What is a rapid reaction between oxygen and fuel that produces thermal energy?

4.4.1 Waste sources and types

Wood-treating plants generate significant amounts of waste wood in the form of bark, sawdust, wood chips, and trimmings. Provided this scrap does not come into contact with treatment chemicals, it is a good source of biomass fuel that can be used to generate electricity and steam. When mixed with treated wood and/or process sludge, both special engineering and operational practices are needed to ensure an ultra-high degree of combustion efficiency in order to prevent the formation of dioxins, furans, and PAHs.

Given the typically low moisture (10%), sulfur (<0.3%), nitrogen (<0.4%), and ash (<2%) contents, as well as a high heating value (>21 kJ/g) and bulk density compared to other biomass fuels, many treated woods, such as telephone poles, transmission poles and railroad ties, are a potentially attractive renewable fuel for co-firing in industrial wood-waste boilers. Such treated woods are available in large quantities and often have very high landfill disposal costs (up to $80/ton) to utilities and other industries. Process sludge, on the other hand, is not a good source of renewable fuel for co-firing. These wastes contain many inert materials that lower combustion efficiency, and toxic metals, and often have a high moisture content, which further lowers combustion efficiency. Up until the early 1990s with the introduction of the boiler industrial furnace (BIF) rules, many wood-treating facilities indiscriminately co-fired wood-waste boilers with this type of waste without any regard for the pollution generated. Today there is ongoing concern that wood-treating facilities that do continue to burn treated wood wastes generate significant pollution.

The possibilities for treated wood recycling are limited, because the treating chemicals can easily diffuse in undesirable areas like the interiors of buildings, or in soil and groundwater. The production of particleboard from treated wood is also questionable due to the same concerns. Similar reasons inhibit the use of treated wood residues in landspreading. Here, the danger of biocide release by uncontrolled leaching needs to be considered.

Combustion of either treated wood or process sludge results in the formation of both fly and bottom ashes that will contain not only dioxins, furans, and PAHs, but heavy metals as well. The term fly ash refers to the lighter particulate matter that is emitted from the stack of the boiler, whereas bottom ash is the heavier particulate matter that settles out on to the grate of the furnace. Fly ash can be controlled using mechanical types of air pollution controls such as multiclones, wet scrubbers, electrostatic precipitators, or fabric filters (called baghouses). The ash itself may be considered toxic as it will contain heavy metals. Creosote coal tars are derived from coal, which contains a wide range of metals including arsenic, cadmium, chrome, and lead. These tend to partition during combustion, concentrating in the bottom ash of the boiler. As such the ash must be managed responsibly either in secure landfills or by vitrification methods.

Combustion is a complex series of chemical reactions, but from a physical standpoint may be described as the rapid combination of oxygen with a fuel, such as natural gas or wood, resulting in the release of heat. Most fuels contain carbon and hydrogen, and the oxygen usually comes from air. Combustion generally consists of the following overall reactions:

Carbon+Oxygen→Carbon dioxide+Heat

Hydrogen+Oxygen→Water vapor+Heat.

Stoichiometric or perfect combustion is obtained by mixing and burning exactly the correct proportions of fuel and oxygen so that no oxygen remains at the end of the reaction. If too much oxygen is supplied, the mixture is lean and the reaction is oxidizing. This results in a flame that is relatively shorter. If too much fuel is supplied, the mixture is rich and the reaction is reducing. This typically results in a flame that is relatively longer and sometimes smoky.

In terms of chemistry, combustion is a chemical reaction in which an oxidant reacts rapidly with a fuel to liberate stored energy as thermal energy, most often in the form of high-temperature gases. Small amounts of electromagnetic energy (light), electric energy (free ions and electrons), and mechanical energy (noise) are also produced during the combustion process.

Conventional hydrocarbon fuels contain primarily hydrogen and carbon, in elemental form or in various compounds. Their complete combustion produces carbon dioxide (CO2) and water (H2O); however, small quantities of carbon monoxide (CO) and partially reacted flue gas constituents (gases and liquid or solid aerosols) also form. Most conventional fuels also contain small amounts of sulfur, which is oxidized to sulfur dioxide (SO2) or sulfur trioxide (SO3) during combustion, and noncombustible substances such as mineral matter (ash), water, and inert gases.

Flue gas is the product of complete or incomplete combustion and includes excess air (if present), but not dilution air.

Fuel combustion rate depends on three factors:

the rate of the chemical reaction of the combustible fuel constituents with oxygen;

the rate at which oxygen is supplied to the fuel (the mixing of air and fuel); and

the temperature in the combustion zone.

The reaction rate is fixed by fuel selection. Increasing the mixing rate or temperature increases the rate of combustion.

When engineers refer to complete combustion of hydrocarbon fuels, they infer that all hydrogen and carbon in the fuel is oxidized to form H2O and CO2. Generally, for complete combustion to be accomplished excess oxygen or excess air must be supplied beyond the amount theoretically required to oxidize the fuel. Excess air is usually expressed as a percentage of the air required to completely oxidize the fuel.

In stoichiometric combustion of a hydrocarbon fuel, the fuel is reacted with the exact amount of oxygen required to oxidize all carbon, hydrogen, and sulfur in the fuel to CO2, H2O, and SO2. Therefore, exhaust gas from stoichiometric combustion theoretically contains no incompletely oxidized fuel constituents and no unreacted oxygen (i.e. no CO and no excess air or oxygen). The percentage of CO2 contained in products of stoichiometric combustion is the maximum attainable and is referred to as the stoichiometric CO2, ultimate CO2, or maximum theoretical percentage of CO2.

Stoichiometric combustion is seldom if ever realized in practice in simple or conventional combustion equipment because of imperfect mixing and finite chemical reaction rates. For economy and safety, most combustion equipment is designed to operate with some excess air. The purpose of this is to ensure that fuel is not wasted and that combustion is complete despite variations in fuel properties and in the supply rates of fuel and air. The amount of excess air to be supplied to any combustion equipment depends on four primary factors:

the expected variations in fuel properties and in fuel and air supply rates;

the equipment application;

the degree of operator supervision required or available;

the control requirements for the machine used to burn fuel.

As a general rule, in order to achieve maximum efficiency, combustion at low excess air is desirable.

The term incomplete combustion refers to a condition when a fuel element is not completely oxidized during combustion. For example, a hydrocarbon may not completely oxidize to carbon dioxide and water, but may form partially oxidized compounds, such as carbon monoxide, aldehydes, and ketones.

Conditions that promote incomplete combustion include such factors as:

insufficient air and fuel mixing (causing local fuel-rich and fuel-lean zones);

insufficient air supply to the flame (providing less than the required quantity of oxygen);

insufficient reactant residence time in the flame (preventing completion of combustion reactions);

flame impingement on a cold surface (quenching combustion reactions);

flame temperature that is too low (slowing combustion reactions).

When incomplete combustion occurs the combustion process uses fuel inefficiently, and it can be hazardous because carbon monoxide and various products of incomplete combustion are generated as air pollution.

For chemical engineers in particular, combustion is a challenging subject because the combustion reactions are complex. The reaction of oxygen with the combustible elements and compounds in fuels occurs according to fixed chemical principles, including:

chemical reaction equations;

law of matter conservation – the mass of each element in the reaction products must equal the mass of that element in the reactants;

law of combining masses – chemical compounds are formed by elements combining in fixed mass relationships;

chemical reaction rates;

oxygen availability – oxygen for combustion is normally obtained from air, which is a physical mixture of nitrogen, oxygen, small amounts of water vapor, carbon dioxide, and inert gases.

For practical combustion calculations (i.e. within everyday engineering accuracy), dry air consists of 20.95% oxygen and 79.05% inert gases (nitrogen, argon, and so forth) by volume, or 23.15% oxygen and 76.85% inert gases by mass. For calculation purposes, nitrogen is assumed to pass through the combustion process unchanged (although small quantities of nitrogen oxides form and, in the case of incomplete combustion, nitrated PAHs are formed).

Other important factors in combustion are:

flammability limits;

ignition temperature;

heating value.

Fuel burns in a self-sustained reaction only when the volume percentages of fuel and air in a mixture at standard temperature and pressure are within the upper and lower flammability limits or explosive limits (UEL and LEL). Both temperature and pressure affect these limits. As the temperature of the mixture increases, the upper limit increases and the lower limit decreases. As the pressure of the mixture decreases below atmospheric pressure, the upper limit decreases and the lower limit increases. This is a well-understood principle and one that has been experimentally determined by many investigators and reported by the National Fire Protection Association (NFPA) as well.

The term ignition temperature refers to the lowest temperature at which heat is generated by combustion faster than heat is lost to the surroundings and combustion becomes self-propagating. Table 4.11 provides some typical values for the ignition temperatures of common materials. The fuel–air mixture will not burn freely and continuously below the ignition temperature of a material unless heat is supplied, but chemical reaction between the fuel and air may occur. Ignition temperature is affected by a large number of factors.

Table 4.11. Typical ignition temperatures of common fuels in fuel–air mixtures

SubstanceTemperature (°F)Activated coke1220Carbon monoxide1128Hydrogen968Methane1301Propane871n-Butane761Ethylene914Acetylene763–824Sulfur374Hydrogen sulfide558

Combustion produces thermal energy or heat. The quantity of heat generated by complete combustion of a unit of specific fuel is constant and is termed the heating value, heat of combustion, or caloric value of that fuel (Table 4.12). The heating value of a fuel can be determined by measuring the heat evolved during combustion of a known quantity of the fuel in a calorimeter, or it can be estimated from chemical analysis of the fuel and the heating values of the various chemical elements in the fuel.

Table 4.12. Typical heating values of materials found in common fuels

SubstanceHigher heating value (Btu/lb)aSpecific volume (ft3/lb)bCarbon (to CO)3950–Carbon (to CO2)14,093–Carbon monoxide434713.5Hydrogen61,095188Methane23,87523.6Ethane22,32312.5Propane21,6698.36Butane21,3216.32Acetylene21,50214.3Sulfur (to SO2)3980–Sulfur (to SO3)5940–Hydrogen sulfide653711

Higher heating value, gross heating value, or total heating value are terms that include the latent heat of vaporization and is determined when water vapor in the fuel combustion products is condensed. Conversely, lower heating value or net heating value is obtained when the latent heat of vaporization is not included. When the heating value of a fuel is specified without designating higher or lower, it generally means the higher heating value in the USA. (The term lower heating value is mainly used for internal combustion engine fuels.)

Heating values are usually expressed in Btu/ft3 for gaseous fuels, Btu/gal for liquid fuels, and Btu/lb for solid fuels. Heating values are always given in relation to a certain reference temperature and pressure, usually 60, 68, or 77°F and 14.696 psia, depending on the particular industry practice.

With incomplete combustion, not all fuel is completely oxidized and the heat produced is less than the heating value of the fuel. Therefore, the quantity of heat produced per unit of fuel consumed decreases, implying lower combustion efficiency.

Not all heat produced during combustion can be used effectively. The greatest heat loss is the thermal energy of the increased temperature of hot exhaust gases above the temperature of incoming air and fuel. Other heat losses include radiation and convection heat transfer from the outer walls of combustion equipment to the environment.

Now let's focus on the burning of wood. The combustion of wood has three requirements: fuel, air, and heat. If any one of these is removed, burning ceases. When all three are available in the correct proportion, combustion is self-sustaining because the wood itself releases more than enough heat to initiate further burning.

The rate at which wood burns is controlled by the amount of air. A lack of air causes wood to smolder and produce pollutants. Too much air will cool the fire and waste heat.

Another very important aspect of combustion is the energy content of the wood fuel, i.e. the Btu (British thermal units) content. Energy content is greatly affected by the moisture content and weight of the wood. For example, hardwood and softwood at 50% moisture will contain about 4700 Btu per pound. The same wood at 20% moisture will contain about 6200 Btu per pound. Hardwood has about twice the weight as softwood and twice the heat content. The same is true with wood chips – 4000 Btu per pound green (50% moisture content) and 7400 Btu per pound dry (10% moisture content).

Taking these factors into consideration, let's examine the stages of wood burning. Wood combustion may be simply described as undergoing three distinct stages of burning, where all three stages occur simultaneously:

Stage 1. The wood is heated to evaporate and drive off moisture.

Stage 2. Starting at about 500°F (260°C) wood begins to break down chemically and volatile matter is vaporized. The vapors contain 50–60% of the heat value of the wood. These vapors have to be heated to 1100°F (593°C) at a minimum in order to burn. If not, smoke is generated that can coat heat exchange surfaces and chimneys with creosote, and produce harmful PAHs and dioxins.

Stage 3. Once the volatile gases are released, the remaining material (charcoal) burns at temperatures above 1500°F (815°C).

The process described is idealized, but it gives us a simplified working framework to better characterize the nature of wood combustion.

A final term we introduce is that of destruction and removal efficiency (DRE), which is most appropriately related to incineration. Incineration is different from normal combustion in the sense that the objective is to completely consume fuel or rather any toxins contained in the fuel or harmful products produced as a result of the combustion process. This term is important because some wood-treating facilities operate wood-waste boilers for the purpose of generating steam and chemicals can enter the feed to boilers that are used for this purpose. A primary measure of incineration performance is the characteristic known as DRE for polyorganic hydrocarbons (POHCs). DRE is an explicit RCRA performance standard that requires the achievement of 99.99% DRE performance (“four nines”) for non-polychlorinated biphenyl (PCB) and nondioxin waste. Dioxin-containing or dioxin-forming wastes require incineration facilities that are capable of achieving 99.9999% DRE (“six nines”). When chemicals such as creosote or PCP are introduced into the fuel by means of chemical spills, process sludge from treating cylinders, or treated wood scrap, PAHs, dioxins, dibenzofurans, and other hazardous air pollutants can be generated. These pollutants need to be controlled through appropriate combustion controls, by retrofitting the combustion equipment with air pollution controls, by preventing chemicals from entering the fuel feed, and through continuous emissions monitoring.

Incineration under controlled conditions is considered the most appropriate destruction method for treated wood wastes. Oehme and Zuller (1995) and Chagger et al. (1998) have noted that the advantages of incineration include:

volume reduction;

lower net CO2 emissions (the carbon dioxide released during combustion of biomass equals that taken up during growth; full or partial replacement of fossil fuels with wood or biomass would therefore reduce net emissions of CO2);

the possibility of substituting other fuel types for energy production (because of the good calorific value and low sulfur content);

the removal of residues of environmental concern.

As noted, it is well known that polychlorodibenzodioxins and polychlorodibenzofurans are always formed during wood combustion via precursors like phenols and lignin or via de novo reactions in the presence of particulate carbon and chlorine. When waste wood is burnt, the levels of dioxins and furans in the flue gas emissions are significantly lower than those obtained from other sources. Even nontreated wood contains small amounts of chlorine, which means that dioxin emissions can only be minimized, but not eliminated (Salthammer et al., 1995). According to new EC regulations regarding the classification of biomass and of waste combustion systems, many biomass residues (e.g. nondangerous wood residues) will be subject to severe dioxin emission regulations, because a perfect separation of “clean” wood and “nonclean” varieties (i.e. from demolition, containing laminates or particleboard) is often impossible at the post-consumer level. This compromises a large part of the potential use of biomass combustors, in particular in small-scale applications.

The severity of pollution depends on both the composition and concentration of the waste streams that are co-fired and the combustion dynamics, which are limited by the boiler design and the types of pollution controls employed. Here we make an important distinction in that wood-waste boilers typically used by the industry are classical combustion systems that differ from incinerators. The role of combustion equipment is to burn a relatively clean fuel in an efficient manner for the purpose of generating steam and electricity. It is a practice that aims to convert waste to energy. Incineration has a very different objective. The objective of incineration is to apply energy to efficiently and completely destroy certain chemicals of concern. Incineration does not or should not concern itself with how efficiently a waste is converted into a useful energy form, but rather its objective is to supply as much energy as possible in order to destroy chemical toxins. Historical practice by the industry sector shows that it has generally not bothered to make this distinction, but rather has abused legitimate waste to energy projects by attempting to incinerate wastes such as sludge, arguing these sources to be fuel supplements.

A primary measure of incineration performance is the characteristic known as DRE for POHCs. This explicit RCRA performance standard requires the achievement of 99.99% DRE performance (“four nines”) for non-PCB and nondioxin waste. Dioxin-containing or dioxin-forming wastes require incineration facilities that are capable of achieving 99.9999% DRE (“six nines”). Unless the six nines of destruction can be demonstrated though stack testing, an industrial wood-waste boiler cannot be considered an appropriate device for burning treated wood or process sludge.

There is considerable controversy in the literature on reported emission factors for treated wood-waste burning. We believe that only emission factors based on stack testing measurements followed by continuous emissions monitoring (CEM) should be employed to ensure that pollution emissions do not pose harm to communities. The foregoing discussion summarizes emission factors reported in the literature. While we have relied on such data in ascertaining historical impacts on communities, uncertainties in the application of these factors only permit ranges of probable emissions to be calculated to within scientific certainty. In this day and age, where stack testing and monitoring instrumentation are sophisticated and precise, it is irresponsible not to employ such tools to accurately quantify, report, and control stack emissions.

MILD oxy-fuel combustion

The presented study focuses on MOFC technology, which is a next step in clean coal technologies. MOFC combines the advantages of moderate and intense low-oxygen dilution (MILD) combustion and oxy-fuel combustion (OFC) to achieve efficient and environmentally justified CO2 capture from fossil fuel-based power generation. The advantages of MOFC are:

It increases the efficiency of the coal-fired boiler;

It increases the purity of CO2 in the flue gases;

It reduces the oxygen consumption of the boiler by using lower oxidizer excess;

It reduces the energy consumption associated with CO2 recirculation.

Therefore using MOFC decreases the overall net energy efficiency penalty associated with CO2 capture from coal-fired power plants. MOFC boiler design also gives an opportunity to include membrane ASUs with heat integration on required high temperature levels. Thus, within this research, a preliminary thermodynamic analysis of an integrated MOFC power plant with CO2 capture is presented. The data for the new boiler design were obtained by CFD modelling [103], while other technological modules of the integrated MOFC power plant were modelled by process modelling software. Data concerning CO2 transport, utilization and storage were obtained from available databases and literature.

10.12.5.2 Nr Creation

Fossil-fuel combustion created only about 0.6 Tg N year− 1 in 1890 through production of NOX (Table 5). Crop production was primarily sustained by recycling crop residue and manure on the same land where food was raised. Since the Haber–Bosch process was not yet invented, the only new Nr created by human activities was by legume and rice cultivation (the latter promotes Nr creation because rice cultivation creates an anaerobic environment that enhances N fixation). Although estimates are not available for 1890, Smil (1999) estimates that in 1900, cultivation-induced Nr creation was ~ 15 Tg N year− 1. Additional Nr was mined from guano (~ 0.02 Tg N year− 1) and nitrate deposits (~ 0.13 Tg N year− 1) (Smil, 2000).

Thus, in 1890, the total anthropogenic Nr creation rate was ~ 15 Tg N year− 1, almost all of which was from food production. In contrast, the natural rate of Nr creation was ~ 220 Tg N year− 1. Terrestrial ecosystems created ~ 100 Tg N year− 1, and marine ecosystems created ~ 120 Tg N year− 1 (within a range of 87–156 Tg N year− 1) (Galloway et al., 2004). An additional ~ 5 Tg N year− 1 was fixed by lightning. Thus, globally, human activities created about 6% of the total Nr fixed and about 13% when only terrestrial systems are considered.

One century later, the world's population had increased by a factor of ~ 3.5, from about 1.5 billion to about 5.3 billion, but the global food and energy production increased approximately 7- and 90-fold, respectively. Just as was the case in 1890, in 1990 (and now), food production accounts for most new Nr created. The largest change since 1890 has been the magnitude of Nr created by humans. Smil (1999) estimates that in the mid-1990s, cultivation-induced Nr production was ~ 33 Tg N year− 1. The Haber–Bosch process, which did not exist in 1890, created an additional ~ 85 Tg N year− 1 in 1990, mostly for fertilizer (~ 78 Tg N year− 1) and the remainder in support of industrial activities, such as the manufacture of synthetic fibers, refrigerants, explosives, rocket fuels, and nitroparaffins.

From 1890 to 1990, energy production for much of the world was transformed from a biofuel to a fossil-fuel economy. The increase in energy production by fossil fuels resulted in increased NOx emissions – from ~ 0.6 Tg N year− 1 in 1890 to ~ 21 Tg N year− 1 in 1990. By 1990, over 90% of the energy produced created new Nr. There was substantial atmospheric dispersal. Thus, in 1990, Nr created by anthropogenic activities was ~ 140 Tg N year− 1, an ~ 9-fold increase over 1890 even though there was only an ~ 3.5-fold increase in the global population. With the increase in Nr creation by human activities came a decrease in natural terrestrial N fixation (from ~ 100 to ~ 89 Tg N year− 1) because of the conversion of natural grasslands and forests to croplands, etc. (C. Cleveland, personal communication).

Coal Combustion

Most coal combustion processes are used to convert water in a boiler to high-pressure steam, which is then used to drive turbines for electric power generation (Figure 13) or used in industrial and commercial heating applications. Other uses of combustion, without the need for a boiler, include the heating of limestone and other ingredients in a kiln for cement manufacture and the production of heat, char, and carbon monoxide for the processing of iron ores by direct reduction methods.

Coal combustion for electric power production mostly involves ignition of finely ground or pulverized coal (pulverized fuel or pf), blown with air through a jet into a furnace chamber. The heat energy from the combustion is then used to produce high-pressure steam, which is in turn used to drive the associated turbine and generator system (Figure 14). Other types of combustion processes include burning of coarsely crushed or broken coal (solid coal) in a loose granular bed, or forcing a strong updraft of air through a layer of crushed coal to lift the particles and burn them under fluidized bed conditions. Stabilized slurries of fine coal in oil or water, known respectively as coal–oil mixtures (COM) and coal–water mixtures (CWM), may also be burned in combustion operations, using similar techniques to those used for combustion of oil and other liquid fuels.

2.2.3 Fossil fuel burning and atmospheric deposition

The by-products of fossil fuel combustion principally, exhaust from motor vehicles (mobile source) and electric power generation (fixed source), are major sources of N to coastal waters in many regions. Nevertheless, the relative importance of these combustion sources of reactive N varies around the world. For example, in Asia, road transport accounted for nearly 28% of N oxide emissions in 1990 that was 45% in Europe in 1998 (Bradley and Jones, 2002). However, Asia contributes to larger share of N oxide emissions in coal burning than Europe, which relies more on oil, natural gas, etc. for electric power generation. Nitrogen-based trace gases such as nitric oxide released during fossil fuel combustion may be directly deposited onto the coastal waters. Additionally, those deposited on landscape as acid rain or dry pollutants can run off into surface water and thus reach coastal ecosystems. Direct atmospheric deposition of nitrogen may contribute between 1% and 40% of the total nitrogen inputs to coastal ecosystems and is most pronounced near to emission sources (Fig. 2.6). This also depends on the size of the coastal ecosystem relative to its watershed; the larger the size, the greater will be the percentage of N that is deposited (Korpinen and Bonsdorff, 2015). In contrast to P, the amount of N exported into coastal waters from nonagricultural landscapes, including forests, can be substantial. In Chesapeake Bay, atmospheric depositions comprise 12% and 6.5% of TN (total nitrogen) and TP (total phosphorus) inputs, respectively (Kemp et al., 2005). With relatively larger watersheds than the size of the coastal ecosystem, the greater source will be run off of N deposited in landscapes. This may be more important than direct deposition and more difficult to quantify (Nixon and Fulweiler, 2009).

As fuel combustion releases reactive or biologically available N into the atmosphere, it can easily travel great distances before deposited on land and water. Hence, the source of atmospheric N deposited in coastal watersheds can be far from the coast and even outside the watershed area that drains into a bay or estuary. The area from which various materials may be put into the atmosphere and reach a given estuary is called the airshed of that estuary (Fig. 2.5). For example, the airshed of Chesapeake Bay is 6.5 times larger than the watershed of the bay, which is nearly 17 times larger than the bay (Nixon and Fulweiler, 2009). In addition to fossil fuel burning, agriculture contributes to N in the atmosphere in two ways: from cultivation of N-fixing crops as well as from industrial production of inorganic N fertilizers by the Haber–Bosch process (Smil, 2002). The production of reactive N from agriculture is five times greater than that from fuel combustion (Galloway et al., 2002). On a global scale, atmospheric nitrogen deposition has strongly increased during the Anthropocene with centers mainly in densely populated and intensively used regions (Fig. 2.6).

The Coming Century of Struggle to Lower the Risks of Climate Change

In 2015 carbon emissions from fossil fuel combustion, though not land use change, stabilized. This is one of few encouraging developments, as the Anthropocene progresses (Butler, 2016b). Discouragingly, however, the growth in the atmospheric concentration of greenhouse gases accelerated in 2016, despite this partial leveling of emissions. While this growth is mainly attributed to the very strong El Niño that persisted into the second half of 2016, it is likely that a minor contribution to this increase was from reinforcing feedbacks, especially from thawing tundra and increased wetlands in the Arctic. Average global surface temperatures in 2016 reached a new high of 14.8°C, 1.3°C above the preindustrial level.

In coming decades, several linked struggles relevant to climate change will continue; some could become desperate. Many of these struggles are relevant to population health, via various pathways, one of which is food security, the focus of this article. One such struggle, already alluded to, is the race between emissions from direct anthropogenic (human) sources and those from earth system feedbacks, such as a climate change induced drying of the Amazon rainforest, worsened by deforestation. To date, the consensus is that the share from these feedbacks in the rising greenhouse gas concentrations is minor, though increasing. However, a spectre which shadows humanity is that greenhouse gas emissions due to feedbacks, such as from melting permafrost, could in coming decades rival and then exceed emissions directly released by human activity. If this “emission transition” (see glossary) were to occur, it would likely render ineffectual the global switch to cleaner energy, (the energy transition—see glossary) even if then advanced and ambitious, and trigger aggressive geoengineering (see glossary) in a desperate attempt to limit the harm of and block additional, runaway climate change.

A second race is between the declining cost of fossil fuel-sparing energy systems (including energy storage) and the influence of the fossil fuel industry and its lobbyists and propagandists (Obama, 2017). The price of comparatively clean energy is the main determinant of its displacement of coal and oil, assisted by subsidies and consumers’ consciences. While falling, the price of alternatives to fossil fuel is still too high to fully compete with the widely subsidized price of fossil fuels, but prices are converging. Today, on a global scale, most new power installations rely on renewable energy sources. The fall in oil prices was assisted by new fracking technologies that in turn helped to provoke Saudi Arabia to increase oil production, in an attempt to retain market share. This price fall has delayed the transition to clean energy, hopefully not fatally (Obama, 2017).

Energy from the combustion of biomass, the dominant fuel in many rural, low-income settings is even cheaper than from coal and requires less expensive infrastructure to use for cooking. Such combustion harms air quality on an enormous scale, significantly damages health, puts further pressure on the environment but may have a net cooling effect. Energy from solar and wind is likely to remain more expensive for cooking than from biomass for the foreseeable future.

A third major competition is between the physical Earth system and the global social system. The concerted and widespread opposition to action to slow climate change, embodied, for example, by the beliefs and influence of US President Trump, is strong evidence that parts of the global social system lag the recognition by the natural scientific community of the grave threat to long-term prosperity which climate change constitutes. As a result, not only food systems but also peace is at risk in an increasing number of settings.

While it is still possible that society will respond to the emerging crisis of global environmental change in a constructive way (when and if it becomes unavoidable), many lines of evidence suggest otherwise. For example, though sea level rise in places such as Miami, Florida, USA, is increasingly problematic, economic and political drivers continue to attempt to disguise its risk to investors and insurers—even if insurers are harder to fool. Although conflict in places such as Syria and Yemen is attributed, by some, as aggravated by a scarcity of natural resources, including of water, influential schools of thought within political science continue to deny such relationships.

What is a rapid reaction between oxygen and fuel called?

What type of reaction is fuel and oxygen?

What is it called when fuels burn in oxygen to release energy?

What does the reaction of fuel and oxygen produce?