A CART technique to adjust production from longwall coal operations

under ventilation constraints

C. Özgen Karacan ⇑,

⇑ Corresponding author. Tel.: +1 412 386 4008.
 

E-mail address: cok6@cdc.gov (C.Ö. Karacan).
 

Gerrit V.R. Goodman

National Institute of Occupational Safety and Health (NIOSH), Office of Mine Safety and Health Research (OMSHR), Dust Control, Ventilation, and Toxic Substances Branch,

Pittsburgh, PA 15236, United States

abstract

Methane emissions in longwall coal mines can arise from a variety of geologic and production factors,

where ventilation and degasification are primary control measures to prevent excessive methane levels.

However, poor ventilation practices or inadequate ventilation may result in accumulation of dangerous

methane–air mixtures. The need exists for a set of rules and a model to be used as guidelines to adjust

coal production according to expected methane emissions and current ventilation conditions.

In this paper, hierarchical classification and regression tree (CART) analyses are performed as nonparametric modeling efforts to predict methane emissions that can arise during extraction of a longwall panel.

These emissions are predicted for a range of coal productivities while considering specific operational,

panel design and geologic parameters such as gas content, proximate composition of coal, seam height,

panel width, cut height, cut depth, and panel size. Analyses are conducted for longwall mines with and

without degasification of the longwall panel. These models define a range of coal productivities that

can be achieved without exceeding specified emissions rates under given operating and geological

conditions.

Finally, the technique was applied to longwall mines that operate with and without degasification system to demonstrate its use and predictive capability. The predicted results proved to be close to the

actual measurements to estimate ventilation requirements. Thus, the CART-based model that is given

in this paper can be used to predict methane emission rates and to adjust operation parameters under

ventilation constrains in longwall mining.

1. Introduction

Methane emissions during longwall mining can adversely affect

the safety of underground coal miners. The combination of methane emitted from the coal transported out of the mine by the conveyer belts and methane emissions from the face and ribs is

directly discharged into the mine ventilation air. The methane

emitted from fractured strata, if not controlled by existing degasification methods, can migrate to the caved zone and finally to the

ventilation system. Therefore, the ventilation system must be

appropriately designed based on the existence of any existing

degasification system and must have sufficient capacity to maintain methane levels within statutory limits.

The ability to predict methane emissions is important to provide adequate ventilation air to dilute and render harmless high

gas levels that threaten mine safety. However, accurate prediction

of methane inflows into the ventilation system is complex due to

the large number of variables that impact these potential emission

sources. These variables include, but are not limited to, lithology

and stratigraphy of the mining horizon, depth of the mined coalbed, seam thickness, coalbed reservoir properties, gas content coal

seam, coal rank, longwall panel dimensions, face advance rates,

existence of any methane drainage, conveyor speed, and timedependent changes in these parameters.

Due to the complexity and number of parameters that can influence the magnitude of methane emissions, it is not easy to model

these emissions in a longwall environment. Lunarzewski (1998)

suggested an empirical model and linked methane emissions to

different stages in the life of a coal mine and to coal production,

which were regressed to measured emissions using two empirical

constants. This approach summarized methane emissions as a

function of coal production and life of the mine only and combined

other important variables into empirical constants. Although simple to use, this approach was site-specific due to the empirical

constants and required a long period of data collection before a

correlation could be established for predictions.

Numerical modeling efforts, either based on boundary element

(Lunarzewski, 1998), finite element (Tomita et al., 2003), or finitedifference reservoir models (Karacan et al., 2005, 2007) have
improved the prediction of methane emissions. In addition to the

efforts in predicting in-mine emissions, several studies have also

investigated performance of vertical and horizontal boreholes in

reducing gas content of coal seams for controlling methane gas

in underground coal mines (e.g. Keim et al., 2011; Packham et al.,

2011; Sang et al., 2010). An extensive review of coal mine methane,

its control and prediction techniques and their importance for coal

mine safety and greenhouse gas production is given in Karacan et

al. (2011).

However, numerical models require expertise and, in most

cases, specialized and expensive software packages In order to address that challenge, Karacan (2008, 2009) developed an artificial

neural network (ANN) and principle component analysis (PCA)

based approach to model and predict ventilation methane emissions from US longwall mines, to diagnose the need for degasification, and to suggest the best degasification combination for an

operation. These models have been published as methane control

and prediction software (MCP), which can be downloaded at the

following site: http://www.cdc.gov/niosh/mining/products/product180.htm (Karacan, 2010). These modeling techniques predict

methane emissions and perform sensitivity analyses to adjust

ventilation or coal production quantities. However, they are mostly

implicit in that the functional relationships between various

parameters and outcomes are either too complex or not defined

in a simple way for visual recognition by the user. Hence, this

paper employs a nonparametric modeling approach, CART (classification and regression tree) analyses, to define the relationships

between methane emissions levels and their causative factors so

that production can be adjusted to not overwhelm the existing

ventilation.

2. Data set used in CART modeling of longwall methane

emissions

Classification and regression tree (CART) analysis is a nonparametric modeling technique that produces ‘‘trees’’ that are formed

by a collection of rules based on the values of certain variables in

the modeling data set. The data set used in this work is similar

to the data set used in Karacan (2008, 2009) and was compiled

from the same sources described in those references. That work

also gives a detailed discussion of these variables and how they

may impact both the emissions of methane into the ventilation

system and the selection of an appropriate degasification system.

An effective degasification system, although of secondary

importance to ventilation for controlling methane emissions,

can greatly affect an operation’s productivity. Therefore, the data

set used in this work was first searched for the existence of a

degasification system in the operations that were included in

the database. Based on this information, the database was categorized into two groups for separate CART analyses: (1) cases where

no degasification system was employed and (2) cases where at

least one or a combination of gob gas ventholes, horizontal

borehole (either in-mine or surface), and vertical degasification

borehole was used. The number of cases included 126 longwall

mines without degasification and 354 longwall mines with

degasification.

Tables 1 and 2 give the parameters and their ranges used in the

current CART modeling work for mines without degasification and

with degasification, respectively.

Table 1

Variables used for CART analyses for longwall mines without degasification. The table gives basic statistics conducted on 126 samples.

Variable and unit Definition Minimum Maximum Mean Std. deviation

SH (in) Seam height 39.0 96.0 72.3 14.7

CH (in) Cut height 53.0 120.0 74.4 15.9

PW (ft) Panel width 400.0 1100.0 799.8 151.0

PL (ft) Panel length 3000.0 13000.0 7625.0 2470.7

OB (ft) Overburden depth 300.0 1850.0 671.3 335.0

ENT Number of gateroad entries 2 5 – –

CD (in) Cut depth 29.0 42.0 32.8 3.6

FCS (ft/min) Face conveyor speed 215.0 357.0 267.8 34.6

SLS (ft/min) Stage loader speed 247.0 517.0 329.4 44.9

T-GAS (scf/ton) Total gas content of coal 9.6 249.6 123.2 58.7

SULP (%) Sulfur content of coal 0.6 4.5 2.0 1.1

LD-GAS (scf/ton) Lost and desorbed gas content 6.4 211.2 83.9 44.1

R-GAS (scf/ton) Residual gas content 3.2 96.0 39.3 29.0

HC (BTU/lb) Heat content of coal 10,300 13,300 12,200 720

ASH (%) Ash content of coal 4.0 15.0 8.2 3.4

CP (M tons/day) Daily coal production 0.3 20.0 7.7 4.3

Variable Minimum Maximum Average Std. deviation

Ventilation methane emission (MM scf/day) 0.1 10.3 2.5 2.3

To eliminate the site dependency

of the current work, variables dealing with geographical locations

were excluded from the data set and other variables were included

to improve data variability and predictive capability. These new

variables were ash content of coal, sulfur content, heat value,

overburden depth, panel length, cut depth, lost and desorbed gas

content, and residual gas content. Ash and sulfur contents are

complementary information to gas content (higher ash and sulfur

usually indicates lower gas contents). Overburden depth and heat

value are indicative of caving pattern (together with panel width)

and rank (high heat value indicates higher rank), respectively. Cut

depth affects production rate while panel length affects methane

reservoir size.

Simple analyses between the mean values of parameters in

Tables 1 and 2 show how these values differ with and without

the presence of a degasification system (Fig. 1). Mean methane

emissions when using a degasification system are 172% higher

than mines without a degasification system. Although this sounds

paradoxical, it can be interpreted that mines operating at greater

depths, in higher rank coals, and in gassier seams usually have

higher emissions and, consequently, use degasification to supplement their main ventilation system. The larger average differences

between degasification (D) and no degasification (ND) for overburden (OB, 69.3%), coal height (HC, 7.7%), total gas content (T-GAS,

115.3%), and lost and desorbed gas content (LD-GAS, 155.3%) prove

this comment. Moreover, mines with a degasification system and

with higher emissions have higher coal production (23.4%) and

operate in thicker seams (SH, 14.9%) with increased cut depths

(CD, 7.6%) and with faster face conveyors (SLS, 10.6%; FCS, 5.5%).

One exception is that average sulfur content is 15% lower for

operations using degasification because higher rank coals are usually associated with lower sulfur contents.
Table 2

Variables used for CART analysis for longwall mines with degasification. The table gives basic statistics conducted on 354 samples.

Variable and unit Definition Minimum Maximum Mean Std. deviation

SH (in) Seam height 50.0 276.0 83.1 37.6

CH (in) Cut height 43.0 156.0 79.3 19.2

PW (ft) Panel width 465.0 1060.0 832.0 145.5

PL (ft) Panel length 1400.0 13000.0 7682.7 2348.7

OB (ft) Overburden depth 400.0 2700.0 1136.8 508.1

ENT Number of gateroad entries 3 5 – –

CD (in) Cut depth 27.0 44.0 35.3 4.4

FCS (ft/min) Face conveyor speed 187.0 450.0 282.4 42.9

SLS (ft/min) Stage loader speed 220.0 600.0 364.3 67.6

T-GAS (scf/ton) Total gas content of coal 70.4 585.9 265.3 136.8

SULP (%) Sulfur content of coal 0.4 4.3 1.7 1.1

LD-GAS (scf/ton) Lost and desorbed gas content 44.8 542.1 214.1 153.2

R-GAS (scf/ton) Residual gas content 6.4 97.4 51.2 28.8

HC (BTU/lb) Heat content of coal 9400 14,900 13,100 1025

ASH (%) Ash content of coal 4.0 29.0 9.3 5.5

CP (M tons/day) Daily coal production 0.5 28.2 9.5 5.8

Variable Minimum Maximum Average Std. deviation

Ventilation methane emission (MM scf/day) 0.1 19.2 6.8 4.2

Fig. 1. Percentage differences between mean values of variables in mines operating

with a degasification system (D) and without a degasification system (ND).

3. CART modeling technique

3.1. Brief description of CART

CART represents a computational–statistical algorithm that

generates predictions in the form of a decision tree. The CART

procedure can be defined as a method of partitioning data into terminal nodes (child nodes) by a sequence of binary splits starting at

a parent node. Binary split means that each node has the potential

of splitting into only two new nodes within a split level. CART

repeats the partition for each child node, continuing recursively

until the homogeneous level in the required generic node is obtained or a given stopping criterion is verified. Typically, the modeling algorithm will stop if either the maximum tree depth set by

the user is reached or if no more splits can be made because there

is no significant predictor variable left to split the node.

The CART splitting algorithm in each node is based on the concept that each child node must be more ‘‘pure’’ than the original

parent. ‘‘Pure’’ is a concept linked to the values of a given variable

which leads to zero variance between the splitting steps (Bevilacqua et al., 2003). The splitting procedure builds a tree structure

based on a set of ‘‘if–then’’ rules that guide the decision maker.

In starting the analyses and decision making, the variable choices

and specifications should be causal and rational, just as with any

traditional statistical methods.

The decisions using CART modeling are usually better than

simple regressions because the variables are allowed to change

based on hierarchical prioritization and are also allowed to interact

and have different values under different conditions. CART modeling is used in materials science (Li, 2006), mechanics (Bevilacqua

et al., 2003), clinical epidemiology (Li and Rapkin, 2009), intensive

care medicine (Abu-Hanna and de Keizer, 2003), and modeling of

environmental contaminants (Vega et al., 2009).

3.2. Application of CART in modeling methane emissions and

validation methods

In this work, all of the variables listed in Tables 1 and 2 were

used for CART modeling of emissions from longwall mines operating without (ND) and with (D) a degasification system, respectively. Methane emissions from the ventilation system were

selected as the dependent variable and various maximum tree

depths (level of branching) were implemented to find the optimum

tree size. During these trials for both categories, tree depths ranging from 5 to 10 were employed. For interpretation of results and

cross validation of the predictive capability of various tree models,

predicted results were tested with Kolmogorov–Smirnov (KS)

goodness-of-fit tests with a 5% significance level (a) and with

regression analyses using a 95% confidence level.

Tables 3 and 4 give the results of regression analyses and KS

tests for goodness-of-fit tests between the original data and the

predicted values from model building and cross-validation. Tree

depths of 8 and 10 were employed in Tables 3 and 4, respectively

as these values were determined to be the best depths for CART

modeling of methane emissions. The d- and p-values are indicative

parameters of the KS test determining whether the null hypothesis,

that the data follow a specified distribution with a significance

level (a), should be accepted or rejected. The hypothesis regarding

the proposed distribution is rejected if the d-value test statistic is

greater than the critical p-value obtained for the corresponding

significance level (a) and if the critical p-value is less than the
significance level. If the null hypothesis is accepted, the value of

‘‘p’’ is the risk of rejecting that hypothesis. In this work, the null

hypothesis states that the distribution of CART-predicted emission

values is close to that of the distribution of actual values. Tables 3

and 4 indicate acceptance of the null hypothesis meaning that the

data predicted by CART analyses adequately represent the distribution of methane emission data used for modeling and crossvalidation.

Table 3

Regression and Kolmogorov–Smirnov (KS) goodness-of-fit test statistics of predicted data with the original data in model building and crossvalidation steps. MSE and RMSE stand for mean square error and relative mean square error, respectively. Tree-depth of 8 was selected to predict

methane emissions from mines using a degasification system in their operation.

Two-sample Kolmogorov– Model building Cross-validation Regression Model building Cross-validation

Smirnov test results results test results results

d-Value 0.062 0.233 R2 0.95 0.86

p-Value 0.450 0.384 MSE 1.728 3.040

Null hypothesis Accept Accept RMSE 1.314 1.744

Table 4

Regression and KS goodness-of-fit test statistics of predicted data with the original data in model building and cross-validation steps. MSE and RMSE

stand for mean square error and relative mean square error, respectively. Tree-depth of 10 was selected to predict methane emissions from mines

operating without a degasification system installed.

Two-sample Kolmogorov– Model building Cross-validation Regression Model building Cross-validation

Smirnov test results results test results results

d-Value 0.059 0.433 R2 0.96 0.84

p-Value 0.515 0.515 MSE 0.401 0.676

Null hypothesis Accept Accept RMSE 0.633 0.822

The CART-predicted data and the actual methane emissions

data were compared using linear regression. These analyses were

conducted for both model building and cross-validation phases.

The purpose of this approach was to find the regression coefficient

(R2

) between actual and predicted values, which under ideal conditions should be 1, mean square error (MSE), and relative mean

square error (RMSE). The right-hand sections of Tables 3 and 4

show the results of regression tests which indicate high regression

coefficients (>0.8) between predicted and actual values as well as

low MSE and RMSE. These tree depths produced accurate predictions of methane emissions, but at the expense of generating

unnecessarily large trees with many splits (rules).

Figs. 2 and 3 show the results given in Tables 2 and 3 for regression between actual and predicted values in CART building and

KS-test, respectively.

Fig. 2. Comparison of actual and CART-predicted methane emissions for mines operating with (A) and without (B) degasification. The regression coefficients are based on

both model building and cross-validation analyses.

Fig. 3. Comparison of cumulative relative frequency distributions of actual and predicted methane emission data in two-sample KS goodness-of-fit test for mines operating

with (A) and without (B) degasification.

In each of these figures, (A) plots are for the

methane emissions from mines that utilize degasification system

and (B) figures are for the ones that do not use degasification at

all in support to ventilation. These figures and Tables 3 and 4, show

that the actual and predicted values and their cumulative relative

frequency distributions are close to each other.
Table 5

The rules extracted from the terminal nodes of the 8-level CART procedure for predicting methane emissions from longwall mines using a degasification system (D).

Rule Ventilation SH CH PW PL OB ENT CD FCS SLS T-GAS SULP LD-GAS R-GAS 

methaneemission(MM scf/day) (in) (in) (ft) (ft) (ft) (in) (ft/min) (ft/min) (scf/ton) (%) (scf/ton) (scf/ton) 

1 0.105 117–132 0.4–0.9 44.8–145.6 

2 0.400 2850–5785 2.9–4.25 44.8–145.6 

3 0.625 96–117 730–820 0.4–0.9 44.8–145.6 

4 0.750 69–72 5785–10,000 4.0 2.9–4.25 44.8–145.6 

5 1.033 1.3–1.5 44.8–145.6 

6 1.100 96–117 610–730 0.4–0.9 44.8–145.6 

7 1.200 923.5–947 0.9–2.9 44.8–75.134 

8 1.600 1.3–1.5 44.8–145.6 

9 1.700 64–69 5785–10,000 4.0 2.9–4.25 44.8–145.6 

10 1.800 58–76 630–730 280–382 44.8–145.6 

11 1.800 5350–10,000 1.5–3 44.8–145.6 

12 1.809 568–923.5 0.9–2.9 44.8–75.134 

13 1.857 5785–10,000 4.0 2.9–4.25 44.8–145.6 

14 1.900 625–750 220–233 0.9–2.9 75.134–145.6 

15 2.033 76–80 630–730 280–382 44.8–145.6 16 2.233 3300–5625 37.5–44 198.4–542.12 

17 2.550 75–79.5 4.0 38.5–44 435–447.5 2.185–3 44.8–145.6 18 2.860 730–1000 8000–9150 280–382 44.8–145.6 19 3.350 585–625 220–233 0.9–2.9 75.134–145.6 

20 3.480 730–1000 9150–10,000 280–382 44.8–145.6 

21 3.667 4200–5350 1.5–3 44.8–145.6 

22 3.825 75–79.5 4.0 38.5–44 447.5–460 2.185–3 44.8–145.6 23 3.867 233–260 0.9–2.9 75.134–145.6 24 3.867 7125–11,500 43–44 198.4–542.12 

25 3.900 862.5–1025 3.0 38.5–44 509–550 2.185–3 44.8–145.6 26 4.050 700–862.5 3.0 38.5–44 509–550 2.185–3 44.8–145.6 27 4.257 400–1400 198.4–542.12 

28 4.333 730–1000 280–382 44.8–145.6 29 4.462 1.5–3 44.8–145.6 

30 4.675 650–850 30–38.5 212–335 44.8–145.6 

31 5.100 79.5–84 4.0 38.5–44 2.185–3 44.8–145.6 

32 5.540 72–76.5 3.0 38.5–44 280–509 2.185–3 44.8–145.6 33 5.800 70.5–76 3.0 38.5–44 1.7–2.185 44.8–145.6 34 5.883 76.5–78 38.5–44 280–509 2.185–3 44.8–145.6 35 5.900 750–825 650–850 30–38.5 212–335 44.8–145.6 36 6.271 382–467 44.8–145.6 37 6.500 66–70.5 3.0 38.5–44 1.7–2.185 44.8–145.6 38 6.620 850–1100 30–38.5 212–335 370–420 44.8–145.6 39 6.733 4250–4900 28–37.5 258.5–350 198.4–542.12 

40 6.824 1400–2700 198.4–542.12 

41 6.950 8000–9500 850–1100 30–38.5 212–335 44.8–145.6 42 7.075 4.0 38.5–44 1.7–2.185 44.8–145.6 43 7.143 825–1000 650–850 30–38.5 212–335 44.8–145.6 44 7.533 65–69 28–37.5 220–258.5 428.865–542.12 

45 7.580 54–67.5 4900–7500 28–37.5 258.5–350 198.4–542.12 

46 7.704 7125–11,500 37.5–43 198.4–542.12 47 7.970 850–1100 30–38.5 212–335 287–370 44.8–145.6 

48 8.100 76–81 975–1050 30–38.5 335–400 44.8–145.6 

49 8.529 4950–6750 575–1475 28–37.5 258.5–350 198.4–542.12 

50 8.750 7100–11,300 1475–2050 28–37.5 258.5–350 198.4–542.12 

51 9.313 9500–12,000 850–1100 30–38.5 212–335 44.8–145.6 

52 9.650 67.5–72 4900–7500 28–37.5 258.5–350 198.4–542.12 
53 10.233 70–76 975–1050 30–38.5 335–400 44.8–145.6 12,800–14,040 8.904–28.219

54 10.786 96.5–111 582.5–950 28–37.5 220–258.5 198.4–542.12 3.973–19.452

55 10.967 6750–8000 575–1475 28–37.5 258.5–350 198.4–542.12 6.164–19.452

56 11.150 5625–7125 37.5–44 198.4–542.12 3.973–19.452

57 11.525 930–975 30–38.5 335–400 44.8–145.6 12,800–14,040 8.904–28.21958 12.529 4000–7100 1475–2050 28–37.5 258.5–350 198.4–542.12 6.164–19.452

58 12.575 65–69 28–37.5 220–258.5 198.4–428.865 3.973–19.452

60 12.636 69–96.5 582.5–950 28–37.5 220–258.5 198.4–542.12 3.973–19.452

61 16.050 69–111 560–582.5 28–37.5 220–258.5 198.4–542.12 3.973–19.452

4. Rule-based results from CART modeling and discussions

CART models with tree depths of8 and 10 were selected for

mines with degasification (D) and without degasification (ND),

respectively. These models resulted in 127 rules forD and 57 rules

for ND, where these rules are the total number of rules for both

internal nodes and for terminal nodes. As stated earlier in Section

3.1, the decisions in the CART technique depend on the internaland terminal-node concept, and the final rules are based on how

terminal nodes evolve from internal ones. At each level of split,

the internal nodes produce two child nodes asa result ofa binary

split. Duringa binary split, the variance in the data is minimized in

sucha way that the child nodes of the previous level become

parents in the next level until the level of variance in the data

cannot support another split. This is the level that establishesa

‘‘terminal node’’ and is the level of least variance in the data.

4.1. Rules and results for predicting emissions for mines usinga

degasification system (D)

As mentioned before, the terminal nodes contain the most

homogeneous data and are the ‘‘purest’’ in the CART. Out of 127

nodes and associated rules that describe the conditions under

which certain methane emission levels could be expected, only

the terminal nodes (61 nodes) were selected for rule-based decision making and interpretation. Each terminal node is associated

witha set of ‘‘rules’’ that recorda sequence of splitting criteria that

lead to the formation of that specific node. These rules can predict

methane emissions intoa ventilation system while the interactions

of different predictors can be analyzed and interpreted.

Table5 gives all 61 rules with parameter intervals resulting in

various predicted methane emissions levels froma longwall mine

usinga degasification system for different coal productivities. This

table is formatted so that the rules are given in rows in the direction of increasing methane emission. Also, the empty cells in the

table indicate that those parameters are not included in the

corresponding rules as ‘‘rule-making’’ variables during the CART

procedure. Qualitatively, this table shows that the coal quality

parameters related to gas content, coal rank, and coal production

are included in almost all rules; whereas cut depth and conveyor

speed become more prevalent at methane emissions exceeding

1.9 MM scf/day. The other parameters and their determining intervals are included in the rules based on the level of emissions and

the values of other parameters. In these 61 rules, SH, R-Gas,

T-Gas, and ASH are not included in the rules at all, indicating their

insignificant influence on the determination of methane emissions.

These rules can be used to definea specific emissions level fora

range of coal production. As an example, Table6 contains sets of

rules for generating daily coal productions between 8.904M tons/

day and 28.219M tons/day with seam degasification. This shows

that methane emissions can vary from 1.800 MM scf/day to

11.525 MM scf/day. In this case, the coal’s lost and desorbed gas

content (LD-Gas) does not change and the higher methane emissions originate from the magnitudes of other parameters contained

in the rules. The rules given in Table6 are shown graphically in

Fig. 4. Ina similar fashion, the rules can be gathered to analyze

the conditions that lead to other values of daily coal productions.

4.2. Rules and results for predicting emissions for mines operating

without degasification (ND) A CART with a tree depth of 10 created 53 nodes leading to predicted methane emissions at longwall mines that do not use a

degasification system in their operation.A node-selection criterion

similar to the ‘‘D’’ case in Section 4.1 was applied in this situation,
which populated 27 rules for obtaining various emissions levels.

These 27 rules are given in Table 7.

Table 6

Rules for achieving coal production between 8.904 M tons/day and 28.219 M tons/day and predicted methane emissions levels from 1.800 to 11.525 MM scf/day (with

degasification).

Predicted methane emission (MM scf/day) Rules to obtain a daily coal production from 8.904 and 28.219 M tons/day

1.800 CH [58, 76]; PW [630, 730]; SLS [280,382]; HC [9400, 12,800]; LD-GAS [44.8, 145.6]

2.033 CH [76, 80]; PW [630,730]; SLS [280, 382]; HC [9400,12,800]; LD-GAS [44.8,145.6]

2.550 SLS [435, 447.5]; CH [75, 79.5]; ENT [4]; SULP [2.185, 3]; CD [38.5,44]; HC [12,800,14,040]; LD-GAS [44.8,145.6]

3.825 SLS [447.5, 460]; CH [75,79.5]; ENT [4]; SULP [2.185,3]; CD [38.5,44]; HC [12,800,14,040]; LD-GAS [44.8,145.6]

3.900 OB [862.5, 1025]; SLS [509, 550]; ENT [3]; SULP [2.185, 3]; CD [38.5,44]; HC [12,800, 14,040]; LD-GAS [44.8, 145.6]

4.050 OB [700,862.5]; SLS [509,550]; ENT [3]; SULP [2.185,3]; CD [38.5,44]; HC [12,800, 14,040]; LD-GAS [44.8,145.6]

5.100 CH [79.5,84]; ENT [4]; SULP [2.185,3]; CD [38.5,44]; HC [12,800,14,040]; LD-GAS [44.8,145.6]

5.540 CH [72, 76.5]; SLS [280,509]; ENT [3]; SULP [2.185, 3]; CD [38.5,44]; HC [12,800, 14,040]; LD-GAS [44.8, 145.6]

5.800 CH [70.5,76]; ENT [3]; SULP [1.7,2.185]; CD [38.5,44]; HC [12,800, 14,040]; LD-GAS [44.8, 145.6]

5.883 CH [76.5,78]; SLS [280,509]; ENT [3]; SULP [2.185, 3]; CD [38.5,44]; HC [12,800, 14,040] LD-GAS [44.8,145.6]

6.271 SLS [382, 467]; HC [9400,12,800]; LD-GAS [44.8,145.6]

6.500 CH [66, 70.5]; ENT [3]; SULP [1.7,2.185]; CD [38.5,44]; HC [12,800, 14,040]; LD-GAS [44.8, 145.6]

7.075 ENT [4]; SULP [1.7,2.185]; CD [38.5, 44]; HC [12,800,14,040]; LD-GAS [44.8,145.6]

8.100 CH [76, 81]; PW [975,1050]; FCS [335, 400]; CD [30,38.5]; HC [12,800, 14,040]; LD-GAS [44.8,145.6]

10.233 CH [70,76]; PW [975, 1050]; FCS [335, 400]; CD [30,38.5]; HC [12,800,14,040]; LD-GAS [44.8,145.6]

11.525 PW [930, 975]; FCS [335, 400]; CD [30,38.5]; HC [12,800, 14,040]; LD-GAS [44.8, 145.6]

Fig. 4. A tree diagram for coal production between 8.904 and 28.219 M tons/day and the rules that produce various methane emission rates (in bold).

Table 7 shows the rules that lead to predicted methane emissions rates between 0.300 and 8.867 MM scf/day into the ventilation system of a longwall mine without degasification. The table

shows that coal production significantly impacts methane emissions into the mine’s ventilation system and that production

should be adjusted with ventilation constraints and with the values of other parameters related to coal, mine size and operation.

Besides production, the ash content, gas content, cut depth, and

stage loader speed seem to be indicative parameters that can be

used to make this adjustment based on the values of other variables. However, the absence of cut height and the heat value of coal

(rank indicator) in any of the rules suggest that these parameters

do not significantly affect methane emissions.

Table 8 gives the rules for a daily coal production between 0.274

and 14.110 M tons/day without using a degasification system. This

table shows that methane emissions between 0.3 MM scf/day and
Table 7

The rules extracted from the terminal nodes of the 10-level CART procedure for predicting methane emissions from longwall mines not using a degasification system (ND).

Rule Ventilation SH CH PW PL OB ENT CD FCS SLS T-GAS SULP LD-GAS R-GAS 

methaneemission

(MM scf/day) (in) (in) (ft) (ft) (ft) (in) (ft/min) (ft/min) (scf/ton) (%) (scf/ton) (scf/ton) 

1 0.300 565–800 29–38.5 247–295 1.4–1.5 6.4–119.626 

2 0.433 300–565 29–31 247–387.5 6.4–119.626 

3 0.729 300–565 29–31 247–387.5 6.4–119.626 

4 0.750 565–800 29–38.5 247–295 1.5–4.5 6.4–119.626 3.2–12.83 

5 0.800 5240–8500 300–487.5 29–38.5 387.5–517 

6 0.800 300–565 4.0 31–38.5 247–387.5 6.4–119.626 

7 1.100 896–992 29–38.5 247–387.5 6.4–119.626 

8 1.233 300–565 3.0 31–38.5 247–387.5 6.4–119.626 

9 1.267 565–800 29–38.5 247–295 1.5–4.5 6.4–119.626 12.8–68.6

10 1.400 896–992 29–38.5 247–387.5 6.4–119.626 

11 1.550 55–72.5 38.5–42 

12 1.629 565–800 29–38.5 295–387.5 6.4–119.626 

13 1.700 1150–1575 29–38.5 247–387.5 179.2–236.8 119.626–211.2 

14 1.775 573–716 29–38.5 247–387.5 6.4–119.626 

15 1.850 75–96 5240–8500 487.5–650 29–38.5 387.5–517 

16 1.900 1575–1850 29–38.5 247–387.5 179.2–236.8 119.626–211.2 

17 2.200 716–896 6000–7500 29–38.5 247–387.5 6.4–119.626 

18 2.667 716–896 7500–9000 29–38.5 247–387.5 6.4–119.626 

19 2.950 54–75 5240–8500 487.5–650 29–38.5 387.5–517 

20 3.575 1575–1850 29–38.5 247–387.5 179.2–236.8 119.626–211.2 

21 3.880 29–38.5 247–387.5 236.8–249.6 119.626–211.2 22 4.350 72.5–90 38.5–42 

23 4.350 8500–10,688 29–38.5 387.5–517 

24 5.150 291–325 

25 5.225 8500–10,688 29–38.5 387.5–517 

26 7.200 38.5–42 

27 8.867 254–291 
3.880 MM scf/day can be realized by mining a coal with a relatively

low gas content, high ash content, in shallow overburden; properties that typify a low rank coal. Other parameters should be within

the specified ranges to achieve the range of coal productions

shown in this table. Similar tables can be constructed for other

ranges of coal production and methane emissions.

Table 8

Rules for achieving coal production between 0.274 M tons/day and 14.110 M tons/day and predicted methane emissions levels from 0.300 to 3.880 MM scf/day (without

degasification).

Predicted methane emission (MM scf/day) Rules to obtain a daily coal production from 0.274 to 14.110 M tons/day

0.300 SULP [1.4,1.5]; SLS [247, 295]; OB [565, 800]; ASH [4, 11.8]; LD-GAS [6.4,119.626]; CD [29, 38.5]

0.750 R-GAS [3.2,12.8]; SULP [1.5,4.5]; SLS [247, 295]; OB [565,800]; ASH [4,11.8]; LD-GAS [6.4,119.626]; CD [29,38.5]

0.800 OB [300,487.5]; PL [5240,8500]; SLS [387.5,517]; CD [29,38.5]

0.800 ENT [4]; CD [31,38.5]; OB [300,565]; ASH [4,11.8]; LD-GAS [6.4,119.626]; SLS [247, 387.5]

1.233 ENT [3]; CD [31,38.5]; OB [300,565]; ASH [4,11.8]; LD-GAS [6.4,119.626]; SLS [247, 387.5]

1.267 R-GAS [12.8,68.65]; SULP [1.5,4.5]; SLS [247, 295]; OB [565,800]; ASH [4,11.8]; LD-GAS [6.4,119.626]; CD [29,38.5]

1.629 SLS [295,387.5]; OB [565, 800]; ASH [4,11.8]; LD-GAS [6.4,119.626]; CD [29, 38.5]

1.700 OB [1150,1575]; T-GAS [179.2,236.8]; LD-GAS [119.626,211.2]; SLS [247, 387.5]; CD [29,38.5]

1.775 PW [573, 716]; ASH [11.8,15]; LD-GAS [6.4,119.626]; SLS [247,387.5]; CD [29, 38.5]

1.850 SH [75, 96]; OB [487.5,650]; PL [5240,8500]; SLS [387.5,517]; CD [29, 38.5]

2.200 PL [6000,7500]; PW [716, 896]; ASH [11.8,15]; LD-GAS [6.4,119.626]; SLS [247,387.5]; CD [29, 38.5]

2.667 PL [7500, 9000]; PW [716, 896]; ASH [11.8,15]; LD-GAS [6.4,119.626]; SLS [247,387.5]; CD [29, 38.5]

2.950 SH [54, 75]; OB [487.5,650]; PL [5240,8500]; SLS [387.5,517]; CD [29, 38.5]

3.880 T-GAS [236.8, 249.6]; LD-GAS [119.626,211.2]; SLS [247,387.5]; CD [29, 38.5]

Fig. 5 is a tree diagram that was generated using the rules given in

Table 8 for coal production between 0.274 and 14.110 M tons/days

for operations without degasification.

Fig. 5. A tree diagram for coal production between 0.274 and 14.110 M tons/day and the rules that produce various methane emission rates in MM scf/day (in bold).

This figure shows that the

highestmethane emissionwithcoal productionwithinthis specified

range is 3.880 MM scf/day and can originate with a cut depth (CD)

between 29 and 38.5 inches, in a coalbed with loss and desorbed

gas content in the range of 119.3–211.2 scf/ton, and with a total

gas content in the range of 179.2–236.8 scf/ton. Also, stage loader

speed should be between 247.0 and 387.5 ft/min. If the operational

and coal seam characteristics are within these ranges for coal

production up to 14.110 M tons/day and the mine is not using any

degasification, then the ventilation air quantity should be based on

predicted emissions of 3.88 MM scf/day. Similar analyses and

interpretation can be performed for other emission rates.
5. Examples of the application of the technique

This section provides two examples and demonstrates the

application of the CART technique and how the rules can be used

to determine the methane emissions from the ventilation system

of two mines and thus what the ventilation air requirements

should be. This demonstration shows the 2006 operation of the selected mines.

5.1. Example from a mine utilizing degasification system (D)

5.1.1. The mine and its relevant data

The selected operation mines metallurgical coal of medium to

low volatile rank from a longwall operation. As an aid to its ventilation system, the mine employs a coal gas degasification program

that combines vertical, horizontal, and gob gas ventholes. Fig. 4

shows the daily average coal production, methane emissions from

the mine’s ventilation system and methane drained between 2002

and 2005. Fig. 6 shows that during this 5-year period, the mine

averaged a daily coal production of 5–6 M tons/day and emitted

daily average methane rates of approximately 7.5–11.0 MM scf/

day from its ventilation system, while draining significant amounts

of methane varying between 8 and 24 MM scf/day using its degasification system.

Fig. 6. Average daily coal production and methane emission values for the specific

mine that utilized degasification system between 2002 and 2005.

Thus, knowing that this particular mine produced 2.56 million

tons of coal in 2006, which corresponds to an average coal production of 7.008 M tons of coal per day, our aim in this example application was to measure what the methane emission from the

ventilation system would have been in 2006 and what the minimum average ventilation air quantity should have been to keep

the mine’s average methane level under 1%. In order to proceed

with this example, the rules given in Table 5 are more appropriate

because this mine uses a degasification system (D).

Average fundamental properties relevant to maturation, rank,

composition, and heat value of the produced coal by this mine

on an ‘‘as received’’ basis are given in Table 9.

Table 9

Average properties of the coals produced by the selected mines that are used in the

examples.

Coal properties Coal produced by the Coal produced by the

mine (D) mine (ND)

Average vitrinite 1.25 0.75

reflectance (%Ro)

Heat value (BTU/lb) 13500.0–14300.0 13500.0–14000.0

Rank Low-medium vol. bit. High vol. bit.

Ash (%) 9.6 9.4

Moisture (%) 0.6 1.14

Sulfur (%) 0.6–0.8 0.9

Hydrogen (%) 4.7–4.9 5.03

Carbon (%) 76.8–81.1 78.6

Nitrogen (%) 1.6–1.7 1.1

Oxygen (%) 4.1–4.2 8.5

These values are

determined in the laboratory by petrography as well as by elemental and proximate analyses. Although these values require

non-mining related expertise to determine, they are known by almost all mine operators. In addition to the basic properties of the

produced coal given Table 9, the 2006 longwall census reported

that the particular mine operated with the conditions listed in

Table 10.

Table 10

Longwall size and operational parameters of the selected mines that are used in the

examples.

Parameter Description Mine with (D) Mine with (ND)

SH (in) Seam height 50.0 48.0

CH (in) Cut height 72.0 78.0

PW (ft) Panel width 850.0 1000.0

PL (ft) Panel length 12500.0 10000.0

OB (ft) Overburden depth 1900.0 900.0

ENT Number of gateroad entries 4 4

CD (in) Cut depth 36.0 42

FCS (ft/min) Face conveyor speed 305.0 347.0

SLS (ft/min) Stage loader speed 390.0 450.0

Gas content of the mined coal seam has a significant impact on

emissions into the ventilation system in a longwall mine. Reducing

the gas content of the coal is the main reason for degasification efforts prior to and during mining. Almost all the rules presented in

Tables 5 and 7 for D and ND cases, respectively, contain lost and

desorbed gas content data. In this example application, because

the gas content of the coal was not available, NIOSH’s MCP 2.0

(Karacan, 2010) was used to determine the total and desorbable

gas content of the coal extracted by this operator. Using the rank,

moisture, and ash average heat value given in Table 9, the total

and desorbable gas contents were determined to be 519.5 scf/ton

and 422.1 scf/ton, respectively. The difference between the two

values corresponds to the combination of lost and residual gas

content.

5.1.2. Evaluation of the rules (D) with the data to predict methane

emission potential into the ventilation system for the studied mine

Evaluation of the rules given in Table 5 starts with examining

the goals to be achieved and the constraints at hand. The goal is

to produce an optimum quantity of coal safely. The constraints

can be operational conditions that can be adjusted, such as FCS,

and those that cannot be changed, such as OB or PW, or physical

and chemical properties of the coal. However, the ultimate constraint in terms of safety is the ventilation quantity, which is determined based on critical factors, such as methane emissions. The

physical conditions of the mine and the power of main fans to

provide that amount is another question of interest; however, once

the methane emission is known, based on a set of goals and constraints, then either ventilation should be adjusted accordingly,

or the adjustable constraints or goals should be changed to meet

the requirements.

The mine in question produced an average of 7.008 M tons of

coal per day in 2006. According to this productivity goal, rules 1–

9, 12–14, 16, 19, 23–24, 29, 44, 46, 49–50, 54–56 and 58–61 can

be applicable because this goal is within the ranges given as CP

in these rules. However, some of these rules are not applicable

for the constraints of this mine and cannot be changed. For

instance, rule 12 and similar rules are applicable for the coals with

lower rank (based on HC) or for those that have less LD-GAS. Similarly, some of these rules are applicable for OB depths less than
1900 ft. Therefore, the rules that do not include the HC (14000.0–

14500.0 BTU/lb), OB (1900.0 ft) and LD-GAS (>422.1 scf/ton)

should be eliminated. This examination reduces the applicable

rules to 16, 24, 44, 46, 50, 54, 56 and 58–61.

Further examination on the remaining rules using other constraints given in Table 10, such as PL, CD, CH, and FCS reduces

the applicable rules even further. For this examination, one should

look for the rules that contain the range of values given in Table 10

for the operational parameters of this mine. For instance the CD,

CH, and FCS used in this mine are 36 in, 72 in and 305 ft/min,

respectively. Therefore, rule 44 and others that do not qualify for

the operational constraints that survived in the previous step have

been eliminated in this stage.

Elimination of rules as briefly described and applied to the

study mine leaves only two rules that can most likely be applicable

to the study mine: rules 46 and 50 which indicate that, under the

given circumstances, the methane emissions into the ventilation

system can be 7.704 MM scf/day and 8.750 MM scf/day, respectively. These can be considered as the low and high limits of the

likely methane emissions under the given constraints and the

values for which the ventilation amounts should be planned

accordingly. These predictions, as well as the actual emissions

measured from this mine, are given in Fig. 7.

Fig. 7. A comparison of predicted and actual methane emissions into the study

mine that uses degasification (D) for 2006 using Rule 46 and Rule 50.

This figure shows that

the predictions obtained, using the rule-based system introduced

in this paper, actually encompass the measured methane emission

rate of 8.7 MM scf/day in 2006. Therefore, a ventilation flow

between 530,000 scfm and 610,000 scfm is required to keep the

methane concentration at the 1% level.

The air rates predicted for this mine to keep its methane

concentration at the 1% level were also compared with the actual

ventilation rates documented in US EPA (2010) for 2008–2009 periods. This report is based on monthly and quarterly methane and

ventilation air flow sampling studies conducted by MSHA at main

fans of gassy underground coal mines in the United States. This report indicated that the example mine studied in this paper operated

with an average ventilation air flow of 640,600 scfm to keep the

methane levels at 1%. This comparison suggests that the ventilation

air rates that were predicted under certain operation and productivity constraints are very close to the actual measurements.

5.2. Example from a mine that does not utilize degasification system

(ND)

5.2.1. The selected mine and its relevant data

This mine produces coal for mainly steam production and metallurgical purposes and does not use any degasification system.

Rank of the coal is high volatile bituminous. Fig. 8 shows the daily

average coal production and methane emissions from the mine’s

ventilation system between 2002 and 2005.

Fig. 8. Average daily coal production and methane emission values for the specific

mine that did not utilize a degasification system (ND) in its operation between 2002

and 2005.

Fig. 8 shows that during this 5-year period, mine’s daily coal production dropped from

7.1 M tons/day to 2.6 M tons/day. In accordance with this decrease

in production, mine’s methane emission from ventilation system

has decreased from 3.3 MM scf/day to 1.0 MM scf/day too. Average

coal production and methane emission during this period were

4.1 M tons/day and 2.32 MM scf/day, respectively.

This mine produced 933 thousand tons of coal in 2006, which

corresponds to an average coal production of 2.55 M tons of coal

per day. Methane emission from the ventilation system for 2006

and what the minimum average ventilation air quantity should

be to keep the mine’s average methane level under 1%, can be estimated using the same approach in the previous example. In order

to proceed with this example, the rules given in Table 7 can be

used since this mine does not use a degasification system (ND).

Average fundamental properties relevant to maturation, rank,

composition, and heat value of the produced coal by this mine

on an ‘‘as received’’ basis are given in Table 9, as well. In addition

to the basic properties of the produced coal given Table 9, the

2006 longwall census reported that the particular mine operated

with the conditions listed as (ND) in Table 10.

Gas contents of the coal produced by this mine were also predicted by using NIOSH’s MCP 2.0 (Karacan, 2010). The total and

desorbable gas contents of the coal were predicted as 157.2 scf/

ton and 104.6 scf/ton, respectively by using the rank, moisture,

ash and average heat value given in Table 9. The difference

between total and desorbable gas contents is 52.6 scf/ton, which

corresponds to the combination of lost and residual gas content.

5.2.2. Evaluation of the rules (ND) with the data to predict methane

emission potential into the ventilation system

Evaluation of the rules given in Table 7 starts with examining

the goals and the constraints, as it was in the previous example.

The mine in this example produced an average of 2.55 M tons of

coal per day in 2006. According to this productivity (CP) goal, rules

3, 10, 16, and 24–27 will be eliminated. In addition, of the remaining rules, some of them do not comply with the properties of the

coal that is mined. For instance, when the ash content of the coal

is considered, rules 7, 14, 17, 18 can be eliminated. Similarly, SULP

and LD eliminates 1, 4, 9, 13, 20 and 21. This examination related to

CP and coal properties reduces the applicable rules to 2, 5, 6, 8, 11,

12, 15, 19, 22, 23.

Further examination on the remaining rules using other constraints given in Tables 10, such as OB eliminates rules 2, 5, 6, 8,

15, 19 as these rules are for much shallower depths, and SH eliminates 22 and 11. Therefore, only rules 12 and 23 carry the closest
data that survive the elimination process. These rules show expected ventilation methane emission values of 1.629 MM scf/day

(1131 scfm) and 4.350 MM scf/day (3021 scfm). These values can

be considered as the lower and upper limits of possible methane

emission under the given constraints and the values for which

the ventilation amounts should be planned accordingly. These values, as well as the actual emission measured from this mine, are

given in Fig. 9.

Fig. 9. A comparison of predicted and actual methane emissions into the study

mine that operated without a degasification system (ND) for 2006 using Rules 12

and 23.

This figure shows that reported methane emission

from ventilation system for 2006 is 0.9 MM scf/day (625 scfm),

which would require at least ~ 62,500 scfm of air for 1% methane

in the ventilation system. On the other hand, predicted ventilation

emissions for this mine would require ~115,000 scfm and

~ 300,000 scfm, for 1.629 MM scf/day and 4.350 MM scf/day methane emissions, respectively.

As in the previous example, the air rates predicted for this mine

to keep its methane concentration at the 1% level were also compared with the actual ventilation rates documented in US EPA

(2010). This report indicated that this mine operated with an average ventilation air flow of 345,000 scfm and measured methane

concentrations were ~ 0.48%. For 1% methane concentration, the

air requirement is ~166,000 scfm, which is almost the mid-value

of the air rate predicted. Therefore, although the mine have not

used methane drainage, the air quantity was enough to provide

safe mining without overwhelming the ventilation system.

6. Conclusions

The magnitude of methane emitted into the working environment is dependent upon a number of geologic and operational

parameters. Control of these emissions is critical to protect the

safety of the underground workforce. This work used classification

and regression tree analyses (CART) to predict methane emissions

for longwall operations based on the levels of a number of parameters such as coal gas content, proximate coal analysis, seam and

mining heights, cut depth, and panel size. The analyses were

conducted for operations with and without the use of coal seam

degasification.

CART analyses of operations using coal seam degasification identified 61 rules to predict methane emissions ranging from 0.105 to

16.050 MM scf/day for coal productions from 0.822 to 28.219 M

tons/day. These analyses revealed that parameters related to coal

quality were included in almost all rules, and operational parameters such as cut depth and face conveyor speed were prevalent when

emissions levels exceeded 1.9 MM scf/day. The variables of seam

height, residual gas, total gas, and ash content were not included

in any of the rules, indicating their minimal impact on methane

emissions. Similar assessments for those operations not employing

coal seam degasification identified a total of 27 rules for predicting

methane emissions ranging from 0.30 to 8.867 MM scf/day and coal

productions from 0.274 to 20.000 M tons/day. Emissions between

0.30 MM scf/day and 3.88 MM scf/day can be realized by mining a

coal with a relatively low gas content, high ash content, and shallow

overburden, properties that typify a low rank coal. Emissions in

excess of 3.88 MM scf/day were predicted when cut depth and panel

length increased.

This method was applied to two large longwall mines operating

with and without degasification to control methane emissions. For

the mine utilizing degasification, comparison of operation and

geologic conditions narrowed down the 61 rules for operations

using degasification to only 2. The methane emissions rates of

7.704 MM scf/day and 8.750 MM scf/day predicted by rules 46

and 50, respectively, provided a lower and an upper bound on

the actual emissions rate of 8.7 MM scf/day. These predicted emissions corresponded to ventilation rates of 530,000 scfm and

610,000 scfm, respectively, to maintain methane levels at 1%. The

calculated ventilation rates were compared with the actual rates

that the example mine exhausted from its ventilation system during its 2008–2009 operation. The comparison showed that this particular mine operated with an average ventilation rate of

640,600 scfm with an average methane concentration of 1%. On

the other hand, for the mine operating without a degasification

system, coal and operational parameters resulted in only 1 possible

rule that can be applicable to estimate methane emissions and

required ventilation amount. The rule-based decision process

determined that the mine would likely have 1.629 MM scf/day.

This amount would require an air flow rate of 115,000 scfm to keep

methane at most 1% level. However, the mine was already providing ~345,000 scfm air to the mine and was able to keep methane at

0.48% level. It should be noted that cutting the air rate by half will

bring the concentration ~1% and air rate to closer to the amount

calculated for this concentration. These results suggest that this

method is not only practical, but also accurate enough for predicting methane and ventilation rates.

Disclaimer

The findings and conclusions in this paper are those of the

authors and do not necessarily represent the views of the National

Institute for Occupational Safety and Health. Any commercial

product mentioned in this paper is not endorsed by NIOSH.

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