12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, Estoniaare considered not to cause sulphur corrosionproblems, especially not with low-sulphur wood fuels.Certainly this design specification must be reconsideredin case of changed fuel properties.The maximum pyrolysis production for each load pointis restricted by the maximum steam extraction rate andby the boiler‘s maximum burning power. The maximumpossible pyrolysis yield logically requires highestpossible fuel input since heat must be provided both, fordrying and pyrolysis. Conversely, this means that thesteam enthalpy exceeds the demand of the DHN. Thisis because the boiler temperature is controlled bymeans of the evaporator- and superheater tubes in theboiler walls. If now, the heat input in the boiler is kept ona higher level as usual the water amount needed todissipate the heat from the boiler walls is onlydecreasing to a certain amount (resulting from areduced temperature after the economizer).Consequently, in order to match the DH load, this heatmust now be ―dissipated‖ in the pyrolysis heatexchanger (19) or in the dryer (24). By iteration the DHload is matched by adjusting dryer load, correlated splitoffto the pyrolysis heat exchanger and fuel input. In allcases the boiler load (characterised by the fuel heatinput) is restricted to 100%. So, the overload back-upcapacity of the boiler is maintained. With this setup thepyrolysis yield constantly increases with the decrease ofthe DH levels down to 60%. The maximum flow off thedryer (and thus its capacity) is be restricted by thepressure prevailing in the feedwater tank, which in turnis given by the extraction pressure after the turbinestage (11). The pressure decreases with falling livesteam parameters and steam massflow. Hence, there isa pressure dependant maximum enthalpy flow that canbe fed into the feedwater tank until saturation state isreached for the mixture of the condensates from the DHexchanger (13) and the dryer (24). In order to overcomethis restriction the feedwater tank pressure has beenincreased load-dependently to a maximum of 2 barsmatching its design pressure. However, due to thereason mentioned above, for loads below 60% the heatthat would need to be ―dissipated‖ in the dryer (in orderto match the DH load) would result in such a high dryercondensate heat flow which again would bring thefeedwater beyond saturation state. Hence for thosecases the boiler load is gradually decreased, resulting inlower pyrolysis yields. The lowest DH load level that canbe represented is 28.6% of the plant‘s full load.Compared to a minimum load of 50% in the base case– which is given by the minimum fuel input required forstable combustion conditions in the boiler-, theintegrated process offers possibilities to increase theoperating hours of the CHP plant considerably.Fig. 2: CHP Plant with integrated pyrolysis and steam drying171