Air Pollution Prevention and Control

Bioreactors and Bioenergy

Christian Kennes, Maria C. Veiga, Christian Kennes, Maria C. Veiga

• Publisher: Wiley
• Year: 2013
• Print ISBN: 9781119943310
• eBook ISBN: 9781118523353
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Over the past two decades, the use of microbes to remove pollutants from contaminated air streams has become a widely accepted and efficient alternative to the classical physical and chemical treatment technologies. This book focuses on biotechnological alternatives, looking at both the optimization of bioreactors and the development of cleaner biofuels. It is the first reference work to give a broad overview of bioprocesses for the mitigation of air pollution. Essential reading for researchers and students in environmental engineering, biotechnology, and applied microbiology, and industrial and governmental researchers.

Part I

Fundamentals and Microbiological Aspects

Chapter 1

Introduction to Air Pollution

Christian Kennes and María C. Veiga

Department of Chemical Engineering, University of La Coruña, Spain

1.1 Introduction

This book describes the different biodegradation processes and bioreactors available for air pollution control as well as other alternatives for reducing air pollution, mainly by using more environmentally friendly fuels and biofuels, such as ethanol, hydrogen, methane or biodiesel. Only the bioreactors and (bio)fuels most widely used or studied over the past decade are reviewed in this book. Bioreactors, for which not much significant research or many new developments have occurred over the past decade, have been described in other book chapters [1] and are not included in this book.

1.2 Types and sources of air pollutants

Two major groups of pollutants can be considered in terms of air pollution: particulate matter and gaseous pollutants. The latter may be subdivided into volatile organic compounds (VOCs) and volatile inorganic compounds (VICs). The best available treatment technology will depend on the composition and other characteristics of the emissions to be treated. The most significant contaminants and their origin are shown in Figure 1.1, in terms of emission percentages, in 2006 by source category for the 27 member states of the European Union. The member states are (year of entry in brackets) Austria (1995), Belgium (1952), Bulgaria (2007), Cyprus (2004), Czech Republic (2004), Denmark (1973), Estonia (2004), Finland (1995), France (1952), Germany (1952), Greece (1981), Hungary (2004), Ireland (1973), Italy (1952), Latvia (2004), Lithuania (2004), Luxembourg (1952), Malta (2004), The Netherlands (1952), Poland (2004), Portugal (1986), Romania (2007), Slovakia (2004), Slovenia (2004), Spain (1986), Sweden (1995) and the United Kingdom (1973).

Figure 1.1 Distribution of EU-27 total emission estimates for different pollutants, by source category, in 2006.

Table 1.1 and Table 1.2 compare the annual emission estimates for both the European Union (EU-27) and the United States, considering anthropogenic land-based sources only [2]. Natural sources of emission and other possible sources such as navigation have not been included, as comparable information for Europe (EU-27) and the United States often could not be obtained. Although some recent data were sometimes not available for the United States and needed to be extrapolated [2], it is still possible and accurate to conclude that the results follow in both cases a similar trend for the different pollutants, in terms of both the relative total emission of each pollutant and the source of pollution. However, some differences may still be found when analysing the tables in detail, mainly in the case of carbon monoxide (CO) emission. For example, in Europe, almost 43% of CO emissions come from mobile sources (vehicles and transportation in general), while this represents as much as 85% in the United States. Conversely, CO from combustion sources represents about 44% in Europe, while it is only 7% in the United States.

Table 1.1 2006 emission estimates for different pollutants, by source category, in the European Union (EU-27) (10⁶ kg yr⁻¹). Reprinted under the terms of the STM agreement from [2] Copyright (2012) Elsevier Ltd.

Table 1.2 2006 emission estimates for different pollutants, by source category, in United States (

kg yr⁻¹). Reprinted under the terms of the STM agreement from [2] Copyright (2012) Elsevier Ltd.

1.2.1 Particulate matter

Particulate matter can be defined as a small solid or liquid mass in suspension in the atmosphere. Primary particles are directly emitted from a polluting source, while secondary particles are formed in the atmosphere as a result of reactions or interactions between pollutants and/or compounds present in the atmosphere, usually volatile organic compounds, nitrogen oxides or sulphur oxides as well as water. A water droplet of acid rain, carrying sulphuric acid (

) or nitric acid (

) produced from nitrogen oxides (NOx) or sulphur oxides (SOx), would be classified as particulate matter. Different terms can be used for particles (e.g. dust, smoke, mist or aerosol) depending on their nature and characteristics.

Although many particles are not spherical, for the sake of simplicity and for engineering calculations, nonspherical particles are often assimilated to spheres of the same volume as the original particle. Particle size, then, refers to the corresponding particle diameter.

Typically, the size (diameter) of particulate matter found in polluted air or waste gases may vary between about

and a few hundreds of micrometres (

), although smaller and larger particles may also be found. Larger particles do, however, settle quite fast, and in that way are quickly eliminated from the atmosphere. In order to give an idea of the scale,

is a common size for viruses, while coal particles, flour or cement dust may be around 10⁺² µm. The sizes of the latter may, however, vary considerably, between only a few micrometres and about 1 mm. The same is true for water droplets, for example mist or raindrops, with sizes ranging between a few micrometres up to more than 1 mm. Particles of

are considered large particles. Particulate matter is classified as

for sizes up to

, and

for smaller sizes up to

.

The effect of particles on health is more important in the case of smaller particles, for instance those below

, as they will more easily reach the lungs than larger particles. Some particles may carry heavy metals and carcinogenic molecules. They can also cause disorders of the respiratory system, asthma, bronchitis and even heart problems. Besides, particles can reduce visibility and be involved in acid precipitations, or acid rain, described later in this chapter.

1.2.2 Carbon monoxide and carbon dioxide

According to data of the European Environment Agency and the US Environmental Protection Agency (EPA), the highest emission of gaseous pollutants to the atmosphere corresponds to emissions of CO, in both the European Union and the United States (Table 1.1 and Table 1.2). Close to 50%, or somewhat more, of the total anthropogenic emission of pollutants corresponds to CO. Large amounts may be produced by natural sources as well. On average, mobile sources account for about 85% of the total CO emissions in the United States. It reaches 42.6% in Europe; another 43.7% come from combustion processes in stationary sources (Figure 1.1). Considering that a large part of mobile sources are vehicles such as cars and trucks, it becomes obvious that CO pollution will be more significant in urban areas. As mentioned, the second largest source of CO emission, after motor vehicle exhaust, corresponds to stationary combustion processes and other industrial production processes. Its main origin is the incomplete combustion of fossil fuels or other materials such as wood. Combustion is the result of a reaction between oxygen and a fuel. Carbon dioxide, water and heat will be produced if the reaction is complete and if the fuel contains only carbon, hydrogen and, eventually, oxygen atoms, such as in the example of methane (a major chemical present in natural gas):

1.1

Carbon monoxide, instead of carbon dioxide, will be formed when the combustion is not complete, as shown in this reaction:

1.2

Several reasons may be involved in this incomplete reaction. The most important ones are the amount of available oxygen, temperature, reaction time and turbulence. The theoretical amount of oxygen needed for complete combustion can be calculated from the stoichiometric equation. However, some excess air is generally recommended for ensuring complete oxidation, but not too much, since excess air needs to be heated as well. Increasing the temperature and residence time in the combustor will be favourable to complete combustion, as well as increasing turbulence in order to achieve intimate mixing between the oxygen and fuel.

Carbon monoxide is not of significant conce...

Table of contents

• Cover
• Title Page
• Copyright
• List of Contributors
• Preface
• Part I: Fundamentals and Microbiological Aspects
• Part II: Bioreactors for Air Pollution Control
• Part III: Bioprocesses for Specific Applications
• Part IV: Environmentally-Friendly Bioenergy
• Part V: Case Studies
• Index