13.1: Introduction

Combustion is the principal technology that powers the energy economy. Simply stated, combustion is at the heart of our everyday lives, from the provision of electricity to our home and place of work, to the automobiles we drive, to the propulsion of jet aircraft we fly. Combustion is also the principal source of the environmental impact we experience, from climate change to degraded urban air quality.

The following four principal forces are driving the paradigm shifts from our dependency on combustion to alternative technologies for the generation of electricity and powering of vehicles:

1. Degraded urban air quality (1943): The first evidence of persistently degraded urban air quality in the United States was chronicled in the Los Angeles Times, describing a tenacious haze that seemed to irritate eyes and cause many to cough (Figure 13.1.1). Today, urban regions throughout the world (for example, in India, China) are affected by degraded air quality.

Figure 13.1.1 Los Angeles 1943: Degraded urban air quality. Reproduced with permission from Getty Images.

2. Finite petroleum resources (1980s): Automobile companies recognized that petroleum was finite and demand may outweigh discovery in the next millennium.
3. Climate change (1990s): The world recognized that anthropogenic sources may be affecting the climate, leading to the signing of the UN Framework Convention on Climate Change in 1992 (Chapter 10).
4. Fuel independence (2001): The assault on the World Trade Center enhanced the urgency to reduce US dependence on foreign sources of petroleum.

Box 13.1 Atmospheric Pollutants

In this chapter, two groups of anthropogenic emissions (CO₂ and criteria pollutants) are considered. The formal designation of criteria pollutants (ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, lead, and particulate) was established in 1970 by the US Clean Air Act based on demonstrated health and environmental impacts established by a series of “criteria” studies. Some of the criteria pollutants (“primary” criteria pollutants) are emitted directly from the exhaust of combustion and other sources, while other criteria pollutants (“secondary” criteria pollutants) are formed in the atmosphere from reactions of primary criteria pollutants. The concentration of criteria pollutants emitted in the exhaust is very low (often less than 10 parts of pollutant per million parts [ppm] of exhaust), but when the emissions accumulate from the large population of sources in an urban basin, they result in a health impact.

In 2009, carbon dioxide was classified by the US Environmental Protection Agency (EPA) as a pollutant that poses a danger to human health and welfare. The typical concentration in the exhaust of a combustion source is approximately 120,000 ppm (that is, 12% of the volume). Unlike criteria pollutants, which affect public health within hours to days of exposure near the source of their emission, CO₂ has a more insidious impact, taking years to generate demonstrable and unambiguous climate change worldwide.

Combustion

Figure 13.1.2 Combustion.

Depending on the type of engine, either air is compressed to a high pressure and fuel is added, or a fuel-air mixture is compressed to a high pressure. In both cases, the fuel-air mixture is then ignited, initiating a combustion process (essentially “burning” the fuel-air mixture) that transforms the energy bound in the fuel (for example, gasoline) to high-temperature gas (thermal energy). The high-pressure, high-temperature gas then pushes on a piston (to power the transmission in a traditional gasoline vehicle, or generate electricity in a gasoline hybrid vehicle) or expands through a turbine (to generate electricity for the home and business). From this process, depicted in Figure 13.1.2, you can intuitively deduce that (1) the efficiency (the percentage of energy bound in the fuel that is transformed to useful power) will be limited by the friction associated with all of the mechanical steps, and (2) criteria pollutants will be formed because of combustion chemistry and emitted in the exhaust.

When you consider the role of combustion in everyday life, the examples seem limitless (for example, cooking; heating water; space heating; generating electricity; propelling aircraft and rockets; and powering automobiles, buses, trucks, locomotives, and ships). Simply stated, combustion is interwoven into the fabric of both the quality of life and the economics of the world’s markets.

Figure 13.1.3 Combustion impacts. Image of earth reproduced with permission from Science Photo Library.

In Figure 13.1.3, the relationship between combustion and the environment is illustrated. Fuel and air are injected into a chamber, ignited to liberate the energy bound in the fuel into thermal energy, and expanded to produce a useful product.

Unfortunately, combustion has an exhaust as a by-product composed of criteria pollutants that degrade urban air quality (affecting the public health) and carbon dioxide (affecting the world’s climate). Notably, the amount of criteria pollutant mass in the exhaust is minuscule and was historically ignored until the first consequences to public health in modern times surfaced in 1943 (Los Angeles) and 1952 (London).* It is as if Nature incorporated environmental impacts in the combustion of fossil fuels to counsel the world’s population that combustion is not sustainable.

Why is it that such a minuscule emission of a few chemical criteria pollutant molecules affects the urban air basin, and a larger but still relatively modest emission of CO₂ affects the world’s climate? Consider that the atmosphere is evenly distributed in a thin layer around the Earth, barely 10 miles in depth. In Figure 13.1.3, the purple sphere in the image represents the volume of all the air if it were gathered together, relative to the volume of the Earth. The image conveys the surprisingly small air resource upon which life on Earth depends, and the relatively small volume of air into which products of combustion are injected. Within this small volume, CO₂ and other greenhouse gases (GHGs) accrue to affect climate, and secondary criteria pollutants are formed and primary criteria pollutants amass to degrade urban air quality. As noted in Figure 13.1.3, combustion is responsible for over 90% of the world’s emission of CO₂ and criteria pollutants.

In addition to contaminating the air resource with CO₂ and criteria pollutants, the combustion process has an impact not widely recognized: namely the consumption of oxygen from the air. For every tankful of gasoline in your car, a ton of air (2,000 pounds) passes through your engine, and 400 pounds of oxygen are consumed. Given the finite resource of oxygen in the atmosphere, this is sobering. While Nature appears to be replenishing the oxygen removed to date, an increasing demand for oxygen could lead to an additional point of environmental stress. Fortuitously, the evolving transition from a classic “combustion-dominant construct” to a “renewable-dominant construct” will, in parallel with reducing the emission of CO₂ and criteria pollutants, serve to mitigate the likelihood of this environmental stress.

The electric grid

Figure 13.1.4 The classic electric grid.

A principal role of combustion is the generation of electricity. The electric grid is represented in Figure 13.1.4 in its classic form. Electric power is generated at large, central power plants in the general range of 100 to 1,000 megawatts (MW). While hydro and nuclear contribute to varying degrees, combustion fueled by fossil fuels (natural gas, oil, or coal) has historically been the dominant strategy for the generation of electricity.

Figure 13.1.5 The emerging electric grid.

The classic form of the electricity grid, however, is not the only way in which electricity can be provided to houses, businesses, and factories. Figure 13.1.5 illustrates the following four potential paradigm shifts from the classic to the future electric grid.

1. Use distributed generation (DG), the generation of power at the point of use (Figure 13.1.5 ①). This could take the form of fossil fuel power plants such as gas turbines, solar panels, fuel cells, or ground source heat pumps that extract heat from under the ground. The advantages of this paradigm are threefold:
   • Avoiding transmission losses. By generating electricity at the point of use, the loss in energy due to conveying electricity from central power generators to the urban loads, estimated to be in general 7%, is avoided.
   • Increasing reliability. Generating electricity at the point of use increases the reliability of the electricity supply to the customer. Should the grid experience an outage, for example, DG can power critical circuits (at a minimum) and, if needed, power all circuits.
   • Capturing and using exhaust heat. With generation at the point of use, the heat in the exhaust can be captured and used to serve thermal loads (such as steam, hot water, and chilled water) and thereby displace electricity and natural gas that would otherwise be required for these purposes. This gives rise to high overall efficiencies that can exceed 90%. Terms used to describe this attribute are combined heat and power (CHP) and combined cooling, heat, and power (CCHP).
2. Provide direct current power. The clean power generators emerging for the DG market (for example, photovoltaic panels, fuel cells, and micro-turbine generators) produce direct current (DC) that is converted to alternating current (AC) with a concomitant loss of energy estimated to be 10%. Then, the AC power is converted back to DC (with another estimated loss of 10%) to serve DC loads in a building, examples of which are lighting, personal computers, and servers. By serving these loads directly with DC, DG can avoid the conversion inefficiencies.
3. Deploy renewable power generation. The third paradigm shift is the deployment of renewable solar and wind resources in central generation, as well as the deployment of solar in distributed generation (Figure 13.1.5 ②). The advantage of this paradigm is the displacement of the fossil fuel generation of power, by utilizing the sun as the fuel resource, and the transition from combustion to a sustainable future that supports a clean, inexhaustible fuel supply (the sun) and protection of the environment. In California, for example, the penetration of renewable solar and wind resources has increased dramatically in the past decade (exceeding 30%) and is on course to meet a target of 60% in 2030 (Figure 13.1.6). California’s renewable energy policies are discussed further in Chapter 9.

Figure 13.1.6 California annual renewable percentage estimates. Data from California Energy Commission 2018.

In contrast to traditional central generating plants that produce electricity continuously around the clock, renewable solar and wind resources vary diurnally—that is, the power produced varies throughout the day due to the presence and angle of the sun and the availability and strength of the wind. They also experience intermittencies, such as from a cloud momentarily shading a photovoltaic resource and dropping the generation, or a burst or drop in wind momentarily increasing or decreasing generation from a wind source. Diurnal variation refers to the daily cycle, while intermittencies are short-term and less predictable.

Renewable resources also have a low capacity factor, defined as the percentage output divided by the maximum (often called “name plate”) output over a month, year, or other period of time. For example, traditional central plants have capacity factors of approximately 50%, whereas renewable resources have capacity factors of approximately 25% (solar) and 32% (wind). The capacity factors of 24/7 base load generators** are below 100% because of load following (that is, plant operators or controllers turning down the generation to match the load), whereas the capacity factors for renewable resources are low because of the diurnal variation.

Renewable resources cannot load follow, generating instead whenever the “fuel” (sun or wind) is available. As a result, renewable wind and solar are “must take” resources, and other technologies must be used to meet the load demand. If the load is less than the renewable generation capacity, either the excess energy must be stored (for example, in electric batteries, as pumped hydro, or in the generation of hydrogen), or the renewable generation resources must be curtailed. Curtailment is the action of reducing (in the extreme, turning off) the renewable wind or solar generation resource when load on the grid (that is, demand) is insufficient to utilize the electricity that would otherwise be produced.

4. Improve energy storage. A fourth paradigm shift is the deployment of battery storage at both the central and distributed generation levels (Figure 13.1.5 ③) to buffer and manage (1) the diurnal variation and intermittencies associated with wind and solar renewable resources, (2) uncontrolled vehicle charging loads,*** and (3) the demand for rapid ramping of spinning reserves**** with the goal to provide a resource that can absorb an increase in generation in the absence of load and also discharge energy when the load exceeds the generation capacity.

Figure 13.1.7 Lithium-ion battery.

The most pervasive electric battery technology used today, from cell phones to multi-megawatt applications, is the lithium-ion (Li-ion) battery (Figure 13.1.7). Just like your flashlight battery, the Li-ion battery stores energy (by charging on demand) and dispatches energy (by discharging on demand).

While the electrolyte allows lithium ions to flow in both directions, electrons are rejected by the electrolyte and must instead flow through an external circuit from one electrode to the other. When the battery is fully charged, all of the lithium ions are in the anode. When the battery is discharging (Figure 13.1.7a), the lithium ions travel through the electrolyte to the cathode while the electrons travel through the external circuit and energize a load (for example, a lightbulb). When the battery is charging, energy from a power source (for example, the grid) creates a flow of electrons from the positive cathode back to the negative anode.

Anodes in a Li-ion battery are typically composed of a carbon material that is able to absorb and store the electric charge. The cathode is an oxide of lithium such as lithium nickel manganese cobalt oxide, or lithium manganese oxide.

In the future, energy storage technologies may be required in addition to electric batteries to (1) absorb the enormous amount of otherwise curtailed energy, (2) provide the ramp rates (rate at which the generation resource responds to load change) required for both the absorption and reuse of the energy, (3) store the energy for months (for example, from one season to another), and (4) counter the self-discharge associated with electric batteries. While pumped hydro is expected to complement electric batteries, opinions differ as to whether additional, more flexible and highercapacity energy storage technologies (for example, flow batteries and/or hydrogen “batteries”) will be required.

*Ramifications of combustion exhaust were observed centuries before, an example of which is “fumifugium” (Evelyn 1661).

**A base load generator is an electric power plant that provides a constant supply of electricity to meet the minimum load demand.

***Uncontrolled vehicle charging loads result from the charging of plug-in electric vehicles (PEVs) with no control over key variables (for example, the time of day the charging occurs, the duration of the charging, and the rapidity with which charging occurs). As the population of PEVs grows, control over these variables will be required to protect grid resources (for example, transformers) and assure that generation resources are available to meet the charging load.

****Spinning reserves refers to rotating machinery (for example, gas turbines) that are spinning but generating little or no electricity and ready thereby to immediately (with a short delay) generate electricity if called upon. (This is similar to an aircraft with engines idling at the beginning of takeoff.)