International Journal of Mining Engineering, 1983, 1, 71-77
TECHNICAL NOTE
Nitrogen and carbon dioxide occurrence in UK coal seams Introduction
The principal constituents of firedamp, the combustible gas evolved from coal seams, namely hydrocarbons, carbon dioxide and nitrogen, can be accommodated in substantial quantities on the internal surfaces of the coal material. At extreme pressures bituminous coals haw~ the potential to absorb of the order of 20 m 3 t -1 methane, 10 m 3 t -1 nitrogen or 30 m 3 t -1 carbon dioxide (CEC, 1980). Early investigations by Graham showed that air-flee gaseous mixtures emitted from coal seams may contain up to 5% carbon dioxide and on average about 10% by volume of nitrogen (Graham and Shaw, 1927). Although carbon dioxide must have been produced during the formation of coal from original vegetation, there is no evidence that large quantities are retained in mature, bituminous coal seams. The generally accepted explanation of exceptionally high carbon dioxide concentrations such as those recorded in Silesia and southern France is that they were introduced from deep-seated faults or intrusive igneous sources (Patching, 1970). Analyses of firedamp drained from coal seams often show a wide variation in nitrogen content even after allowing for the quantities associated with any oxygen in the proportions found in air. However, because some of the oxygen may have reacted with coal it cannot be discerned how much of the excess nitrogen was initially derived from air. That nitrogen is present in coal seams is nevertheless indisputable owing to its occurrence under pressure in natural gases derived from coal seams. The principal aim of this study was to evaluate the quantities of nitrogen and carbon dioxide contained in UK seams in general. In particular, the possibility of nitrogen being emitted as a result of coal extraction was considered worthy of investigation as a factor relevant to the interpretation of oxygen deficiency in mine air. Oxygen deficiency calculations presuppose all nitrogen in mine airways to have originated from the atmosphere. Such calculations are used in determining the carbon monoxide/oxygen deficiency ratio for monitoring spontaneous combustion, and mine climate studies may also require a measure of oxygen deficiency as a basis on which to estimate heat produced through oxidation. This investigation was greatly aided by the extensive National Coal Board surface borehole exploration programme, which enabled flesh samples of coal core to be gathered from a number of different coalfields. A technique developed for measuring the hydrocarbon content of coal seams (Creedy, 1978) was extended to include the determination of
Key words: Coal mining; mine gases; mine ventilation
72
Creedy and Pritchard
gaseous nitrogen, carbon dioxide and oxygen. The sampling procedure involved sealing pieces of surface borehole core or coalface lumps in gas-tight Perspex vessels, then purging with helium to displace atmospheric gases.
Calculation of gas evolved
The total initial quantities of the various gases evolved from a sampled coal lump were obtained as the sum of the volumes of gases evolved during rate of desorption measurements, plus the estimated quantity of gas evolved before sampling and the volumes of gas remaining in the sample on completion of the rate of desorption measurements. The calculation of the quantity of each gas lost prior to sampling (V~) was achieved by using a method of least squares to fit the desorption results to a simplified form of Airey's empirical degassing equation, namely V s = (Vo/t~/2)tl/2- V i
(1)
where Vs represents the cumulative quantities of gas evolved into the desorption vessel, V0 is the initial gas content, t is the total desorption time from the instant the sample began to degas in the borehole, and t k is a time constant index of emission. The time constant of emission, which is used in firedamp prediction formulae, is defined as the time in which 63% of the volume of the gas is desorbed (Airey, 1968). The time constant index t k calculated from Equation 1, although not equivalent to Airey's time constant, is nevertheless a useful parameter for comparing sample gas emission rates. Measurements on surface borehole core samples and coalface lumps were carried out in a similar manner, except that with the latter no attempt was made to estimate the quantities of gas lost before sampling, the time at which degassing was initiated being an unknown factor.
Nitrogen and carbon dioxide contents determined from surface borehole cores
Seam nitrogen and carbon dioxide contents were determined on coal cores from boreholes in the Lancashire, Nottinghamshire, Selby, Staffordshire and Warwickshire coalfields. Of the boreholes visited, Darlaston, situated near Stone (Staffordshire), was the most comprehensively sampled. Nitrogen content seemed invariant to depth. However, at Brookhouse Farm borehole (Lancashire) nitrogen content increased by about 0.07 m 3 t -~ per 100 m but, because of problems with atmospheric contamination during the analysis of this sample set, such a result may be merely fortuitous. Averaging all the data (73 samples) yielded a mean nitrogen content of 0.47 m 3 t -1 with a standard deviation of 0.15 m 3 t -~. The average of the measured carbon dioxide contents was 0.13 m 3 t -1 with a standard deviation of 0.08 m 3 t 1. Carbon dioxide content appeared to decrease markedly with depth as illustrated in Fig. 1.
Nitrogen and carbon dioxide occurrence in U K coal seams
73
100.5-
~ }
tO-
c"4
0.4-
" 0.5-
0.1-
%
o 02-
0.1-
66~
860
1060 depth{m}
lZbO
0.0010.1
1.0
Fig. 1. Variation of carbon dioxide content with depth, Darlaston Borehole.
10
100
1000
Desorption time (h)
Seam
Fig. 2. Desorption of seam gases from two freshly sampled coal cores.
Rates of emission of firedamp constituents from coal samples Desorption measurements were made over a number of weeks on some freshly sampled coal cores. To facilitate comparison of the degassing rates of nitrogen, carbon dioxide, methane and ethane, the data was fitted to Airey's (1968) full empirical emission equation: ~Zt
=
V0{1 - exp[-(t/to)n]}
(2)
where Vt is the volume of gas emitted in time t from coal of initial gas content V0, to is the time constant of emission and n an emission constant. The experimental data fitted the empirical emission equation extremely well; correlation coefficients and calculated emission parameters are listed in Table 1. The desorption curves are shown in Fig. 2. Values of the emission constant n (Table 1) are greater than theory predicts; however, theory takes no account of possible interactions between different gases. The fact that the coal was contained in a helium atmosphere may also have influenced the emission characteristics. The results show that nitrogen is emitted at a greater rate from coal than methane or ethane. The emission of carbon dioxide from coal has not been considered in detail as the quantities measured were small and hence any desorption results are likely to have large uncertainties. Nevertheless, indications are that carbon dioxide desorbs at a slower rate than nitrogen.
Nitrogen emission from coal seams The rate at which nitrogen is emitted from broken coal will depend on the composition of the surrounding gas. Within crack systems in the coalface, nitrogen will desorb readily and
Creedy and Pritchard
74 Table 1. Desorption of gases from coal cores into a helium atmosphere.
Sample no.
Gas
Correlation* coefficient
Methane Ethane Nitrogen Carbon dioxides
0.9992 0.9910 0.9973 0.9937
669 8504 242 228
0.64 0.64 0.74 0.63
6.64 0.82 0.36 0.07
Methane Ethane Nitrogen Carbon dioxides
0.9996 0.9869 0.9950 0.9875
1063 10725 682 264
0.56 0.56 0.80 0.68
6.67 0.80 0.64 0.09
t0(b )
n
G a s content (m 3 t-1)t
*Indicates suitability of fit of measured data to a linearized form of Airey's emission equation. ?Ash-free basis. SVery low concentrations involved therefore large uncertainty in the results. Table 2. Gas content measurements on coalface samples. Degree of degassing (%)
Gas content (m 3 t-l) * Colliery
Seam
N2
CO 2
CH 4
C2H 6
CH 4
Net
Kellingly
Beeston
0.14 0.17 0.22 0.22
0.02 0.02 0.02 0.01
3.75 5.41 4.30 4.52
0.63 0.77 0.77 0.73
34 5 24 20
72 66 56 56
Sherwood
Deep Hard/Piper
0.01
--
2.71
2.51
46
98
Sutton Manor
Wigan 4 ft
0.07 0.07 0.14
0.08 0.06 0.07
5.29 5.52 7.32
1.43 1.53 1.55
29 26 2
86 86 72
*Ash-free basis. ?Assumes initial nitrogen content = 0.5 m 3 t -1. be flushed out by the large volume of desorbing methane. The partial pressure of nitrogen will thus be negligible and given sufficient time all the nitrogen will be emitted. A similar desorption e n v i r o n m e n t m a y prevail within piles of freshly cut coal. However, when exposed to mine air, fine coal in particular m a y re-adsorb nitrogen f r o m the atmosphere. The partial pressure of nitrogen in the a t m o s p h e r e is about 0.08 M P a and, were sorption equilibrium established, bituminous coals would contain approximately 0.3 m 3 t -1 of nitrogen, although low rank coals ( > 3 6 % volatile matter) m a y only adsorb half that
Nitrogen and carbon dioxide occurrence in U K coal seams
75
100-
50-
0
0
5'0
160
CH4 degassed(°/o)
Fig. 3. Relationship of degree of methane loss to degree of nitrogen loss from coalface lumps.
volume. For purposes of comparison it may be noted that, depending on rank, coal can adsorb 21-4 times more methane than nitrogen at a given gas pressure. Ratios of methane time-constant indices to nitrogen time-constant indices calculated from the simplified emission, Equation 1, lay in the range 2.5-2.8 for the Selby, Nottinghamshire and Lancashire samples and 3.5-4.1 for the Warwickshire and Staffordshire samples. Regional differences in the ratios may perhaps reflect variations in coal properties or firedamp composition. However, nitrogen is clearly more readily emitted from coal than methane. By writing Equation 1 first in terms of nitrogen, then in terms of methane, dividing the two expressions and assuming a typical initial nitrogen content of 0.5 m 3 t -t, it can be shown that after any period of time within the first few days of emission the volume of nitrogen emitted equals the volume of methane emitted/initial methane content × f ( a factor which depends on the ratio of the time-constant indices of the two gases, has a value in the range of 1 to 1.3). Theoretical studies (Airey, 1982) indicate that considering all sizes of coal, about 65 per cent of the initial methane content is emitted before the coal leaves its district of origin. The implication of inserting this result into the empirical expression relating nitrogen and methane emissions is that most, if not all, of the inherent nitrogen content of the coal is released into the airway of the working district. This conclusion was somewhat tentative because the empirical expression is not strictly applicable either to desorption for prolonged periods or to small sizes of coal and was derived using measurements on gases desorbing into a helium atmosphere. Corroborative evidence was gathered by measuring the gas contents of some coalface lumps (Table 2) and plotting nitrogen loss against methane loss as percentages (Fig. 3). A curve drawn from the origin through the plotted points indicates that a coalface lump which has emitted about half of its methane will have lost practically all of its nitrogen. Assuming for practical purposes that cut coal desorbs all of its nitrogen on the coal face and in the return airway, and making generous allowance for the fact that additional nitrogen may be discharged from adjacent seams, the net contribution to the concentration of nitrogen in the outbye return air is likely to be less than 0.1%.
76
Creedy and Pritchard
Nitrogen enrichment and oxygen deficiency in mine air An occurrence of spontaneous combustion within a mine may be first detected from a rise in carbon monoxide and a reduction in oxygen relative to its natural proportion in air. The CO/O2 deficiency ratio, if interpreted correctly, may be used to monitor the progress of a heating (Coles and Thirlaway, 1956; Criddle, 1961). Provided that oxygen deficiency is significantly greater than 0.03% (the error introduced as a result of 0.1 per cent extraneous nitrogen), nitrogen emitted from coal probably does not contribute a significant error. Nitrogen emission from coal is not the only process that may cause nitrogen and oxygen proportions in mine air to deviate from those normally found in the atmosphere. Throughout the mine, chemical reactions connected with corrosion of metals and oxidation of coal, especially freshly cut fine coal, will tend to remove oxygen from the ventilation air and yield varying quantities of carbon monoxide and carbon dioxide. From mine air analyses alone it would, therefore, be difficult to deduce whether nitrogen enrichment was due to addition of seam nitrogen or removal of atmospheric oxygen. Isolation of the latter component by attempting to monitor the oxidation products CO2 and CO would be difficult due to the difficulty of accounting for CO2 release from coal, the solubility of CO2 in water, possible adsorption of the gases on coal, fixation or release of the gases by bacterial action and expiration of CO2 and CO by miners.
Conclusions Some bituminous coals, mostly of high volatile rank, sampled from various coalfields contained on average 0.5 m 3 t -1 nitrogen and 0.13 m 3 t -1. Nitrogen is released more readily than methane during coal winning but the volumes of nitrogen released are small compared with ventilation quantities and are only likely to be of significance for detailed oxygen deficiency studies.
Acknowledgements The authors wish to thank Mr P.G. Tregelles, Director of Mining Research and Development of the National Coal Board for permission to publish this paper. Grateful thanks are expressed to the NCB geologists, together with the NCB's and contractor's drilling teams, for their willing co-operation and assitance in the sampling of surface boreholes. Thanks are also due to colleagues at M R D E for their assistance with sampling and laboratory work. Opinions expressed in this paper are the authors' and not necessarily those of the National Coal Board.
Nitrogen and carbon dioxide occurrence in UK coal seams
77
References Airey, E.M. (1968) Gas emission from broken coal: An experimental and theoretical investigation, International Journal of Rock Mechanics and Mineral Sciences 5, 475-94. Airey, E.M. (1982) Private communication. Coal Directorate of the CEC (1980)Firedamp Drainage Handbook, Verlag Glfickauf, Essen. Coles, G. and Thirlaway, J.T. (1956) The disappearance of CO in the mine. Transactions of the Institution of Mining Engineers 115, 767-87. Creedy, D.P. (1978) Gas content variations in coal seams, MSc thesis, University College, Cardiff. Criddle, S.J. (1961) The CO/O 2 ratio and spontaneous combustion. Colliery Guardian 202, 664-6. Graham, J.I. and Shaw, A. (1927) The composition of the gaseous mixture given off from coal, Transactions of the Institution of Mining Engineers 73, 529-37. Patching, T.H. (1970) The retention and release of gas in coal - a review. CIM Transactions 73, 328-34.
National Coal Board Mining Research and Development Establishment Burton on Trent, UK Received 14 D e c e m b e r 1982
D.P. CREEDY F.W. PRITCHARD
