Transient modeling of a D-type boiler with natural circulation loop: a study on boiler performance during a step change in load

In this paper, the dynamic modeling of a D-type boiler was performed during a step increase in loading. The D-type boilers are used in industry to produce process saturated steam. In these types of boilers, heat absorption occurs in two sections of the furnace and the convection section. The pipes of the convection section are indirectly exposed to heat, causes the flow direction to change from the upper drum to lower drum and vice versa. One of the special features of this boiler is that the drum circulation loop is natural and as a result, the hydraulic balance equations of the system must be considered. In the present modeling, the equations of water-side were considered in one-dimensional and the equations of flue gas-side were considered in three-dimensional space using the zonal method. The transient continuity and energy equations of the water-side form a system of nonlinear ordinary differential equations which were solved using the Runge–Kutta method. To solve the water and gas-side equations in two-way coupling method, a Fortran computer program has been developed. Investigation of the boiler operating parameters during start-up can help to improve the existing control system. The comparison of the present results with the test data of the desired boiler showed that maximum modelling errors are equal to 4.8, 1, and 6.8 percent for steam pressure, drum water temperature, and outlet steam of boiler during start-up, respectively. Also, the boiler response to a step increase in consumer steam demand is less than 20 seconds.
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