WOOD IN PULVERIZED FUEL POWER PLANTS: CHAR CHARACTERIZATION AND CHAR COMBUSTION
Sustainable process-product development & green chemistry
Sustainable & Clean Technologies - III: Combustion & Emission (T1-6P)
Keywords: wood, char, pyrolysis, combustion
In suspension fired boilers char oxidation is the slowest step in the conversion of wood and thus determines the degree of burnout of the fuel as well as the heat release profile in the boiler, affecting the operation and efficiency of the plant. Our work aims at developing tools for the prediction of wood particles burnout in suspension fired boilers.
In the past, it has been assumed that wood char has the same structure as wood; this work gives clear evidence that this is not the case when high heating rates and high temperature industrial combustion processes are considered.
It is well known that pyrolysis conditions influence both the yield of char and the char reactivity; however wood char reactivity data available in literature differ much. Furthermore it is not clear how and to what extent the morphology (and size) of the char particles depend on pyrolysis conditions.
In this study experimental work was carried out to investigate the influence of high temperature pyrolysis on wood char properties; chars from different wood fuels (bark and pine) were produced at different conditions in a pilot-scale entrained flow reactor (EFR). With this reactor the wood particles experienced rapid heating and high reactor temperatures, which reproduces the conditions in a real suspension boiler and differs substantially from the much lower temperatures and heating rates normally applied in pyrolysis studies. For comparison, some wood chars were produced by thermogravimetry (TGA) at lower heating rate but similar final pyrolysis temperatures.
The produced chars were analyzed by SEM microscopy and their reactivity derived by thermogravimetry. SEM analyses showed that wood particles subjected to rapid heating had lost completely their characteristic fibrous structure; char particles were spherical and porous, and it seemed likely that during devolatilization the particles had gone though a molten phase. Wood particles exposed at higher temperatures resulted in char particles of smaller size.
The oxidation reactivity of the produced wood chars was derived by TGA. Chars have been oxidized during non-isothermal runs and a volumetric reaction model assuming a single first order reaction was used to analyze the experimental mass loss curve. The obtained kinetic parameters could reproduce well the experimental data.
Char produced in inert atmosphere showed a tendency to deactivate when the pyrolysis temperature increased. Moreover, for the chars produced in TGA, an interval of pyrolysis temperature where char reactivity did not depend on pyrolysis temperature was observed. Chars produced under boiler-like conditions in the EFR showed significantly higher reactivity than those produced at milder conditions in the TGA apparatus. This could be due to the major transformation of the particle structure during fast pyrolysis and/or re-arrangement of the mineral matter during fast pyrolysis.
Kinetic parameters obtained from chars produced at boiler-like conditions will allow a more accurate prediction of fuel burnout in pulverized wood power plants.
Presented Monday 17, 13:30 to 15:00, in session Sustainable & Clean Technologies - III: Combustion & Emission (T1-6P).
