Biomass to Renewable Energy Processes

Jay Cheng, Jay Cheng

• Editorial: CRC Press
• Año: 2017
• ISBN:
  9781498778855
• Edición: 2
• Categoría: Physical Sciences
• Categoría: Energy

Biomass to Renewable Energy Processes, Second Edition, explains the theories of biological processes, biomass materials and logistics, and conversion technologies for bioenergy products such as biogas, ethanol, butanol, biodiesel, and synthetic gases. The book discusses anaerobic digestion of waste materials for biogas and hydrogen production, bioethanol and biobutanol production from starch and cellulose, and biodiesel production from plant oils. It addresses thermal processes, including gasification and pyrolysis of agricultural residues and woody biomass. The text also covers pretreatment technologies, enzymatic reactions, fermentation, and microbiological metabolisms and pathways.

1

Introduction

J. Cheng Jay

North Carolina State University

CONTENTS

1.1 Objectives of the Book

1.2 Renewable Energy vs. Fossil Fuel Energy

1.3 Life Cycle Assessment

1.4 Problems

References

1.1 Objectives of the Book

This book is written as a textbook for a graduate course in renewable energy production for both graduate and senior undergraduate students, focusing on the areas of agricultural, biological, chemical, and environmental engineering as well as crop, food, plant, and wood sciences. It is also intended to be a reference book for professional engineers and scientists who are interested in the processes of converting biomass into renewable energy sources.

This book introduces fundamental principles and practical applications of biomass-to-renewable energy processes, including biological, chemical, and thermochemical processes. Chemical properties of a variety of biomass are presented in the book. Resources of biomass that can be utilized for renewable energy production are also presented in the book, including their production and basic characteristics. Logistics of biomass handling such as harvesting, transportation, and storage are included in the book as well. Biological processes include anaerobic digestion of organic waste materials for biogas and hydrogen production, and bioethanol and biobutanol production from sugars, starch, and cellulose. Pretreatment technologies, enzymatic reactions, fermentation, and microbiological metabolisms and pathways are presented and discussed in the book. Chemical process of biodiesel production from plant oils, animal fats, waste oils and fats, and microalgae is described and discussed in the book. Thermal processes include combustion, gasification, and pyrolysis of woody biomass and agricultural residues. Finally, engineering principles of biomass combustion, gasification and pyrolysis, and potential end products are discussed in the book.

1.2 Renewable Energy vs. Fossil Fuel Energy

Energy consumption has increased steadily since the 1900s as the world population has grown and more countries have become industrialized. Fossil fuel, especially crude oil, is currently the predominant energy source around the world. Table 1.1 shows the top five crude oil consuming countries in 2012 (Energy Information Administration, 2017). However, the reserves of fossil fuel are limited and will be depleted in the near future at its current consumption rate. Although there are debates about the exact year of peak oil production, it is generally believed that it will occur before 2025, after which significant reduction of oil production should be expected. Moreover, burning fossil fuels causes environmental concerns such as greenhouse gas (GHG) emission, which is generally believed to be the major reason for global climate change. Because the world economy depends on oil, the consequences of inadequate oil availability could be severe. Therefore, there is a great interest in exploring alternative energy sources. The negative environmental impact of burning fossil fuels reminds us that the alternative energy source should be sustainable and environment friendly. Energy production from biomasses such as crop, herbaceous, woody, and organic waste materials have a great advantage over fossil fuels. The former is renewable annually or in several years, while the latter needs thousands or millions of years for reproduction. The energy production from biomass releases CO₂, which is believed to be a major GHG and cause of global climate change, but the CO₂ is utilized for biosynthesis during the growth of biomass. Thus, using biomass for energy production can have a balanced CO₂ production and consumption or little net CO₂ release, compared to a huge discharge of CO₂ from burning fossil fuels. Currently, energy production from biomass is only a small portion of the total energy production. In 2016, the source of energy consumption in the United States includes 80.7% fossil fuels (petroleum, natural gas, and coal), 6.9% nuclear energy, 5.8% biomass, 2.5% hydroelectric power, and 4.0% other renewable energy (Energy Information Administration, 2017). The percentage of biomass energy has significantly increased in the last 10 years. The resources of biomass materials such as crops, grasses, wood, agricultural residues, and organic wastes are quite abundant, so there is a great potential to substantially increase energy production from biomass materials.

Ethanol production from dedicated crops or agricultural residues is one form of renewable energy that addresses the critical need for sustainable fuels. There is already a well-developed market for ethanol in the U.S.; almost 15 billion gallons (57 billion liters) of ethanol were produced in 2015 according to the Renewable Fuels Association. Ethanol is primarily used as an additive to gasoline to improve emissions and boost octane. Biogas is generally produced from organic waste materials such as sewage sludge, agricultural wastes, industrial wastes, and municipal solid wastes. Proper treatment of these waste materials is necessary to protect our environment from pollution. At the same time, biogas production from treatment facilities provides a renewable energy source. A number of commercial biogas production plants have been in operation utilizing sewage sludge, animal manure, and municipal solid wastes and the resulting biogas is used for either electricity generation or direct combustion for heat production. Biodiesel is produced from vegetable oils, animal fats, and waste oils, and is used as an alternative to petroleum diesel for buses and trucks. The major benefits of producing and using bioenergy generated from biomass are as follows:

TABLE 1.1
Top Crude Oil Consuming Countries in the World in 2012

Country	Crude Oil Consumption (million barrels/day)
USA	18.5
China	10.3
Japan	4.7
India	3.6
Russia	3.2

1. Energy Independence: Crude oil or natural gas reserves are located in limited regions in the world. Many countries have to rely on importing oils from these limited oilproducing countries. For example, the US Energy Information Administration reported that the US imported 56% of its overall petroleum demand in 2003, two-thirds of which were from the politically unstable Persian Gulf. The reliance makes the US vulnerable to supply disruptions and non-market related price instability. This jeopardizes the nation’s energy and economic security; according to a report from the US Congress, a dependence on foreign oil for more than 50% of a country’s needs is a peril to the country (NREL, 2000). On the other hand, biomass is almost everywhere. Renewable energy production from biomass could protect oil-deficit countries from depending on foreign oil and generate local jobs.

2. Air Quality: Oxygenated fuels such as ethanol typically promote more complete combustion as compared to fossil fuels. More complete combustion translates into fewer emissions, particularly carbon monoxide (CO) emissions (National Science and Technology Council, 1997).

3. Water Quality: Ethanol is replacing methyl tertiary butyl ether (MTBE) as the preferred octane enhancer in the US. This is because of concerns over MTBE’s persistence in the environment, and its possible negative effects on water quality and human health. Ethanol breaks down quickly, and spills of ethanol do not pose a critical threat to the environment.

4. Greenhouse Gas (GHG) Emissions: The combustion of fossil fuels results in a net increase in the emission of GHGs (primarily carbon dioxide) into the atmosphere. According to the US Energy Information Administration, the US transportation sector is responsible for approximately one-third (1/3) of all carbon dioxide emissions. The impact of GHGs on global climate change is of increasing concern around the world. Use of biogas, bioethanol, and biodiesel as energy sources can significantly reduce the net GHG emissions. For example, use of E85 (85% ethanol and 15% gasoline) can reduce the net emissions of GHGs by as much as 25% on a fuel cycle basis as compared to gasoline (Wang, 1999).

1.3 Life Cycle Assessment

The purpose of the life cycle assessment (LCA) of renewable energy production is to evaluate the environmental impacts of a product or process. LCA normally involves energy balance and GHG emissions. Energy balance determines the net energy production of a renewable energy product or process, while GHG emissions indicate its impact on climate changes. The first step in conducting an LCA is to define a boundary for the assessment, e.g., from corn cultivation to ethanol combustion for corn-based fuel ethanol production process. All the unit operations inside the boundary need to be analyzed for energy input and output, as well as GHG emissions. Again using corn-based fuel ethanol production process as an example, the unit operations generally include corn cultivation, harvest, and transportation to the ethanol plant; conversion of corn to fuel ethanol; ethanol fuel distribution; and combustion of fuel ethanol to provide energy. Corn cultivation involves mainly land preparation, seeding, fertilization, irrigation, and weed control. Corn harvest includes corn kernel collection and residue handling. Conversion of corn to fuel ethanol consists of corn grinding, saccharification, fermentation, distillation, dehydration, and byproduct processing. Similar procedures can be applied to the life cycle assessment of lignocellulose-based ethanol and biodiesel production processes. For biogas production from organic waste materials, life cycle assessment usually needs to include the cost of the waste disposal to prevent environmental pollution as an offset to the biogas production process.

TABLE 1.2
Impact of Common Greenhouse Gases (GHGs) to the Global Climate Change

Greenhouse Gas	Global Warming Potential or CO2 Equivalent
CO2	1.0
CH4	21
N2O	310

Source: U.S. Environmental Protection Agency/U.S. Greenhouse Gas Inventory Program/Office of Atmospheric Programs, Greenhouse Gases and Global Warming Potential Values, 2002.

The major greenhouse gas emission in the energy production processes is CO₂. Other GHGs related to the unit operations of renewable energy production include methane (CH₄) and nitrous oxide (N₂O). The impact of the GHGs on global climate change can be expressed with global warming potential (GWP) or CO₂ equivalent. Their typical values are listed in Table 1.2.

In general, biogas production from organic waste materials needs little energy input or has a high net energy production. It also has a high impact on GHG emission reduction. A big challenge for biogas as a renewable energy source is its low energy density, which makes it economically unfavourable to transport from one location to another. Both corn and lignocelluloses-based ethanol productions are generally believed as net energy producing processes. However, corn-based ethanol production has limited reduc...