Advanced Mine Ventilation: Respirable Coal Dust, Combustible Gas and Mine Fire Control

By Pramod Thakur

Advanced Mine Ventilation presents the reader with a unique book providing the theory and applications for designing mine ventilation with computers, controlling respirable coal dust and diesel particulate matter, combustible gas control and, mine fire management. The book summarizes the latest knowledge created in the past 40 years in these areas. Authored by an expert in the field with 50 years' experience, the book is a great combination of theory and applications. The mine ventilation section provides computer programs (both FORTRAN and C++) to calculate not only air quantities and pressure losses but also the concentration of any pollutant in all junctions and branches of the mine network. Small particle mechanics and dust control is covered in the second section of the book. The third section on combustible gas control discusses all aspects of mine gases from origin to control. The last section on mine fire control discusses spontaneous combustion, frictional ignitions, mine explosions, and mine sealing and recovery. The book is not only a very good reference book but also an excellent textbook for two graduate level courses in Mining Engineering.

• Provides the latest knowledge on the four related topics of mine environment control; that is, ventilation, dust, gas, and fire in a single volume
• Computer simulation of mine ventilation in both FORTRAN and C++
• State-of-the-art respirable dust control
• Mine degasification and methane production from a coal lease
• Mine fire management

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 23, 2018
• ISBN: 9780081004586

Pramod Thakur

Dr. Pramod C Thakur is the president of a consulting firm, Expert Solutions for Mine Safety (ESMS) LLC, in Morgantown, West Virginia, USA, and an adjunct professor at the West Virginia University. He served the coal industry for 50 years. He began his career designing mine ventilation with Andrew Yule & Co for 8 years. During his last 42 years with CONOCO, CONSOL Energy and Murray Energy, he developed four different techniques for mine degasification and coalbed methane production. He researched respirable dust control and diesel exhaust dispersion at the Pennsylvania State University. These techniques are used not only in the United States but also in many countries overseas. Appointed by the Governor of West Virginia, he served as a commissioner for 18 years and wrote the WV Diesel Regulations that serves as a model for the global coal industry. He is a distinguished alumnus of IIT(ISM), India, and a Centennial Fellow of the Pennsylvania State University. He is a member of the Society of Mining, Metallurgy, and Exploration (SME), USA, since 1969. He has written four books on mine ventilation and coalbed methane control and published more than 50 papers in technical journals. He has been awarded the Howard Hartmann and Howard Eavenson Award for excellent work in mine ventilation engineering and mine health and safety by the SME.

Book preview

Advanced Mine Ventilation

Respirable Coal Dust, Combustible Gas and Mine Fire Control

Pramod Thakur, Ph.D.

President, Expert Solutions for Mine safety, LLC, Morgantown, West Virginia, USA

Table of Contents

Cover image

Title page

Copyright

Dedication

Preface

Section One. Mine Ventilation

1. Underground Coal Mine Atmosphere

1.1. Introduction to Coal Mining

1.2. Underground Mine Atmosphere

1.3. Properties of Air

1.4. Pollutant Control Strategy

1.5. Enforcement of Ventilation Standards

1.6. Definition of Air (Gas) Properties [9]

Problems

2. Air Flow in Mine Airways

2.1. Introduction

2.2. Derivation of Basic Fluid Flow Equation

2.3. Traditional Equations for Pressure Loss Calculation in Mines

2.4. Determination of Mine Airway Friction Factor, K

2.5. Air Flow in Ventilation Duct/Pipes

2.6. Shock Losses in Mine Airways

2.7. Mine Characteristics Curve

2.8. Ventilation Airways in Series/Parallel

2.9. Calculation of Air Horsepower

Problems

3. Turbulent Dispersion of Pollutants in Mine Airways

3.1. Mine Ventilation Systems

3.2. Generalized Mass Transfer Model

3.3. Instantaneous Stationary Point Source

3.4. Continuous Stationary Point Source

3.5. Dispersion of Respirable Dust From a Heading

3.6. Dispersion in a Leaky Roadway

3.7. Concentration Growth in a Roadway With Uniformly Distributed Source

Problems

4. Estimation of Ventilation Air Quantity

4.1. The Modern Mine Layout

4.2. Methane Emissions

4.3. Mildly Gassy Coal Seams

4.4. Moderately Gassy Coal Seams

4.5. Very Gassy Coal Seams

4.6. Limitations on the Longwall Face Width Owing to Face Methane Emissions

4.7. Limitations on the Longwall Face Width Owing to Gob Methane Emissions

4.8. Air Quantity Requirements for Development Headings

4.9. Estimation of Total Required Ventilation Air

4.10. Standards of Volumetric (Ventilation) Efficiency

Problems

5. Ventilation Network Analysis

5.1. Network Analysis for Air Quantities and Pressure

5.2. Introduction to the Ventilation Network Analyzer

5.3. Verification of the Ventilation Network Analyzer in a Working Mine

Problems

6. Mechanical and Natural Ventilation

6.1. Radial Flow Fans

6.2. Fan Characteristics

6.3. Axial Flow Fans

6.4. Fan Laws

6.5. Fan Testing

6.6. Matching a Fan to Mine Characteristics

6.7. Natural Mine Ventilation

Problems

Section Two. Respirable Coal Dust Control

7. Health Hazards of Respirable Dusts

7.1. Growth of Coal Workers' Pneumoconiosis

7.2. A Basis for Respirable Dust Standard

7.3. Prevalence and Cessation of Coal Workers' Pneumoconiosis

7.4. Lifestyle Intervention Program

8. Characteristics of Respirable Coal Dust Particles

8.1. Settling Velocity of Small Particles Due to Gravity (Stoke's Formula)

8.2. Aerodynamic Shape Factor for Dust Particles

8.3. NonSettling Fraction of Respirable Dust

8.4. Size Distribution of Respirable Dust Particles

8.5. Determination of Mass Distribution for Fine Coal Dust Particles

8.6. Chemical Composition of Respirable Coal Dust

9. Generation of Respirable Coal Dust

9.1. A Mathematical Model for Respirable Dust Generation

9.2. Sample Preparation and Experimental Details

9.3. Yield of Respirable Dust

9.4. Dependence of Respirable Dust Index on the Properties of Coal

9.5. Statistical Analysis of Data

9.6. Results of Similar, Subsequent Studies

9.7. Impact of Cutting Bit Wear on Respirable Dust Production

10. Respirable Dust Control

10.1. Theory of Dust Suppression and Collection

10.2. Collection of Dust Particles by Filters

10.3. Dust Control in Continuous Miner Section

10.4. Dust Control in Longwall Faces

10.5. Dust Control for Roof Bolters

10.6. Personal Protective Equipment

10.7. Optimization of Water Sprays

10.8. Use of Surfactant to Improve Dust Control

10.9. Electrostatic Charging of Water Particles for Improved Dust Collection

11. Diesel Exhaust Control

11.1. Health Hazards of Diesel Particulate Matter

11.2. Diesel Particulate Matter Standards

11.3. Diesel Exhaust Control Strategy

11.4. Diesel Exhaust Dilution

11.5. Diesel Equipment Maintenance and Training of Personnel

11.6. West Virginia Diesel Regulations—A Model for Coal Industry

12. Respirable Dust Sampling and Measurement

12.1. Early Dust Measuring Instruments

12.2. Gravimetric Personal Dust Samplers

12.3. Dust Concentration Measurement by Light-Scattering Instruments

12.4. The Tapered Element Oscillating Microbalance Instrument (A Personal Dust Monitor)

12.5. Respirable Dust Sampling Strategy

12.6. Threshold Limits for Various Dusts Prevailing in Mines

12.7. Diesel Particulate Monitor

Section Three. Combustible Gas Control

13. Origin of Gases in Coal Mines

13.1. Introduction

13.2. Properties of Gases in the Mine Atmosphere

13.3. Characteristics of Coal

13.4. Characterization of Methane from Coal

13.5. Coalbed Methane—An Energy Source

14. Reservoir Properties of Coal Seams

14.1. Gas Content of Coal

14.2. Coal Matrix Permeability

14.3. Diffusivity of Methane in Coal

14.4. Reservoir Pressure

Problems

15. Premining Degasification of Coal Seams

15.1. Coal Seam Reservoir Parameters

15.2. Premining Degasification

15.3. Application of In-Mine Horizontal Drilling

15.4. Application of Vertical Wells With Hydraulic Fracturing

15.5. Application of Horizontal Boreholes Drilled From Surface

15.6. Optimum Widths of Longwall Panels

15.7. Field Observations of Optimum Longwall Panel Width

Problems

16. Postmining Degasification of Coal Mines

16.1. The Gas Emission Space

16.2. European Gob Degasification Methods

16.3. US Gob Degasification Method

16.4. Gas Capture Ratios by Vertical Gob Wells

16.5. Gob Well Production Decline

Problem

17. Floor Gas Emissions and Gas Outbursts

17.1. Floor Gas Emissions

17.2. Gas Outbursts

17.3. Parameters Indicating a Propensity to Gas Outbursts

17.4. Prevention of Gas Outburst

18. Gas Transport in Underground Coal Mines

18.1. Construction of Pipeline

18.2. Gas Leakage Detection and Safeguards

18.3. Other Preventive Measures for Safe Gas Transport

18.4. Ventilation

18.5. Corrosion of Steel Pipelines for Methane Drainage

18.6. Compressors

18.7. Surface Discharge of Gas

18.8. A Typical Application for Mine Safety and Health Administration Approval of a Gas Pipeline System

19. Measurement and Monitoring of Mine Gases

19.1. Detection Methods

19.2. Monitoring of Mine Gas

19.3. Wireless Communication and Monitoring System

19.4. Special Arrangements for Monitoring in Mines Liable to Spontaneous Combustion

20. Economics of Coal Mine Degasification

20.1. Safety in Mines

20.2. Reduced Cost of Mining by Improved Productivity

20.3. Revenues From Drained Methane

20.4. Gas Production From Coal Seams—A Stand-Alone Business

20.5. Economic Analysis

Problem

Section Four. Mine Fire Control

21. Spontaneous Combustion of Coal

21.1. Spontaneous Combustion of Coal

21.2. Detection of Spontaneous Combustion

21.3. Mine Design for Coal Seams Liable to Spontaneous Combustion

Problems

22. Prevention of Frictional Ignitions

22.1. Coal Seam Degasification

22.2. Ventilation

22.3. Wet Cutting or Water-Jet-Assisted Cutting

22.4. Machine Design Parameters

22.5. Summary and Conclusions

22.6. Frictional Ignitions Caused by Belt Conveyors

23. Gas and Dust Explosions

23.1. Gas Explosions

23.2. Dust Explosions

23.3. Prevention of Gas Explosions

23.4. Stone Dust Barriers for Explosion Propagation Prevention

Problems

24. Mine Sealing and Recovery

24.1. Mine Sealing

24.2. Inertization of the Sealed Area

24.3. Sampling the Sealed Mine Atmosphere and Interpretation of Data

24.4. Recovery of the Sealed Mine

Problem

Appendix A

Appendix B. Ventilation Network Analyzer in Fortran IV

Appendix C: Ventilation Network Analyzer in C++ With Input and Output

Appendix D: The Input and Output Data for the Hypothetical Mine

Index

Copyright

Woodhead Publishing is an imprint of Elsevier

The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States

The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom

Copyright © 2019 Elsevier Ltd. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

Library of Congress Cataloging-in-Publication Data

A catalog record for this book is available from the Library of Congress

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

ISBN: 978-0-08-100457-9

For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Joe Hayton

Acquisition Editor: Maria Convey

Editorial Project Manager: Charlotte Kent

Production Project Manager: Swapna Srinivasan

Cover Designer: Victoria Pearson

Typeset by TNQ Technologies

Dedication

This book is dedicated to my family:

Parents: Late Radha and Jagdish Thakur

Wife: Meera, and

Our sons: Drs. Gautam and Anand Thakur

For their love and support.

May it keep all coal miners safe around the world.

Preface

Maintaining a healthy and safe air environment in an underground coal mine is a sine qua non for efficient coal production. There are three main hazards associated with coal mining air environment. First, coal mining process creates many respirable dusts, such as, coal and silica dust, and diesel particulate matter (DPM) which can be hazardous to human health in high concentrations. Some of these dust clouds can also be explosive.

Secondly, coal seams inherently contain many combustible gases, such as, methane and ethane that can become explosive when mixed with insufficient volumes of air. Several thousand fatalities have occurred because of methane and dust explosions in the coal mines of the world since the beginning of coal mining some 200 years ago. The above two problems are minimized by mine ventilation. Large volumes of air, often 20 tons of air for each ton of coal mined, are circulated through the mine workings to dilute the gas and dust concentrations to safe levels. Proper design of mine ventilation is thus crucial to mine safety.

Thirdly, all coal seams have a tendency for spontaneous combustion (slow oxidation of coal) when it comes in contact with air. If no corrective actions are taken, the coal can catch fire leading to the loss of the mining section and sometimes, the entire mine.

The purpose of this book is to address the four related issues in detail to make coal mining a safe and profitable business. In modern, highly productive coal mines, ventilation alone cannot provide adequate control of pollutants. Special techniques for minimizing the risks of respirable dust, combustible gases and mine fires are needed.

Most of the existing knowledge on mine ventilation has been derived from the following three books published 50–60  years ago:

1. Mine Ventilation: ed. A Roberts (1960).

2. Mine Ventilation and Air Conditioning: ed H Hartmann (1961). The book was reprinted in 1982 and 1997 with only minor changes.

3. Mine Ventilation: A Skochinsky and V. Kamarov (1969)

These books provide good basic knowledge of mine ventilation (and air conditioning in deep metal mines) but they could not cover the new developments in all four critical areas mentioned above. A tremendous amount of research and innovation over the past 50  years have resulted in:

1. Digital (computerized) design of mine ventilation able to compute air flows, pressure losses and the concentrations of many pollutant in all junctions and airways of the mine.

2. Adequate control of respirable dust such that coal workers' pneumoconiosis (CWP) has been eliminated in some countries and minimized in most countries.

3. Efficient drainage of methane pre-mining and post-mining such that mine explosions are minimized, if not, eliminated. Each large coal mine has become a potential field for gas production with added profit.

4. Design of coal mines such that it minimizes the risk of spontaneous combustion of coal and consequently reduces the chances of mine fire.

Advanced Mine Ventilation discusses these four topics in four sections named:

1. Coal Mine Ventilation

2. Respirable dust control

3. Combustible gas control

4. Mine fire control

The author had the unique opportunity to research and work on all four topics over his 50  years' service to the coal industry.

The coal mine ventilation section has six chapters comprising, (1) Underground coal mine atmosphere, (2) Air flow in mine airways, (3) Turbulent dispersion of pollutants in mine airways, (4) Estimation of ventilation air quantity, (5) Ventilation network analysis, and (6) Mechanical and natural ventilation.

The respirable dust control section has six chapters comprising, (7) Health hazards of respirable dusts, (8) Characteristics of respirable coal dust particles, (9) Generation of respirable coal dust, (10) Respirable dust control, (11) Diesel Exhaust control, and (12) Respirable dust sampling and measurements.

The combustible gas control section has eight chapters comprising, (13) Origin of gases in coal mines, (14) Reservoir properties of coal seams, (15) Pre-mining degasification of coal seams, (16) Post-Mining degasification of coal mines, (17) Floor gas emissions and gas outburst, (18) Gas transport in underground coal mines, (19) Measurement and monitoring of mine gases, and (20) Economics of coal mine degasification.

The mine fire management section has four chapters comprising, (21) Spontaneous combustion of coal, (22) Prevention of frictional ignitions, (23) Gas and dust explosions, and (24) Mine sealing and recovery.

Advance mine ventilation is a unique book that is not only an excellent reference book for the practicing mine engineer but also a great text book for two graduate level courses in Mining Engineering program. I recommend teaching mine ventilation and respirable dust control in one semester , and combustible gas control and mine fire management in the second semester.

I owe thanks and gratitude to a number of people for helping me to learn the contents of this book and actually practicing it in the industry for the past 50  years.

Coal mine ventilation: to late Dr. H Hartmann of the University of Alabama who invited me to write chapters in his books on Mine ventilation and air conditioning.

Respirable dust control: to Dr. AK Sinha and late Dr. R Stefanko under whose guidance I did my MS (Characterization of coal dust particles) and Ph.D. theses (Computer-aided analysis of diesel exhaust dispersion in mine airways) respectively.

Combustible gas control: to late William Poundstone and late Eustace Frederick of CONSOL Energy for providing support in discovering and applying various methane control techniques in the mines of CONSOL Energy.

Mine fire management: to late L David Hughes of Andrew Yule and Co and late Donald Mitchell, a friendly consultant, who taught me the elements of mine fire fighting enabling me to fight and control many fires on my own.

My thanks are also due to Joyce Conn for typing the manuscript and Sowmya Devraja for drafting all figures. Finally my thanks are due to Natasha Welford and Charlotte Kent of Elsevier for their help and encouragement in completing this text.

Pramod Thakur, Ph. D.

Section One

Mine Ventilation

Outline

1. Underground Coal Mine Atmosphere

2. Air Flow in Mine Airways

3. Turbulent Dispersion of Pollutants in Mine Airways

4. Estimation of Ventilation Air Quantity

5. Ventilation Network Analysis

6. Mechanical and Natural Ventilation

1

Underground Coal Mine Atmosphere

Abstract

Fossil fuels comprise nearly 90% of the proved reserve of global energy. Coal is the major component of fossil fuels, containing nearly 90% of the total fossil fuel energy. In spite of some environmental concerns, global coal production will continue to rise to meet the rising demand for electricity, especially in developing countries such as India and China. The provision of an adequate air environment in underground coal mines to promote the health, safety, and comfort of mine workers has always been and will continue to be a prime requisite for successful mining operations. The chapter identifies the main air pollutants in coal mine air and lists their threshold limit values. Because ventilation of the mine workings by forced air is the primary method of diluting the contaminants in mine air, the properties of air are discussed in detail. Gas laws controlling the behavior of air are discussed. A pollution control strategy is developed for major pollutants, such as respirable dust, methane, and diesel exhaust components. The three E's of safety, namely, Engineering, Education, and Enforcement, are listed. The enforcement of mine ventilation regulations is briefly discussed. Finally, air(gas) properties that will be frequently mentioned later in the book are clearly defined.

Keywords

Air properties; Definition of gas properties; Gas laws; Mine ventilation; Threshold limit values

Chapter Outline

1.1 Introduction to Coal Mining

1.2 Underground Mine Atmosphere

1.3 Properties of Air

1.3.1 Gas Laws Related to Air

1.3.1.1 Boyle's Law

1.3.1.2 Charles' Law

1.3.1.3 Dalton's Law of Partial Pressure

1.3.1.4 Graham's Law of Diffusion

1.3.1.5 Air Density at Higher Altitude

1.3.1.6 Pressure Versus Fluid Head

1.4 Pollutant Control Strategy

1.5 Enforcement of Ventilation Standards

1.5.1 Mine Ventilation Regulations

1.5.1.1 US Federal Regulations

1.5.2 Maximum Concentration of Explosive Gases in Coal Mine Air

1.5.3 Some Highlights of Code of Federal Regulations 30, Parts 70 and 75

1.5.3.1 Respirable Dust Measurement

1.5.3.2 Methane Measurements

1.5.3.3 Minimum Air Requirements

1.5.3.4 Permissible Electrical/Diesel Equipment

1.6 Definition of Air (Gas) Properties

1.6.1 Atomic Weight

1.6.2 Avogadro's Number

1.6.3 British Thermal Unit

1.6.4 Critical Temperature and Pressure

1.6.5 Density

1.6.6 Dew Point

1.6.7 Enthalpy or Heat Content

1.6.8 Entropy

1.6.9 Mole Volume

1.6.10 Relative Humidity

1.6.11 Specific Heat

1.6.12 Viscosity

Problems

References

1.1. Introduction to Coal Mining

Coal is the most abundant and the cheapest fossil fuel in the world today. Over the past 200  years, it has played a vital role in the growth and stability of world economy. The current world human population of about 7300 million consumes 5  ×  10²⁰  J of energy per year. It is likely to increase to 7.5  ×  10²⁰  J/year in the next 20 years. Fossil fuels at present provide 87% of all energy consumed. Nuclear and hydro power provide 12%. Solar, wind, and geothermal energy barely provide 1% [1] as shown in Table 1.1.

Barring a breakthrough in nuclear fusion, fossil fuels will remain the main source of energy in the foreseeable future, as they have been in the past 200  years. Ninety percent of all fossil fuel energy in the world is in coal seams. It is, therefore, natural to anticipate that coal's share in the energy mix will increase. At present, coal provides 26% of global energy demand and generates 41% of the world's electricity. Coal deposits are widespread in 70 countries of the world. Coal is a very affordable and reliable source of energy. The total proved, mineable reserve of coal exceeds 1  T tons to a depth of about 3300 feet. Indicated reserves (mostly nonmineable) to a depth of 10,000 feet range from 17 to 30  T  tons [2]. Current (2015) world coal production is about 8000 million ton/year. Coal production from top 10 countries are shown in Table 1.2 [3].

Total tonnage mined in these 10 countries comprises nearly 90% of global production. Coal production may continue to increase if they start converting coal into synthetic gases and liquid fuels, such as diesel and aviation fuels.

Coal mining is done in two ways: surface mining and underground mining. Most thick and shallow deposits of coal are mined by surface mining methods. The depth of surface mining is generally less than 200–300 feet. The overlying soil and rocks are removed to expose coal before it is mined out. Nearly 50% of the global production of coal is obtained by surface mining. The mine is open to the atmosphere, hence no ventilation is needed.

Table 1.1

a E  =  10¹⁸.

b Z  =  10²¹.

c reprocessing not considered. 1000  J  =  0.948 BTU.

Table 1.2

a 1 metric ton  =  1.1 short tons.

Adapted from World Coal Statistics.

However, most of the coal deposits are deeper than 300 feet and thus are mined by underground mining methods. Mine shafts or inclines are dug to access the coal seam. A series of tunnels are driven to create a large block of coal, called a longwall panel, that is mined by machines. Coal is transported out of the mine by conveyor belts and hoists (Fig. 1.1).

1.2. Underground Mine Atmosphere

The underground mine atmosphere has many pollutants. They are mostly solids (respirable dust) and gases (such as methane, carbon dioxide, etc.). Liquid pollutants, such as mists, are not an issue in the mining industry.

The provision of an adequate air environment to promote health, safety, and comfort of mine workers has always been and will continue to be a prime requisite for successful coal mining operations. Although the definition of an adequate environment varies from country to county, it generally means the provision of sufficient circulating air, often at specified velocities, to maintain at least 19.5% oxygen in the working areas; concentrations of solids (respirable dust) and gaseous pollutants, such as methane, carbon dioxide, etc., below specified limits; and heat and humidity below specified limits. Because most coal mines are shallow (less than 3000  ft deep), temperature and humidity control is not warranted. It is a concern only in deep metal mines (up to 15,000  ft) for copper, silver, and gold.

Table 1.3 shows the maximum allowable concentrations of these atmospheric pollutants in US underground coal mines [4].

Figure 1.1  A typical underground coal mine layout.

1.3. Properties of Air

Because large quantities of air, sometimes 20 tons of air for each ton of coal mined, is circulated through the mine airways, it is important to know its properties. The chemical composition of air is as follows:

Air is a physical mixture of these gases with a specific gravity of 1.00. It is a colorless, odorless, and tasteless gas that supports life and combustion via its oxygen content. Some important properties of air are listed in Table 1.4.

Table 1.3

a Coal dust concentration is limited by silica concentration in the respirable dust by the following formula: respirable coal dust  =  10/silica concentration in %, mg/m³.

b Applicable only in Pennsylvania and West Virginia in the United States. It is the most stringent standard in the world for diesel particulate matter.

Table 1.4

a STP Standard temperature of 70°F and pressure 29.92 inches of Hg.

1.3.1. Gas Laws Related to Air

Air just like all other gases follows many laws of physics that are essential to understand its behavior. Only the most pertinent laws will be discussed here.

1.3.1.1. Boyle's Law

The volume of air, V is inversely proportional to pressure, P at constant temperature, T. For a given volume of gas changing from volume V1 and pressure P1 or to volume V2 and pressure P2,

(1.1)

1.3.1.2. Charles' Law

The volume of air (gas) is directly proportional to the absolute temperature, T at constant pressure.

Mathematically,

(1.2)

Combining Eq. (1.1) and Eq. (1.2) we get the generalized gas law:

(1.3)

where R is the gas constant with a value of 53.35  ft-lb/lb mass °R.

1.3.1.3. Dalton's Law of Partial Pressure

It states that the pressure exerted by a mixture of gases is equal to the sum of separate pressures that each gas would exert if it alone occupied the whole volume.

Mathematically,

(1.4)

where P is the pressure and P1, P2, P3, etc are partial pressures. In normal air, there are only two gases, dry air and water vapor.

where Pa  ,  Pv are partial pressures of air and vapor.

An example:

Let us assume P (barometric pressure) is 30 inches of Hg

Partial pressure of vapor is 0.5 inch Hg

Calculate the density of dry air if the temperature was 70°F

Pa = P – Pv=30–0.5=29.5 inch Hg

Or Pa=2085.8lb/ft²

Hence density of dry air;=ρ=0.0738lb/ft³

Density of the moist air is also calculated by using another equation:

(1.5)

where D is 1.3258 if pressure is measured in inches of mercury; P1  =  barometric pressure; Pv  =  vapor pressure; T  =  dry-bulb temperature in Rankine.

This yields the density of dry air in the above example as 0.0743  lb/ft³, which is quite close to the previous volume of 0.0738  lb/ft³.

1.3.1.4. Graham's Law of Diffusion

.

(1.6)

In other words, a gas lighter than air will diffuse faster than one heavier than air.

For example, methane has a specific gravity of 0.55 and carbon dioxide has a specific gravity of 1.5 compared with air, hence methane will diffuse 1.65 times faster than carbon dioxide.

1.3.1.5. Air Density at Higher Altitude

The air density is normally measured at sea level, and it decreases as the altitude increases. The temperature also normally goes down as the altitude increases. Madison [5] provides a mathematical relationship as follows:

(1.7)

where W2 is the density at height, H over the sea level; W1 is the density at the sea level.

An example:

Calculate the density of air, W2 on top of Mt. Everest at 29,000ft where

1.3.1.6. Pressure Versus Fluid Head

In coal mining practices, ventilation pressures are small in magnitude and hence they are measured by inches of water or mercury. Water and other liquid pressures are measured in pounds per square inch.

The equation for conversion is shown in Eq. (1.8).

(1.8)

where p is the pressure in  lbs/ft²; W1, W2 are the density of the fluid in  lb/ft³; H1, H2 are height of the fluid column in feet.

An example:

Atmospheric pressure of air is measured in inches of mercury. Typically it is 30 inches Hg.

Convert it into inches of water.

Density of mercury is 13.6; water is 1.00.

Using Eq. (1.8), atmospheric pressure  =  30  ×  13.6  =  H2 • 1.

Hence, H2  =  408 inches or 34 feet of water.

Example 1:

A mine fan is running at 10 inches of water. Convert it into lb/ft².

=52lb/ft²

It is important to remember that:

1 inch of water=5.2lb/ft²

1 inch of Hg=13.6 inches of water

1 psi=2.036 inches of Hg=27.7 inches of water.

1.4. Pollutant Control Strategy

In modern, highly mechanized and productive coal mines, it is not possible to dilute the respirable dust and gases generated by the mining process to safe levels by ventilation alone. Engineering control of a pollutant requires the following strategy:

1. Minimize the generation of dust or gas at the source.

2. Suppress the dust/gas at the source.

3. Collect or contain the pollutant at the source.

4. Dilute the remaining pollutant by ventilation to safe levels.

Thus in the case of respirable dust, the generation of dust can be minimized by pretreating the coal with water and using sharp cutting bits to cut coal. Next, well-designed water sprays can be used to suppress the dust at the site. The dust that gets airborne can be next collected by a scrubber on site. Finally, the remaining airborne dust can be diluted to a safe level of less than 1.5  mg/m³ by adequate ventilation air.

Similarly for methane in coal, the emission of gas can be minimized at the source by drainage of gas ahead of mining. Water sprays on cutting machines create a good mixing of gas and air. Finally, enough air is circulated to dilute the gas to less than 1% by volume and render it safe.

For diesel exhaust, the control strategy is as follows:

1. Select engines and fuel (low sulfur) that have a low specific diesel particulate matter emission, preferably less than 5 gm/bhp-hr.

2. Make the exhaust go over a catalytic convertor, where harmful gases, such as CO are converted to CO2.

3. Next, let the exhaust go through a filtration system where most particulates are collected with an efficiency of 90%–95%.

4. Finally, dilute the exhaust with enough air to render it harmless.

1.5. Enforcement of Ventilation Standards

The final goal of a safe coal mining air environment is achieved by the 3  E's.

Engineering: The purpose of this book is to provide the engineering control principles.

Education: It is the domain of academic institutions. Technology transfer must be a part of the education process.

Enforcement: The basic requirement for engineering control must be enforced.

An agency of the US federal government does the inspection and enforcement in coal mines. It is called Mine Safety and Health Agency or MSHA in the United States.

1.5.1. Mine Ventilation Regulations

Each coal mining country has its own ventilation regulations, but they are very similar. Large countries, such as the United States, have not only federal regulations that apply to all states but each state also has its own state regulations for local situations that may not be adequately covered by the federal regulations. State regulations generally are stricter than federal regulations.

1.5.1.1. US Federal Regulations

Federal regulations for all mining activity are specified in the Code of Federal Regulations (CFR). It is divided into 50 titles. Title 29 of the CFR regulates tunnel construction work. Title 30 is devoted to mineral resources and is the most pertinent for coal mining. CFR 30, part 70 deals with health standards, whereas CFR 30, part 75 deals with safety standards in coal mines [6].

All ventilation standards (as shown in Table 1.1) are established by the CFR Title 30 or the threshold limits established by the American Conference of Governmental Industrial Hygienists [7]. Some excerpts from these vast documents are provided here to illustrate the safety measures. Reference should be made to the original documents for details.

1.5.2. Maximum Concentration of Explosive Gases in Coal Mine Air

The legal maximum concentration of several gases in coal mines are limited by MSHA as shown below:

These limits are much lower than the minimum explosive limits of these gases in air. It will be discussed in detail later in the book.

1.5.3. Some Highlights of Code of Federal Regulations 30, Parts 70 and 75

1.5.3.1. Respirable Dust Measurement

Respirable dust is theoretically defined as particles smaller than 5  μm, but it is actually measured by the amount collected by the approved sampling instrument on a filter. The standard for respirable dust requires that the average concentration (of five consecutive shifts) to which a miner is exposed be at or below 1.5  mg/m³. In addition, the respirable dust concentration in the intake air to the same working section should be below 1  mg/m³.

When the respirable coal dust contains more than 5% quartz, the dust standard is lowered by the following formula:

Thus if silica concentration is 10%, the respirable dust standard is reduced to 1  mg/m³. Additional details on dust measurement will be provided later in the book.

Both the mechanized mining units and the designated areas are sampled for an effective control of respirable coal dust. Respiratory equipment, either a filter type respirator or a supplied-air type device, is provided to a miner required to inspect dusty areas with more than 1.5  mg/m³ of dust concentration.

1.5.3.2. Methane Measurements

30 CFR 75 requires frequent checks on methane levels. Each working area is examined within 3  h of start of work for methane concentrations. A certified person uses an approved device to measure and record all methane readings in a working area.

The second examination is on-shift measurement of both methane and oxygen. At the working face, a reading should be taken every 20 minutes. In addition, each mining machine is fitted with a methane detector. At 1.5% the machine gives a visible (yellow light) and audible alarm. At 2% a red light comes on and electric power to the machine is cut. The machine is not restarted until excess methane is cleared and safe levels of methane are obtained.

1.5.3.3. Minimum Air Requirements

Minimum air quantities, often at specified velocities, are required to make sure all contaminants are adequately diluted.

Federal government requires a minimum of 6000 CFM for the development sections and 9000 CFM for longwall sections. These air quantities may be grossly inadequate if the coal seam is moderately or highly gassy [8]. State regulations of West Virginia require larger air quantities because mines are gassier.

1.5.3.4. Permissible Electrical/Diesel Equipment

To further secure the safety of coal mines, all electrical or diesel equipment working inbye of last open cross-cut must be permissible. Such equipment will not ignite an explosive mixture of methane and air. This is verified in laboratory before a ‘permissible’ certificate is issued. The 30 CFR is a large document and goes in detail to secure mine safety. Reference can be made to it for additional information.

1.6. Definition of Air (Gas) Properties [9]

For a clear understanding, various properties of air and gases are defined in the following section.

1.6.1. Atomic Weight

Atomic weight is the relative weight of the atom on the basis of oxygen as 16. For a pure isotope, the atomic weight rounded off to the nearest integer gives the total number of neutrons and protons making up the atomic nucleus. These weights expressed in grams are called gram atomic weights.

1.6.2. Avogadro's Number

Avogadro's law says that equal volumes of different gases at the same pressure and temperature contain the same number of molecules. The number of molecules in 1  g-molecular weight of a substance is 6.02  ×  10²³ (±1%).

1.6.3. British Thermal Unit

It is the quantity of heat required to raise the temperature of one pound of water by 1° of Fahrenheit at, or near, its point of maximum density (39.1°F).

1.6.4. Critical Temperature and Pressure

Critical temperature is that temperature above which a gas cannot be liquefied by pressure alone. The pressure under which a substance may exist as gas in equilibrium with liquid at the critical temperature is the critical pressure.

1.6.5. Density

It is the concentration of matter, measured by mass per unit volume. It is expressed as lb/ft³.

1.6.6. Dew Point

It is the temperature at which condensation of vapor in the air takes place.

1.6.7. Enthalpy or Heat Content

It is a thermodynamic quantity equal to the sum of internal energy in a system plus the product of the pressure–volume work done on the system.

Thus:

H=E+pv (Btu/lb)

Where

H=enthalpy or heat content

E=internal energy of the system

p=pressure

v=volume

1.6.8. Entropy

It is the capacity factor for isothermally unavailable energy. The increase in the entropy of a body, ds, during an infinitesimal stage of a reversible process is equal to the infinitesimal amount of heat, Q, absorbed divided by the absolute temperature of the body, T.

Thus for a reversible process:

1.6.9. Mole Volume

The volume occupied by a mole or a gram molecular weight of any gas at standard conditions is 22.414  L.

1.6.10. Relative Humidity

It is the ratio of the quantity of water vapor present in the atmosphere to the quantity which would saturate it at the existing temperature. It is also the ratio of the pressure of water vapor present to the pressure of saturated water vapor at the same temperature.

1.6.11. Specific Heat

Heat required to raise the temperature of unit weight of a gas by 1°F at constant pressure (Cp) or volume (Cv) and is measured in Btu/lb°F.

1.6.12. Viscosity

It is the drag or shear resistance of air to motion. It is measured in lbs/ft². This is also called absolute (μ) viscosity. Absolute viscosity divided by mass density is called kinematic viscosity (υ).

Problems

1.1 Calculate the height of a column of dry air equivalent to 1 psi pressure. Assume standard conditions for atmospheric pressure and temperature.

1.2 Repeat the above calculation for water.

1.3 Calculate the density of air at 10,000ft. Assume air density at sea level is equal to 0.075lb/ft³.

1.4 Calculate the maximum allowable respirable dust concentration in a coal mine if the respirable dust contains (a) 10%, (b) 15%, and (c) 20% silica.

1.5 Calculate the specific gravity of methane, ethane, propane, hydrogen, carbon dioxide, nitrogen, and oxygen if the specific gravity of air is 1.00. (Hint: specific gravity is proportional to the molecular weight).

References

[1] World Energy Reserves and Consumptions, http://en.wikipedia.org/wiki/world_energy_consumption/; 2013.

[2] Landis E.R, Weaver J.W. Global coal occurrences: hydrocarbons from coal. AAPG Study of Coal Geology. 1993;38:1–12.

[3] World Coal Statistics. World coal association. 2013. http://worldcoal.org/.

[4] Thakur P.C. Gas and dust control. In: Darling P, ed. Chapter 15.4 SME mining engineering handbook. 2011:1595–1609.

[5] Madison R.D. Fan engineering. Buffalo, New York: Buffalo Forge Company; 1999.

[6] Anon. Code of federal regulations Title 30. Washington, DC: Mineral Resources, US Government Printing Office; 1980.

[7] Anon. TLVs threshold limit value for chemical substances and physical agents in workroom environments. Cincinnati, Ohio: American Conference of Governmental Industrial Hygienists; 1979.

[8] Thakur P.C, Zachwieja J. Methane control and ventilation for 1000-ft wide longwall faces. In: Proceedings of Conference on Longwall USA, Pittsburgh, PA, USA. 2001:167–179.

[9] Hodgman C.D, et al. Handbook of chemistry and physics. Cleveland, Ohio, USA: The Chemical Rubber Publishing Company; 1962:3125–3193.

2

Air Flow in Mine Airways

Abstract

The chapter deals with the fundamentals of fluid flow in pipes and mine airways. Basic pressure loss equation is derived and various ways to estimate the friction factor, λ, are presented. In fully turbulent flow only the degree of roughness determines the friction factor. However, mine airways are very different from conventional pipes. Atkinson's equation is developed further, and a large collection of friction factors from the US and British coal mines are presented. Mine airways are typically rectangular and also have a lot of obstructions that cannot be theoretically analyzed. Actual data with experience alone can yield reliable results. Airflow in ventilation ducts made of steel or fiberglass are discussed. Shock losses owing to obstructions and changes in airway directions are also discussed. Total resistance of a mine to airflow, R, is used to create a characteristic curve for the mine. It is useful in determining the correct fan size for the mine (to be discussed later in the book). The concept of equivalent orifice is mathematically analyzed and used to determine the area of a regulator in a mine airway to restrict the air flow to a predetermined value. Mine airways change the cross section many times owing to geology or ground control conditions. Such airways are treated as airways in series. Similarly, often two to five airways are needed in parallel to carry a given volume of air. Both series and parallel airways are mathematically analyzed for air flow distribution and pressure losses. Finally, an equation is provided to estimate the horsepower needed to run a fan that can provide a prescribed ventilation air volume at a required pressure differential.

A coal