Oxy-Fuel Combustion for Power Generation and Carbon Dioxide (CO2) Capture

L Zheng

• Publisher: Woodhead Publishing
• Year: 2011
• ISBN:
  9780857090980
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

Oxy-fuel combustion is currently considered to be one of the major technologies for carbon dioxide (CO2) capture in power plants. The advantages of using oxygen (O2) instead of air for combustion include a CO2-enriched flue gas that is ready for sequestration following purification and low NOx emissions. This simple and elegant technology has attracted considerable attention since the late 1990s, rapidly developing from pilot-scale testing to industrial demonstration. Challenges remain, as O2 supply and CO2 capture create significant energy penalties that must be reduced through overall system optimisation and the development of new processes.Oxy-fuel combustion for power generation and carbon dioxide (CO2) capture comprehensively reviews the fundamental principles and development of oxy-fuel combustion in fossil-fuel fired utility boilers. Following a foreword by Professor János M. Beér, the book opens with an overview of oxy-fuel combustion technology and its role in a carbon-constrained environment. Part one introduces oxy-fuel combustion further, with a chapter comparing the economics of oxy-fuel vs. post-/pre-combustion CO2 capture, followed by chapters on plant operation, industrial scale demonstrations, and circulating fluidized bed combustion. Part two critically reviews oxy-fuel combustion fundamentals, such as ignition and flame stability, burner design, emissions and heat transfer characteristics, concluding with chapters on O2 production and CO2 compression and purification technologies. Finally, part three explores advanced concepts and developments, such as near-zero flue gas recycle and high-pressure systems, as well as chemical looping combustion and utilisation of gaseous fuel.With its distinguished editor and internationally renowned contributors, Oxy-fuel combustion for power generation and carbon dioxide (CO2) capture provides a rich resource for power plant designers, operators, and engineers, as well as academics and researchers in the field.

• Comprehensively reviews the fundamental principles and development of oxy-fuel combustion in fossil-fuel fired utility boilers
• Provides an overview of oxy-fuel combustion technology and its role in a carbon-constrained environment
• Introduces oxy-fuel combustion comparing the economics of oxy-fuel vs. post-/pre-combustion CO2 capture

1

Overview of oxy-fuel combustion technology for carbon dioxide (CO₂) capture

L. Zheng, CanmetENERGY, Natural Resources Canada, Canada

Abstract:

This chapter provides an overview of the concepts, main components, background, advantages, and challenges of oxy-fuel combustion technology for power generation and carbon dioxide (CO₂) capture. Brief descriptions of other carbon capture technologies and their comparisons with oxy-fuel technology are outlined. A concise description of each chapter in this book is included.

Key words

oxy-fuel combustion

carbon capture technologies

clean coal technology

1.1 Introduction

Oxy-fuel combustion is currently considered to be one of the major technologies for carbon dioxide (CO₂) capture. This book focuses on the development of oxy-fuel combustion technologies using coal as fuel.

1.1.1 Coal as an energy source

Coal plays a very important role in our day-to-day lives. In a comprehensive report published in 2008, the International Energy Agency (IEA) predicted that the demand for coal will surpass oil in absolute terms between 2030 and 2050, and will become the predominant fuel for the world (IEA, 2008–1). Currently, about 40% of the world’s electricity is generated with coal (WCI, 2009) making it the largest fuel source for power generation (IEA, 2008–1). In the two largest CO₂ emitting countries, China and the United States of America, more than 77% and 50% of the electricity, respectively, is generated with coal (WCI, 2009). The improvement of global living standards and continuous economic growth will require increased use of energy. From 2000 to 2006, IEA reported that worldwide demand for electricity increased from 12,641 TWh to 15,665 TWh, a stunning rise in demand of nearly 24%. This growth is expected to continue at an average annual rate of 2.5%; coal generation is projected to produce 14,600 TWh of electricity by 2030 – more than double its current contribution of approximately 6300 TWh (IEA, 2008–2).

The attraction of coal as a fuel source is due to several factors. First, it is abundant: even under rapid growth scenarios, known coal reserves can continue to meet our energy needs for at least the next 100 years (Lackner and Sachs, 2005). Indeed, some studies have suggested that there is more than 190 years’ worth of coal available, almost four times that of oil and gas combined (WCI, 2009). Unlike oil and gas, coal is well distributed in the world, making it easily accessible and very reliable. Coal is also one of the most affordable energy resources at one to two US dollars per MM Btu; by contrast, oil and gas costs are in the range of 6 to 12 US dollars per MM Btu (MIT, 2007). This combination of attributes makes it very likely that coal will continue to be a critical fuel source well into the future.

1.1.2 Developments in clean coal technology

Coal as an energy source has a number of negative environmental impacts, including (but not limited to) the release of particle matter (PM), oxides of sulphur and nitrogen (SO_x and NO_x), carbon monoxide (CO), and trace metals such as mercury. It is a long procedure from the point of realization of the need to control emissions, to pass appropriate emissions control regulations and standards, to develop control technologies, and to effect their implementation. With the gradual installation of each add-on unit for emissions reduction, the environmental impacts associated with coal combustion have been greatly reduced. Installation of electrostatic precipitators (ESP) and/or baghouses, initiated in the 1970s, allows for flue gas flyash reductions above 95%. Flue gas desulfurization (FGD) technologies, such as wet scrubbers introduced in the 1980s, are capable of 90% SO_x removal. Since the 1990s, low NO_x burners have been employed to reduce nitrogen oxide formation. Combined with selective catalytic reactors (SCRs), it is now possible to reduce NO_x emissions by more than 90% (EPA, 2006). Rapid developments are currently taking place to address the issues of fine particulate matter less than 2.5 μm (PM_2.5) and mercury emissions. Clean coal combustion technologies have become major business concerns in coal utilization. It has been reported that the capital and operating costs of emission control systems of a typical 500 MW_e coal-fired power plant are roughly 47% and 57% of the total respective costs (Marin et al., 2003).

Of increasing concern, coal combustion is also one of the largest sources of anthropogenic CO₂ emissions. In 2006, about 42% of the world energy-related CO₂ emissions were attributable to coal use (IEA, 2008–2). On an annual basis, a typical 500 MW_e coal-fired power plant emits about three million tonnes of CO₂ to the atmosphere (MIT, 2007), the equivalent of the total CO₂ emissions from 374,000 passenger cars (EPA, 2000).

The concern about CO₂ emissions from coal-fired units has prompted intensive research into its control technologies. Some reductions can be achieved by upgrading the coal by washing, drying and briquetting. Optimized operating conditions on excess air and stack temperature reductions lead to better efficiencies and, hence, lower emissions. Furthermore, up to 25% CO₂ emission reduction can be obtained through the utilization of supercritical and ultra supercritical boiler technologies (Beér, 2007).

One of the most feasible options to stabilize CO₂ levels in atmosphere is carbon (CO₂) capture and storage (CCS). CCS is a process in which CO₂ is removed from emission gases, transported, and stored (sequestered) in a location where it is isolated from the atmosphere. The United Nations Intergovernmental Panel on Climate Change (IPCC) has noted that ‘CCS has the potential to reduce overall mitigation costs and increase flexibility in achieving greenhouse gas emission reduction’ (IPCC, 2005). Due to the large quantity and concentrated nature of CO₂ emissions from coal-fired power generation stations, these emitters have become the focus of CCS development.

1.1.3 Carbon capture technologies

Currently, there are three major CO₂ capture technologies that have reached the level of industrial-scale demonstration. These three technologies are:

• Post-combustion capture: a chemical solvent such as amine or ammonia is used to scrub CO₂ out of the combustion flue gas.

• Pre-combustion capture: solid fuel is gasified with oxygen to produce a gaseous fuel consisting mainly of carbon monoxide (CO) and hydrogen (H₂). A water–gas shift reaction is employed to convert CO and water to H₂ and CO₂ and a physical sorbent is then used to capture CO₂.

• Oxy-fuel combustion: pure oxygen is used for fuel combustion, thereby producing a CO₂-enriched flue gas ready for sequestration once water is condensed from the flue gas and other impurities are removed.

The post-combustion capture approach is the same as the approaches for control of particulate matter, SO_x, and NO_x. This approach involves adding unit operations after combustion, making post-combustion capture an attractive option for retrofitting existing plants or building a CO₂ capture-ready plant. For postcombustion technology, a liquid solvent such as monoethanolamine (MEA) or ammonia is used in an absorption tower to scrub CO₂ from the flue gas. The CO₂-rich solvent is then pumped to a stripper or regeneration tower where heat is used to separate CO₂ from the solvent. The captured CO₂ is then compressed and transported for storage. Chemical solvents are already widely used in refineries, natural gas processing, and petrochemical plants to capture CO₂; demonstrations of solvent applicability for coal-fired power plants are currently underway. Because air is used for combustion, most (70%) of the flue gas is N₂ with comparatively little (< 15%) CO₂, hence large equipment is needed for post-combustion capture processes. Most post-combustion capture technologies are capable of capturing more than 90% of the CO₂ at very high purity levels. A major challenge for post-combustion capture technologies is the intensive energy needed to regenerate solvent or to cool the flue gas when the chilled ammonia process is employed. Furthermore, stringent flue gas limits for SO_x, NO_x, and flyash are necessary to minimize solvent usage when amines are used.

Coal gasification is a central technology for pre-combustion CO₂ capture. In gasification, coal is reacted with oxygen and steam at high temperature and pressure to produce a synthetic gas (syngas). Usually, less than 50% of the oxygen required for complete combustion is used in gasification; thus, the coal is only partially oxidized to CO and H₂, the major syngas components. To capture CO₂ from a gasification process, a water–gas shift reactor is used to convert CO and water to H₂ and CO₂. The CO₂ is then separated with physical sorbent. Gasification technology is widely employed in the chemical industry. However, its application in the power sector is very limited due to the high cost associated with it for electricity generation. At present, there are only four commercial-scale power generation plants in the world that gasify coal (Ratafia-Brown et al., 2002). The process that utilizes coal gasification for power g...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Energy
• Foreword
• Natural Resources Canada: Ressources naturelles Canada
• Chapter 1: Overview of oxy-fuel combustion technology for carbon dioxide (CO2) capture
• Part I: Introduction to oxy-fuel combustion
• Part II: Oxy-fuel combustion fundamentals
• Part III: Advanced oxy-fuel combustion concepts and developments
• Index