Jaakko Saastamoinen

Simultaneous drying and pyrolysis of solid fuel particles

Drying and devolatilization are studied at combustion temperatures. The surface temperature of particles at the end of drying can significantly exceed the temperature when devolatilization starts, implying that drying and pyrolysis may partly overlap. Devolatilization is controlled by heat transfer, when the particle size is large. The critical particle size at which heat transfer dominates chemical kinetics is discussed. A model for calculating the intrinsic rate of generation of volatiles in the regime of heat transfer control is presented. A novel isotherm migration method is used for the computation of simultaneous drying and pyrolysis inside a fuel particle. It applies to the study of heat transfer in a one-dimensional geometry with moving phase-change boundaries, internal fluid flow and mass generation, including steep temperature and density profiles, as frequently encountered in combustion.

Simultaneous pyrolysis and char combustion

Simultaneous pyrolysis and char combustion of < 500 μm coal and peat particles in an entrained-flow reactor was studied experimentally and theoretically at 700–1100 °C and at oxygen contents < 21 vol%. A method based on near infrared radiation was developed to measure the particle temperatures. Simultaneous pyrolysis and char oxidation occurs when oxygen reaches the char particle surface during the devolatilization stage. The overlap of homogeneous and heterogeneous combustion is favoured by low temperature and high oxygen content, as well as by a decrease in particle size. A simplified method of calculating simultaneous pyrolysis and char oxidation is presented. Model calculations show that under some conditions a maximum combustion rate may be achieved with a specific particle size, since the contribution of faster homogeneous combustion is increased relative to slower heterogeneous combustion.

Evaluation of char reactivity data by different shrinking-core models

Abstract Carbon reactivity was measured in two entrained flow reactors, two drop tube furnaces and one fluidized bed. The results were interpreted by using three shrinking-core models. A number of minor differences between the models were identified and the resulting deviations could mostly be ascribed to non-measurable parameters such as particle specific heat capacity as a function of temperature, boundary layer gas composition and Nusselt and Sherwood numbers, together with bulk flow equations and numerical procedure. Using all experimental data, the calculations gave different overall activation energies ranging from 107 to 130 kJ mol −1 . The differences between the model calculations with respect to overall kinetics were small but significant under realistic conditions for combustion and for the experiments performed. A comparison between individual experiments showed that for experiments performed with small coal particles, long residence times or preheated particles, good agreement between model results was obtained. This was not so for larger particles, short residence time and no preheating, since data taken under transient conditions impair the results. Guidelines are given for experimental design for making reactivity measurements on solid fuels in experimental reactors.

Determination of heats of pyrolysis and thermal reactivity of peats

Heats of pyrolysis of four Finnish peats were determined by differential scanning calorimetry (d.s.c.), and combustion enthalpies were compared by differential thermoanalysis (DTA) in an air atmosphere. Weight loss data obtained by thermogravimetric analysis (TGA) were combined with the enthalpy data. The heats of pyrolysis of the peats varied between 120 and 250 kJ kg−1. These values were used to model the combustion and pyrolysis of small peat particles (typical in pulverized fuel combustion) and large peat particles (typical in grate combustion). TGA and DTA together gave useful information about the reactivity of peat in an oxidizing atmosphere with changing temperature, despite the similar shapes of the derived TGA (DTG) and DTA-curves. The pyrolysis and combustion behaviour of the peat with the lowest degree of decomposition was similar to that of cellulose, and the differences in the peats were apparent with all the methods used (d.s.c., DTA and TGA). The initial temperatures of pyrolysis and combustion were lower with peats of high ash content (inorganic material) and the combustion proceeded through more steps than with the peats of low ash content.

Analytical solutions for steady and unsteady state particle size distributions in FBC and CFBC boilers for non-breaking char particles

Abstract Continuous analytical solutions for the particle size distributions of char in steady and unsteady states in fluidized beds, when the inlet fuel feed is presented by monosize, lognormal, Rosin-Rammler or gamma distributions, are derived from a population balance model. The stationary size distribution is directly related to the rate of reduction of the particle size. Combustion and attrition reduce the particle size. Thus, it is possible to extract the dependence of the rate of reduction of radius (affected by a fuel’s reactivity and attrition) on radius from a measured steady-state particle size distribution. Unsteady particle size distributions are derived for impulse, step and square pulse changes in the fuel feed, when the oxygen level in the reactor is maintained constant.

Combustion Characteristics of Fuels: Experiment Scale-Up From Bench Scale Reactors to Commercial Scale CFB Boiler

The equipment scale-up towards larger CFB units requires accurate knowledge of the process and combustion behavior of fuels. Unit sizes of 300 MWe are in operation and plans for larger units have been made. Shift from natural circulation to once through steam cycle requires more precise knowledge of the dynamic behavior of the fuel since there is no steam drum. The combustion of inhomogeneous fuels, as well as, special demands for dynamic process behavior poses new challenges to boiler manufactures. Nowadays, dynamic models are used to develop and analyze the dynamic behavior of the combustion process. Testing all the dynamic changes in the full-scale reactor would be both expensive and risky. Therefore, bench and pilot scale experiments, combined with dynamic models of the combustion processes, give a good basis to study behavior of larger scale units. At the same time models also increase knowledge of different process relations. The main objective of this paper is to present results of scale-up experiments from the bench scale, via pilot scale, to full-scale boilers. Further, how the combustion and reactivity of fuels in the full-scale boilers can be studied with the aid of small-scale experiments and simulations. Dynamic experiments were carried out with three reactors of different scale. Calculation and simulation models have been developed to illustrate the combustion in the reactors; e.g. heat release profiles, fuel reactivity and particle size distribution. Results from the dynamic experiments are used to adjust the computer models.Copyright

Study of Operation of a Pilot CFB-Reactor in Dynamic Conditions

The development of a high efficiency CFB power plant (once-through supercritical CFB technology) and the use of alternative fuels require advanced methods of control and knowledge of the dynamic behavior of the furnace. Dynamic response analysis is needed for design of control algorithms in load changes. The operation of a pilot CFB-reactor in dynamic conditions and in load changes is analyzed experimentally and by modeling at different process conditions. Reactivity parameters for different fuels can be extracted from simple dynamic measurements and then used in computations studying operation in load changes. Dynamic studies are also required to see the necessary requirements for the fuel quality and fuel feed system to maintain stable operation. For high volatile coals the fuel feeding must be steadier to keep the variation in the outlet oxygen concentration at some range than with coals with low reactivity or alternatively greater air coefficient is needed to prevent too low O2 concentrations, which can cause an increase in CO emissions. The fuel quality can be characterized by the fluctuation of oxygen concentration in flue gases in steady operation conditions, which depends on the fluctuations in the combustion and in the fuel feed and on operational conditions. The amplitude of the fluctuations was studied. For advanced controls, it is necessary to know the factors affecting the process dynamics, such as reactivity and the behavior of char inventory in bed. This information is also necessary in developing and optimizing the CFB boiler considering emissions, combustion process and furnace scale up.Copyright

Modeling of particle size distribution of limestone in sulfur capture in air and oxy-fuel circulating fluidized bed combustion

© 2016 Saastamoinen et al. This work is published by Dove Medical Press Limited, and licensed under a Creative Commons Attribution License. The full terms of the License are available at http://creativecommons.org/licenses/by/4.0/. The license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 2016 Saast moinen et al. This work is published and licensed by e Medical Press Limited. The full terms of this license are availabl at https://www.dovepress.com/terms. p p and incorporate the Creative Commons Attribution – Non Commercial (unp rted, v3.0) Licen e (http://creativecommons.org/licenses/by-nc/3.0/). By accessing the work you hereby accept the Terms. Non-commercial uses of the work are permitted without any further permission from Dove Medical Press Limited, provided the work is properly attributed. For permission for commercial use of this work, please see paragraphs 4.2 and 5 of our Terms (https://www.dovepress.com/terms.php). Energy and Emission Control Technologies 2016:8 41–52 Energy and Emission Control Technologies Dovepress