Coal Is Dirty, but Where It Is Burned Especially Matters
Coal abatement actions for pollution reduction often target total coal consumption. The health impacts of coal uses, however, vary extensively among sectors.
Here, we modeled the sectorial contributions of coal uses to emissions, outdoor and indoor PM2.5 (particulate matter with an aerodynamic diameter of less than 2.5 mm) concentrations, exposures, and health outcomes in China from 1970 to 2014. We show that in 2014, residential coal accounted for 2.9 percent of total energy use but 34 percent of premature deaths associated with PM2.5 exposure, showing that effects were magnified substantially along the causal path.
The number of premature deaths attributed to unit coal consumption in the residential sector was 40 times higher than that in the power and industrial sectors. Emissions of primary PM2.5 were more important than secondary aerosol precursors in terms of health consequences, and indoor exposure accounted for 97 percent and 91 percent of total premature deaths attributable to PM2.5 from coal combustion in 1974 and 2014, respectively.
Our assessment raises a critical challenge in the switching of residential coal uses to effectively mitigate PM2.5 exposure in the Chinese population.
Energy production and consumption dominate emissions of major air pollutants.(1) Among various energy types, coal is the most important in China and subject to both energy supply and pollutant emissions due to its abundance across the country (1.7 Eg in 2018).(2,3) In 2019, annual coal consumption reached 3.24 Pg in China, accounting for 54.5 percent of the global total.(4) In 2014, large amounts of coal were used in coal-fired power plants (2.11 Pg) and industry (1.21 Pg) whereas residential consumption was much lower (0.15 Pg).(5−7)
According to highly sector-resolved emission inventories, 43 percent, 60 percent, and 54 percent of total anthropogenic emissions of primary PM2.5, SO2 (sulfur dioxide), and NOx (nitrogen oxides) in China were attributed to coal combustion in 2014.(8−10) The results of source apportionment studies based on field observations also suggest that coal is one of the most important sources of ambient PM2.5.(11,12) Given its significant contributions to air pollution, coal has been heavily targeted through air pollution mitigation actions.(13−15)
As a result, a large number of coal-fired power plants have been shut down or replaced by petroleum and natural gas powered plants, cutting approximately 0.13 Pg of coal consumption during 2013–2016.(16) Meanwhile, particulate matter removal, desulfurization, and denitrification (DeNOx) facilities have been widely adopted in coal-fired power plants and industrial boilers, leading to substantial emission reductions.(17)
Recently, a clean heating campaign was launched to replace 60 percent of rural residential coal facilities with natural gas or electricity facilities in the North China Plain, which has the potential to abate 42 percent of population exposure to PM2.5 in the targeted rural areas.(18)
Although coal is widely considered a major emission source in China,(19) emission factors (EFs or quantities of pollutants emitted per unit of fuel) of most air pollutants from coal burning vary widely across sectors and facilities due to differences in combustion conditions and end-of-pipe control techniques.(20,21) For example, it was revealed that EFs of PM2.5 generated by bituminous coal used in cook stoves (9.7 g/kg) could be 1 order of magnitude higher than those measured from industrial activities (1.1 g/kg).(8)
Consequently, simply cutting total coal consumption without considering EF differences among sectors cannot cost-effectively mitigate emissions from coal burning. A few studies have addressed this issue by simulating emissions and health impacts through exposure to ambient PM2.5 and have reported relatively less important effects of residential coal.(19,22) However, without taking indoor exposure into consideration, the health impacts of residential coal are likely to be underestimated(18,23) because people spend much more time indoors.(24−26)
To our knowledge, such a study quantifying health impacts associated with indoor and outdoor PM2.5 exposures from different coal burning sources has never been carried out.
The present study aims to quantitatively assess the effects of coal used in various sectors on health risks of human exposure to PM2.5 in both indoor and outdoor environments from 1970 to 2014, with residential sources emphasized. The specific methodology used is presented in the Methods section. In brief, we modeled ambient PM2.5 levels using a state-of-science atmospheric chemical transport model(27) and calculated indoor PM2.5 concentrations using a statistical model.(23,28)
On the basis of both outdoor and indoor PM2.5 exposure, health impacts from 1974 to 2014 were quantified and distinguished for coal and noncoal emissions and for coal used in the power, industrial, and residential sectors(29) to lend scientific support to emission mitigation strategies subject to coal combustion.
Power plants, industry, and residential uses were main sectors related to coal combustion in mainland China. Coal consumptions in power and industrial sectors were collected from International Energy Agency (IEA).(5) Rural residential coal consumptions were derived by interpolating and extrapolating data from a nationwide survey in China.(6,30) Urban residential coal consumptions were collected from statistic yearbooks by Shen et al.(7)
Detailed coal types in power plants and industry include bituminous coal, anthracite, and cooking coal, and those for residential uses include bituminous coal, honeycomb briquettes, and patent fuel.(5,7,30)
The emissions of primary PM2.5, PM10 (particulate matter with an aerodynamic diameter less than 10 μm), BC (black carbon), OC (organic carbon), SO2, NOx, NH3 (ammonia), and CO (carbon monoxide) from coal combustion were calculated by multiplying coal consumptions by their respective EFs.
In the calculation of the mean EFs in power and industrial sectors, the influences of different control measures, including methods of coaling (pulverized coal, circulating fluidized bed, and traditional stoker), particulate matter removal measures (cyclones, wet scrubber, electrostatic precipitators, and fabric filter), desulfurization measures (flue gas desulfurization), and DeNOx measures (low nitrogen burner and selective catalytic reduction), on the mean EFs were taken into consideration.
However, the influences of residential stove upgrades on the EFs were not considered in this study.
Mean EFs were derived from the PKU-emission inventories (Peking University Emission Inventories).(31) Emissions from other combustion (noncoal anthropogenic and natural combustion sectors) and industrial process sectors were obtained from the PKU-emission inventories.(31) Emissions of noncombustion NH3 in agriculture sector and NMVOCs (nonmethane volatile organic compounds) were collected from EDGAR (Emission Database for Global Atmospheric Research)(32) and HTAP (Hemispheric Transport of Air Pollution).(33)
Changes of the emissions of major air pollutants from coal combustion are determined by the increase/decrease of coal consumption, switching from lump coal to honeycomb briquette in the residential sector, and promotion of control measures (pulverized coal, circulating fluidized bed technology, particulate matter removal measures, desulfurization measures, and DeNOx measures). To distinguish contributions from each factor to accumulative emission changes, each factor was successively changed from the baseline year 1970 to 2014, and the corresponding changes of emissions were defined as accumulative effects of each factor.
Absolute and relative changes in major energy consumption from 1970 to 2014 in mainland China are shown in Figure 1.(5−7,30) Mainly due to rapid industrialization, urbanization, and population growth, annual energy consumption increased 5.6-fold from 18 EJ to 120 EJ. In 1970, the residential sector used 59 percent of all energy, and coal accounted for 47 percent of the total followed by biomass fuels (41 percent).
Annual coal consumption increased 8.8-fold from 0.35 Pg (47 percent of total energy consumption) in 1970 to 3.47 Pg in 2014 (73 percent) due to the rapid expansion of infrastructure and manufacturing.(5−7,30) During this period, the total population increased by 63 percent, and residential coal consumption increased slightly from 0.13 Pg to 0.15 Pg, due to combined effects of population increase, urbanization, and residential energy transition.(7,30)
In comparison to those of other sectors, the growth rate of residential coal consumption was much slower, and the relative contribution of residential energy to total energy use dropped to merely 7.5 percent in 2014.(23) Meanwhile, the consumption of renewable energy, gases (natural gas, liquefied petroleum gas, and biogas), and petroleum increased substantially 40-, 41-, and 11-fold from 1970 to 2014, respectively.(5−7,30)
Figure 1. Temporal variation in the consumption of major energy types in mainland China from 1970 to 2014. The results are presented either as absolute (A) or relative (B) terms.
Coal burning produces a variety of air pollutants, including primary PM2.5 as incomplete combustion products;(20) OC and BC as components of primary particulate matter;(20) SO2 from oxidized sulfur;(21) and NOx as a high temperature combustion product.(21) These species are major components or precursors of PM2.5 in the air. Figure S4 shows annual emissions of these pollutants from coal burning and their contributions to total emissions from all anthropogenic sources in China.(31)
The temporal trends of the five pollutants differed. Emissions of primary PM2.5 from coal generally followed patterns of coal consumption and were affected by the installation of particulate matter removal facilities.(17) SO2 and NOx emissions passed their peaks in approximately 2006 and 2011, respectively, due to desulfurization and DeNOx efforts driven by two amendments of national emission standards.(58,59)
Generally, relative contributions of coal to emissions did not increase as fast as absolute contributions due to even faster increases in emissions from noncoal sources. For example, due to the rapid expansion of the motor vehicle fleet,(60) NOx emissions from transportation increased approximately 30-fold from 0.16 Tg/yr in 1970 to 4.57 Tg/yr in 2014.(10) By 2014, the relative contributions of coal to total anthropogenic emissions of SO2, NOx, primary PM2.5, BC, and OC were 60 percent, 54 percent, 43 percent, 32 percent, and 42 percent, respectively, all leading among various energy types.
Figure S5 compares relative coal emission contributions of the power, industrial, and residential sectors for 1970 to 2014. Although residential coal contributions to emissions have decreased across the board, they still accounted for 29 percent of total primary PM2.5 emissions from coal burning in 2014. This decrease was much slower than the declining contribution of residential coal to total coal consumption because no end-of-pipe abatement facilities were available for residential stoves.
In addition to coal consumption, pollutant emissions are affected by other factors,(20,61) which are quantitatively assessed in Figure 2 as cumulative contributions to emissions changes. The quantified driving forces include coal consumption, transitioning from lump coal to honeycomb briquette use, coal pulverization, circulating fluid bed technology promotion, particulate matter removal, desulfurization, and DeNOx.
Sectors involved are designated by subscripts p, i, and r for power plants, industry, and residences, respectively. In addition to increases in coal consumption observed for power plants (+9.5 Tg/yr), coal pulverization resulted in a significant increase in PM2.5 emissions (+8.6 Tg/yr) from 1970 to 2014 due to increased EFs of PM2.5.(62)
The increased emissions generated from coal used in power plants and industry were largely offset by extensively installed particulate matter removal facilities and mainly electrostatic precipitators (−11 Tg/yr) and fabric filters (−7.1 Tg/yr).(8) Unlike PM2.5, changes in BC and OC emissions were primarily driven by changes in residential coal use and by power generation after 2000 to certain extent.
Changes in SO2 and NOx emissions were caused by rapid increases in coal consumption in nonresidential sectors (63 Tg/yr SO2 and 20 Tg/yr NOx),(9,10) which were partially offset by desulfurization (−51 Tg/yr SO2) and DeNOx (−7.8 Tg/yr NOx).(9,10) Overall, net cumulative contributions of coal (particularly residential coal consumption) to temporal changes in emissions of primary PM2.5, OC, and BC were significant.
Figure 2. Cumulative effects of major factors driving temporal changes in emissions of primary PM2.5, OC, BC, SO2, and NOx from coal combustion from 1970 to 2014.
Factors include coal consumption (Q), transitioning from lump coal to honeycomb briquette use (T), coal pulverization (P), circulating fluidized bed technology promotion (B), particulate matter abatement using cyclones (C), particulate matter abatement using wet scrubbers (W), particulate matter abatement using electrostatic precipitators (E), particulate matter abatement using fabric filters (F), desulfurization (D), low nitrogen burner promotion (L), and selective catalytic reduction promotion (S). Subscripts represent the power (p), industrial (i), and residential (r) sectors.
Emissions of air pollutants affect both ambient and indoor air quality. National annual mean PM2.5 concentrations in ambient and indoor air in 1974, 1984, 1994, 2004, and 2014 were modeled. The contributions of coal and noncoal anthropogenic emissions to PM2.5 exposure were quantified, and coal used in power plants, industry, and households was classified (Figure S6A).
Noncoal emissions were responsible for increases in ambient PM2.5 from 4.4 ± 3.8 μg/m3 in 1974 to 6.0 ± 5.0 μg/m3 in 2014, but coal emissions-driven PM2.5 levels doubled in the same period from 2.7 ± 2.2 μg/m3 in 1974 to 5.8 ± 4.9 μg/m3 in 2014. The larger increase in ambient PM2.5 from coal emissions was mainly associated with the power and industrial sectors, induced by rapid industrialization.(2) The process slowed after 2004 due to widely installed cleanup facilities.(17)
By contrast, the contribution of residential coal has not changed much over time, which is in line with a previous report showing that the consumption of rural residential coal as an intermediate phase of the residential energy ladder changed little from 1992 to 2012.(30)
This is taken from a very long article. Read the rest here: pubs.acs.org
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Dano S.
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After reading this article, you’d never know that clean coal technology has been around for decades and has been at the point where there are zero emissions for quite some time now. Very thorough job informing us all. Thanks for your transparency.
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SteveT
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@Dano. Have a +100 ticks upvote.
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Doug Harrison
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Of course no mention here of the protests/riots that occurred in Wuhan in 2019 over air pollution which, it was later shown, caused a severe depletion of vitamin D in the population and so the large number of deaths that occurred subsequently from respiratory diseases.
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