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°“√»÷°…“°“√¥—°®—∫‰Œ‚¥√‡®π —≈‰ø¥å‚¥¬„™âÀ‘πªŸπRemoval of H2S using Raw Limestone and Spray-Calcined Limestonein a Fixed Bed Reactor at High Temperatures
*»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡ ‡∑§‚π∏“π’ µ.§≈ÕßÀâ“ Õ.§≈ÕßÀ≈«ß ®.ª∑ÿ¡∏“π’ 12120 ‚∑√. 0-2577-1136 ‚∑√ “√. 0-2577-1138Environmental Research and Training Center, Department of Environmental Quality Promotion. Technopolis. Klong 5 Klong Luang, Pathumthani 12120 e-mail: nittaya@deqp.go.th**Department of Fuel and Energy, University of Leeds, UK.
N. Milne* and W. Nimmo**
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·ª√√Ÿª‡ªìπ CaO Ÿß∂÷ß 95% CaO “¡“√∂¥Ÿ¥´—∫
Drop Tube Reactor (DTR)
Fixed - Bed Reactor (FBR)
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ABSTRACTThe effectiveness of raw and spray-
calcined limestone for flue gas desulphu-
risation, in particular, H2S removal from coal
gasif icat ion, has been invest igated by
experimental studies using drop tube reactor
(DTR) and fixed-bed flow reactor (FBR). The
limestone was ground and sprayed into a
DTR for calcination at 1073 and 1323 K.
Sulphidation experiments were performed in
a FBR at temperature range from 873 to 1173
K. For calcination, the higher the temperature,
the greater conversion at shorter residence
time. The surface area, pore size and structure
have influence on the rate of decomposition
of CaCO3 to form CaO. 95% conversion was
obtained at 1323 K. Sulphidation of the raw
limestone (CaCO3), gave less performance in
sulphur conversion than the calcined form
(CaO). The results from this study indicated
that calcined forms of limestone can be
applied for flue gas desulphurisation from
coal-fired power plants. However, the efficiency
depends very much on the characteristics of
the materials used, the mechanisms of
conversion, and the process conditions.
1. IntroductionCoal is the most abundant, safe,
secure and economical fossil fuel. However,
environmental impact from coal-fired power
plants has been very much concerned.
Sulphur Oxides (SOX) emission from fossil
fuel combustion systems has a significant
impact to the environment as it causes acid
rain, which could damage the ecosystem and
human health. Therefore, the implementation
of measures to control the emission of SO2 is
an essential part in limiting such impact.
At present, the retrofitting of large coal-fired
power stations with flue gas desulphurisation
(FGD) units has been the focus of the strategy
in the U.K. for the reduction of sulphur
emissions1. Current technologies involve the
use of calcined limestone as an agent for
sulphur absorption, which is injected into
the flue gas at the appropriated conditions of
temperature and Ca/S ratios. It is proposed
that similar technologies using limestone
may be applied to the removal of sulphur from
the product gas2-7 as part of the development
of the next generation of clean coal gasification
systems, which is so called the integrated
gasification-combined cycle (IGCC). In IGCC,
the coal is gasified in a restricted air (or more
commonly oxygen) supply in the reaction
vessel and the resultant fuel gas is then burnt
with air in the combustion chamber of a gas
turbine. During gasification, impurities arising
from sulphur, nitrogen and chlorine in the coal
are present in their reduced forms (H2S, COS,
NH3 and HCl), which can be removed to
a very low level using conventional technology.
Desulphurisation at high temperatures (up
to 1200 ÌC) would make a substant ia l
contribution in improving the thermal efficiency
of electric power generation in IGCC systems
which can be as much as 2-3%2. Amongst
the possible sorbents for H2S removal at high
temperatures, calcium-based materials have
the advantage of being cheap, abundantly
available, and commonly used as bulk
chemical. Limestone, dolomite, and calcium
hydroxide are the most common calcium-
based sorbents used in this application.
Recently, the use of other materials has been
suggested as alternatives to l imestone
injection, namely the carboxylic acetate salts
of Mg and Ca (CMA), but the studies have been
mainly concern with SO2 capture.
»Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡ §-93
propably due to the onset of the sintering of
CaS crystals forming a new, less prenetrable
coating around the limestone particles. Under
these conditions the rate of sulphidation is
limited by the rate of diffusion of H2S through
the sintered product layer. This is probably
the main reason for the poor performance
of limestone as an absorption medium for
H2S removal.
At temperatures greater than about
900 ÌC - at the conditions mentioned above,
the limestone will calcine at rates faster than
sulphidation to form CaO which reacts relatively
quickly to give CaS. There is evidence to
suggest that at temperatures just above the
calcination temperature the CaO is consumed
almost as rapidly as it is formed (Fenouil and
Lynn, 1995, Part 2)4. At temperature about
50 ÌC above the calcination temperature the
calcination rate is fast enough not to limit the
sulphidation process. Experiments have
shown that at these temperatures (about
950 ÌC), the rate of sulphidation is not affected
by the initial form of the sorbent, whether it
be raw limestone or pre-calcined limestone.
2.3 H2S decomposition and the water-gas equilibrium
The principal components of the gas
mixture which comprise the products of coal
gasification, namely CO2, CO, H2O, H2 exist
in equilibrium
CO + H2O → CO2 + H2 (4)
With a temperature dependent
equilibrium constant,
K = [CO2] [H2] , equilibrium constant. [CO][H2O]
H2S from coal sulphur may decompose
if the temperature is high enough,
H2S → H2 + S (5)
Additionally, H2S may react with CO2
This study is aimed to investigate the
sulphidat ion per formance of calcined
limestone and raw limestone as an agent
for H2S removal from the flue gas from coal
combustion. Spray pyrolysis/calcination was
performed using a drop tube reactor (DTR)
for calcination of limestone to produce CaO,
and the CaO was then sulphided under
controlled conditions of temperatures and
gas composition in a fixed bed reactor (FBR).
The sulphidation efficiency was compared with
that of limestone and the calcined form.
2. Background
2.1 CalcinationLimestone consists of predominantly
CaCO3 and undergoes calcination at a
temperature of which is determined principally
by the CO2 partial pressure in the gas mixture
surrounding the particles.
CaCO3 → CaO + CO2 (1)
Limestone will calcine under an inert
atmosphere at about 700 ÌC (Borgwardt,
1985)23 and at about 900 ÌC (Fenouil and
Lynn, 1995, Part 3)5 under pure CO2, at 1 bar.
The calcination process opens up pores in
the lime particles enhancing their reactivity
towards H2S absorption due to greater surface
availability.
2.2 SulphidationH2S can be removed from coal gas by
CaCO3 or CaO to form CaS.
CaCO3 + H2S → CaS + H2O + CO2 (2)
CaO + H2S → CaS + H2O (3)
Under an atmosphere of 2 mol% H2S,
5 mol% H2O and 88 mol% CO2 (Fenouil and
Lynn, 1995, Part 1)3 the lowest temperature
at which sulphidation of CaCO2 takes place, at
1 bar, was found to be less than 10% under
these conditions. At temperatures greater than
660 ÌC the rate of sulphidation decreased;
§-94 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡
CO2 + H2S → COS + H2O (6)
It may be necessary to be aware of
the possible affect of these equilibria on the
actual gas composition surrounding the
sorbent samples under test, since the initial
feed composition will not remain unchanged
in the reactor. The actual composition may be
computed and/or measured by extracting gas
samples for analysis by gas chromatography.
The kinetic data obtained will therefore
pertain to the sorbent gas environment.
2.4 The effect of O2
Observed increases in the rate of
limestone conversion in the presence of O2
(Nimmo and Agnew, 1999)1 may be explained
by the formation of SO42- ions which can break
the metastable CaS crust and enhance the
rate of conversion of CaCO3 (Fenouil and Lynn,
1995 Part 1)3. The presence of small amounts
of oxygen has been reported as apparently
enhancing sulphidation. Enhanced sulphidation
rates may be explained by the misinter-
pretation of sulphidation measurements due
to the formation of CaSO4 via,
CaS + 2O2 → CaSO4 (7)
The enhancement is likely to be due
to the higher molecular weight of CaSO4 or to
some catalytic effect exercised by CaSO4
(Heesink and Swaaij, 1995)7. Therefore, under
sulphidation conditions, it is important that
sweep gases should be oxygen-free to avoid
errors in kinetic studies. Oxygen may be
removed from carrier/reactant gases by, for
example, beds of copper-based catalyst
pellets.
The presence of CO (1%) in the
sweep gas has been found to prevent CaSO4
formation from the oxidation of CaS by CO2
under atmosphere of CO2/N2 (Fenouil and
Lynn, 1995)4.
2.5 CaO sinteringLimestone-derived CaO will begin to
reduce in surface area due to sintering at
temperatures greater than 700 ÌC under inert
atmosphere (Borgwardt, 1985)23. The onset
of sintering of CaO produced from ultra-pure
CaCO3 occurs at a temperature of about
900 ÌC. The reason for this may lie in the
presence of latt ice defects caused by
impurities in the limestone-derived CaO.
Reduction in porosity and internal surface
area will influence the accessibility of H2O
and CO2 to accelarate the rate of CaO
sintering due to catalytic effects (Borwardt,
1989; Mai and Edgar, 1989). The effect of
H2 and CO on sintering may be indirect
through the water gas shift reaction (reaction
(4)), therefore, increasing H2 will increase
the H2O content and increasing CO content
will increase the CO2 content. Reduced
sulphidation rates in the presence of H2 and
CO have been attributed to competitive co-
adsorption on the CaO surface (Heesink and
van Swaaij, 1995)7.
2.6 CaS sinteringCaS sintering is dependent on the
composition of the atmosphere surrounding the
particles (Fenouil, 1995)3. Under atmosphere
containing CO2, CaS was observed to sinter at
temperatures above 850 ÌC, but under N2
structural difference was observed under
examination by scanning electron microscope
(SEM). This observation was confirmed by
surface area measurement. Sintering of CaS
is characterised by a smoothing surface of
individual grains and even a merging of grain
boundaries can be observed. Carbon dioxide
appears to catalyse the sintering of CaS even
when the CO2 concentration is 5% by volume.
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3. Experimental Equipment andAnalytical Methods
3.1 Drop Tube Reactor (DTR).A schematic diagram of the drop
tube furnace (2000 mm x 40 mm i.d.) is
shown in Figure 1. The furnace consisted of
six heated sections rated at 0.5 kW each
of which were l inked to provide three
independently controllable temperature zones.
The limestone was ground in a sealed
limestone hopper (Figure 2). Then the particles
were fed into the reactor through a water
cooled spray injection system, utilising an
internally mixing, twin fluid atomiser and a
constant-pressure feed system, to ensure
a steady flow of solution to the nozzle of
about 10 ml/min. Atomising air was fed at
a rate of about 3 ml/min, and carrier air was
fed at up to 40 l/min. The flow of liquid was
varied with the carrier gas flow to produce
difference residence times at each port in the
reactor. Quenching, due to the evaporation of
water from the spray at the top of the DTR,
meant that calcination conditions (1073 and
1323 K) prevailed only in the bottom two-thirds
of the tube, which was controlled to give the
desired range of reaction temperatures. The
time the particles spent in this section was
taken as the residence time, up to 0.8 s.
The low temperature at the top also meant
that the reactor could not be operated with
a uniform temperature throughout its length;
therefore, a fixed profile (Figure 3) was
used, with the temperature varied in the last
section only.
Solid particle sampling from the DTR
was performed using a sampling probe, which
was inserted into the gas flow, and a portion
of the flow was directed through a two-stage
sample recovery system. Whilst the reaction
temperatures were still at over 400 K, particles
greater than 3-4 mm were removed by a
cyclone separator with a heated catchpot
(353 K). Then the remaining fine materials
were trapped just downstream by a poly
(tetrafluoroethylene) filter.Figure 1 : Schematic diagram of the drop
tube reactor (DTR).
Figure 2 : Limestone feeder
Figure 3 : Temperature profiles in the DTRwith a common profile in evaporation zone up
to 1073 K but differing profiles in the calcination tofinal temperatures of 1073 and 1323 K.
§-96 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡
3.2 Fixed Bed Reactor (FBR).Sulphidat ion experiments were
performed in a fixed bed reactor operating
under dif ferent conditions. Calcination,
sulphidation, and sintering experiments have
been reported using bench-scale laboratory
reactor3, 23, where small batches of sample,
<50 mg, were reacted under controlled
conditions of temperature and gas composition.
The reactor used in this study was based
on these proven designs (Figure 4) and
operated at atmospheric pressure. Gas flows
were accurately metered using mass flow
controllers. The concentration of H2S was
maintained at about 2% throughout the
temperature range of concern (873-1173 K)
by the inclusion of H2 to prevent H2S decom-
position. The gas mixtures used in the
sulphidation studies in the FBR are shown
in Table 1 at the two relevant reactor
temperatures obtained from equilibrium
calculations. Checking of gas composition
was performed by extracting gas samples
for analysis by gas chromatography. An
important feature of the design permitted
the solid sample to be withdrawn from the
hot zone of the furnace and cooled under
nitrogen.
Tests were performed for different
residence times so the rates of sulphidation
could be obtained from conversion data using
TGA analysis. Experiments were performed
under differential conditions using high gas
flow rates and small solid sample weights
to ensure that the inlet and outlet gas
concentrations were as close as possible.
These conditions ensured that the particle
reactions were not affected by changes in
the ambient gas concentrations due to, for
example, CO2 evolution under calcination
conditions. The particles were dispersed in
a substrate of quartz wool, so that interparticle
effects were minimised.
The sulphidation per formance of
sprayed-calcined limestone was compared
with that of raw limestone (Omyacarb) in the
FBR at the same conditions of temperature
and residence times.
3.3 CaO Sulphidation and TGA Analysis.A method was developed to analyze
the material from sulphidation experiments
using the fixed-bed reactor by TGA, in a
two-stage programme that enabled the
determination of carbonate and sulphide in the
sample, and oxygen by difference, as follows:
Figure 4 : Schematic diagram of fixed bed reactor used in sulphidation studies.
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(1) Heating of solid to 750 ÌC under
N2 and holding for 15 min.
CaCO3 + CaO + CaS → 2CaO + CaS + CO2
(2) Heating from 750 to 900 ÌC in air
and holding for 25 min.
CaO + CaS + air → CaO + CaSO4
From a known initial mass of sample,the degree of sulphidation can be calculated.From procedure 1, the mass of carbonatepresent can be calculated in the initial samplefrom the mass loss, and from procedure 2,the mass of sulphide present can be calculatedfrom the mass gain. Hence the mass of CaOin the original sample can be calculated bydifference:
mCaO = mtotal - (mCaS + mCaCO3) (1)
From this, the percent conversion to
sulphide can be calculated:
X = (MCaS/Mtotal)100 (2)
Where m = mass of compound n, M
= molar quantity of compound n, and X = %
conversion.
3.4 Particle Characterisation.Malvern Mastersiser instruments were
used to set up the atomiser nozzles andmeasure the size of the spray-formed CaOpar ticles. Samples from the latter wereprepared by suspending the CaO powder indry ethanol, and particle separation wasensured by mild vibrational agitation beforeparticle sizing was performed. Surface areameasurements were made using a TechmationQuantasorb instrument using the three-pointBrunaur, Emmett, and Teller (BET) theory, inwhich N2 is absorbed at three different partialpressures to give three different coveringvolumes and fitted according to the BETisoterm.
Scanning electron microscope images
were produced using a Camscan 4 instrument
linked to a PC for storage and electronic
manipulation.
3.5 Limestone Samples.The limestone used in the experiments
presented here were sourced in Spain as part
of a larger study on the performance of
different sorbents for H2S removal at high
temperature in the next generation of clean
coal gasification systems. The particle size
range used in the calcination and sulphidation
studies was 75-106 µm.
4. Results and Discussion
4.1 Particle Structure.Evidence for macropore formation
during calcination can be seen when the poresize distribution is examined. The BET surfaceareas of the calcined CaO from limestoneobtained at 1323 K was 23 m2g-1, comparedwith uncalcined values of 0.3 and 4 m2g-1.The results indicate that the calcined formof limestone has greater proportion of thesurface area of pores greater than about100 Å. However, these data alone cannotaccount for the greater capacity of dolomitefor sulphur capture. Examination of sulphidedand raw samples was per formed usingscanning electron microscopy (SEM) toexamine the macrostructure of the particles.An SEM images of the raw limestone, spray-calcined form, and sulphided spray-calcinedlimestone are presented in Figure 5, 6 and 7,respectively.
Figure 5 : Sub-10 µm diameter particles of rawlimestone showing crystal structure
§-98 »Ÿπ¬å«‘®—¬·≈–Ωñ°Õ∫√¡¥â“π ‘Ëß·«¥≈âÕ¡ °√¡ à߇ √‘¡§ÿ≥¿“æ ‘Ëß·«¥≈âÕ¡
4.2 Calcination.Batches of calcined limestone were
prepared in the drop tube reactor at a
temperature of 1023 and 1323 K. The result
showed that the higher the temperature,
the greater the rate of calcination, indicated
by greater conversion at shorter residence
times. The final degree of conversion was in
the region of 95%. These results led to the
assumption that the rate of calcination is
proportional to the initial surface area of the
uncalcined material and treated in a manner
similar to that given by Borgwardt, 198523 (21)
where the rate of calcination of small particles
can be described by
ln(1-x) = -ksSgt (4)
where x is the fraction converted to
CaO, ks is the rate constant of the surface
reaction (mol cm-2 s-1), Sg is the BET surface
area (cm2 mol-1), and t is the residence time
in the reactor.
The calcination rate constant for each
temperature was extracted from a plot of ln
(1-x) versus t. Nimmo et al.1 has shown that
the per formance of calcinat ion and
sulphidation between two different sizes
(<38 µm and 75-125 µm) are comparable.
This is due to the fact that the difference
between the size fraction is not great. The rate
constants for limestone calcination are shown
in Table 1. 95% conversion was obtained in
this study at 1323 K (1050 ÌC).
4.3 Sulfidation.Samples previously calcined in the
DTR to 70-95% conversion (at 1023 and 1323
K) were sulphided at 873, 1073 and 1173 K
in the FBR. The performance of the calcined
materials is shown in Figures 8. The reproduc-
ibility of the sulphidation results was ±5%
of the quoted values. It is evidenced that
the percentage conversion depended on
temperature, partial pressure of the sulphur-
containing gases, sur face area of the
materials (particle size and morphology), and
pore size. Nimmo et al.1 also observed that
not only the surface area of the material has
an influence on the sulphidation, the pore
size of the calcined form is also to be
considered. The larger pore sizes will be less
prone to blockage by buildup of a sulphide layer,
thereby permitting deeper penetration of H2S
into the particle and resulting, ultimately, in
Figure 6 : Sub-10 µm diameter particles of spray calcinedlimestone at 1323 K showing conversion from CaCO3 to CaO.
Figure 7 : Sub 10 µm diameter particles of sulphidedspray-calcine limestone at 873 K
Table 1 Calcination surface rate constants, ks, for limestone (mol cm-2 s-1).
Temperature (K) Limestone (Omyacarb)
1073 1.75 x 10-6
1323 7.07 x 10-6
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greater degrees of particle conversion. As
residence time increased in the fixed bed
reactor (H2S), sulphidation increased rapidly
in the first 10 minutes. After 10 minutes, 17%
conversion was obtained. After 20 minutes,
20% conversion was found.
The apparent loss in sulphide
conversion observed at 1173 K has been
attributed to partial oxidation of the CaS.
This proposal was tested with Derbyshire
limestone at 1073 and 1173 K, with samples
sulphided for 10 min. (This apparent loss in
conversion is only observed at temperature
above 1073 K.) Samples obtained from the
FBR were analysed with the TGA method
described previously for sulphide conversion.
However, on completion of the oxidation, the
sample was heated further under nitrogen to
decompose the CaSO4 formed. Sulphide
content was calculated, with total sulphur
being obtained from the dissociation weight
loss. Bases on TGA experiment, CaSO4 assumed
to decompose via:
CaSO4 → CaO + SO3
5. Conclusions
Calcination:5.1 The higher the temperature, the
higher percentage of decomposition of the
limestone carbonate.
CaCO3 → CaO + CO2
5.2 The surface area, and pore size
and structure have influence on the rate of
decomposition.
Sulphidation:5.3 Sulphidation of the raw limestone
(CaCO3) gave less performance in sulphur
conversion than the calcined form (CaO).
CaO + H2S → CaS + H2O
(CaCO3 + H2S → CaS + H2O + CO2)
5.4 Rapid conversion during the first
10 minutes in the FBR was observed.
5.5 Current research is now using
calcium acetate, Ca (CH3COO)2, solution to
produce small CaO particles.
In general:5.6 The results from this study
indicated that calcined forms of limestone
can be applied for flue gas desulphurisation
from coal-fired power plants. However, the
ef f ic iency depends very much on the
characteristics of the materials used, the
mechanisms of the conversion, and the
process conditions, which further research
should be investigated.
6. AcknowledgementThanks to Dr E. Hampartsoumian
and staff at the Department of Fuel and
Energy, and the Department of Materials,
University of Leeds, for their technical help.
References1. Nimmo, W., Agnew, J., Hampartsoumian,
E., and Jones, J.M., Removal of H2S byspray-calcined calcium acetate. Ind. Eng.Chem. Res. 1999, 38, 2954-2962.
2. Adanez, J., Garcia-Labiano, F., de Diego,L. F., Fierro, V. H2S removal in entrainedflow reactors by injection of Ca-basedsorbents at high temperatures. EnergyFuels 1998, 12,726.
3. Fenouil, L. A., Lynn, S. Study of Calcium-based sorbents for high-temperature H2S
Figure 8 : Sulphidation of precalcined limestone. FBRtemperature = 873 K. DTR calcinations temperature = 1323 K.
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removal. 1. Kinetics of H2S sorption by uncalcinedlimestone. Ind. Eng. Chem. Ress. 1995, 34,2324.
4. Fenouil, L. A., Lynn, S. Study of calcium-based sorbents for high-temperature H2Sremoval. 2. Kinetics of H2S sorption bycalcined limestone. Ind. Eng. Chem. Res.1995, 34, 2334.
5. Fenouil, L.A., Lynn, S. Study of calcium-basedsorbents for high-temperature H2S removal.3. Comparison of calcium-based sorbentsfor coal gas desulfurization. Ind. Eng.Chem. Res., 1995, 34, 2343 - 2348.
6. Yrjas, P., Iisa, K., Hupa, M. Limestone anddolomite as sulfur absorbents underpressurised gasification conditions. Fuel1996, 75( 1), 89.
7. Heesink, A. B., van Swaaij, W.P.M. TheSulphidation of calcined limestone withhydrogen sulphide and carbonyl sulphide.Chem. Eng. Sci. 1995, 50(18), 2983.
8. Steciak, J.; Zhu, W.; Levendis, Y.A., Wise,D.L. Effectiveness of calcium (magnesium)acetate and calcium benzoate as NOx
reduction agents in coal combustion.Comb. Sci. Technol. 1994, 102(1-6), 193.
9. Steciak, J., Levendis, Y. A., Wise, D. L.Ef fectiveness of calcium magnesiumacetate as dual SO2-Nox emission controlagent. AIChE J. 1995, 41( 3), 712.
10. Steciak, J., Levendis, Y.A., Wise, D. L.,Simons, G.A. Dual SO2-NOx concentrationreductions by calcium salts of carboxylicacids. J. Environ. Eng. ASCE 1995, 121(8), 595.
11. Levendis, Y.A., Zhu, W., Wise, D.L., Simons,G.A. The ef fect iveness of calciummagnesium acetate as an NOx sorbent incoal combustion. AIChEJ. 1993, 39(5), 761.
12. Atal, A., Steciak, J., Levendis, Y.A. Combus-tion and SO2-Nox emissions of bituminouscoal par ticles treated with calcium-magnesium acetate. Fuel 1995, 79(4), 495.
13. Shukerno, J.L., Steciak, J., Zhu,W., Wise,D.L., Levendis Y.A., Simons, G.A., Gresser,J.D., Gutoff, E. B., Livengood, C.D. Controlof air toxin particulate and vapour emis-sions after coal combustion utilizingcalcium magnesium acetate. Resour.Conserv. Reccl. 1996, 16 (1-4), 15.
14. Weng, W., Baptista, J.L. A new synthesisof hydroxyapatite. J. Eur. Ceram. Soc.1997,17(9 ), 1151.
15. Palasantzas, I. A., Wise, D.L. Preliminaryeconomic analysis for the production ofcalcium magnesium acetate from organicresidues. Resour. Conserv. Recycl. 1994,11(1-4), 225.
16. Dosoretz, C.G., Jain, M.K., Grethein, H.E.Oxidat ive fermentat ion of calcium-magnesium lactate to calcium magnesiumacetate deicing salt. Biotechnol. Lett. 1992,14(7), 613.
17. Oehr, K.H., Barrass, G. Biomass derivedalkaline carboxylate road deicers. Resour.Conserv. Recycl. 1992, 7(1-3), 155.
18. Wise, D.L., Augustein, D. An evaluation ofthe bioconversion of woody biomass tocalcium acetate deicing salt. Sol. Energy1988, 41( 5), 453.
19. Sasaoka, E., Uddin, M.A., Nojima, S. Novelpreparation method of macroporous limeand l imestone for high-temperaturedesulfurization. Ind. Eng. Chem. Res. 1997,3(9), 3639.
20. Taniguchi, Y., Hayashida, T., Kitamura, T.,Fujiwara, Y. Vanadium-catalysed aceticacid synthesis from methane and carbondioxide. Stud. Surf. Sci. Catal. 1998, 114,439.
21. Fujiwara, Y., Kitamura, T., Taniguchi, T.,Hayashida, T., Jintodu, T. Transition-metal catalysed acetic acid synthesis frommethane and carbon dioxide. Stud. Surf.Sci. Catal. 1998. 119, 349.
22. Kurioka, M., Nakata, K., Jinkotu, T., Taniguchi,T., Takaki, K. Palladium-catalysed aceticacid synthesis from methane and carbonmonoxide or dioxide. Chem. Lett. 1995,3, 224.
23. Borgwardt, R.H. Calcination kinetics andsur face area of dispersed limestoneparticles. AIChE J. 1985, 31, 103.
24. Zhang, S. C., Messing, G. L., Borden,M. Synthesis of spherical particles by spraypyrolysis. J.Am. Ceram. Soc. 1990, 73(1), 61.
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