Measuring The Gas Content of Coal
Coalbed gas content measurements are commonly used in mine safety as well as coalbed methane resource assessment and recovery applications. Gas content determination techniques generally fall into two categories: (1) direct methods which actually measure the volume of gas released from a coal sample sealed into a desorption canister and (2) indirect methods based on empirical correlations, or laboratory derived sorption isotherm gas storage capacity data.
The total gas content by the in direct methods is based on the empirical formula given by Meinser and Kim. The quantity of gas is determined by Meisner and Kim formula with using the moisture content, volatile content, volume of gas adsorbed on wet coal, fixed carbon, thickness of coal and temperature.
Meinser (1984) observed that the amount of methane gas (VCH4) is related to volatile matter (daf).
VCH4 = −325.6 × log (V.M/37.8)
Estimation of in-situ gas content of the coal will be evaluated by using Kim's (Kim 1977) equation
V = (100 −M − A) /100 ×
Where,
V = Volume of methane gas adsorbed (cc/g)
M = Moisture content (%)
A = Ash content (%).
Vw/Vd = 1/(0.25 ×M + 1)
Vw = Volume of gas adsorbed on wet coal (cc/g)
Vd = Volume of gas adsorbed on dry coal (cc/g)
The values of K and N depend on the rank of the coal and can be expressed in terms of ratio of fixed carbon (FC) to Volatile matter(VM)
K = 0.8 (F.C /V.M) + 5.6 Where
F.C = Fixed carbon (%)
VM = Volatile matter (%)
N = Composition of coal (for most bituminous coals, N = (0.39 - 0.013 × K)
b =Adsorption constant due to temperature change (cc/g/◦C).
T = Geothermal Gradient × (h/ 100) + To
T = Temperature at given depth
To = Ground temperature
h = Depth (m)
Estimation of methane content in coal seams by Karol curve
In the absence measured methane content of coalbeds, and production data from coalbed methane wells, gas content can be estimated using the Eddy curve. Eddy and other construct a series of curve the give the estimated maximum producible methane content of coalbed as a function of depth and rank.
The estimation of methane content of a coalbed is determined from the Eddy curve by locating the average depth of each coal seam on the depth axis. A normal line is extended upward from the depth axis (feet) to intersect the specific coal rank curves. A line from the point on the curve is extended normal to the lost and desorbed gas axis(cm3/gm). The intersection of the line and the axis is the estimated methane content of the coal seam.
Read more about this topic: Coal Bed Methane Extraction
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