Clean Coal Technologies Research Papers (original) (raw)

To develop accurate predictive models for coal combustion or gasification, it is necessary to know the rate and amount of volatiles released in pyrolysis as a function of the particle temperature history. The volatiles, which can account for up to 70% of the coal's weight loss, control the ignition, the temperature and the stability of the flame in combustion and the temperatures and product distributions in gasification or mild gasification. In addition, the pyrolysis process controls softening, swelling, particle agglomeration, char reactivity, and char physical structure; soot formation (which can dominate radiative energy transport) is controlled by the tar produced in pyrolysis.
The development of predictive capabilities requires the understanding and description of a complicated mix of heat and mass transfer proccesses and chemical reactions. Unfortunately, progress is hampered by a lack of agreement on the rates of coal pyrolysis processes. For example, at particle temperatures estimated to be 800°C, rates reported in the literature (derived using a single first-order process to define weight loss or tar evolution) vary from about I sec-I to more than 100 sec-I with values in between. It is important that a more precise knowledge of pyrolysis rates and mechanisms be obtained. There are many possible explanations for such discrepancies. For example, the coal samples differ widely and are themselves heterogeneous. Samples may contain a mixture of macerals and minerals and even pure macerals themselves may be a mixture of similar components (e.g. molecules bound in a macromolecular network, mixed with smaller unbound guest molecules of the same composition) or heterogeneous components from apparently different origins (aromatic ring clusters with small peripheral groups linked by bridges, mixed with long chain aliphatics). In addition, the heating rates used in the experiments vary from fractions of a degree C/sec to tens of thousands of degrees C/sec. However, closer examination of the data show that wide differences in rates are reported even for similar coals at comparable heating rates.
On the other hand, the discrepancies could arise because of the difficulties in interpreting the complicated mixture of heat transfer, mass transfer, and chemistry which comprises the pyrolysis process. Heat transfer is from the hot reactor to the coal particles whose physical shape, heat capacity, emissivity and thermal conductance are changing with temperature. In addition, the evolving volatile products can alter the region surrounding the sample, thus changing the heat transfer environment.
The chemistry of pyrolysis includes the decomposition of individual functional groups in the coal to produce light gas species, and the decomposition of the macromolecular network to produce smaller fragments which can evolve as tar. The network decomposition is a complicated mixture of bridge breaking, crosslinking, hydrogen transfer, substitution reactions, concerted reactions, etc. The light species and light network fragments are then conveyed by mass transport processes to the exterior of the coal particle. The mass transport processes include diffusion in the decomposing solid or liquid, vaporization of the light network fragments, gas phase diffusion and pressure-driven convective transport. The transport can occur within pores, by bubble movement or a combination of these.
To study pyrolysis, a variety of reactors have been employed depending on the aim of the experiment. Simple weight loss measurements were performed at low heating rates in crucibles (ASTM) or TGA apparatuses and at high heating rates in heated grids or entrained flow reactors. Species evolution kinetics have been measured in heated grids, entrained flow reactors, fluidized beds, TGAs, and mass spectrometry probes. Pyrolysis experiments with direct particle temperature measurements have been performed using laser heating with particle temperature measurements by two-color pyrometry, and heating by contact with hot gases with particle temperature measurements by FT-IR Emission/Transmission spectroscopy or two-color pyrometry. In the majority of cases, particle temperatures have been calculated indirectly from a knowledge of the surrounding ambient temperatures.
The results of these experiments have been interpreted with a number of assumptions on the heat transfer (or particle temperature) in the reactor and a variety of pyrolysis models. The models vary from simple weight loss models which lump chemistry and mass transport using one or two competing reactions to more detailed species evolution models and even more complicated general mechanistic models which include chemistry, mass transport, and/or heat transport. The diversity ofrates results from jitting data from different experiments and different coals with different models and different heat transfer assumptions. In most cases, the heat transfer assumptions were not verified with direct particle temperature measurements.
When we consider a kinetic rate reported in the literature, we must, therefore, consider the experiments and the model used to determine the rate. All the models fit the data used to derive them, even though the reported rates vary between three orders of magnitude at comparable temperatures. The key question is, however, can the model predictions be extrapolated to other conditions for the same experimental apparatus or other reactors? In many cases, the answer is no. Thus, the usefulness of the model and rates are limited to curve fitting the data used to derive them.
To derive models and rates which are more generally applicable, it is essential to re-examine the experiments themselves, the heat transfer assumptions, and the models to find the reasons for the discrepancies. The review is focused primarily on pyrolysis under conditions where internal temperature gradients are unimportant because the presence of temperature gradients within the particle makes the extraction of kinetic rates significantly more difficult. This limits the consideration of pyrolysis studies at high heating rates to those where pulverized-sized coal particles were used. The review is concerned primarily with the evaluation of kinetic rates and broadly defined mechanisms for major volatile species evolution and does not consider specific pyrolysis models which have been proposed except with regard to how this influences the kinetic parameters.