save water and sewage charges.
It can save 2.3 million m3 of fresh water every year, and save a total of 3.9 million yuan in water and electricity costs; reduce drainage by 2.3 million m3 and reduce sewage costs by 150,000 yuan every year; use less calcium carbonate 810t (equivalent to 900t of limestone).
Reduce the purchase cost of limestone by 55,000 yuan; reduce the electricity cost and maintenance cost of coal mill and circulating pump by 10,000 yuan in total; increase the pipeline, valve and construction cost by 100,000 yuan in total:
After the power plant uses the circulating water drainage as the desulfurization process water, an additional investment of 100,000 yuan is required, but the annual cost savings is about 4.11 million yuan.
Conclusion
Circulating water sewage is used as water for wet desulfurization process, which not only saves water resources but also reduces sewage, and has significant economic and environmental benefits. Be aware of the risk of scaling when cooling water for equipment.