1987

Aho, Martti

Peat, wood and straw contain more pyrolysable compounds and generally less ash than coals of different rank. Moreover, the moisture content in sod peat, milled peat and wood chips is much higher than that in coals during hunting. Special burners and therefore needed for biomass-based fuels, and to assist the development of these burners, basic research is required to characterise the burning and emission properties of the fuels. A review of the literature encouraged a pyrolysis and combustion study of peat and wood under well-controlled conditions. First a study was made of the temperatures required to initiate the pyrolysis and to oxidise the fuels completely. The temperatures required for complete oxidation in air were </= 5 500 °C, low compared with the values for coals. The pyrolysis of peat started at lower temperature than that of wood but the burnout temperature was equal to or higher than that of wood-based fuels because of the lower content of pyrolysable compounds in peat. The pyrolysis and combustion study of wood as a single particle was first studied with regular-shaped willow cylinders because such cylinders are easily prepared in wet form without disturbing the original moisture balance inside the cylinder. The ratio of volume to surface area strongly affected the shapes of the weight loss curves during pyrolysis under isothermal conditions. The density and reactivity of the char residue was dependent on the original water content in the cylinder. At 500 °C in air the cylinders ignited after drying, partial pyrolysis and a high degree of mass loss. At 700 - 800 °C ignition occurred, while the sample retained nearly its original water content. The cylinders banned incompletely with no flame at 500 °C in 10.5 % O2. At 800 °C the reduction of oxygen content decreased the pyrolysis rate only slightly because pyrolysis is a mainly chemically controlled reaction with temperature dependence. The pyrolysis rates of peat pellets and sod peat were studied and the elemental composition in tar and char from sod peat was followed. The relative amount of hydrogen increased in tar and decreased in char with increasing temperature. But the tendencies in the concentrations of C, O and N were not inverse in tar and char with increasing temperature. Increase in air flow rate, from 0.1 to 0.3 m/s strongly increased the combustion rate of sod peat layer. The increase was small with a room-dried birch chip layer. Preheating of the air increased the reaction rate of all the samples. The maximum combustion powers obtained were about the same for all the fuels, but the duration times of the maximum combustion power were much shorter for wet wood than for peat. Two experiments were performed with peat pellets, with constant flow rate of air and of 15.8 % O2, and the different behaviour of the ash from the pellets was instantly observed. Despite the almost identical measured ash melting points of the ash from sod peat and peat pellets, the ash from the pellets formed a slag at predominantly lower temperature than the ash from sod peat during layer combustion. This confirms the importance of size, density and porosity of the fuel particle. With peat, the maximum conversion percentage of fuel nitrogen to nitrogen oxides was strikingly higher in single particle combustion (> 20 %) than in layer combustion (~ 10 %), where the concentration of nitrogen oxides dropped sharply after the critical lower limit in oxygen concentration was reached. This observation suggests that the emissions of nitrogen oxides could be reduced by regulating the oxygen concentration in critical parts of the combustion zone.