CONFIGURING A DISTRICT HEATING PLANT UNDER RECONSTRUCTION: OPTIMUM QUANTITY OF BOILER UNITS

Abstract

Relevance. An over-dimensioning district heating plant is costly, as there will be more losses during low-load periods and more capital bound in the investments. During the highload (peak load) periods, a regular heat plant might not be able to deliver enough heat to the system. To ensure that the customers still receive their heat, the peak production plants are often created to top-up during peak-load periods. As these peak-load plants are often small in size and constructed only to run for short periods of time, they are often fossil-fuel fired, or fired by other highly refined, and hence expensive fuels. The aim of this work is to develop a tool (a multi-step approach) capable of adjusting the secondary load to match the optimum load structure to save primary energy sources, and meanwhile ensure the demand and supply balance. Methods. The optimum scheme is selected by evaluating and comparing several schemes, considering the constraints from the original system configuration, which results in a limited number of independent variables. We dealt with sequencing control for multiple boilers as required by load demand that is desirable when two or more boilers are installed in parallel. This can be accomplished by utilizing a steam flowmeter or water temperature sensor, when applicable, to energize an additional boiler when the load cannot be met by the boilers already on-line. The difference in the efficiency between boilers located in different places can be appreciable if the fuel input to a boiler is modulated. This increase in efficiency is due to the increase in the ratio of heat exchanger surface area to heat input as the firing rate is reduced. The mathematical model should be solved with a programming software, such as Statistica 6. Results. Three different geographical locations were considered. The authors have calculated the mean annual values for electricity accounted 3,68 roubles per kWh, and values for the days of highest plant activity accounted 5,48 roubles per cubic meter of gas, for comparison with an acceptable concentration level for the exposed population. The results indicate that for thermal energy value varying under six scenarios (from 2,5+2,5 to 2,5+1,6+1,0 MW configuration) the situation is completely different when implementing the optimization model for Kazan and Kharkiv. It can be also seen that 2,5+1,6+1,0 MW configuration (and in particular Kharkiv and Omsk: related to the warmest and coldest locations respectively) is more advantageous from the economic point of view. Conclusions. In this work a multi-step approach was used to evaluate the economic aspects of six heat-only boiler plant energy-recovery configurations. The aim of the developed methodology was to supply useful information to a designer. The obtained results were interesting, and they can certainly be justified in practice. In particular, the results showed a low energy-production cost for production of heat, as well as an opportunity to minimize environmental impacts. The economic analysis results, however, show that the proposed method is quite helpful, because of the considering price difference between gas and electrical energy distributions.

Keywords

• primary energy
• district heating
• natural gas
• boiler
• production
• generation
• optimization
• minimization
• costs