Table Of ContentThe Institute of Energy's Second International Conference on
COMBUSTION & EMISSIONS CONTROL
Proceedings of The Institute of Energy Conference held in London, UK,
on 4-5 December 1995
Organised by The Institute of Energy
Co-sponsored by:
Associazione Termotecnica Italiana
British Flame Research Committee
Institution of Chemical Engineers
Institution of Mechanical Engineers
Institution of Plant Engineers
National Society for Clean Air and Environmental Protection
The Combustion Engineering Association
The Combustion Institute
The Institute of Physics Combustion Physics Group
The Institution of Gas Engineers
World Coal Institute
and in association with
American Society of Mechanical Engineers
Australian Institute of Energy
Canadian Institute of Energy
L'Institut Francais de l'Energie (Paris)
The Fuel Society of Japan (Tokyo)
Southern African Institute of Energy
Venn Deutscher Ingenieure (VDI-Gesellschaft Energietecknik)
Published by
Special Distributor
energy Pergamon
An imprint of Elsevier Science Ltd
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Pergamon, an imprint of Elsevier Science Ltd ISBN 0 08 0421393
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Organising Committee
Chairman Professor Ν Syred, Professor of Mechanical Engineering
University of Wales, Cardiff
Committee Members
Dr M Bigg HM Inspectorate of Pollution
Dr C J Clark BP Oil, Oil Technology Centre
Mr M J Cooke Consultant
Dr R M Davies Consultant
Professor J S Harrison University of Leeds
Dr A Ν Hayhurst University of Cambridge
Mr A Milani ENI, Italy
Dr A Sanyal International Environmental & Energy Consultants
Dr J Ρ Smart Fuel and Combustion Technology International
Professor A Williams University of Leeds
Mr Ν G Worley Institute of Energy
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Combustion and Emissions Control II
COAL SELECTION FOR NOx REDUCTION IN PULVERISED FUEL COMBUSTION
J.R. Gibbins, F.C. Lockwood, C.K. Man, and J. Williamson, Imperial College, London
G.J. Hesselmann, Babcock Energy Ltd, Renfrew
B.M. Downer and N.M. Skorupska, National Power PLC, Swindon
A major factor affecting a coal's performance in air-staged low-NOx burners is
the amount of nitrogen remaining in the char after de volatilisation. Current
standard proximate devolatilisation tests do not apply realistic heating conditions
for PF combustion, but a recently-developed high-temperature wire-mesh reactor
now allows relatively simple captive-sample measurements at heating rates of 104
K/s. Char nitrogen data is reported for devolatilisation temperatures from 400°C
to 18U0°C, including values for a range of UK and world-traded coals for which
NOx measurements from three full-scale low-NOx utility plants and/or a pilot
combustor are available. The most general correlation between char nitrogen and
combustion NOx is observed for a peak preparation temperature of 1800°C with
0.15 s (or 2 s) hold time or 1600°C with 2 s hold, conditions which give the
maximum release of nitrogen from the char.
(Keywords: coal, combustion, NOx, devolatilisation, char, test)
INTRODUCTION
Pulverised coal combustion in large utility boilers can result in the production of significant amounts of
oxides of nitrogen (NOx - principally NO in this type of plant). Although some of this NOx is formed
from nitrogen in the combustion air, the principal source of NOx is generally the nitrogen contained in
the coal itself since the latter is available in more reactive form. NOx is not the only possible end product
for coal nitrogen, however, which can also form molecular nitrogen, N . In this respect nitrogen differs
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from most other pollutant sources in coals (e.g. sulphur, chlorine, heavy metals) in having the potential to
yield a totally non-polluting and readily-disposable residue. A simplified scheme for the formation of NOx
and N from coal nitrogen during combustion is shown in Figure 1 (based on Pohl and Sarofim(l)).
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Because of the relatively low capital and operating costs involved, combustion modifications to maximise
N formation and minimise NOx formation are attractive, either on their own or in conjunction with further
2
NOx reduction techniques applied after the main fuel combustion has taken place (e.g. reburn, SNCR,
SCR). The main principal in minimising NOx formation during combustion is to minimise the oxygen
supply to the burning coal for as long as possible. In practice this means reducing oxygen availability
during coal devolatilisation and the accompanying homogeneous combustion of the volatiles. Although
this may result in slightly longer reaction times these near-burner processes are intrinsically fast enough
(order 0.1 s) to permit some increase within current plant geometries (i.e. with acceptable flame lengths).
In contrast heterogeneous combustion of the residual char is a relatively slow process (order 1 s) which
requires the presence of excess oxygen to achieve acceptable rates and final burnout. As a result, NOx
formation from the char Ν is unlikely to be reduced significantly with air-staged low-NOx combustion
systems.
The intrinsic nature of the coal is the main factor affecting nitrogen distribution between volatiles and char
during devolatilisation. As natural materials coals exhibit widely different combustion behaviour due
principally to:
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Combustion and Emissions Control II
(a) origin - coals were formed from a wide variety of plant materials over a range of geological periods,
and
(b) rank - the extent of geological maturation the coal has undergone which will vary principally with age,
depth of burial and the temperatures which the coal has experienced.
Origin was not a consideration when industrial activities relied primarily on indigenous coals from a limited
geographic area and usually a single period of origin (e.g. Carboniferous in UK and Eastern US). For coals
of similar origins a wide range of properties of industrial interest correlate with rank, as illustrated by the
well-known Seyler chart for UK coals. Because of this broad cross-correlation of properties many
characterisation tests (e.g. carbon content, vitrinite reflectance) can be used to indicate rank and hence, by
analogy with the known performance of other coals of similar rank from the same origin, the combustion
behaviour.
Although most rank-indicating tests are clearly not directly related to combustion processes, the proximate
volatile matter content (or the fuel ratio Johnson(2), the proximate fixed carbon divided by the volatile
matter) might be considered to represent a more direct measure of a coal's devolatilisation behaviour during
combustion. This probably is the case for slow combustion of lump coal on a grate, but it is well-known
that volatile yields under the rapid heating conditions applied in PF combustion are significantly higher
than proximate yields. A previous wire-mesh study Gibbins et al(3) has confirmed that this is due to a
combination of the absence of retrograde char-forming reactions within the bed of coal in a proximate
analysis crucible when dispersed samples of finely ground coal are heated, and a greater release of
volatiles from the particles themselves at higher heating rates. The enhancement is not constant, however,
nor does it appear to vary in a consistent manner with rank. It therefore appears that, in as far as
proximate volatile matter or fuel ratio does give some indication of PF combustion behaviour, this is also
primarily due to the proximate VM being another reasonable indicator of rank within a series of coals from
a similar origin.
With an increase in the use of world-traded coals and international participation in power station
construction and operation projects, the commercial demand for information on the combustion behaviour
of a wide range of overseas coals has increased. In this case correlations based on the results of empirical
'ranking' tests on indigenous coals have been found to be inadequate, particularly when comparing Northern
Hemisphere coals with Southern Hemisphere (Gondwanaland) coals Carpenter and Skorupska(4). In
principle new sets of rank-based correlations could be derived for each geological 'family' of coals. In
practice this approach is difficult to implement because of the number of different correlations that would
have to be derived. Even then the origin of a coal may not be easy to determine and blends of coals from
significantly different sources can be encountered. A more satisfactory approach is the development of new
tests which subject coal particles to conditions comparable to those that will be encountered during PF
combustion. In this case the results can be used to indicate performance directly.
'Drop tube' entrained flow reactors are probably the most widely-used small-scale test facilities for the
devolatilisation behaviour of coals under PF combustion conditions (4). A small flow of powdered coal
is injected into a stream of (usually pre-heated) inert or weakly oxidising gas and passed down an
externally-heated tube. Typically peak temperatures are limited to 1450°C (if relatively low cost silicon
carbide heating elements are to be used) and particle residence times to 500 ms (to keep the tube length
around 2m) although more realistic conditions are possible (e.g. up to 1700°C with 3 s residence time in
a recent design Hutchings and Williamson(5)). The char sample is typically collected in a water-cooled
probe at the bottom of the reactor tube. Volatile yields can be deduced either from the difference between
the weights of coal fed and char collected or from the relative ash (or specific elemental Fletcher(6))
contents of the coal and the char, assuming the ash behaves as an inert tracer. In both cases, tars
condensing with the sample may cause difficulties and a small amount of oxygen can be added to the inert
sweep gas (usually nitrogen or argon) to promote tar breakdown Thompson and Stainsby(7). The
char/volatile nitrogen partition can be determined from the total weight loss and the measured nitrogen
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Combustion and Emissions Control II
content of the residual char. NOx formation in air-staged combustion of coal blends has been observed
to correlate with char nitrogen yields from drop tube measurements, with an apparent conversion of about
30% of the char Ν to NOx Nakamura et α/(8).
While drop tube reactors have yielded many insights into coal combustion behaviour, and arguably provide
the most realistic conditions for small-scale experiments, they have some limitations to their use as a
routine test. In the simpler designs, temperatures and residence times are near the lower margin of what
might be considered appropriate for PF combustion. Total volatile yields (and hence char/volatile Ν
partition) can also be difficult to determine accurately for coals with a low ash content or volatile ash
constituents, while total char collection is always difficult to guarantee. To overcome some of these
limitations, a captive sample test has been developed based on the high-temperature version of the wire-
mesh reactor. This paper describes the apparatus, examines the nature of volatile and nitrogen release from
coals devolatilised at elevated temperatures and shows the correlation between wire-mesh measurements
and NOx formation in full size utility plant and test combustors.
APPARATUS AND TEST METHODS
The key characteristic of the wire-mesh apparatus is the use of a captive sample of coal held between two
layers of mesh which also act as an electrical resistance heater. The coal sample can then be weighed
before and after heating to establish the weight loss directly. Historically, peak temperatures achievable
using WM apparatus have been limited to around 1100°C by the stainless steel mesh used, and heating rates
to less than 10,000 K/s because of power supply and control limitations (4). As a result of a four-year
development programme at Imperial College, peak temperatures have been extended to 2000°C using
molybdenum mesh. A battery-powered DC power supply and a feedback control system based on a 486
PC allow heating rates up to 50,000 K/s. Temperatures are measured using a two-colour infra-red
pyrometer calibrated against W3 thermocouples. A flow of helium across the wire-mesh sample holder
prevents volatiles redepositing; the apparatus is limited to operation under non-oxidising conditions by the
properties of the mesh material. The new apparatus is shown in Figure 2; additional technical details will
be published elsewhere Pendlebury et α/(9).
After grinding and sieving to 125-150 μιη for compatibility with the molybdenum mesh (63 μιη wires χ
106 μηι aperture), coal samples are dried overnight in a nitrogen-purged oven at 105°C and then stored
under nitrogen until required. Typically 5-10 mg of the sample coal are used for each devolatilisation test.
Duplicate determinations are generally undertaken to establish reproducibility; yields are usually repeatable
to within 2% of the original sample mass. The residual char from the WM runs can be almost entirely
recovered from the mesh. Sufficient char (typically 2-7 mg) is available for duplicate CHN measurements
using a Carlo Erba 1106 microelemental analyser. The use of reference chars as internal standard materials
was found necessary to achieve reproducible char Ν measurements.
RESULTS AND DISCUSSION
The effect of temperature on volatile yields and volatile nitrogen yields from a UK bituminous coal is
shown in Figure 3. With zero hold time at peak temperature, total volatile and nitrogen yields are roughly
proportional for temperatures up to 1400°C. With extended hold times at 1000°C little further nitrogen
release takes place, but at 1400° C a significant amount of additional volatile nitrogen is evolved Gibbins
et al(\0). The latter phenomenon is illustrated in more detail in Figure 4, showing nitrogen release from
Daw Mill coal as a function of isothermal hold time at 1450°C. Nitrogen evolution is accompanied by a
reduction in char hydrogen content, and ceases when the hydrogen content falls to negligible levels.
The nature of nitrogen release can probably be interpreted as a two-stage process. Part of the nitrogen is
evolved rapidly with the tars and other primary volatile products, but with little or no selectivity as the
concentration of nitrogen in the volatiles remains roughly constant. This behaviour is consistent with the
coal nitrogen being carried out of the coal within relatively large coal component fragments "broken away'
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Combustion and Emissions Control II
as volatile products (although subsequent secondary cracking reactions of these fragments is also a likely
possibility). A second phase of nitrogen evolution then occurs when the temperature is sufficiently high
for the condensed aromatic structures in the residual char to be disrupted. By definition (since they have
already survived primary devolatilisation) these must be quite thermally stable, hence the elevated
temperatures required. Large aromatic molecules are unlikely to be released from the char, and both
experimental evidence Freihaut and Seery(ll) and a simple consideration of the possible volatile products
strongly suggest that the bulk of this nitrogen from secondary devolatilisation is evolved as HCN. This
is also consistent with the inter-relationship between nitrogen and hydrogen release from the char.
Given the sensitivity of the char/volatiles nitrogen split to devolatilisation conditions, some difficulty exists
in selecting the most appropriate test conditions to use for predicting char nitrogen contents under low-NOx
combustion conditions. Residence times will clearly be short (order 100's of ms), but particle temperatures
will vary depending on particle size, trajectory etc. Particles will also attain very high temperatures (order
2000°C) as they ignite at the start of heterogeneous combustion, making the upper temperature limit for
devolatilisation, as opposed to heterogeneous char combustion, difficult to define. Because of the problems
of making m a priori prediction for the most appropriate conditions, a range of devolatilisation conditions
have been used to test coals for which NOx emission levels from either full-size utility plants and/or a 45
MW single burner rig were available, Lam et al(\2), King(13). In all cases, measured NOx emissions for
3% 0 in the flue gases are plotted against calculated Vernon(14) combustor exit NOx emissions for zero
2
conversion of volatile nitrogen and 100% conversion of char nitrogen to NOx.
The test conditions initially selected, somewhat arbitrarily, as a compromise between what could
conveniently be achieved with an earlier prototype version of the wire-mesh apparatus and estimates of
actual conditions, were heating at 1000 K/s to 1450°C with 150 ms hold at peak temperature Man et α/(15).
As Figure 5 shows, a reasonable correlation is achieved with published NOx data for a number of coals
(12) (corrected to 3% flue gas 0 where necessary (14)), but several discrepancies are also observed Man
2
et al(\6). One of these is for the S. African coal (SAF) burnt in the utility boiler, but this coal produced
much less NOx in the single burner trials and, from observations made at the time, it appears likely that
fouling in the furnace chamber leading to enhanced thermal NOx was at least partly responsible for the
higher plant NOx levels. Although plant thermal NOx levels will obviously vary somewhat between coals
and exact values are not known, differences in thermal NOx do not appear able to explain the case of the
Colombian coal (COL), which gave a lower plant NOx value than predicted from the nitrogen content of
chars prepared under these conditions.
Since it was considered that the devolatilisation conditions used might be unrealistic, principally because
of too low a temperature and heating rate, the coals were re-tested at a heating rate of 10,000 K/s to
1600°C, again with 150 ms hold. It was thought that the Colombian coal might need these more severe
conditions to approach an asymptotic nitrogen yield, while the other coals might have already achieved
near-ultimate yields and would thus undergo relatively little further nitrogen loss. In fact all of the coals
tested exhibited some additional nitrogen release, resulting in little improvement in the correlation for the
Colombian coal (16).
To examine the correlation for conditions under which maximum rapid-heating Ν yields from the char were
almost certain to be achieved, all coals were then additionally tested at 10,000 K/s to 1800°C and with
2 s hold at peak temperature, although it was recognised that these conditions would be more severe,
particularly with regard to heating time, than would be encountered in actual plant. In this case, as shown
in Figure 6 (16), results for all coals now appear to follow a consistent trend. Overall char nitrogen
contents are, however, much reduced and either higher conversion rates of char nitrogen to NOx or a higher
volatile/thermal NOx contribution would have to be invoked to interpret the observed trends.
To investigate the apparently anomalous results for the Colombian coal, a more detailed study of the effect
of peak temperature and hold time was undertaken for it and the US coal, which gave similar NOx
emissions in the same utility plant. The results are shown in Figure 7 (16), with total volatile yield, volatile
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Combustion and Emissions Control II
nitrogen yield and char hydrogen content plotted as a function of temperature for both 150 m s and 2 s hold
times. In this case 10,000 K/s heating was also used for the 1450°C values - results were very close to the
1000 K/s measurements and consistent with the lower heating rate giving a slightly increased time at
temperature. Comparison between the Colombian and US coals confirms that significant differences in
nitrogen release patterns exist. In particular, much less nitrogen (and hydrogen) is released during primary
devolatilisation of the former (i.e. at 1450°C with 0.15 s hold). Therefore although the extended hold time
at 1450°C gives a similar increase in volatile nitrogen yields from both coals, this still leaves the
Colombian coal at a much lower total volatile nitrogen release.
That the values for 1600°C with 0.15 s hold are similar to those for 1450°C with 0.15 s hold is perhaps
not surprising, given that only an extra 15 ms of heating time between 1450°C and 1600°C (plus a slightly
longer period during cooling) are available to add to the effect of the higher temperature. Again
differences in the nitrogen distribution from primary devolatilisation predominate. Secondary
devolatilisation of both chars can eventually run to completion at 1600°C, however, as evidenced by the
very low char hydrogen contents after 2 s hold. This results in the combined primary and secondary
devolatilisation nitrogen yields from the two coals now being comparable.
At 1800°C, the rates of the secondary devolatilisation processes are so high in both coals that they are
completed by 150 ms, with essentially no further reaction taking place over the next 1.85 s. This confirms
that similar char nitrogen contents can be obtained for these coals, to match the similar plant NOx
emissions, without requiring unreal ι s tically long heating times. More intense heating conditions are
required than 150 ms at 1600X, but either heating time and/or temperature could be increased to achieve
parity. Based on the present data the hold time will fall rapidly from 2 s with increasing temperature.
Given that the hydrogen content is already close to zero after 150 ms at 1800°C, and that negligible
differences are observed between 1600T and 1800°C with 2 s hold, it can also probably be concluded that
no continuing increase in yields can be expected for temperatures beyond 1800°C.
On the basis of the results above, 104 K/s heating to 1600°C with 2 s hold has been selected as suitable
conditions for a routine wire mesh test for estimating the NOx formation potential of a coal. While perhaps
not as close to actual boiler conditions as a shorter holding time at 1800°C, this operating mode still
achieves the maximum feasible nitrogen release and applies less thermal stress to the apparatus. Figure 8
shows the previous utility plant and 45 MW single burner NOx measurements (plus three additional data
points for another utility plant) plotted against NOx values for complete conversion of the char Ν obtained
with these heating conditions. As expected, a reasonable correlation is obtained.
Also shown in Figure 8 are data points for an additional range of UK coals and a single Polish coal (POL)
with combustion NOx measurements obtained in a 160 kW test combustor located at Babcock Energy Ltd's
Technology Centre at Renfrew operated with a model conventional aerodynamically air-staged low-NOx*
burner, and, for a sub-set of the coals, with additional furnace air staging Hesselmann and Marshall(17),
Hesselmann( 18). This facility consists of a 5 m long 0.5 m diameter refractory lined furnace arranged
vertically with the burner firing downwards. Overfire air and gas/solid sampling ports are located along
the furnace length allowing the full range of combustion conditions pertinent to large utility combustors
to be investigated fully. Combustion NOx values with the low-NOx burner follow a similar trend to plant
and larger-scale single burner data for the UK coals. Somewhat elevated NOx levels are measured for the
Polish coal with the low-NOx burner. These are attributed to the much slower volatile release rate
observed for this sample in the early stages of combustion, with the effects possibly amplified by the
relatively small scale and/or the absence of adjacent burners. Under furnace air staging conditions, with
significantly more time available for devolatilisation, the Polish coal exhibits similar behaviour to the UK
coals. NOx measurements for the UK coal A are for a reduced thermal load (100 kW) in both combustion
modes. The value shown for combustion in the low-NOx burner has been corrected to full load (160 kW),
with an estimated uncertainty of +/- 50 ppm from the value shown. In the absence of sufficient load effect
data no correction has been applied to the combustion NOx measurements for coal A in furnace air staging
mode, although some increase from the plotted value can probably be expected.
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Combustion and Emissions Control II
With furnace air staging (0.9 primary zone stoichiometry) exit NOx levels are significantly reduced, but
the slope of the correlation line (i.e. the char Ν to NOx conversion ratio) remains approximately unchanged
at about 25%. The reduction is then apparently due almost entirely to a drop in the intercept of the
correlation line, corresponding to the combined volatile and thermal NOx contribution. Although more data
points are obviously required before firm conclusions can be drawn, if true this trend has several interesting
implications. With respect to aerodynamic air-staged low-NOx burners it implies that some volatile and
thermal NOx is being formed in the flame envelope that inevitably exists between the inner reducing
zone(s), corresponding to the fuel and primary air flow, and the secondary/tertiary air flows, That the level
of this NOx formation is not related to coal properties, together with previous results showing an increasing
sensitivity to excess air levels for burner designs with less effective internal air staging (13), suggests that
burner aerodynamics is the main determining factor. It also appears that in 'second generation' internally
air-staged low NOx burners, char nitrogen contents are already close to the minimum that can be achieved.
Hence, when furnace air staging is used to apply more severe (i.e. longer) heating during the reducing
stage, char nitrogen contents cannot be reduced significantly. Performance improvements can still be
derived, however, from the more complete conversion of potential volatile NOx precursors to N that can
2
be achieved before the addition of the secondary air, and possibly also from reductions in peak flame
temperatures leading to less thermal NOx.
CONCLUSIONS
A captive sample wire-mesh test has been developed which can apply coal devolatilisation conditions
comparable to those likely to be encountered in PF combustion. Comparison with results from full size
utility boilers has shown that both the bench-scale high temperature wire-mesh apparatus and the 160
kW test facility can give a good indication of large scale combustion performance.
The nitrogen content of wire-mesh chars from a range of world-traded and UK coals correlates with
measured NOx emissions from three different full-size boiler plants fitted with low-NOx burners, a 45
MW single burner rig and the 160 kW combustor, provided that sufficiently severe heating conditions
are employed for secondary devolatilisation of the char to proceed nearly to completion.
The wire-mesh apparatus test conditions (and by implication plant conditions) required for agreement
imply peak particle temperatures in the region of 1600-1800°C for feasible residence times. Results
also suggest that present 'second generation' internally air-staged low-NOx burners may already be
achieving close to the maximum possible release of nitrogen from the char under reducing conditions.
Convenient test conditions to give complete secondary devolatilisation and hence maximise rapid-
heating nitrogen release appear to be 10,000 K/s heating to 1600°C with 2 s hold. These conditions
achieve similar yields to 0.15 and 2 s hold at 1800°C.
A combination of measured wire-mesh char Ν contents and 160 kW combustor data suggest that
reductions in thermal/volatile NOx formation may be more significant than reductions in char Ν content
or char Ν to NOx conversion in contributing to the overall improvements in exit NOx levels that can
be achieved with furnace air staging.
ACKNOWLEDGEMENTS
The development of the wire-mesh apparatus was supported by the UK Science and Engineering Research
Council under Grants GR/F89817 and GR/J14127. Its application for industrial test purposes is currently
being investigated with funding from the UK Department of Trade and Industry under ETSU Contract
C/00/00078, Babcock Energy Ltd and National Power PLC. The authors are grateful to National Power
PLC and Babcock Energy Ltd for permission to incorporate plant and test rig combustion data in this paper
but emphasise that this does not necessarily imply an endorsement by these organisations of the coal testing
techniques described, nor the conclusions drawn.
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Combustion and Emissions Control II
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