Research On Modeling And Optimization Control On The Bed Temperature Of Large-Capacity Circulating Fluidized Bed Boiler

As an efficient clean coal combustion technology, Circulating Fluidized Bed(CFB) boiler combustion technology has become a beneficial supplement to conventional coal-fired boilers, and has broad prospects for development. In the small-capacity circulating fluidized bed boiler, on-field operation has long been plagued by problems of low rate of usage of combustion automatic control system. On one hand, what people know about CFB is not enough for them to get in-depth understanding of the mechanism inside CFB, on the other hand, there is great room for improvement and optimization of its automatic control system. As large scaling becomes a trend of CFB boiler, it is even more urgent to get to know better about combustion system and design the control system well. On-going inside CFB boiler is a distributed parameter, time-varying, nonlinear, multivariable coupling complex process. Bed temperature is one of the core control performance index for CFB combustion system. It is coupled with a number of factors, and the most serious of which is its coupling effect with main steam pressure. To ensure safe and stable operation of CFB boiler, it is of great importance to build the bed temperature model properly and control it reasonably and effectively.The dissertation takes following several aspects of research work as its main contents:1. On the basis of the knowledge of combustion, heat transfer and flow process inside the CFB boiler and previous research results, large-scale CFB boiler is divided into left and right side along the boiler center line. On each side, it is further divided into dense phase zone, transition zone and dilute phase zone alone the furnace height. By establishing balance equations respectively in each zone for oxygen volume, carbon residue, bed material flow and dynamic energy, the dynamic bed temperature model is set up. By means of disturbance response and comparing between the modeling results and the on-field historical data, the effectiveness of the model is verified. On this basis, the impact the unbalanced lateral bed material flow between both sides of CFB boiler poses on the variation of bed temperature is studied and verified.2. As for the strong coupling characteristics between bed temperature and main steam pressure, of which the process is a strong coupling, large delay, large inertia and stable two-input two output system, the method of multivariable decoupling internal model control is introduced for better control of the system. The paper discusses in detail the design of controller based on multivariable P-norm decoupling IMC structure and multivariable V-norm decoupling IMC structure respectively. The related problems, like model reduction and delay term approximation methods are discussed too. For the P-norm decoupling controller design, this paper proposes to alter the form of decouple filter so as to speed up the system response. For the V-norm decoupling controller design,the method which gives consideration to both decoupling and zero steady-state error is studied. The extending of the Enhanced IMC(EIMC) to multivariable control,combining with the V-norm decoupling structure is applied during the controller design. In the design process of both methods, a new way, using the integrated control performance optimizing index(ICP), to tune the filter parameters is developed. The ICP optimizing index take into consideration both the system output and the controller output. Apply both methods to the decoupling controlling of CFB boiler combustion system, good decoupling results are yielded and better system performance is achieved.