A.N. Hayhurst

the effect of CO2 on the kinetics and extent of calcination of limestone and dolomite particles in fluidised beds

Measurements have been made of the rates of calcination of limestone particles (diam. 0.4–2.0 mm) in a fluidised bed, electrically heated to a well-defined temperature. Experiments were conducted at atmospheric pressure and also at pressures of 3, 6, 12 and 18 bar, for bed temperatures varying from 1073 to 1248 K. The fluidising gases were air, or occasionally nitrogen, containing up to 20 vol. % CO2. The results indicate that under these conditions the rate of calcination of such limestone particles is controlled by chemical reaction at a sharp interface between CaCO3 and CaO. The temperature of this reaction zone is only a few degrees (< 15 K) below that of the fluidised bed. The rate of calcination is found to be of the form: k(peCO2 - piCO2 - PyI) kmol/m2 s, where peCO2 and piCO2 are, respectively, the partial pressures of CO2 for equilibrium at the temperature of the reaction interface and in the fluidising gas, k is the rate constant associated with the reverse carbonation reaction (CO2 + CaO → CaCO3), P is the total pressure, and yI is a constant, which depends on the temperature of the bed. Values of k were measured. They appear to be independent of temperature, indicating that carbonation proceeds without an associated activation energy. It is hard to explain the appearance of yI (an effective mole fraction for CO2) in the above rate expression. The calcination of one dolomite has been briefly studied and calcination times etc. measured. In general, this appears to be a two-stage process, with the calcination of the MgCO3 component being insensitive to pressure, unlike the CaCO3. The value of k for CO2 + MgO → MgCO3 is similar to that for the calcium case.

Regeneration of sintered limestone sorbents for the sequestration of CO2 from combustion and other systems

Abstract The capacity of particles of CaO, produced by calcining limestone, to reactively absorb CO2, degrades with the number of cycles of carbonation and calcination. A novel method of reactivating the stone in humid, ambient air is described. Typically, a calcined limestone has a carrying capacity for CO2 which falls from ∼79% (on the basis of moles of CO2 per mole of CaO) to only about 20–30% after 30 cycles of regeneration and reuse. This new technique enables the carrying capacity to be restored to ∼55%, thereby improving the economics of sequestrating CO2 using a calcium-based sorbent.

Emissions of nitrous oxide from combustion sources

Abstract Nitrous oxide (N 2 0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N 2 O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N 2 0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO 2 , H 2 O and NO in combustion gases react in a sampling vessel to produce N 2 O, has revealed that, in fact, less than 5 ppm of N 2 0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N 2 O levels of ca. 50ppm in their off-gases. The gas-phase reactions of N 2 0 in flames are reviewed first. It is clear that in most cases N 2 0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N 2 0 are thermal decomposition to N 2 and O 2 and also radical attack in e.g. N 2 O + H → N 2 + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH 3 ; the reactions are NCO + NO → N 2 0 + CO and NH + NO → N 2 0 + H. The levels of N 2 O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH 3 at producing N 2 0. The removal of N 2 O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts. When coal burns in a fluidized bed, most of the N 2 O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH 3 are produced from nitrogenous material released during devolatilization; these two compounds give N 2 0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O 2 on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N 2 0, in addition to CO, CO 2 and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N 2 O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N 2 O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N 2 O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N 2 O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N 2 O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N 2 O emissions from fluidized bed combustors are identified.

Does solid carbon burn in oxygen to give the gaseous intermediate CO or produce CO2 directly? some experiments in a hot bed of sand fluidized by air

Abstract Relatively large spheres of non-porous graphite (diam. 2–12 mm) have been burned in electrically heated beds of sand, fluidized by air. The rate of reaction of the carbon was derived from measurements of the concentrations of CO and CO2 in the gases leaving the bed. The carbon particles temperature was monitored continuously during its combustion using a very fine thermocouple inserted in the graphite sphere. The temperature of the burning particle rose whilst its diameter shrank. The observations can be interpreted in terms of CO being the only product of oxidation for graphite particles burning between 1000 and 1400 K in such a fluidized bed. The oxidation of CO is inhibited by the proximity of sand, which provides a large surface area for radicals (e.g. OH and HO2) to recombine. Nevertheless, CO does burn to CO2 close to the carbon at the higher temperatures studied (1400 K), but at ∼ 1100 K it mainly diffuses away before burning in say a rising air bubble. In this case less heat is fed back to the burning carbon. The use of tiny sand particles can give small air velocities over the carbon particle, so that convection of CO away from the carbon is reduced and the full enthalpy of combustion of carbon to CO2 is donated to the carbon. It seems likely that, just as radicals are important in oxidising CO to CO2, they are also involved in burning solid carbon to CO.

In situ gasification of coal using steam with chemical looping: a technique for isolating CO2 from burning a solid fuel

AbstractIn chemical looping combustion (CLC), a metal oxide (generalised as MeO) is used to oxidise a gaseous fuel in: (2n+m) MeO+C n H 2m →(2n+m) Me+mH2O+nCO2. Pure CO2 can then be obtained by cooling the off-gases to condense the steam. The reduced form of the metal oxide, Me, is then transferred to a different reactor, where it is re-oxidised by air in: Me+1/2O2→MeO. The gas from this reactor is N2 containing unused O2. The net effect of these reactions is that the fuel has been burned, with the total heat evolved being the same as for combustion of the fuel in air; however, the resulting CO2 is now pure and not mixed with the nitrogen from the air. This paper shows how it is possible to use CLC with a solid fuel, such as coal char, provided a gasification agent like steam is introduced into the reactor. The gasification agent transfers solid carbon to gaseous CO, which, like the H2 also formed, can be reacted with a solid, e.g. Fe2O3, carrying oxygen, to yield CO2 and H2O. On the basis of a limited se...

The order, Arrhenius parameters, and mechanism of the reaction between gaseous oxygen and solid carbon

Abstract The rates of oxidation of tiny particles (initial diam. ∼115 μm) of a nonporous graphite have been measured in beds of silica sand fluidized by different mixtures of O2 + N2 at 1 atm. Temperatures of 973–1173 K were used. Before burning, the graphite was fully characterized by B.E.T. analysis. Such a fluidized bed has several advantages for measuring the rate of burning of solid carbon: it proved straightforward to ensure that combustion was kinetically controlled, and that such small particles were both isothermal and at the same temperature as the fluidized bed. Also, the oxidation of CO to CO2 is inhibited by the large area of sand, which removes radicals, such as HO2 and OH. It is concluded that in this situation oxidation occurs in Cs + 1/2 O2 → CO (I) whose rate was measured to be half-order in O2 over the temperature range used and for a variety of concentrations of O2. Writing the rate of consumption of carbon in reaction (I) as k1 [O2]1/2 per unit area of carbon, it was found that k1 = 1.1 × 104 exp (−179 kJ mol−1/RT) mol1/2 s−1 m−1/2 correct to 50%. The implications of this are that initially O2 chemisorbs dissociatively on carbon to yield tightly bound oxygen atoms. These are not desorbed; instead, they appear to be in equilibrium with less tightly bound atoms of oxygen, which desorb to give mainly gaseous O2 or, less probably, CO. In such a fluidized bed, CO subsequently burns in the gas phase (usually in bubbles) far away from the original carbon particle. The graphite particles used here were initially not spherical; their shape was found to remain fairly constant during burning, as expected for combustion being kinetically controlled. In combustors other than fluidized beds, the radical HO2, or even OH well above 1300 K, might be alternative reactants instead of O2 in (I).

Does carbon monoxide burn inside a fluidized bed ? A new model for the combustion of coal char particles in fluidized beds

Beds of silica sand were fluidized by mixtures of C3H8, CH4, or CO with air. Starting from cold the way such a bed behaved before it reached a steady state was observed visually. In addition, high-speed cine films were taken, as well as measurements of the loudness of the noise emitted. These beds behave in a way indicating that such hot gas mixtures at up to 1000°C do not burn in the interstices between the sand particles. Instead, combustion occurs either above the bed or in the ascending bubbles. Measurements of the aiameter (dig) of a bubble made immediately prior to ignition confirmed that the ignition temperature (Tig) of the bubble varies with dig ∝ exp(EigRTig), so that larger bubbles ignite at lower temperatures. It proved possible to generate combustion of these gas mixtures in the particulate phase by adding Pt-coated catalyst pellets. This leads to a new model for the burning of char particles in a fluidized bed. In the model, char is first oxidized to CO with the reaction Cs + 12O2 → CO occurring mainly inside the pores of each particle. The resulting CO burns either above the bed or in bubbles rising up the bed, but not in the particulate phase. Considerable uncertainties exist as to the correct values of Nusselt and Sherwood numbers, as well as of, e.g., the intrinsic rate constant for the initial production of CO. However, the model is capable of predicting the temperatures observed for char particles burning in fluidized beds. Some of the problems of O2 diffusing inside the pores of a char particle and then reacting to give CO are addressed.

Mass spectrometric sampling of ions from atmospheric pressure flames—III: Boundary layer and other cooling of the sample

Abstract It is shown that when a flame is sampled supersonically, cooling of the sample can occur in two regions: the boundary layer on the high pressure side of the sampling orifice and also in the subsequent supersonic and near-adiabatic expansion into the vacuum chamber. It is possible to divide such a sample into two parts: that which is affected by the boundary layer and a central core which is not. Boundary layer thickness is found to decrease as orifice diameter is increased, with the result that for a tiny sampling hole nearly all the sample passes through the boundary layer. Conversely, the sample is nearly all core and unaffected by the boundary layer when a relatively large orifice is used. Estimates of the residence times of flame gas in these regions are made, as well as the likelihood of the sample composition undergoing change. For this, computations are made of the fall in temperature of the sample, as it passes through the boundary layer. The cooling there is found to depend, amongst other things, on the orifice diameter and is greatest for small holes. The fractional temperature drop in the boundary layer is found to be appreciable under some conditions, being occasionally as large as 0.4.

The amounts of NOx and N2O formed in a fluidized bed combustor during the burning of coal volatiles and also of char

Abstract Three quite different coals were burned batchwise in electrically heated beds of sand fluidized with O2 and N2 at either 800 or 900°C. The coal was injected into the bed either as a single piece (1 or 2 cm in size) or as 1.0 g batches of smaller particles (either 1.4–1.7 or 2.0–2.4 mm in diameter). The concentrations of NOx (i.e., [NO] + [NO2]), N2O, CO, and CO2 in the off-gases were measured as functions of time whilst the coal was burning. It proved relatively easy to separate the emission of any one of these gases into that from the initial, devolatilization stage of combustion and that from the subsequent burning of the char. The first stage of coal combustion, when the volatile matter burns, turns out to be very important. For a low rank coal, 70%–90% of all the N2O produced appears while the coal is undergoing devolatilization. The fraction drops to 40% for a coal with a low volatile content. The figures for NOx emissions range from 55% of the NOx arising from the volatiles in a low rank coal to 25% for a coal of high rank. In general, the effects of bed temperature and coal size were much less important than a coals volatile content. Interestingly, changing the bed temperature altered the ratio [N 2 O] [NO x ] in the off-gases and not the total quantity of oxides of nitrogen emitted. Lower concentrations of O2 resulted in slightly less N2O and NOx being produced. The rates of production of NOx and N2O during combustion of the volatiles were found to be proportional to one another. This seems to derive from a lack of mixing of the volatiles. The fact that large (> 1 cm) coal particles float on top of a fluidized bed during devolatilization is important and, e.g., can result in large particles producing less NOx and N2O during devolatilization than tiny particles. The observations made during devolatilization conform to effectively all the fuel-nitrogen in the volatile matter being converted to HCN. Nitric oxide is then produced most probably heterogeneously on the sand from CN radicals, which alternatively can yield NCO radicals. N2O is generated during devolatilization by the reaction NCO + NO → N2O + CO occurring, probably in the gas phase. As for the burning of char in a fluidized bed, the particle size (> 1 mm) here is large enough for there to be control by mass transfer of oxygen to a particle. Oxidation is thus confined to the exterior of the char. It appears that CO, from burning char, is important, together with reaction between NO and solid carbon, in converting NO to N2. This is why the yield of NO from char-N is less than that from volatile-N.

The combustion of carbon monoxide in a two-zone fluidized bed☆

Abstract The oxidation of CO to CO2 has been studied in a two-zone fluidized bed of silica sand heated externally to 1000°C. The lower, fuel-rich, part of the bed is fluidized by a primary mixture of N2 + CO, whereas the upper zone is fluidized by O2 + N2, premixed before being introduced through a sparger located roughly in the middle of the bed. First, the mixing characteristics of such a bed were studied and the cross-flow factor was measured for secondary bubbles in the upper fuel-lean zone. Then with mixing the burning of CO + O2 occurring in the upper zone, concentration profiles were measured throughout the bed and freeboard. These studies indicate that CO and O2 do not react in the particulate (emulsion) phase, but do react in the bubble phase as well as in the freeboard. The addition of CO2 confirms that the strong inhibition of reaction in the particulate phase is a chemical effect and not a heat transfer effect, as has been suggested. In fact, combustion of CO is quenched in the particulate phase primarily by radicals recombinining and also by electronically excited species being deactivated on the huge available surface areas of solids, although homogeneous chain termination is also possible, as with the addition of CO2. The implications of there being no combustion of gases within the particulate phase of a fluidized bed are discussed, especially with respect to coal combustion in these systems. The rate constant for CO and O2 reacting in the bubble phase and freeboard was determined.