Study On Real-time Monitorning Models Of Ash Fouling And Soot-Blowing Optimization Scheme For Boiler’s Heating Surfaces In Thermal Power Plant

As the boiler in thermal power plant used the coal as fuel, it is inevitable that the ash fouling deposited on the heating surfaces in the process of operation which may cause the degradation of heat transfer performance, the increase of exhausted gas temperature and the drop of boiler efficiency, even give rise to the safety accident like the erosion of tube wall, tube explosion and boiler shutdown. Even though the soot-blowing was an effective method to eliminate the ash fouling, the timing quantitative blowing mode used by the plant was inappropriate. It may lead to the too frequent blowing that causes the waste of steam, or lead to blowing missing the best time that causes the continuous drop of transfer efficiency. Therefore, how to make the real-time monitoring of ash fouling on heating surfaces and the formulation of soot-blowing optimization scheme come true has become an important research topic for thermal power plant to implement the policy of energy saving and emission reduce. In this paper, the monitoring model of ash fouling on heating surfaces was established to realize the real-time monitoring of the ash fouling, and the soot-blowing optimization scheme was developed according to the monitoring results, so the problem about the treatment of ash fouling in thermal plant could be solved.The paper defined the fouling ratio as the ash fouling norm, and deduced the calculation flow of fouling ratio by heat transfer model and heat balance principle to establish real-time monitoring model of ash fouling for heating surfaces. Among these models, the monitoring model of ash fouling on convective and semi-radiation heating surfaces used the heat transfer coefficient as the characteristic parameter for fouling ratio, and introduced the lumped parameter dynamic model to reduce the influence of variable load under variable load condition. The monitoring model of slagging in furnace used the thermal effective coefficient of water wall as the characteristic parameter for fouling ratio, considered the weakening of the furnace flame radiant energy in the transfer process, and introduced an method for calculating the furnace outlet flue gas temperature and furnace fouling ratio by combining the furnace and semi-radiation heating surface’s heat transfer calculation aiming at the problem that the current calculating method of furnace outlet gas temperature lacking real-time performance. The sectionalized monitoring model of ash fouling on tri-sectional rotary air preheater used the utilization coefficient as the characteristic parameter for fouling ratio. The monitoring model of air preheater can realize the sectionalized monitoring for the hot and cold section of air preheater, and solved the problem that existing method can only carry out overall monitoring of air preheater. The formulation of soot-blowing optimization scheme used the critical fouling ratio as the judging standard, and considered the safety of actual operation of boiler. The calculation model of the critical fouling ratio based on the premise that the soot-blowing time was invariant was established according to the principle of minimum operating loss.According to the established real-time monitoring models of different heating surface and the formulated soot-blowing optimization scheme for whole boiler, the paper used the offline data collected from boiler’s DCS system to carry out calculation and model validation. The result shows:(1) The calculation results of fouling ratio under the stable load condition by the monitoring models of ash fouling on convective and semi-radiation heating surfaces, slagging in furnace and ash fouling on tri-sectional rotary air preheater are in line with the actual ash fouling degree, so the established models can meet the requirement for real-time monitoring of ash fouling. (2) The calculation results of fouling ratio under the fast variable load by the monitoring models of ash fouling on convective and semi-radiation heating surfaces and slagging in furnace are oscillating and deviated from the actual ash fouling degree, the paper considers that it is due to the unreasonable interval of collecting data. And the established monitoring model of ash fouling on tri-sectional rotary air preheater is unsuitable for variable load condition. (3) Because of the time of variable load condition just accounting for a tiny proportion of total operation time and soot-blowing operation cannot being activated when boiler load is variable, therefore, the monitoring models established in the paper can meet the requirement for real-time monitoring of ash fouling as a whole. (4) Under the normal operation load condition, the low temperature superheater is easiest to be polluted while the platen superheater is difficult to be polluted. The establishment of soot-blowing optimization scheme for whole boiler need considering not only the critical fouling ratio but also the load condition, exhausted gas temperature, superheated steam temperature, reheated steam temperature and attemperation water, combining with the safety of boiler operation.