From MZJ’s textbook in progress…
Concrete Manufacturing
Concrete is mixture of aggregate (sand, gravel, and crushed stone) and a paste (water and Portland cement). The paste binds the aggregate together, making a hard surface. Concrete is used for roads, foundations, buildings,
runways, sidewalks, driveways, and a variety of other purposes.
Joseph Aspdin (1788 to 1855) of Leeds, England, invented Portland cement in the early 19th century. He formed it by burning powdered limestone and clay on his kitchen stove. Today, cement contains limestone, shells, or chalk, all of which contain CaCO3(s), mixed with clay, shale, slate, blast furnace slag, silica sand, or iron ore. These ingredients are heated to 1500 degrees C to form a hard substance that is ground into a fine, powdery cement.
The concrete industry produces about 5 percent of the world’s CO2 emissions. About half of the emissions during concrete production are from energy use and the other half are from chemical reaction during cement manufacturing. The chemical emissions rate of CO2 from cement manufacturing is 900 kg-CO2 per tonne-cement. These emissions arise due to the reaction,
CaCO3(s) + clay + heat à clinker (SiO2, Fe2O3, Al2O3, CaO) + CO2
Where the CO2 originates from calcium carbonate. The clinker is then mixed with gypsum (CaSO4-2H2O) to form cement by
Clinker + gypsum à cement
The cement is subsequently mixed with water to form a paste, which is combined with the aggregate to form concrete.
Three ways of reducing emissions from concrete manufacturing are to
- Use an alternative to concrete that doesn’t emit CO2 as part of the chemical process,
- Make concrete that traps CO2, and
- Recycle concrete.
A Type of Concrete That Emits no CO2
One commercialized alternative to concrete is Ferrock, or iron carbonate (FeCO3) (Stone, 2017; Build Abroad, 2016). Ferrock is derived by first mixing waste steel dust containing iron (FeO) with crushed glass containing
silicon dioxide (SiO2), limestone (CaCO3), kaolinite or another clay, stabilizers, promoters, and a catalyst into a mixer at room temperature. The mixture is then poured into a mold containing seawater. The filled mold is put into a curing chamber, where CO2 from a furnace is injected. The iron, CO2, and saltwater react together to form Ferrock and molecular hydrogen (H2). When the final product dries, it is about five times harder than and more flexible than cement. The production of Ferrock not only avoids the chemical CO2 emissions and most of the energy emissions of concrete production, but it also sequesters CO2 and produces hydrogen, which can be used for other applications.
Sequestering CO2 in Concrete
Trapping CO2 from combustion emissions, as done in Ferrock, is a method of offsetting chemically produced CO2 emissions from the concrete production process. Trapping CO2 within concrete itself is another option (e.g., Carbon Cure, 2018). The clinker in cement contains CaO. When the cement is mixed with water to form a paste, the CaO reacts with water to form calcium hydroxide, Ca(OH)2, by CaO + H2O à Ca(OH)2
If CO2 captured from any source is mixed with the clinker, it will react to form CaCO3 + H2O within the cement.
Upon drying, the solid CaCO3 strengthens the cement. Even if the cement breaks, the CO2 will not break free because it is a solid bound to the cement.
Cement also contains calcium silicate hydrate (3CaO-SiO2-4H2O). CO2 can react with this chemical to form calcium carbonate, which is bound in the cement, by CO2 + 3CaO-SiO2-4H2O à CaCO3 + 2CaO-SiO2-4H2O
Concrete Recycling
A third method of reducing CO2 emissions from concrete manufacturing is concrete recycling. Concrete structures or roads are often demolished. Historically, such concrete has been sent to a landfill. However, if the
concrete is uncontaminated (free of trash, wood, and paper), it can be recycled. Rebar in concrete can also be recycled, as magnets can remove it. The rebar can then be melted and used for other purposes. The broken concrete is crushed. Crushed concrete is often used as gravel in new construction projects or as aggregate in new concrete.
Remaining CO2 emitted chemically during the cement formation process may need to be captured chemically upon emissions. However, whether it is captured or whether the funds for the capture equipment are used instead to purchase WWS electricity generators to replace fossil fuel power plants, thereby reducing both CO2 and air pollution simultaneously, needs to be evaluated on a case-by-case basis.
Controlling Non-Energy Air Pollution and Climate Relevant Emissions
WWS technologies address energy-related emissions. However, some emissions that affect human health and climate are not energy-related emissions but are still necessary to eliminate or reduce to decrease their impacts on human health and climate. These include open biomass burning, methane emissions from agriculture and waste, halogen emissions from leakage and draining, and nitrous oxide emissions from fertilizers, industry, and
wastewater treatment. These sources of emissions and methods to control them are discussed briefly.
2.9.1. Open Biomass Burning and Waste Burning
Open biomass burning is the burning of evergreen forests, deciduous forests, woodland, grassland, and
agricultural land, either to clear land for other use, to stimulate grass growth, to manage forest growth, to satisfy a
ritual, by accident (campfires, debris burning, cigarettes), or by arson. About 17 percent of all global CO2 and air
pollution emissions worldwide are from open biomass burning (Jacobson, 2014). Although, such burning may be
natural or anthropogenic in origin, humans cause around 93 percent of it today, and nature causes the rest
(Jacobson, 2014).
Agricultural fields are often burned after harvest to remove straw leftover to clear the field for a new harvest
during the next spring. Sugarcane fields are usually burned before harvest to remove the outer leaves around the
sugarcane stalks to facilitate the sugarcane extraction during harvesting.
Waste burning is the burning of trash, such as in a landfill (Figure 2.34), open pit, garbage can, or backyard
incinerator. Such waste burning is illegal in many countries but still occurs in many others.
Biomass burning produces not only gases that warm the climate, including CO2, CH4, and CO, but also climatewarming
particles, including black carbon (BC) and brown carbon (BrC). BC and BrC, along with other particle
components emitted during biomass burning (ash, other organic carbon, and sulfate), cause substantial health
impacts to people and animals who breathe them in. In addition, the oxides of nitrogen and organic gases from biomass burning result in elevated levels of ozone, formaldehyde, and other gaseous pollutants that affect human health. Waste burning emits the same chemicals as biomass burning but also emits toxic chemicals from the burning of plastics, paints, varnishes, pesticides, medical waste, and chemical byproducts.
While some argue that biomass burning followed by regrowth of vegetation results in no net increase in CO2 to the air, that contention is incorrect. It is incorrect because, even though CO2 released upon burning is offset by CO2 used to regrow the vegetation in the first place, the time lag between burning and regrowth (from 1 to 10 years for savannah and 80 years for a forest, for example, mathematically always results in elevated CO2 in the air (Jacobson, 2004). The contention is also misleading because biomass burning emits black carbon, brown carbon, water vapor, methane, carbon monoxide, ozone precursors (organic gases and oxides of nitrogen), and heat, all of which increase global temperatures and are not recycled as CO2 is. As such, biomass burning causes net global warming (Jacobson, 2014).
The only solution to biomass burning is to stop it. No technology exists to control its emissions. An alternative to burning agricultural waste straw is to till it into the soil. An alternative to sugarcane burning is to cut away the
leafy parts of the sugarcane before harvest and mix it into the soil. Since humans cause more than 90 percent of all open biomass fires worldwide, biomass burning is largely (but not completely) preventable through government policies restricting burning and discouraging the conversion of forest land to agricultural land or another land use type.
Similarly, the only method of reducing the impacts of landfill waste burning is to stop it because its emissions cannot be trapped. If waste is burned in an incinerator, many of its emissions can theoretically be controlled with
emission control technologies; however, no technology eliminates all the emissions, including all the CO2. Thus,
even incinerators with emission controls result in significant pollution. The best control of waste burning is to
eliminate it by recycling the waste or keeping it in a landfill. Waste should not be dumped into the oceans, since
the plastics and many other materials do not degrade for centuries. The accumulation of plastics in the oceans has
caused an environmental catastrophe, resulting in, for example, the Great Pacific Garbage Patch, a plastic
wasteland in the middle of the Pacific Ocean between California and Hawaii that is three times the size of France.
2.9.2. Methane From Agriculture and Waste
Methane is a long-lived greenhouse gas present in the atmosphere that selectively absorbs certain thermalinfrared
wavelengths of radiation, trapping some of that radiation near the surface of the Earth. It causes about 12
percent of global warming (Table 1.1).
Anthropogenic sources of methane include not only open biomass burning and energy-related natural gas leaks
and fossil-fuel combustion, but also biological sources enabled by human activity. A 100 percent WWS energy
infrastructure will eliminate methane emissions from energy production. Controls of biomass burning (Section
2.9.1) will reduce that source of methane. This section focuses on controlling methane from human-enabled
biological sources.
The root biological source of most methane is methanogenic bacteria, which live in anaerobic (without oxygen)
environments. Methanogenic bacteria consume organic material and excrete methane. Ripe anaerobic
environments include the digestive tracts of cattle, sheep, and termites; manure from cows, sheep, pigs, and
chickens; rice paddies; landfills; and wetlands. Of these, emissions from cattle, sheep, manure, rice paddies, and
landfills are human-enabled emissions.
The main method of reducing methane from bacteria in digestive tracks and manure is to reducing human
consumption of meat and poultry. This requires people changing their diets, which can have additional health
benefits.
Methane released from manure can also be captured with a methane digester. A methane digester, or anaerobic
manure digester, is an airtight tank in which manure is placed after water is separated from it. The manure is
heated and stirred to simulate the inside of a cow’s stomach. Methane rises from the manure to the top of the tank,
where it is captured in a bag or piped out of the digester to a storage tank.
In a 100 percent WWS world, the use of captured methane should be used only for steam reforming of methane
(Section 2.2.2.1) to produce hydrogen for use in a fuel cell, not for methane combustion. Whereas, CO2 emissions
from steam reforming are similar to those of methane combustion, emissions of all other pollutants from steam
reforming are small (Colella et al., 2005, Table 4) in comparison with those from natural gas combustion.
Methane from landfills can also be captured. In this case, a network of vertical and horizontal pipes captures the
gas (Figure 2.34). Landfill gas is often a mixture of nearly 50 percent methane, nearly 50 percent carbon dioxide,
and trace amounts of non-methane organic gases. The landfill gas is usually filtered to separate out the methane
from the other gases. Once methane is relatively isolated, it can be used to produce hydrogen by steam reforming
(Section 2.2.2.1).
Rice paddies release a significant amount of methane globally, both through the leaves of rice plants and in the
anaerobic environment of the flooded soil in which rice plants usually grow. On average, rice paddy soil is
flooded for four months of the year. Direct seeding of rice plants, instead of transplanting them into alreadyflooded
paddies, can reduce the time needed for flooding down to one month. This reduces methane emissions
from rice paddies by 15 to 90 percent. A system of pipes can also be used to capture rice paddy methane just as it
does with methane from landfill gas. The methane should be used only to produce hydrogen by steam reforming.
2.9.3. Halogen Emissions
Figure 1.1 indicates that halogens are responsible for about 9 percent of global warming. Halogens are still used
today as refrigerants, solvents, blowing agents, fire extinguishants, and fumigants. They enter the atmosphere
primarily upon evaporation when they leak or when the appliances containing them are drained in a way that
exposes them to the air. Their persistence in the atmosphere and impacts on warming depend on the specific
global warming potential of each halogen (Table 1.2).
The main methods of reducing halogens and their impacts in the atmosphere are (a) substituting lower GWP
halogens or non-halogen compounds for higher GWP halogens to perform the same function, (b) requiring more
stringent standards for sealing halogens in the equipment or appliance they are used in to reduce leakage, and (c)
requiring tougher standards for disposing of halogens at the end of life of the equipment or appliance they are
used in. For these suggestions to be effective, they need to be implemented and enforced worldwide.
2.9.4. Nitrous Oxide and Ammonia Emissions
Nitrous oxide is a strong greenhouse gas with a high global warming potential and long lifetime (Table 1.2). N2O
is produced by bacteria and contributes to about 4.3 percent of gross anthropogenic global warming (Table 1.1).
67 to 80 percent of anthropogenic nitrous oxide originates from agriculture (Ussiri and Lal, 2012). In particular,
nitrogen-containing fertilizers emit N2O. In addition, the cultivation of legumes (plants in the pea family) results
in the conversion of atmospheric molecular nitrogen (N2) to N2O, which is released to the air, but to a lesser
extent than with fertilized crops. A third agricultural source of N2O is the solid waste of domesticated animals.
Bacterial metabolism in nitrogen-containing fertilizers also results in the emission of ammonia (NH3). NH3 is a
gas that dissolves in liquid aerosol particles to form the ammonium ion (NH4+), which reacts with other
chemicals inside the particles. NH4+ inside of liquid particles also causes the particles to swell by encouraging
water vapor to condense. Swollen particles reduce visibility and attract more toxic chemicals, increasing health
problems for those who breathe in the aerosol particles.
Some methods of reducing nitrous oxide and ammonia emissions from fertilizers are (a) using less nitrogen-based
fertilizer, (b) cultivating leguminous crops that don’t require fertilizer in the crop rotation, and (c) reducing tillage
to reduce the breakdown of organic fertilizer, thereby reducing reaction and release of chemicals.
The remaining anthropogenic sources of N2O are fossil fuel combustion, open biomass burning, industrial
processes, and treatment of wastewater. Transitioning fossil energy to 100 percent WWS will eliminate fossil
combustion sources of N2O. Section 2.9.1 describes how open biomass burning sources will be controlled.
The two main industrial sources of N2O are the production of nitric acid for use in fertilizers and the production
of adipic acid for use in the production of nylon fibers and plastics. To date, N2O emissions from adipic acid
production have been reduced effectively with emission control technologies in several production plants, so the
expansion of such technologies to adipic acid plants worldwide will help reduce N2O emissions. Similarly, N2O
emission control technologies for nitric acid production plants are available (NACAG, 2014) and can be
implemented worldwide with stringent policies.
The source of N2O in wastewater is organic material from human or animal waste. Modulating the dissolved
oxygen content in the wastewater treatment process can control the N2O content of wastewater (e.g., Boiocchi et
al., 2016; Santin et al., 2017).
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