Distributed Parameter Modeling For Evaporation System In Supercritical Once-through Boiler Based On Combustion On-line Monitoring

With the enlargement of the supercritical boiler capacity, exploring the relationship of heat flux distribution and working fluid characteristics and establishing a distributed parameter model for the evaporation system have important influence on boiler safe operation. Because of the complexities of combustion behavior in a furnace and the heat transfer at supercritical pressures, it is very difficult to build this model. In this paper, a distributed parameter model for the evaporation system of different supercritical boiler is developed based on3-D combustion monitoring. The transient parameters distributions of evaporation system are analyzed, such as the heat flux, the metal-surface temperature and the steam quality. Meanwhile, the water wall failure and its prevention measures are studied based on calculation results.By the modular modeling method, the distributed parameter modeling method for evaporation system is developed. For the fue gas model, the radiation flux is obtained based on3-D temperature distribution. For the tube wall model, amixed model that the lumped-parameter model taking the temperature inside the tube wall as the lumped-parameter is combined with the2-D steady state heat conduction model is proposed. For the water/steam model, the simplified thermodynamic properties of water/steam model is proposed, and the non-uniform distributions of thermal conductivity and viscosity coefficient are considered.A simulation research on a subcritical once-through, coal-fired, twin-furnace boiler is performed by using the distributed parameter model for evaporation system. The distributions of the thermal parameters on the four water walls in different cases and distributions of the heat flux, steam quality and metal temperature on the front water wall in the three different cases are obtained, while the hydrodynamic characteristics is studied. The results show that the heat flux and metal temperature in the furnace center is much larger than the boundary region, and the steam quality distribution in front wall presents a W-type curve. Application of low mass velocity optimized ribbed tube makes the water wall have a positive flow response. The distributed parameter model is improved for supercritical spiral water wall. The conversion method is proposed because the distribution of the heat flux is in the vertical direction and the calculation of the working fluid parameters is in the spiral direction. The results show that the spiral water wall has a more beneficial effect than the vertical water wall on the heat flux distribution for critical steam quality. The maximum heat transfer coefficients are located in the pseudo-critical enthalpy region. The enhancement of the heat transfer was reduced by increasing the heat flux and the pressure in the high specific heat region at a supercritical pressure. The reason is that the changes in the thermo-physical properties with temperature slow down with increasing pressure. In addition, the spiral water wall has a negative flow response.The W-shaped boiler structure and combustion feature are analyzed. The heat flux and metal-surface temperature distribution in the front and left water wall are obtained at different loads by solving non-linear equations. The simulation results show that the heat flux distribution on front water wall exist three high heat flux zones, this is due to the arrangement of secondary air box on the arch and staged-air box blew the arch. The height of maximum metal temperature decreases with an increase in load in down-fired boiler. The metal temperature difference is large along the direct of the width on the ash hopper close to the front wall, which will cause thermal stress in lateral direction. The peak of the heat transfer coefficient and the peak of the heat flux are not at the same zone at near-critical pressure, and the peak of the heat transfer coefficient moves to the peak of the heat flux gradually with the increment of load at supercritical pressure. Based on these results, the thermal parameters distributions of evaporation system for a supercritical W-shaped boiler.are analyzed based on3-D combustion monitoring during a safety accident. Combining with the combustion in furnace, heat transfer in boiler and site observation, this paper expounds the accident causes and its remedial measures.In summary, the distributed parameter model in this paper can combine the combustion in furnace with the het transfer in boiler effectively. The distributions of the thermal parameters obtained by this model can provide a early warning for site operation and ensure the safety and the credibility of the boiler operation. The simulation results are helpful for design and modification of the supercritical once-through boiler.