Adjusting of Temperature Levels in Multi Stage Drying System by Means of Outlet Air Measurements

Bio fuel plays an important role in the energy production of the forest industry. The initial moisture of the fuel typically varies between 50 63 w-%. A multi stage drying concept has been developed for the drying of wet bio fuel. To adjust the temperature levels in the multi stage drying system there must be a simple and fast method to define the fuel initial moisture. A method to define the initial moisture based on the measurement of the outlet air moisture was experimentally studied. Regular shaped wood particles having different moisture content were dried in a fixed bed reactor. The difference between outlet and inlet air moisture was measured. According to results, it is possible to make conclusion about the initial moisture of the material by measuring the moisture of the outlet air. INTRODUCTION In Finland, bio fuel represents approximately 19 % of the total primary fuel consumed in forest industry. The most common bio fuels are bark, forest residues and different kinds of waste wood. The moisture content of bio fuel typically varies between 50 and 63 w-% (water per total mass). Furthermore, the fuel can be frozen in winter. By means of fluidised bed technology, it is possible to combust bio fuel having such a high moisture content. Because of the high initial moisture, the effective heating value of the bio fuel is, however, low; this reduces the power production of the power plant. Compared to completely dry fuel, the theoretical decrease in the power production is 12 -21 % for the initial moistures 50 63 w-%, respectively. The variation of the initial moisture additionally makes the operation of the boiler more difficult. To improve the effective heating value of the bio fuel the sufficient final moisture is usually 20 30 w-%. It is additionally important to remember that too dry fuel can cause problems for the operation of the boilers, particularly in existing boilers. For example, the combustion temperatures may rise too high. Since the initial moisture of the fuel varies, the final moisture of the fuel depends on the initial moisture if the drying conditions are always similiar in the dryer. If the variation of the initial moisture is 50 63 w-%, then the moister fuel contains around 70 % more water than the drier one. The heat needed for the evaporation increases as much. The factors most affecting the initial moisture content of the bio fuel are season, weather, storage time, and the type of bio fuel (bark, forest residues et cetera). To achieve the desired final moisture, and to minimise the use of the drying energy it is necessary to adjust the drying conditions on the basis of the initial fuel moisture . The problem is how to define the initial moisture of the fuel on-line in a simple way. The most reliable way to define the initial moisture is the drying of a few fuel samples in an oven. At least two samples are taken from the fuel flow and kept in an oven at 105 C for at least 16 hours but not longer than 24 hours. After the drying, it can be assumed the samples are completely dry and the initial moisture (water per total mass) can be defined as follows: