Richard K. Lyon

Unmixed combustion: an alternative to fire

A new form of combustion has been studied with properties that are greatly different from combustion via fire. Called unmixed combustion, it occurs when fuel and air alternately pass over a catalyst that undergoes oxidation and reduction, storing oxygen from the air and delivering it to the fuel. Examples of catalysts include finely divided Cu/CuO or FeO/Fe2O3 supported on γ-alumina. The full heat of combustion of the fuel is released, the fuel is converted to CO2 and water, and the air is depleted of oxygen, all without any need for the fuel and air to mix. Thus unmixed combustion is an alternative to fire, another way of using fuel and air to generate heat. The properties and characteristics of unmixed combustion are different from those of combustion by fire in a number of ways, some obvious, some subtle, suggesting a number of applications where conventional combustion is not or cannot be used. One example are situations in which it is difficult to provide complete mixing but in which complete combustion is required; e.g., rotary kiln incinerators. These incinerators have a failure mode called “puffing” to which unmixed combustion may be relevant. In the area of pollution control unmixed combustion is capable of burning natural gas and pyridine with zero NOx production; of burning sulfur-containing fuels in a manner that facilitates subsequent removal of the SO2; and of burning coal in a manner that directly provides sequestration ready CO2. Greatly enhanced delivery of heat to surfaces, rapidly supplying heat for cold starting engines, and direct generation of dry inert gases are also possible. Unmixed combustion also allows the delivery of heat uniformly throughout a volume. This method of heat delivery is potentially useful for supplying heat to endothermic reactions carried out in packed beds of catalyst. Experimental evaluation of this technique for steam reforming shows that while conventional steam reforming is a strongly endothermic reaction with an unfavorable equilibrium, the use of unmixed combustion allows a redefinition of the system’s thermodynamics, making the reaction weakly exothermic with a more favorable equilibrium.

The selective reduction of SO3 to SO2 and the oxidation of NO to NO2 by methanol

The article reports the discovery of a new homogeneous gas phase reaction in which methanol converts SO3 to SO2. In the course of this reaction NO is converted to NO2. This new reaction is highly selective in that ppm concentrations of SO3 and NO are converted by equivalent quantities of methanol, even in the presence of large excesses of O2. Both the conversion of SO3 to SO2 and NO to NO2 are reversible in that at any given temperature there is an optimal reaction time; the use of longer reaction times causes decreasing conversion. For the optimal temperature the reaction is also rapid, capable of achieving better than 80% reduction in only 55 ms. The existence of this new reaction was predicted by computer modeling. Subsequent experiments verified the predicted modeling trends. The mechanism by which methanol simultaneously reduces SO3 and oxidizes NO involves methanol functioning as a source of HO2 free radicals, which then initiate the reactions NO + HO2 = NO2 + OH, SO3 + HO2 = HSO3 + O2, and HSO3 + M = SO2 + OH.

A reexamination of the RapreNOx process

The recently proposed RapreNOx process involves the injection of HNCO into combustion effluents in order to reduce their NO content via what was initially assumed to be a homogeneous gas-phase reaction. Although the process is thus similar to the Thermal DeNOx process it had the apparent advantage of operating at temperatures as low as 450°C, as contrasted with the 700°C minimum temperature of Thermal DeNOx. Further, the Thermal DeNOx process had the disadvantage that downstream of the reaction zone unreacted ammonia could react with SO3 to form NH4HSO4 and cause fouling problems, a difficulty thart using HNCO would seem to avoid. In this reexamination of the RapreNOx process we show that the process involves three different modes of NO reduction. The first is catalytic; NO reductions at temperatures significantly below 700°C are found not to occur in the absence of catalytic surfaces. While noncatalytic NO reductions were found to occur at 700 °C in the presence of wet CO, NH3 will also reduce NO at these conditions. Modeling calculations indicate that for this mode of reaction HNCO reduces NO via a complex chain reaction mechanism very similar to that involved in the Thermal DeNOx process. Within this reaction mechanism the interconversion of HNCO and NH3 is sufficiently rapid that the amounts of HNCO and NH3 left at the end of reaction are roughly independent of whether one starts with HNCO or NH3. The modeling calculations also indicate the existence of a third mode of NO reduction by HNCO. In this third mode OH formed by thermal dissociation of H2O reacts with HNCO to form the NCO radical, which reduces NO to N2O. This third mode of NO reduction is predicted to occur at temperatures substantially higher than those used in the Thermal DeNOx process and it is thus potentially suitable for limiting NOx emissions from gas turbines, but the N2O produced in this mode may be a significant disadvantage.

The sulfur retention of calcium-containing coal during fuel-rich combustion

Abstract The fuel-rich combustion of coals containing calcium in various forms has been studied in a tubular downflow reactor to determine whether or not coal-bound sulfur can be efficiently retained as CaS in the recovered solids. Although physical mixtures of coal and limestone gave only limited sulfur retention, it was discovered that, under certain critical conditions, coals in which calcium had been atomically dispersed by ion exchange could be burned with the bulk of the sulfur remaining in the recovered ash/char mixture. The effect of various experimental parameters upon this new sulfur retention process are reported. Char characterization was done using scanning electron microscopy and x-ray diffraction. Analysis of the data indicated that under the conditions of these experiments, the extent of CaS formation was equilibrium limited when ion exchange Ca was used but less than equilibrium for bulk Ca.

Kinetics of the NO−NH3−O2 reaction

In a previous paper (10) it was reported that NO+NH3+O2 mixtures may undergo rapid reaction under conditions such that NO+NH3, NH3+O2, and NO+O2 mixtures were all nonreactive. Thus the NO−NH3−O2 reaction is both a new reaction and an example of an unusual kind of reaction and in this paper its kinetics are elucidated. Experiments were done in a flow system of conventional design, using helium as the carrier gas. It was found that oxidation of NH3 to form NO occurs concurrently with reduction of NO by NH3 and thus [NO] tends toward a steady state value. Via a somewhat complex data reduction procedure it was found that the reduction and oxidation reactions could be formally separated and the rate of the reduction reaction may be expressed as d 1 n [ NO ] − [ NO ] ss d t = − k R [ O 2 ] 1 / 2 [ N H 3 ] 1 / 2 [ He ] − 1 (I) where kR=1.8×1012 exp — (46500/RT)sec−1. The above expression describes the rate of the reduction reaction for the temperature range 872°C to 954°C, the pressure range 1.07 to 2.14 atmo., and for the reactants in such relative proportions that [He]≫[O2]≫ [NH3]≫[NO]. It was also observed that if [NH3] was increased so that it approached [O2], the extent of [NO] reduction occurring during a given reaction time could decrease from a nearly quantitative reduction to little or no reduction. The concentration of NH3 needed to cause this self-inhibition effect was found to increase with increasing [O2], increasing reduction time, and increasing temperature. A free radical reaction mechanism which explains the above results is discussed.

Kinetic modeling of artifacts in the measurement of N2O from combustion sources

Abstract The N 2 O emissions from combustion systems, especially coal-fired utility boilers, has been a matter of increasing concern because N 2 O contributes both to the greenhouse effect and to ozone depletion. Unfortunately, all measurements of these emissions to date have involved sampling procedures that allowed the flue gas to age prior to N 2 O analysis. In this article we report a computer modeling study of the chemical reactions which occur during the aging of flue gas samples. These reactions can produce N 2 O in the amounts observed and there is no need to assume that N 2 O was initially present.

Coal Science: Basic Research Opportunities

More fundamental knowledge of coal (knowledge of its structure and its behavior during conversion processes) is essential before we can generate new technologies necessary for the efficient use of coal in the future. Herein are suggested specific basic research opportunities in the areas of coal characterization, gasification, combustion, and liquefaction, along with an assessment of the impact such research programs could have. Critical characterization needs include qualitative and quantitative determination of the chemical forms of carbon, oxygen, nitrogen, and sulfur and reliable methods for the measurement of surface area, pore volume, and weight-average molecular weights. Mechanistic studies aimed at increasing understanding of the thermal breakdown of the functionalities in coal, the behavior of coal in the presence of molecular and donor hydrogen environments, and carbon gasification and hydrocarbon synthesis reactions starting from carbon monoxide and hydrogen will lay the scientific foundation for the development of new processes for converting coal into clean usable fuels and chemicals.

Oxidation kinetics of wet CO in trace concentrations

Abstract The title reaction is observed at temperatures from 1123°K to 1298°K for pressures of 1.22 to 2.44 atm for initial concentrations of CO as small as 3.1 ppm. Sensitivity analysis demonstrates that at these low CO concentrations the kinetics of the overall reaction are effectively sensitive to only one elementary reaction, CO + OH → CO 2 + H. Thus the good agreement between the observed reaction kinetics and the kinetics predicted by computer modeling confirms the literature value for k CO+OH .

Influence of inert gas pressure on the kinetics of wet CO oxidation

The kinetics of wet CO oxidation have been experimentally studied in considerable detail (see Ref. [1] and references therein) and have been very successfully reproduced by computer modeling I-2]. As a result the process is generally regarded as well understood. In a previous paper [3], however, the authors showed that, contrary to the global rate law determined by Dryer and Glassman [1], the rate of CO oxidation departs from first order in CO at low CO concentration and may essentially stop. This paper reports another complication, the effect of inert gas pressure upon the CO oxidation rate. While this effect may appear surprising and it is certainly not explicitly recognized in the previous literature, one may deduce its existence from elementary considerations via the following argument: Dryers empirical rate law is based on experiments in which CO was oxidized in the presence of large amounts of nonreactive gases, N 2 and CO 2, in an apparatus restricted t o operation at one atmosphere total pressure [1]. The observed rate was first-order in CO, half-order in H20, and quarter-order in 0 2 . It is generally accepted that wet CO oxidation occurs chiefly by the reaction CO + OH--*CO 2 + H. By comparing the known rate constant for this elementary reaction with the rate of the overall oxidation, one may deduce that the OH concentration has a steady state value 100 times its equilibrium value for the conditions of the Dryer experiment. Steady state implies a balance between chain carrier production via the chainbranching reaction cycle and chain carrier removal via three-body processes. The latter may occur by

The Domino Theory of steam gasification of spent shale

Abstract This Paper reports a study of the chemistry which occurs during the steam gasification of spent (i.e. retorted) Green River oil shale. The results suggest that in the presence of steam there is an unusual and strong coupling between the decomposition of the inorganic carbonates, the gasification of organic carbon by steam, and the removal of nitrogen from the shale.