Coal conversion submodels for design applications at elevated pressures. Part I. devolatilization and char oxidation

Abstract

Numerous process concepts are under development worldwide that convert coal at elevated pressure. These developments rely heavily on CFD and other advanced calculation schemes that require submodels for several stages of coal chemistry, including devolatilization, volatiles combustion and reforming, char oxidation and char gasification. This paper surveys the databases of laboratory testing on devolatilization and char oxidation at elevated pressure, first, to identify the tendencies that are essential to rational design of coal utilization technology and, second, to validate two well-known reaction mechanisms for quantitative design calculations.

Devolatilization at elevated pressure generates less volatile matter, especially tar. Low-rank coals are no less sensitive to pressure variations than bituminous coals; in fact, coal quality is just as important at elevated pressure as it is at atmospheric pressure. Faster heating rates do not enhance volatiles yields at the highest operating pressures. The FLASHCHAIN^® predictions for the devolatilization database depict the distinctive devolatilization behavior of individual samples, even among samples with the same nominal rank. The only sample-specific input requirements are the proximate and ultimate analyses of the coal. There were no systematic discrepancies in the predicted total and tar yields across the entire pressure range. Char oxidation rates increase for progressively higher O₂ partial pressures and gas temperatures, but are insensitive to total pressure at constant O₂ mole fraction. Char burning rates become faster with coals of progressively lower rank, although the reactivity is somewhat less sensitive to coal quality at elevated pressure than at atmospheric pressure. An expanded version of the carbon burnout kinetics model was able to represent all datasets except one within useful quantitative tolerances, provided that the initial intrinsic pre-exponential factor was adjusted for each coal sample.

Introduction

Across the globe developers of coal-fired power generators face imperatives to raise conversion efficiencies to compete better with other fuels, especially where CO₂ emissions are being reduced. A multitude of process concepts are under development, as surveyed recently in PECS by Beer [1]. All have one thing in common: Primary conversion of the coal feed at elevated pressure. Entrained coal gasifiers operate at 2.5–3.0 MPa, with temperatures to 2000 °C and overall stoichiometric ratios (SR) of about 0.8. Pressurized fluidized bed combustors (PFBCs) operate at 1.5–2.0 MPa, at about 850 °C and a SR near 1.15. Fluidized bed gasifiers operate at similar pressures and temperatures with SR values as low as 0.7. Pressurized pulverized coal boilers are proposed much less frequently than the other units, but have been incorporated to raise steam by burning the residual char with a small portion of coal feed. In one proposed process [2], the boiler operates at 3.1 MPa with conventional waterwall temperatures. Different versions of these processes are being developed in many of the major industrialized nations, including Japan which imports coals from all the major coal producing regions worldwide. Consequently, they will be fed with coals representing the entire range of coal quality, from lignites to subbituminous to high volatile (hv) bituminous to low volatility coals.

Today, any major technology development effort is almost always supported by computational fluid dynamics (CFD) and/or other design calculation schemes. Such massive calculations are organized into submodels for each of the essential physicochemical stages. There are independent submodels for fluid dynamics, particle dynamics, heat transfer, coal conversion chemistry, and chemistry in the gas phase. We will only consider reaction mechanisms which are essential elements of a submodel for coal conversion chemistry at elevated pressures, particularly the following two steps.

(1) The partitioning of the coal feed into volatiles and char is crucial because volatiles are subsequently converted into ultimate products on much shorter time scales than char. The reaction mechanism responsible for the partitioning is called ‘devolatilization’. It governs the stabilities of flames on the fuel injectors and also affects temperature profiles and all the major emissions. Devolatilization behavior is widely variable, even among different samples of the same type—or ‘rank’—of coal. Devolatilization kinetics are needed in simulations, but the total volatiles yield is the crucial characteristic. The O₂ requirement for volatiles combustion and the associated heat release are also important. Volatiles species compositions are generally ignored in design calculations.

(2) The residual char from devolatilization must be completely converted into ultimate products, simply because fuel costs are the major component of process operating costs. Char oxidation must be described because, even in gasifiers, O₂ is injected to raise the process operating temperature into the target range. A suitable reaction mechanism must automatically adjust the limiting stage to correctly predict the burning rate, beginning with the intrinsic chemical kinetics at low temperatures, then to O₂ transport within the char at moderate temperatures, then to O₂ transport from the bulk gas flow to the external char surface at the highest temperatures. Also, the intrinsic kinetics must also depict the substantial differences among the reactivities of chars from diverse coal types, as well as the loss of reactivity by annealing at temperatures above 1000 °C. Additional factors reduce burning rates during the latest stages of burnout, such as the size reductions that lower particle temperatures, thereby re-instituting chemical kinetic control and, in some special cases, the hindered transport through ash layers [3]. When O₂ is not present, chars are gasified by the combined chemistry of CO₂, H₂O, CO, and H₂ in the process stream. Differences in char reactivity are thought to be even more important in gasification than in oxidation, because the reaction times are so slow that the gasification agents can penetrate deeper into the chars' internal pore structures. A more significant difference is that the concentration of the gasification agents is determined by chemistry in the gas phase that partially oxidizes and reforms the primary volatiles.

Part I of this paper covers both of these issues with one notable exception: only char gasification by O₂ is included. Char reactivities for other gasification agents (CO₂, H₂O, CO, and H₂) are surveyed separately in Part II. This arrangement enables direct references to the substantial databases on devolatilization and char oxidation at atmospheric pressure, to highlight the pressure effects, per se. These same topics were recently discussed in PECS by Wall et al. [4], but with the objectives of thoroughly reviewing the experimental work and surveying some of the major modeling approaches. Our papers are complementary in the sense that reaction mechanisms are emphasized here, and test results are primarily used to evaluate the mechanistic models.

Our ultimate aim is to validate a reaction mechanism for devolatilization that can predict the yields from any coal for heating rates from 10 to 10⁵ °C/s, temperatures from 800 to 2000 °C, and pressures to 3 MPa. This mechanism must also predict the char properties needed to simulate all stages of burnout; viz., the char yield, size, and density of reactive sites. A companion mechanism for char oxidation must predict the burning rates from the onset of ignition to extinction for the same ranges of temperature and pressure, and for O₂ levels up to 100%. The ultimate goal is to establish new benchmarks for the quantitative accuracy of predictions for devolatilization behavior and char oxidation reactivity at elevated pressures by evaluating the model predictions against all the available test results in the English literature that specified the required input for the simulations.

Our research strategy is regarded as classical in many branches of engineering science, but is unique in this area. First, all the datasets on devolatilization and char oxidation at elevated pressure in the literature in English were qualified for their suitability for model validations. Then selected datasets from various sources were combined to clearly illustrate the tendencies for all the important operating conditions, including coal quality. Then the predictions from the reaction mechanisms were evaluated with each dataset, and the discrepancies were compiled into statistics for the ‘best’ representation of the entire database. Although model parameters may have been tuned at various stages in the data evaluations, all model predictions in this paper are based on the ultimate sets of parameter estimation algorithms for both reaction mechanisms.

At the outset, it is worth noting that several essential mechanisms for detailed process simulations involving devolatilization and char oxidation were omitted. Whereas the devolatilization mechanism, per se, is complete except for fragmentation under very high heat fluxes, essential chemistry for the subsequent conversion of volatiles into soot, partial oxidation products, and other reformed species is not considered. All forms of intraparticle gradients are also neglected. Similarly, all the necessary transport and chemical mechanisms to describe char oxidation at the level of individual particles are considered, but subsequent shifting of the primary oxidation products by gas phase chemistry is omitted. Moreover, essential aspects of single-particle burning in fluidized systems are also omitted, including fragmentation and comminution which are usually primary mechanisms for mass loss in bubbling fluidized beds.

This paper is organized in the same way that the research was conducted, except for the addition of a section on design applications after the model validation section. Devolatilization will be considered first, followed by char oxidation.