Study and management of flow and concentration field problems in 600 MW SCR unit

With the increasingly stringent national requirements for environmental protection, the control of pollutants in coal-fired power plants has been gradually strengthened. In order to meet the NOx emission standard, some coal-fired units achieve ultra-low emissions by excessive ammonia injection, which seriously affects the unit operation economy. This paper takes the SCR denitrification unit of a 600 MW coal-fired unit with a Π-type furnace in China as the research object and studies the distribution of flow and concentration fields inside the unit under a 60% load section by means of CFD numerical simulation. It points out the problems of flow and concentration fields at local locations under current operating conditions and proposes the rectification of the corresponding area of the inflow equipment and the idea of a zonal ammonia injection optimization method. It provides a certain reference value for the optimized management of the denitrification system of coal-fired units in the future.


Introduction
As an important part of China's energy structure, coal is the main source of consumption for the domestic power industry.Thermal power units account for 60% of the total installed capacity of power generation in the country.Pollutants, such as nitrogen oxides, sulfuric dioxide and dust produced during coal combustion, are one of the main factors causing air pollution.
The ultra-low NOx emission of thermal power units is achieved through low-NOx combustion technology and SCR denitrification technology.Low-NO x combustion technology can be broadly divided into three categories, low-NO x burners [1] , air combustion technology [2] and fuel graded combustion technology [3] , which reduce NO x generation by changing the temperature, oxygen content and other conditions of fuel combustion.
Selective catalytic reduction (SCR) is the use of vanadium-based catalysts [4] (commonly V 2 O 5 ), and NO x in the flue gas and the reducing agent NH 3 in a certain temperature range (280-420 °C) to generate N 2 and H 2 O. SCR technology has a high denitrification efficiency of more than 90%, and is more widely used.Since SCR technology consumes reductant ammonia in the removal of nitrogen oxides, to achieve ultra-low emission requirements, SCR systems are prone to ammonia escape problems due to excessive ammonia injection [5] , and in severe cases, ammonia hydrogen sulfate blockage can form in the air heater, threatening equipment safety.Factors affecting denitrification efficiency include not only the reactor arrangement [6] , catalyst activity [7] , flue gas temperature [8] , but also closely related to the uniformity of the flow and concentration fields.
Due to the limitations of the field environment, most of the units' field measurement work can only take a certain number of sample points at limited locations to analyze the flow and concentration fields inside the unit.In this paper, a 600 MW coal-fired unit SCR denitrification unit in China is used as the research object.CFD numerical calculation method is used to analyze the flow field and ammonianitrogen mixing inside the denitrification unit in the 60% load section, and suggestions are made for the optimization of the deflector in the denitrification unit and the regulation of the ammonia injection device.

Physical Model
In this paper, a three-dimensional flue model is established at a scale of 1:1.Smoke flows from the outlet of the economizer, and enters the ammonia and nitrogen mixer area.The ammonia injection device is composed of 12 ammonia injection tubes with a diameter of 150 mm, and the vortex mixer is arranged on the upper side of the ammonia injection tube.After mixing, it enters the reaction tower and runs the catalytic reduction reaction in the three catalyst layers to reduce the NO x content in smoke, as shown in Figure 1.The flue gas flow is in the x-direction with a speed of 6.6 m/s into the SCR unit, and the flow is turbulent, with an inlet cross-sectional area of 26.73 m 2 , an inlet flue gas temperature of 350 °C, a flue gas density of 0.6 kg/m 3 , an outlet flue gas temperature of 290 °C and an outlet pressure of -760 Pa.

Calculation method and meshing
The insulation wall and no-slip conditions are used, and the calculation method is chosen based on the semi-implicit method of pressure coupled equation (SIMPLE) [9] .the calculation model is a turbulent kε dual equation model, and the iterative process uses the second-order relaxation factor method.The transport model is used, and each ammonia velocity inlet is set as 5.0 m/s, with an NH 3 volume fraction of 5.0%.The velocity residual convergence condition was 10 -4 .The catalyst layer was simplified using a porous media model.The number of meshes is 4.7 million, where the result of surface meshing is shown in Figure 2.

Figure 2. SCR device surface meshing results
The k-ε dual equation model is mathematically described as follows [10] : where G k and G b are the mean velocity gradient and buoyancy-induced turbulence energy terms, respectively, Y M is taken as 0 in incompressible flow, C 1 , C 2 and C 3 are empirical constants, and S k and S ε are user-defined source terms, here S k =S ε =0.

Flow field distribution calculation results
At 60% of the unit load, the flow field calculation of the SCR device is shown in Figure 3.The highest velocity in the whole domain appears at the lower elbow, and the high velocity flow is beneficial to reduce the phenomenon of ash accumulation through the small flow area.After entering the reactor towel, the flow velocity and uniformity decrease.On the lower side of the rectification grille, the flow velocity on the outer wall of the reactor is 1.0~1.5 m/s higher than that on the inner wall.It is due to the larger space on the upper layer and the limited effect of the deflector plate.The smoke with higher flow velocity enters the reactor and rushes directly to the rear wall.After through three catalyst layers, the velocity distribution at the outlet is more uniform.

Figure 3. z=0 cross-sectional velocity distribution
The flow line distribution is shown in Figure 4.The flow line distribution in the upper layer of the reactor shows a phenomenon that the right side is denser and the left side is sparser, indicating that the flow toward the rear wall is greater and that the rectification grille near the rear wall is more seriously affected by ash erosion.The inclination angle of the flow line in front of the first catalyst layer shows the characteristics of higher in the middle and smaller on both sides.The larger inclination angle (angle of attack in 30-60°) will cause wear on the first catalyst layer and reduce the catalyst efficiency.The middle area module of the first catalyst layer can be repaired or replaced to mitigate the adverse effects.

Uniformity and Deviation Analysis
The velocity distribution of the first catalyst above half meter position is analysed.The velocity distribution is shown in Figure 5.The velocity deviation at this position is calculated (32,000 chosen points), and the velocity deviation is 20.5%, except for the velocity at the rear wall, which is higher than 2.0 m/s, and the internal velocity is between 1.0~1.7 m/s.The local velocity distribution at the centre and lower left side of the cross section is low, which is caused by the uneven velocity along the xdirection.After the smoke through the reducer pipe, the flow rates on the left and right sides are not equal.Although the flow rate is gradually uniform on the distribution, it fails to completely offset the deviation of velocity distribution.Hence, the number of deflectors can be increased at the reducer pipe at the inlet of SCR to improve the velocity conditions before entering the reactor.

Ammonia nitrogen mixed concentration field results
From the standard SCR denitrification reaction equation, the theoretical ammonia to nitrogen molar ratio is 1.To ensure the denitrification reaction process is adequate, the percentage of ammonia can be appropriately increased, but too high will instead cause an increase in ammonia escape, with a crosssection of the upper half meter position of the catalyst.The ammonia nitrogen mixing effect is shown in Figure 6.The deviation of ammonia-nitrogen mixing concentration distribution deviation is 22.1%, which is higher than the design requirement and has a negative impact on the denitrification process.The distribution of the ammonia flow line is shown in Figure 7.The flow line into the reactor has an obvious tendency to flow toward the rear wall, and the inclination amplitude of the flow line of spouts 3 and 4 is more obvious and flows directly to the left rear, which is one of the reasons for the high molar ratio.Areas with low molar ratios, such as some locations near the front wall, were below 0.8, indicating that the ammonia dosage of the corresponding ammonia injection tubes 7, 9, and 11 should be increased.To improve the distribution, the existing ammonia injections can be appropriately partitioned.It should be divided into at least four sections, each corresponding to three injections, the amount of ammonia sprayed in each zone should be converted to the required amount of ammonia-air mixture according to the content of nitrogen oxides in the smoke.

Conclusions
This paper takes the SCR unit of 600 MW unit as the research object, and uses the numerical analysis method to calculate the internal flow field and concentration field.There is still room for optimization of the SCR unit in terms of inflow equipment and ammonia injection method, as the following way: (1) The flow field distribution in the upper layer of the reactor is not uniform, and a certain number of deflectors can be installed in front of the reducer pipe at the entrance of the device to improve the flow conditions of the flue gas entering the reactor.
(2) The calculation found that the flow line inclination of the lower layer of the rectifier grille is large.The middle area of the first catalyst layer will be more serious, so the number of spoiler bars was adjusted and the catalyst module was replaced in time.
(3) Ammonia nitrogen concentration field uniformity is poor.According to the reactor cross-sectional NO x distribution of different conditions, it is recommended that the existing ammonia spraying equipment is divided into at least four sections.The installation of additional automatic regulations achieves automatic control of the sections.

Figure 1 .
Figure 1.Schematic diagram of the SCR device

Figure 4 .
Figure 4. Flow line above the reaction tower

5 Figure 5 .
Figure 5. Velocity contour of upper catalyst cross-section

Figure 6 .
Figure 6.Ratio of ammonia and nitrogen concentration in the upper layer of the first catalyst

Figure 7 .
Figure 7. Distribution of flow lines of ammonia spraying equipment at the vortex equipment