ASH DEPOSITION MODELING IN LOW TEMPERTATURE HEAT EXCHANGERS FOR PULVERIZED COAL FIRED POWER PLANT APPLICATIONS

摘要

Ash deposition is a significant concern in the pulverized coal fired power plant industries because it causes resistance to heat transfer in the heat exchangers which ultimately reduces the efficiency of the power plant. The chemical nature of sustained deposits also causes the material damage to the heat exchanger equipment in form of corrosion and erosion which arises the problem of plant untimely shutdown and maintenance. The factors such as coal properties, boiler design and operating condition influence the ash deposition in the heat exchangers. The transport mechanism through which ash deposition occurs include thermophoresis, inertial impaction, eddy impaction, condensation and chemical reaction. Thermophoretic and inertial impaction deposition are the dominant mechanism in the low temperature sections of the power plant. The low temperature section includes the Gas-to-Gas Cooler (GGC) in this research. The objective of this research is to predict the thermophoretic and inertial impaction deposition rates in the low temperature section of coal fired power plant. Finite Difference Method is implemented to predict the deposition rate. The heat exchanger unit is divided into the finite small cells with equal area. The governing equation is solved for each cell with varying fluid and thermal parameters such as fluid flow rate, temperature distribution, fluid properties, etc. The exit condition of preceding cell is considered the inlet condition for the succeeding cell. The ash deposition rate is calculated for each cell and is integrated over entire unit to calculate the total deposition. The results are simulated using the software MATLAB R2017a. The temperature at inlet for flue gas and water is 410 K and 351 K respectively. Thermophoretic deposition depends upon the temperature gradient between the flue gas and the heat exchanger surface. The calculated temperature gradient varied from 846.7 K/cm to 718.7 K/cm along the heat exchanger unit for gas to gas cooler. The temperature gradient is used to calculate the thermophoretic velocity which is the deposition velocity. The driving factor for the impaction deposition is the ash particle inertia being greater to trace out from the flue gas flow streamlines. The impaction of the ash occurs on the heat transfer unit when the calculated stokes number is greater than 0.125. The heavier particles undergo inertial impaction. The stokes number for particles with diameter from 0-100 urn was calculated in the range 0 to 19 for tube and 0 to 362 for finned surface. The impaction efficiency was calculated as high as up to 80 % for the ash particles. This considers different chemical species of ash. It is observed that the particles with large density are deposited as compared to lower ones. The greater impaction rate and thermophoretic velocity results in the higher amount of ash deposition. The ash deposition modeling will benefit the efficient heat exchanger designs in the future and optimize the timing of the soot blowers use to remove the ash deposits running the power plant with improved efficiency and lower maintenance cost.