Conversion of a high-pressure boiler into a low-pressure operation : Case study for Kuopion Energia's HP2 boiler

Abstract

District heating production in Finland is increasingly adopting more electricity-based production. For operators with aging combustion-based capacity, this creates a complex decision on whether to invest in new combustion-based capacity with a long payback time or to solely rely on electrical alternatives that are subject to electricity price risk. One potential option is to extend the lifetime of existing high-pressure boilers by converting them to low-pressure operation.

This thesis evaluates the technical feasibility of converting an existing CHP-operated high-pressure boiler into a low-pressure operation in order to extend its lifetime. The literature review examines at a general level how a boiler's operating parameters influence the design, structural requirements, and lifetime of a BFB boiler. In addition, the impacts of reduced operating pressure on the lifetime and regulatory requirements of boiler pressure parts are reviewed. Further examination was conducted as a case study for a CHP-operated BFB boiler with a fuel capacity of 250 MW. The evaluation included determining whether the conversion is technically feasible overall, and, if so, identifying the most suitable conversion solution, including the new operating parameters and required modifications to the boiler. In the evaluation, the reference boiler was first modelled using the company's boiler design software. Modelling was then conducted to evaluate the optimal process configuration and parameters.

The literature review showed that the components designed for high-pressure operation retain a significant margin after the conversion and can therefore be assumed to enable lifetime extension. However, to determine how much the conversion could actually extend the lifetime, the existing condition of the components needs to be evaluated with measurements. For the evaluated boiler, the low-pressure conversion was found to be feasible as a forced-circulation hot-water boiler. This allows the utilization of the existing heat transfer components in the furnace, limiting conversion costs. Required modifications include new piping, equipment, and components. Other modifications include demolishing the combustion air preheater and replacing primary air preheating with a water-to-air preheater. In addition, steam-based applications such as sootblowing and thermal water treatment also require modifications, since steam will no longer be available after the conversion. The obtained process parameters were 200 C output temperature and 20 bar output pressure, with feedwater temperature varying between 120 and 168 C depending on boiler load. The mass flow through the boiler circuit is maintained constant, by bypassing the district heating heat exchanger during partial loads. The existing boiler heating capacity of 170 MW can be maintained.

In conclusion, the conversion of the evaluated boiler is considered feasible. It would require significant modifications but could extend the boiler's lifetime with relatively low investment. Cost estimation of converting the evaluated boiler to low-pressure operation is between 10 and 15 % of the price of a new boiler with equivalent heating capacity.