The effect of CaO on emissions of nitric oxide from a fluidised bed combustor

Fuel 158 (2015) 898–907

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The effect of CaO on emissions of nitric oxide from a fluidised bed combustor D. Allen 1, A.N. Hayhurst ⇑ Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK

h i g h l i g h t s  The destruction of NO in NO + CO ? ½ N2 + CO2 is catalysed by surfaces of CaO.  CaO particles in the presence of NO catalyse the Boudouard reaction: 2 CO ? Cs + CO2.  With NO present, CaO can react with carbon at 1100 K and produce CaC2.  CaC2 in a fuel-rich fluidised bed quickly removes NO.  CaC2 in a fuel-lean bed fixes N2 in: CaC2 + N2 ? CaCN2 + C.

a r t i c l e

i n f o

Article history: Received 24 February 2015 Received in revised form 12 May 2015 Accepted 9 June 2015 Available online 15 June 2015 Keywords: Fluidised bed combustion Nitric oxide Limestone Chemical looping Oxy-fuel combustion

a b s t r a c t The effects of adding particles of CaO to a hot bed of quartz sand, fluidised by N2, often containing small amounts of NO and CO, were investigated, especially to study the reduction of NO to N2. These observations indicate that surfaces of CaO are efficient catalysts in the presence of NO of the Boudouard reaction: 2 CO ? Cs + CO2. In addition, CaO catalyses the reaction: NO + CO ? ½ N2 + CO2. The kinetic parameters describing the nett rate constant for the overall conversion of NO to N2 were measured for the first time with CaO as catalyst; the activation energy is 27 ± 8 kJ/mol. This reaction was found to be more important for removing NO than: NO + Cs ? CO + ½ N2 with graphite as the carbon, Cs. It also appears that CaO in a fuel-rich, fluidised bed containing graphite and NO at 1000 K produces CaC2, which is demonstrated here to react readily and very exothermally with NO in: CaC2 + 5 NO ? 5/2 N2 + CaO + 2 CO2, thereby quickly removing NO and producing CO2. The production of CaC2 from CaO and carbon continues for a long time; the mechanism is discussed and probably involves catalysis by NO. In a bed with excess air, NO can be produced by CaC2 fixing atmospheric N2 to give CaCN2, which is oxidised later, i.e.:

CaC2 þ N2 ! CaCN2 þ C; CaCN2 þ 5=2 O2 ! CaO þ 2 NO þ CO2 : Thus the conspicuous interaction between CaO and NO in a fluidised bed is complicated by the continued formation of CaC2, whose subsequent reaction depends on whether conditions are fuel-lean (oxidising) or rich (reducing). Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The advantage of having limestone, or some other calcareous solid, present in a fluidised bed burning coal, biomass or waste is that a solid such as CaCO3 decomposes thermally in: ⇑ Corresponding author. Tel.: +44 1223 334790; fax: +44 1223 334796. 1

E-mail address: [email protected] (A.N. Hayhurst). Present address: BAE Systems, Farnborough Aerospace Centre, GU14 6YU, UK.

http://dx.doi.org/10.1016/j.fuel.2015.06.030 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

CaCO3 ! CaO þ CO2

ðIÞ

at typical temperatures of 1100–1200 K [1–5]. This produces the very porous solid CaO, which subsequently can be used to absorb CO2 from combustion gases [2,6,7] in the reverse of reaction (I). Perhaps the commonest use of calcium oxide to clean up gases is based on the fact that it readily adsorbs and reacts with both sulphur dioxide and trioxide [8–16] in the direct reactions:

CaO þ SO2 ! CaSO3

ðIIÞ

D. Allen, A.N. Hayhurst / Fuel 158 (2015) 898–907

CaO þ SO3 ! CaSO4 :

ðIIIÞ

Quite interestingly, SO2 and CaO also react in a parallel reaction, which produces a mixture of CaS and CaSO4 [8] in:

4 CaO þ 4 SO2 ! CaS þ 3 CaSO4 :

DH0298  373 kJ=mol

etc. Consequently, neither NH3 nor HCN were present, although these species can be very important in a combustor burning coal or biomass [34]. Likewise, the only metal present in these experiments was Ca; this simplified matters immensely.

ðIVÞ

In fact, above 1123 K, CaS and CaSO4 are the only products [8], when SO2 reacts with CaO, i.e. reaction (IV) then dominates (II). To complete the picture it should be noted that CaSO3 starts to become thermally unstable above 1000 K [11], i.e. the reverse of (II) can then occur, depending on the partial pressure of SO2 and the temperature. However, the final products of the thermal decomposition of CaSO3 are usually CaS and CaSO4 in the molar ratio of 1:3; the exact mechanism of this decomposition above 1000 K is uncertain and has been discussed elsewhere [8]. In addition, solid CaSO3 is readily oxidised in O2 to CaSO4 [14–16], just as CaS [16,17] is also oxidised in O2 to CaSO4. The chemistry involved in the removal of SO2 and SO3 by reacting with CaO can become further complicated by, for example, the reverse of reaction (IV) occurring at higher temperatures (>1100 K), possibly in a liquid melt [18]. This complex situation becomes even more involved by its effect on emissions of NOx, whose production and disappearance can both be affected [19,20] by the presence of calcium and sulphur. Likewise, the presence of magnesium can be important [21]. This paper looks at some of these other complications in a fluidised bed containing CaO particles and investigates reactions of nitric oxide with CaO in the presence of carbon monoxide or carbon. These additional reactions have a particular importance deriving from the use of CaO in reactors for chemical looping combustion [3–5,22]. In this general context, it was observed [23–25] some time ago that adding CaO to a fluidised bed combustor can increase emissions of NOx, (i.e. NO + NO2) from the bed. However, there are contrary findings, whereby CaO causes either a decrease [25–27] in the NOx emitted or no change at all [28]. The topic was last reviewed over 20 years ago [25]. Interestingly, previous work indicates that there is a difference [25] between a bubbling fluidised bed, where the CaO particles are in the particulate phase, and a circulating bed, in which these particles are more widely separated from one another throughout the entire riser of the bed. On this point, maybe experiments using an incipiently fluidised bed would provide important information. The literature also indicates that, whenever CaO particles caused an increase in NOx emissions, it was usually in a fluidised bed burning coal under oxygen – rich conditions. A decrease in NOx emissions appears to be associated with either fuel – rich conditions [29] or experiments, in which no combustion was occurring. This latter scenario also applies to the case where the addition of CaO was observed to have no effect on the amount of NOx emitted from the bed. Of course, these observations should be considered together with the fact that the exothermic reaction:

NO þ CO ! 1=2 N2 þ CO2 ;

899

ðVÞ

is catalysed on the surfaces of various particles, such as iron or iron oxide [30] or even carbon [31] or limestone [25]. The oxidising properties of NO, as in reaction (V), are also clear from its reaction with metallic Fe [32,33], producing N2 and an oxide of iron. First, this study looks in more detail at NO and CO reacting heterogeneously via (V) on a surface of CaO in a fluidised bed, because e.g. its activation energy is not known. Here care was taken to avoid the complex chemical reactions producing NO during the combustion of a hydrocarbon or a char particle. In particular, the experiments described below circumvented other chemical complexities, especially those [34] involved in producing NO from the volatile matter or char from e.g. a coal. This was achieved by adding NO to the gas (usually pure N2) fluidising a bed of hot particles and studying the fate of NO in the presence of CaO or graphite,

2. Experimental The apparatus and equipment have been described in detail before [30,35], so only a brief account is given here. The main studies were conducted in a fluidised bed of quartz sand, previously washed and sieved to 425–600 lm and held in a quartz tube (i.d. 135 mm; height 250 mm). The depth of the unfluidised sand was 100 mm, so that when fluidised, the depth and diameter of the bed were comparable, thereby minimising any effects of the hotter quartz walls. The quartz tube stood on a stainless steel distributor plate, through which holes (diam. 0.5 mm) had been drilled on a 12 mm triangular pitch. The bed was heated electrically by four vertical graphite rods, located around the outside of the quartz tube. Altogether these heating rods generated 6 kW. The entire assembly was housed in a thermally insulated box, which did have an opening for visual access to the side of the bed, in addition to a clear view of the top of the fluidised sand. A thermocouple with its tip situated in the fluidised hot sand, 20 mm above the centre of the distributor, was used to control the bed’s temperature to a pre-set value. Other solids, e.g. CaO, were added by quickly throwing small batches of particles on to the top of the hot fluidised sand. Temperatures of 650, 750 and 850 °C in the bed were used. The sand was fluidised with N2, sometimes with small quantities of NO or CO added. Every gas came from a cylinder and was dry. The minimum superficial velocity of the fluidising N2, Umf, i.e. for incipient fluidisation, was measured to be in the range 0.106– 0.120 m/s, depending on the bed’s temperature. The actual superficial velocity, U, was adjusted to give U/Umf = 5.0 in every experiment. This resulted in a strongly bubbling bed without any slugging. The gases leaving the centre of the bed were continuously sampled through a quartz tube (i.d. 4 mm), whose open end was 100 mm above the top of the fluidised sand and on its axis. The sample was dried over fresh CaCl2 and passed to a sequence of two infra-red gas analysers, which continually measured the concentrations of CO and CO2. In series with these instruments was a chemiluminescent analyser (Thermo Electron Model 10) to determine the concentration of NOx (i.e. [NO] + [NO2]) in the gases leaving the bed. In fact, it was found that [NO2]  [NO] in all the experiments described below. All three of these instruments were calibrated using known mixtures of CO, CO2 and NO in excess N2. The residence time of gas in the bed was at the very least 0.2 s, i.e. relatively brief, but some back-mixing of the fluidising gas might have occurred and prolonged this residence time somewhat. A few experiments were performed in a thermogravimetric balance, described previously [8,9,35,36]. For these and other studies, oven – dried particles of CaO (Analar from BDH Chemicals and stored under dry conditions) were sieved to 75–94 lm. There was no evidence that these fairly small particles were elutriated after being added on to the top of a hot fluidised bed. These particles of CaO were found to have a B.E.T. surface area of 14.3 ± 0.6 m2/g. Infra-red spectroscopy of various solids was done using a Perkin Elmer 882 spectrometer with the sample pressed into a KBr disc. Spectra in the range 400–4000 cm1 were obtained and investigated. 3. Results 3.1. Reactions of nitric oxide with calcium oxide and carbon monoxide In a preliminary experiment, a batch of particles of CaO was added quickly to a bed of sand, when fluidised by a mixture of

900

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NO and excess N2 at temperatures of 923, 1023 or 1123 K. It was found that with the short residence times of the gas in the bed, the CaO had very little, or no, effect on NOx emissions. This observation was checked by passing NO (0.07 vol.% in N2) over CaO in a thermogravimetric balance at temperatures of 873–1173 K [35]. It was found that at 1023 K after 2 h the mass of the sample of CaO had increased by 1%. It was also observed that the infra-red absorption spectrum of the CaO had clearly acquired a new peak at a frequency of 1225 cm1. So far this peak has not been identified. The solid product did display paramagnetism with a susceptibility of 3 ± 1  106 mg s. This compares well with the value quoted by Rinck [37] of +2.38  106 mg s for the dioxide, CaO2. There is however a problem here in that it seems that the fact that paramagnetism appears indicates that O 2 ions are formed, instead of O2 2 [38]. In either case, these ions are not particularly stable, but the matter needs tidying up. Possible schemes for forming CaO2 include:

DH0298  þ66 kJ=mol

ðVIÞ

CaO þ 2 NO $ CaO2 þ N2 O; DH0298  þ74 kJ=mol:

ðVIIÞ

CaO þ NO $ CaO2 þ 1=2 N2 ;

Although these fairly slow reactions have no effect on the concentration of NO leaving a hot fluidised bed, they could affect the composition of any CaO present for some time in the bed. The increase noted above of 1% in the mass (at 1023 K, after 2 h, with [NO] = 0.07 vol.%) corresponds to only 3.5% of the CaO being converted to CaO2, assuming the dioxide is responsible for the increase in mass. As for a direct reaction between CO and CaO, little supporting evidence was found from analysing the off-gases from a bed of hot sand fluidised by CO and excess N2, when a batch of CaO particles was suddenly added. Likewise, passing a mixture of CO in excess N2 over CaO in a thermogravimetric balance gave no indication of a chemical reaction. This means that reactions like:

CaO þ CO $ Ca þ CO2 ;

DH0298 ¼ þ352 kJ=mol

CaO þ 5 CO $ CaC2 þ 3 CO2 ;

DH0298 ¼ 46 kJ=mol:

Fig. 1. The concentrations of NOx (i.e. [NO] + [NO2]), CO and CO2 in the off-gases from a bed of sand (425–600 lm) at 1123 K, when fluidised by N2 containing 60 vppm of NO and 1250 vppm CO. As denoted by the arrows, batches of 1 g, then 3 g, followed by 5 g of CaO (75–94 lm) were added to the bed with U/Umf = 5.0.

ðVIIIÞ ðIXÞ

appear unlikely under these conditions. Whether the exothermic reaction (IX) occurs or not will depend on the temperature and the value of ðp5CO =p3CO2 Þ, where pi is the partial pressure of species i. The chemistry of reaction (IX) is sufficiently complex to make its rate relatively slow, so it should not be ruled out entirely. Next, the bed of sand was fluidised by a mixture of gases containing a little NO, together with a much larger amount of CO, both of them in excess N2 at the usual temperatures of 923, 1023 and 1123 K with U/Umf = 5.0. When the bed was in a steady state, a sample of 1 g of CaO was added rapidly to the hot sand, followed by a pause for the CaO to mix with the sand and for the bed to attain a new steady state. Then another 3 g and finally a further 5 g of CaO were added batchwise, whilst the concentrations of NOx, CO and CO2 in the off-gases were continuously recorded, with [NO2]  [NO]. The CaO used was described above; it was the same as that studied in previous thermogravimetric experiments [8,9]. Fig. 1 illustrates the effect of these batchwise additions of CaO on the concentrations [NO], [CO] and [CO2] in the off-gases, with the bed held at a temperature of 1123 K. It is clear that adding CaO, in the presence of CO, reduced the emissions of NO. In fact, more NO was removed, after more CaO had been added to the bed. Concurrently to the reduction in [NO] seen in Fig 1a, there were a decrease in [CO] (shown in Fig. 1b) and also an increase in [CO2], as seen in Fig. 1c. One possibility is that there is a chemical reaction between NO and CO, in the presence of CaO, with CO2 as a product. If CaO were consumed by the reaction, the gas concentrations in Fig. 1 would be expected to return to their original levels,

after the CaO had been used up. However, it appears from Fig. 1 that this is not the case, because [NO] and [CO] attain new, lower, steady-state levels for [NO] and [CO]. However, [CO2] increases to progressively higher constant values after an initial spike, which most probably was caused adventitiously by a little CaCO3 (created rapidly during storage and handling) in each batch of CaO. Such a series of steady state values of [NO], [CO] and [CO2] suggests that CaO is playing a catalytic role. It should be noted that this is not conclusive evidence, because the reduction in the number of moles of e.g. NO in Fig. 1a is far less than the number of moles of CaO added to the bed. However, it will be assumed pro tem that one reaction taking place is the reduction of NO by CO, catalysed by CaO, to produce CO2 in reaction (V). As an aside, it will be seen in Fig. 1 that it takes approximately 50 s for the CaO to mix fully with the sand and the composition of the off-gases to become steady. In addition, the difference between the values of [CO] in the input and outlet streams is roughly twice the subsequent, steady [CO2] measured in the off-gases. Thus the rate of disappearance of CO is roughly twice the rate of production of CO2. Also, the reductions in [NO] are significantly smaller than these changes in [CO] and [CO2]. Thus the largest drop in [NO] in Fig. 1 is 10 ppm, whereas that in [CO] is 250 ppm and so is much larger. This state of affairs might possibly derive from a lack of mixedness in either the bed’s particulate phase or the off-gases. This seems very unlikely given that the measurements in Fig. 1 do settle down 50 s after adding a batch of CaO. Otherwise, it definitely looks as if CO is being converted to CO2 in a way required, or even catalysed by the presence NO. This extra reaction is most probably the well-known Boudouard reaction:

D. Allen, A.N. Hayhurst / Fuel 158 (2015) 898–907

2 CO ! Cs þ CO2 ;

DH0298 ¼ 173 kJ=mol;

901

ðXÞ

which somehow is being sustained or catalysed by the presence of NO. Thus reaction (X) has been assumed to produce solid carbon, Cs, since half as many moles of CO2 appears as CO consumed. It should be stressed that it was noted above that there was no clear evidence for (X) in the absence of NO. The rôle of NO is most probably that of participating in a sequence of heterogeneous reactions between adsorbed species like:

COads þ NOads ! CNOads þ Oads Oads þ COads ! CO2ads CNOads ! NOads þ Cs on the surface of a particle of CaO. These three heterogeneous reactions constitute a possible catalytic mechanism, bearing in mind that they add algebraically to give the Boudouard reaction (X). Radicals such as CNO are well known [39] in these environments, but, of course, alternative mechanistic schemes can be devised. As for the loss of NO in Fig. 1, Kasaoka et al. [40] have previously concluded that the reaction responsible, i.e. (V), is first order with respect to NO and zeroth order in CO, which in any case is present in excess in Fig. 1. Also, it can be seen from Fig. 1a that the rate of reduction of [NO] increased, when more CaO was added to the bed. Thus, Kunii and Levenspiel’s [41] bubbling bed model can be used to derive the expression:

1

     ½NOin 1 UA ¼ þ ln X ðkf 5 SCaO Þ ½NOout

ð1Þ

relating the steady-state inlet and outlet concentrations of NO to the amount of CaO present in the bed. Here, [NO]in, [NO]out, U, A and kf5 are, respectively, [NO] in the feed to the bed and in the off-gases, the superficial velocity of the fluidising gas, the cross-sectional area of the bed and the rate constant for the catalysed forward step of reaction (V), assumed to be of zeroth order in CO. The cross-flow factor, X, for the bed equals (Q/V) (H/Ub), where Q is the volumetric flow-rate of gas through a bubble, i.e. from the particulate to the bubble phase and back again, V is the volume of a bubble, H is the overall depth of the fluidised sand and Ub is the rise-velocity of a bubble. The bubbling bed model assumes the particulate phase is well-mixed, but the bubble phase is not and is actually taken to move in plug flow. This requires the flow of gas through the bubbles to be much greater than that through the particulate phase, i.e. U/Umf > 3 [41]. Here U/Umf was equal to 5.0, so this latter condition is satisfied. Finally, SCaO is the total surface area of CaO in the bed, i.e. the product of the mass of CaO present and the B.E.T. surface area of CaO, measured above as the total surface area per unit mass. Thus the number of moles of NO reacting in (V) per unit surface area of CaO is written as kf5 [NO]; this means kf5 has fundamental units of m/s. Fig. 2 shows plots of 1/ln {[NO]in/[NO]out} versus (1/SCaO). It can be seen that the line of best fit for each temperature is linear, and has a positive intercept on the vertical axis. Thus Eq. (1) for the bubbling bed model appears to be upheld. The cross-flow factors, X, were deduced from the intercepts to be: 0.8 at 923 K, 0.5 at 1023 K and 0.6 at 1123 K. Such magnitudes are acceptable, considering that their associated errors are large and at least 70%. Values of kf5 for catalysis by CaO were derived from the gradients of Fig. 2 and are presented (error  20%) as an Arrhenius plot in Fig. 3. In fact, the associated activation energy and pre-exponential factor were measured, apparently for the first time, to be, respectively, 27 ± 8 kJ/mol and 2.6 ± 0.7  103 m/s. Both these quantities are lower than the equivalent values (85 ± 20 kJ/mol and 0.50 ± 0.13 m/s) for reaction (V), when catalysed by Fe2O3 [30]. The result is that the rate constants for the forward step of reaction

Fig. 2. Plots of: 1/ln {[NO]in/[NO]out)} for the reduction of [NO] in Fig. 1, against 1/ SCaO to check Eq. (1) at the three temperatures shown. Here SCaO is the product of the mass of CaO added to the bed and the B.E.T. area of that sample of CaO.

(V) over unit surface area of CaO are almost an order of magnitude larger than the values for the same reaction over Fe2O3 [30], mainly because of the different activation energies. Reaction (V) has also been shown to be catalysed by graphite [31], when the activation energy is 25 ± 10 kJ/mol. It can therefore be concluded that graphite, Fe2O3 and CaO all provide catalytic surfaces for the reduction of NO by CO via the heterogeneous reaction (V). Reactions involving CO are most applicable to a fuel-rich system, where there will be more of this gas present, although in a fuel-lean bed there can be both oxidising and reducing regions [42]. Therefore, this catalysed reaction (V) between NO and CO goes someway to explaining the observed reduction in [NO] on introducing CaO to a bed operating under fuel-rich conditions [29]. It does not, however, explain the reported increase in NO emissions, when CaO is introduced to a fuel-lean bed (e.g. [43]). Reaction under oxidising conditions will be considered in a subsequent section, along with a study of the combined effects of CaO and carbon on emissions of NO. Graphite was selected as the carbon, because it is pure and free of both inorganic impurities and volatile materials.

3.2. Reaction of nitric oxide with both calcium oxide and graphite The next experiment involved suddenly adding 10 g of graphite particles (from B.D.H. Chemicals, sieved to 2.37–3.35 mm; B.E.T. surface area 9.4 ± 1.4 m2/g) to a steady, hot bed of fluidised sand already containing 12 g of CaO. The fluidising gas was a mixture of N2 and 135 ppm of NO, at a temperature of 1023 K and with U/Umf = 5.0. Visual observation of the top and side of the hot bed indicated that the graphite particles mixed fully with the fluidised sand. The effect of this batch of graphite on NO emissions is shown in Fig. 4, where the arrow denotes when the graphite was thrown into the hot bed of sand containing a little CaO. It can be seen that adding graphite resulted in a very sudden, huge, but sustained reduction of [NO] in the off-gases. Furthermore, this huge reduction of [NO] is substantially greater than that caused by graphite in the absence of CaO. This is seen in Fig. 5, which gives plots of [NO] in the gases leaving a bed of sand without any CaO, but at three different temperatures, when fluidised by a similar mixture

902

D. Allen, A.N. Hayhurst / Fuel 158 (2015) 898–907

Fig. 3. Arrhenius plot of kf5, the rate constant for the forward step of reaction (V), assumed to be zeroth order in CO.

Fig. 4. The measured [NO] in the off-gases from a bed of quartz sand containing 12 g of CaO, when fluidised by N2 containing 135 vppm of NO and suddenly 10 g of graphite particles (2.37–3.35 mm) were thrown on to the top of the bed at 1123 K with U/Umf = 5.0. The arrow denotes when the particles of graphite were added.

of only NO and excess N2, i.e. again without any CO in the fluidising gas. When the bed was steady, 1 g of graphite, was added to the sand, followed sometime later by further 2, 5 and 10 g batches of graphite, adding up to 18 g added to the bed. It is clear that adding graphite in this way temporarily reduces [NO] in the off-gases, but at the most the drop is by 30%, which is much less than in Fig. 4. This short-lived reduction of NO by graphite in Fig. 5 has been shown [31] not to occur by:

NO þ Cs ! CO þ 1=2 N2 ;

DH0298  201 kJ=mol;

ðXIÞ

which seems to be an accepted [44–46] reaction, as reviewed by Johnsson [25]. In fact, there is evidence in Fig. 5 that reaction (XI) does not occur with graphite as the carbon. This is clear from [NO] being always lowered for only 50 s after the addition of each batch of graphite. Subsequently, [NO] always returns to its previous value, indicating that the reaction removing NO is then finished. Actually, these reductions in [NO] in Fig. 5 are caused [31] by O2, originally chemisorbed on the graphite, desorbing as CO, when the graphite entered the hot bed. The resulting pulse of CO then reacts in (V) with NO in the fluidising gas. In Fig. 5, graphite provides the catalytic surface, but in Fig. 1a the heterogeneous catalyst was CaO. Returning now to Fig. 4, we note that there the addition of graphite caused a much more dramatic reduction in [NO] of down to only 15% of its original level and also strikingly prolonged in time. Also, it should be remembered that CaO alone was seen above to have little effect on [NO] in the off-gases, except when CO is present and reaction (V) catalyses the removal of NO and CO. The huge and very long-lasting effect in Fig. 4 thus suggests that at fluidised bed temperatures, there might be a chemical reaction between CaO and graphite. This reaction is actually known [47] to produce calcium carbide, CaC2, in:

CaO þ 3 C $ CaC2 þ CO; DH0298  þ472 kJ=mol:

ðXIIÞ

Any CaC2 formed endothermically in (XII) can then react readily with NO and remove it in the extremely exothermic reactions:

Fig. 5. Emissions of NO on adding consecutive batches of 1, 2, 5 and finally 10 g of graphite particles (2.37–3.35 mm) to the bed of quartz sand, when fluidised by nitrogen containing less than 100 vppm of NO, as shown. The experiment was repeated at 923, 1023 and 1123 K.

CaC2 þ 3 NO ! CaO þ 2 CO þ 3=2 N2 ;

DH0298 ¼ 1074 kJ=mol ðXIIIÞ

CaC2 þ 5 NO ! CaO þ 2 CO2 þ 5=2 N2 ;

DH0298 ¼ 1821 kJ=mol ðXIVÞ

presumably until all the CaC2 is used up. The CaO generated in these two very exothermic reactions is likely to be hot enough to facilitate the endothermic step (XII), thereby making more CaC2 and so prolonging the disappearance of NO. Such a sequence just needs initiating by somehow starting off the first step (XII); afterwards the disappearance of NO continues, provided CaO and carbon are present and form CaC2. However, reaction (XII) is between two solids and is accordingly slightly suspicious, but not impossible; it is usually thought [38,48] to require a temperature of 1800 °C, which

D. Allen, A.N. Hayhurst / Fuel 158 (2015) 898–907

might be provided by the hot CaO produced in (XIII) or (XIV). Also, it was noted above that CaO is heated when catalysing the exothermic reaction (V) between NO in the fluidising gas and CO released by degassing newly added graphite particles. Thus a high temperature might be attained by the CaO particles. An additional possibility for producing CaC2 under fuel-rich conditions is the exothermic reaction (IX), between CaO and the CO liberated when a graphite particle was heated on entering the bed. Such a reaction between CaO and CO would depend on the temperature and whether the local partial pressure of CO was sufficiently high. Of course, reaction (IX) would probably involve (VIII) as a first, endothermic step, which produces Ca metal, whose melting point is 840 °C. The presence of a liquid phase could, of course, facilitate an apparent reaction between two solids. In that case, CaC2 might be formed by graphite reacting with liquid calcium in:

Caliq þ 2 C ! CaC2 ;

DH0298 ¼ 53 kJ=mol

ðXVÞ

Thus the production of CaC2 under the conditions of Fig. 4 is quite possibly not by just one simple reaction (XII); in addition, at least (VIII) and (XV) are maybe contributing. The temperature of the CaO particles is important and should be measured in any future work. Here there is a chance that some of the CaO, which, together with NO, catalyses the exothermic Boudouard reaction (X), becomes ‘‘hot’’ enough to react in (VIII), at least initially. Of course, any carbon produced by the Boudouard reaction can also react exothermally with molten calcium in (XV). Visual observations of the top and side of the bed did not reveal the existence of any conspicuously bright, hot particles. The nett effect of the sequence of (VIII), (X) and then (XV) is seen by algebraically adding them; the result is the overall reaction (IX), which is thus catalysed by NO on a surface of CaO. This would mean that reaction (IX) only proceeds in the presence of NO. In all this, the added graphite is definitely participating and its rôle apparently involves (XII) and (XV), with possibly the more reactive carbon produced in the Boudouard reaction (X) reacting exothermally with NO in step (XI). Finally, it should not be forgotten that the striking reduction of NO observed in Fig. 4 is partly caused by NO reacting catalytically with CO (generated in (XII), (XIII)) in the very exothermic reaction (V); in this case the catalytic surfaces would be provided by both CaO and graphite. Even so, it is now clear that CaC2 might well play an important role removing NO. Certainly, the extent of reduction of [NO] in Fig. 4 is so conspicuous and long-lasting that CaC2 and its reaction with NO must now be considered in more detail. 3.3. Reactions of NO with CaC2 under reducing conditions A sample of CaC2 was obtained from B.D.H. Chemicals. The particles were in the size range 2.1–3.5 mm and their B.E.T. surface area was measured to be 0.4 ± 0.05 m2/g. Although the sample of CaC2 used was the purest available, it still had impurity levels of up to 20 wt%. An EDAX analysis at the Coal Research Establishment (C.R.E., Stoke Orchard), showed the major impurity to be the element oxygen; thus some hydrolysis of the CaC2 to Ca(OH)2 and CaO was suspected. An ash analysis was also provided by C.R.E., Stoke Orchard. This gave the percentages by mass of each impurity in the CaC2 as: SiO2 2.1 wt%, Al2O3 1.3%, SO3 0.9%, Fe2O3 0.2% and Na2O 0.1 wt%. In addition, the following compounds were present in amounts less than 0.1 wt%: MgO, K2O, TiO2, Mn3O4 and P2O5. Thus 5 wt% of the CaC2 sample is comprised of mineral impurities and the presence of these species also contributes to the high level of contamination by oxygen, as already mentioned. A bed of sand was next fluidised with a mixture of N2 containing a trace of NO, again at U/Umf = 5.0 and at temperatures of 923, 1023

903

and 1123 K. Successive batches of 1, 3 and 5 g of CaC2 were quickly added, with intervals between these additions, on to the top of the hot fluidised bed and the concentrations of NO, CO and CO2 in the exit gas recorded. These measurements are shown in Fig. 6 for the temperature of 1123 K. Evidently the addition of CaC2 temporarily lowers [NO], whilst simultaneously producing pulses of CO and CO2. The extent by which NO is reduced is considerable (up to 90% after the 5 g batch of CaC2 had been added) and increases with the amount of CaC2 added to the bed. The numbers of moles of NO, CO and CO2, either reduced or produced during such an experiment were calculated from Fig. 6. This was achieved by performing appropriate integrations on the plots of Fig. 6. For example, the area under one of the peaks of [CO2], when multiplied by the molar flow rate of gas fluidising the bed, gives the number of moles of CO2 produced in the particular peak. The errors in the values derived are estimated to be possibly as high as 10%. Fig. 7 shows the reduction in the moles of NO, when plotted against the amount of CaC2 added in the particular batch. These are all linear plots with zero intercept, indicating that the quantity of NO removed is proportional to the amount of CaC2 added to the bed and also that more NO is reduced at higher temperatures. In fact, it was also found that the quantities of CO and CO2 produced exceeded the amount of NO consumed by almost a factor of 10. This last observation suggests that CO and CO2 are being produced by impurities in the CaC2; this is certainly possible, given that the major impurity in the CaC2 was oxygen. So, before any further analysis of the reaction of NO with CaC2, the amounts of CO and CO2 produced by the high levels of oxygen present as an impurity in the CaC2 were checked. To do this, the above experiment was repeated, but the bed was fluidised only by pure N2. The concentrations of the CO and CO2 emitted during such an experiment at 1123 K, are shown in Fig. 8, which shows that a large amount of CO2 was produced, in fact almost ten times the amount of CO. This result suggests that the oxygen was present in the form of CaCO3, or some other similar compound, which would decompose on heating to give CO2. Given that there was some CO produced, when CaC2 was added to the hot fluidised bed, it is possible that this CO is, partly responsible for lowering [NO]. However, Mellor [49] indicates that there is a direct reaction, like (XIII) or (XIV), between CaC2 and NO, so to investigate this possibility, an experiment was performed in the thermogravimetric balance described previously [8,9,35,36]. A sample of CaC2 was first heated under N2 alone to a constant mass at a temperature of 1123 K in the thermogravimetric balance. During this stage there was a large decrease (30%) in the mass of the sample, and both CO and CO2 were emitted as in Fig. 8. When a steady state had been reached and CO and CO2 were no longer evident in the off-gases, a flow of NO (450 ppm in N2) was introduced over the solid sample and the subsequent decrease in its mass was recorded. The results of this part of the experiment are shown in Fig. 9, where the arrow denotes the entry of NO. Also, Fig. 9 shows both the fall in the sample’s mass in Fig. 9a and the changes in the concentrations of NO, CO and CO2 in the exhaust, for the duration of the reaction of NO with CaC2. The delayed and gradual breakthrough of the [NO] signal to a steady value in Fig. 9b establishes that NO was reduced over CaC2, resulting in a decrease in the mass of the CaC2 shown in Fig. 9a, down to a constant level. Also, some CO was produced (see Fig. 9c), together with a much larger amount of CO2 (see Fig. 9d). Furthermore, the decrease in mass in Fig. 9a corresponds to CaO being produced. This is demonstrated by the fact that Fig. 9a shows the initial mass of solid was 40.3 mg and its final mass was 31.4 mg. Consequently the ratio of the initial and final masses was 1.28. This must be compared to the ratio of the relative molecular masses of CaC2 and CaO, which is actually 1.28. This result is possibly fortuitous, given that the original sample of CaC2 was not pure; however, it does imply that reaction occurs

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via both (XIII) and (XIV), producing, respectively, CO and CO2. In Fig. 9, it looks as if reaction (XIV) takes almost 25 min to go to completion, after which CO2 is no longer detected. However, the production of CO in reaction (XIII) appears to last for only 18 min. These times are for particles of CaC2 with initial ‘‘diameters’’ of 2.1–3.5 mm. That more CO2 is produced than CO indicates that the very exothermic reaction (XIV) dominates (XIII). This means that CaO particles might well attain high temperatures and so continue to produce more CaC2 in the presence of NO and a carbon. This is most probably why the reduction of NO to N2 in Fig. 4 continues for such a very long time. In future work, it would be worthwhile measuring the temperature of the CaO particles. 3.4. Reactions of NO with CaC2 under oxidising conditions So far the reactions of NO under reducing conditions in a fluidised bed have been investigated; now oxidising conditions will be considered. This involved adding batches of 1, 3 and then 5 g of CaC2 to a bed fluidised by NO and air, with a batch added only when [NO] was steady. Later this was repeated, but with the bed fluidised by air alone. Some results of these experiments are shown in Figs. 10 ([NO] = 52 ppm in the fluidising air) and 11 (no NO added) for the temperature in the bed being 1123 K. It can be seen from these two plots of [NO] against time that adding CaC2 to the fluidised bed under these two oxidising conditions actually resulted in increased emissions of NO. However, the ‘‘pulses’’ of [NO] in Fig. 10 are larger than those in Fig. 11. In both cases, sometime after adding particles of CaC2, [NO] did return to its original level. A slight increase in CO2 emissions was also detectable, but not shown in Figs. 10 and 11. Given that the CaC2 used did not contain the element nitrogen, a possible explanation of these observations is that at 1000 °C CaC2 does fix nitrogen [48] from air, yielding CaCN2 in: Fig. 6. The concentrations of NO, CO and CO2 in the off-gases, when 1, 3 and 5 g of CaC2 were added in turn to the bed of quartz sand, when fluidised by nitrogen with 74 vppm of NO. The bed was at 1123 K and U/Umf = 5.0.

CaC2 þ N2 ! CaCN2 þ C;

ðXVIÞ

followed by the subsequent oxidation of CaCN2 to NO and CaO in:

CaCN2 þ 5=2 O2 ! CaO þ 2 NO þ CO2 :

ðXVIIÞ

Reactions (XVI) and (XVII) are plausible suggestions to explain the experimental observations in Figs. 10 and 11, particularly the ‘‘pulses’’ of [NO]; more precise characterisation of these reactions was beyond the scope of this work, but the presence of calcium cyanamide, CaCN2, has been invoked before [34]. Certainly, these experiments show that it is actually possible to generate NO in a fluidised bed, containing CaO, which does apparently react with

Fig. 7. The total number of moles of NO removed from the bed of quartz sand, as in Fig. 6, plotted against the number of moles of CaC2 added batchwise to the bed at the three temperatures shown.

Fig. 8. Plots of [CO] (continuous line) and [CO2] (dotted line) when batches of 1, 3 and then 5 g of CaC2 were added in turn to the bed of quartz sand, when fluidised by pure N2 at 1123 K. Again U/Umf = 5.0.

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Fig. 9. Plots against time of: (a) the mass of CaC2 in the thermogravimetric balance at 1123 K during reaction with NO (450 ppm in N2), (b) [NO] in the off-gas leaving the balance, (c) [CO] in the off-gas, (d) [CO2] in the off-gas. The two vertical arrows denote the moment when NO was added to the N2 flowing over the CaC2.

This fact prompted a study of two other substances, which are likely to be present in large quantities during the combustion of a solid fuel, i.e. solid carbon and CO. Reaction (V) between CO and NO was confirmed above as being catalysed by CaO; the reaction has been observed before [19,20,25,39,50,51], but not studied in any detail. This work has shown that CaO is an even more effective catalyst than Fe2O3 under reducing conditions, for the reduction of NO by CO in (V). Similar effects have been reported [22], whereby CO reduces [NO] from a fluidised bed operated as either an oxy-fuel combustor [52] or a chemical looping combustor of coal in the presence of CaO. Reaction (V) for reducing emissions of NO from a fluidised bed combustor merits further investigation, especially its zeroth order in CO. It is important to compare the values of kf5 derived here with those quoted in Johnsson’s review [25], which were expressed in units of m3 s1 kg1. Given that the surface (B.E.T.) area of the CaO used here was 14.3 m2 g1, the rate constant of kf5 = 1.6  104 m/s Fig. 10. Plots of [NO] against time when successive batches of 1, 3 and 5 g of CaC2 were added in turn to the bed of quartz sand at 1123 K with U/Umf = 5.0 for the bed fluidised with air containing 52 vppm NO.

carbon to produce CaC2. This is in agreement with the observations of those (e.g. [25,29,43]), who have reported an increase in NO emissions, after adding CaO to a bed operating under oxidising conditions. Clearly further work is required. 4. Discussion One aim of this paper was to resolve the apparent discrepancies in the reported effects of CaO on NO emissions from a fluidised bed combustor. Simply isolating the reaction between NO and particles of CaO showed that a slow reaction does occur, but not to any significant extent. That reaction might well involve the formation of CaO2 in possibly (VI) or (VII), but there is no strong evidence for the fairly unstable CaO2 exerting a significant rôle. The literature reveals that CaO only appears to have a significant effect reducing NO emissions in the presence of combustion, and especially of CO.

Fig. 11. Plots of [NO] versus time for the bed of quartz sand fluidised by air alone, i.e. no NO was added, in contrast to Fig. 10. The temperature was again 1123 K with U/Umf = 5.0. Consecutive batches of 1, 3 and 5 g of CaC2 were again to the hot bed, as shown.

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(derived from Fig. 3 for 1123 K) corresponds to kf5 = 2.3 m3 s1 kg1. The values quoted by Johnsson [25] for 1100 K are mainly around 0.8 m3 s1 kg1, i.e. somewhat lower. This amounts to acceptable agreement, given that particle sizes do affect their reactivity and that limestones do have a wide range of B.E.T. areas [4,53], which were not quoted by Johnsson [25]. Also, a limestone’s intrinsic reactivity does vary [4,53], often e.g. depending on metallic compounds as impurities. A reduction in emissions of NO was observed in Fig. 5 on adding graphite to a fluidised bed, but analysis showed that the extent of the reduction of NO could actually be correlated to the amount of CO present in the system. The CO was often produced in this work by the oxidation of graphite by oxygen previously adsorbed on the graphite’s surface. One interesting discovery was that a surface of CaO in the presence of NO catalyses the Boudouard reaction of CO in (X). A set of experiments under reducing conditions considered the interaction between solid CaO and graphite in a bed containing a little NO. It appears that CaO then reacts with carbon at bed temperatures to give CaC2 and CO as products. This possibly occurs in the direct reaction (XII), but alternative schemes were found involving molten Ca metal and NO catalysing reaction (IX). That there is a direct reaction between NO and CaC2, most probably in the very exothermic step (XIV), was demonstrated by experiments using a TGA. Thus the reported reduction [23–25] in [NO], on adding CaO to a bed under reducing conditions, can be explained as a combination of reaction of NO with CO, catalysed by the CaO and reduction of NO by the CaC2 formed from the reaction between carbon and CaO. Under oxidising conditions (i.e. with O2 fed to the bed), CaC2 was observed in Figs. 10 and 11 to actually produce NO, possibly via the formation of CaCN2 in (XVI) and the subsequent oxidation of CaCN2 in (XVII). This in turn would account for reports of increases in NO emissions on adding CaO to a bed operating under O2 – rich conditions [25,29,43]. However, although it does provide an explanation, there may well be other factors, which are also important with regard to reducing [NOx] in a fluidised bed. Even so, this investigation has certainly shown that the conditions, under which a bed is operated (i.e. reducing or oxidising) are important, particularly with regard to the resulting [CO] in the bed, and also the presence of possible catalytic surfaces. This dependence of [NOx] on the overall stoichiometry of a fluidised bed combustor has been noted before [46], so that whether the bed is fuel-rich or lean can lead to decreases or increases in the emissions of NOx.

5. Conclusions The above experiments have made considerable progress by revealing the catalytic behaviour of a surface of CaO in the presence of CO and NO. These experiments aimed at understanding the reactions of NO in a fluidised bed, but one finding was that under these conditions the Boudouard reaction of CO in (X) is catalysed. It was also shown that carbon monoxide reduces NO over the catalyst CaO under reducing conditions via reaction (V); the extent of the reduction is proportional to the amount of CaO present, i.e. the available surface area. With CaO, reaction (V) has a larger rate coefficient than the equivalent reaction over Fe2O3. Its activation energy is identical to that for graphite as the catalytic surface. Still considering reducing conditions, it was shown that, in the presence of NO, CaO can react with carbon to produce CaC2. The details of the reactions involved are not yet totally clear, but NO catalysing reaction (IX) via (VIII), together with the Boudouard reaction (X) and also (XV) seems one possibility. The carbon added

as graphite seems to participate in reactions (XV) and (XII). Also, there is the direct reaction (XIV) between CaC2 and NO, probably giving N2 and CaO as the calcium-containing product. Quite surprisingly reaction (XIV) keeps going for a very long time, because CaC2 is regenerated continuously, as long as CaO and carbon are present. Under oxidising conditions, adding CaC2 to a bed increases NO emissions, very possibly via the fixation of atmospheric nitrogen to give CaCN2, which oxidises in O2 to give NO and CaO. In summary, it appears that a decrease in NO emissions, under reducing conditions in a fluidised bed, can be caused by NO reacting with CO over a catalytic surface, such as carbon, CaO or Fe2O3. The presence of NO not only facilitates the Boudouard reaction (X), but also a reaction between CaO and carbon producing CaC2. In a fuel-rich atmosphere, CaC2 decreases NO emissions, but under oxidising conditions, CaC2 can actually fix N2 and so produce more NO. Acknowledgement This work was supported by the former Central Electricity Generating Board through a S.E.R.C. CASE Studentship to D.A. References [1] Dennis JS, Hayhurst AN. Chem Eng Sci 1987;42:2361–72. [2] Fennell PS, Pacciani R, Davidson JF, Hayhurst AN. Energy Fuels 2007;21:2072–81. [3] Anthony EJ, Granatstein DL. Prog Energy Combust Sci 2001;27:215–36. [4] Blamey J, Anthony EJ, Wang J, Fennell PS. Prog Energy Combust Sci 2010;36:260–79. [5] Abanades JC, Anthony EJ, Wang J, Oakey JE. Environ Sci Technol 2005;39:2861–6. [6] Ives M, Mundy RC, Fennell PS, Davidson JF, Dennis JS, Hayhurst AN. Energy Fuels 2008;22:3852–7. [7] Fennell PS, Davidson JF, Dennis JS, Hayhurst AN. J Energy Inst 2007;80:116–9. [8] Allen D, Hayhurst AN. J Chem Soc Faraday Trans 1996;92:1227–38. [9] Allen D, Hayhurst AN. J Chem Soc Faraday Trans 1996;92:1239–42. [10] Dennis JS, Hayhurst AN. Proc Combust Inst 1984;20:1347–55. [11] Dennis JS, Hayhurst AN. Inst Chem Eng Symp Ser 1984;87:61–8. [12] Dennis JS, Hayhurst AN. Chem Eng Sci 1986;41:25–36. [13] Dennis JS, Hayhurst AN. Chem Eng Sci 1990;45:1175–87. [14] Hu G, Dam-Johansen K, Wedel S, Hansen JP. Prog Energy Combust Sci 2006;32:386–407. [15] Torres-Ordonez RJ, Longwell JP, Sarofim AF. Energy Fuels 1989;3:506–15. [16] Anderson DC, Galway AK. Proc Roy Soc Lond (A) 1996;452:585–602. [17] Davies NH, Laughlin KM, Hayhurst AN. Proc Combust Inst 1994;25:211–8. [18] Davies NH, Hayhurst AN. Combust Flame 1996;106:359–62. [19] Hansen PFB, Dam-Johansen K, Johnsson JE, Hulgaard T. Chem Eng Sci 1992;47:2419–24. [20] Shimuzu T, Tachiyama Y, Fujita D, Kumazawa K-I, Wakayama O, Ishizu K, et al. Energy Fuel 1992;6:753–7. [21] Olanders B, Strömberg D. Energy Fuels 1995;9:680–4. [22] Gao C, Takahashi T, Narisawa H, Yoshizawa A, Shimuzu T, Kim H, et al. Fuel 2014;127:38–46. [23] Vogel GJ (first named author) Annual Report. ANL/CEN/ES-1005, Argonne National Lab, Argonne, Illinois; 1973. [24] Tatebayishi J, Okada Y, Yano K, Ikeda S. Annual Report of Kawasaki Heavy Industries, Japan; 1979. [25] Johnsson JE. Fuel 1994;73:1398–415. [26] Hammons GA, Skopp A. Environmental Protection Agency Report to APCO Exxon Research and Engineering Company, Linden, NJ; 1971. [27] Patsias, Nimmo W, Gibbs BM, Williams PT. Fuel 2005;84:1864–73. [28] Robison EB, Glenn RD, Ehrlich S, Bishop JW, Gordon JS. Report to Division of Control Systems of the Environmental Protection Agency. Pope, Evans and Roberts Inc.; 1970. [29] Leckner B, Amand LE. Report A86 – 157, Department of Energy Conservation, Chalmers University of Technology, Sweden; 1986. [30] Allen D, Hayhurst AN. Proc inst energy 5th int combust conf. Bristol: Adam Hilger; 1991. p. 221–30. [31] Allen D, Hayhurst AN. Fuel 2015;142:260–7. [32] Hayhurst AN, Ninomiya Y. Chem Eng Sci 1998;53:1481–9. [33] Fennell PS, Hayhurst AN. Proc Combust Inst 2002;29:2179–85. [34] Jensen A, Johnson JE, Dam-Johansen K. AIChE J 1997;43:3070–84. [35] Allen D. The removal of gaseous pollutants during coal combustion, Ph.D. Diss., University of Cambridge; 1990. [36] Allen D, Hayhurst AN. Proc Combust Inst 1991;23:935–41. [37] Rinck E. In: Pascal P, editor. Nouveau Traité de Chimie Minérale. Masson et Cie; 1958. p. 399. [38] Shriver DF, Atkins PW, Langford CH. Inorganic chemistry. Oxford: Oxford University Press; 1990. p. 393.

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