Ökad matarvattentemperatur vid biopannan Smurfit Kappa Piteå : Increased feedwater temperature at biomass boiler Smurfit Kappa Piteå

Sammanfattning: Smurfit Kappa Piteå is the largest kraftliner producer in Europe. At the mill they have five different steam producers and one of those is the biomass boiler, which is a steam boiler producing overheated steam at 120 bar and 520C. The purpose of this report is to investigate the possibilities to increase the feedwater temperature from today’s 180C to 210C. The consequences on the system are depending on the temperature in the different process stages and the process temperatures are therefore calculated via a method named The NTU-method which is a suitable method to analyze the heat transfer within an object when the ingoing and outgoing temperature are unknown. The feedwater is preheated with a heat exchanger that is heating the feedwater from 130C to 180C using steam with a pressure of 11 Bar. To increase the temperature to the desired  the approach is to install a complementary feedwater heater that uses steam with a pressure of 27 Bar. The new heater is supposed to heat the feedwater from 180C to 210C while the old heater takes the temperature from 130C to 180C. If the old heat exchanger approaches its end of life span it may have to be replaced with a new one with the same design data. One way to fulfill the required heating is to install one feedwater heater that takes the temperature from 130C to 180C using steam with a pressure of 11 Bar and in the next step install the second feedwater heater that takes the temperature from 180C to 210C using steam with a pressure of 27 Bar. There are two alternatives to installation of complementing high pressure heat exchanger. First alternative A is to install the second feedwater heat exchanger between the economizer and the steam dome. Second alternative B is to install the second feedwater heat exchanger between the already existent heat exchanger and the economizer. The consequence of the installation B is an increased energy loss due to the combustion gases while the consequence of installation A does not involve any increase in energy losses. Therefore, alternative A is wiser. However, whatever alternative that is chosen problem will arise since the temperature margin between the steam dome’s saturation temperature and incoming feedwater temperature needs to be at least 25C. This means that the largest possible pre-warming between the boiler’s workload 50-110% is 21C which not do apply to the whole interval. So, as it seems, the utilization of a second feedwater heat exchanger is not applicable over the whole interval of workloads. The increased feedwater temperature come with other consequences on the system. The whole system will be changed due to the increased temperature and it will affect the details. Some of the pipes must be replaced with new ones that fulfills the restrictions accompanying higher process temperatures. The purpose of the increased feedwater temperature is to increase the mass flow steam to the turbine. From calculations and the extraction-steam-consumption graph the increasing net power outcome from the turbine will increase with about 7.5% due to the increasing feedwater temperature. This will make the turbine produce even more electric energy and generate a bigger income to the factory. The investment cost is in total approximately 25MSEK, including replacing the old feedwater heater with a new one and installing a second feedwater heater. With a payback time of eight years and a life span of ten years it is necessary to question the economic potential of the installation. It provides a hint of the potential of the project. However, one should know that all numbers in this report are estimated from rough calculations.