In many parts of the developing world and economies in transition,
small-scale traditional brick kilns are a notorious source of urban
air pollution. Many are both energy inefficient and burn highly
polluting fuels that emit significant levels of black carbon (BC),
organic carbon (OC) and other atmospheric pollutants into local
communities, resulting in severe health and environmental
impacts. However, only a very limited number of studies are
available on the emission characteristics of brick kilns; thus, there
is a need to characterize their gaseous and particulate matter (PM)
emission factors to better assess their overall contribution to
emissions inventories and to quantify their ecological, human
health, and climate impacts. In this study, the fuel-, energy-, and
brick-based emissions factors and time-based emission ratios of BC,
OC, inorganic PM components, CO,
Artisanal clay brick production using small-scale traditional kilns is a highly polluting activity occurring in developing countries and economies in transition to manufacture building materials. Moreover, traditional brick production is a serious local health hazard to the residents of the poor neighborhoods that typically host brickyards, as well as to brickmakers themselves. Impacts of toxic emissions on brick producers' respiratory health and the environment have been documented in a number of studies (e.g., Zuskin et al., 1998; Co et al., 2009; Martínez-Salinas et al., 2010; Kaushik et al., 2012). Although production zones are clustered at the periphery of – or even within – urban areas, laborers and their families often lack access to adequate public services including clean water, basic sanitation facilities, health services, transport, and education infrastructure. Brick producers often sell the bricks to intermediaries and the economic revenue for producers can be marginal. These conditions contribute to the perpetuation of severe environmental and social injustice problems.
The most current estimates suggest that about 1.5 trillion clay bricks are produced annually, with 90 % of the global production generated by Asian countries, and with only a small fraction (less than 10 %) of global brick production using modern mechanized technology (CIATEC, 2015). However, being predominantly an informal industrial sector, there are substantial uncertainties in the number, types, fuels, and characteristics of kilns used for this activity. The lack of reliable activity data and emission factors makes it difficult to quantify the overall contribution of brick production to local and regional emissions inventories and to assess the ecological, human health, and climate impacts.
Efforts in Mexico to reduce the impacts of brick production include
the promotion of technologically improved kilns and survey-based field
studies to improve the activity data for this sector (Cardenas et al.,
2012). The few data available indicate that fuels and the
characteristics of raw materials vary based on their cost and
availability. The estimated number of brick kilns in Mexico is about
17 000, of which 75 % are the “traditional-fixed” type with
permanent walls that delimit the space of accommodation of the bricks
to be cooked; 22 % are “traditional-campaign” kilns in which the
raw bricks give shape to the kiln, and only
The general steps of brick production include clay preparation, molding, drying, and firing. The firing process itself is divided into burning, smoldering, and cooling stages. Nevertheless, the whole process is artisanal rather than standardized, learned by experience, and locally adjusted depending on the soil characteristics, kiln design, and available fuels. In Mexico, biomass is the predominant fuel used in the production of bricks, although it is often combined with other hazardous and highly polluting materials including waste oils, textiles, tires and plastics (CIATEC, 2015). This results in low efficiency combustion and high levels of gaseous and particulate matter (PM) pollutants that are difficult to quantify in an emissions inventory.
Brick kiln emissions are suspected to be a major source of black
carbon (BC) and other PM components at the local scale in developing
countries. However, there are no reliable estimates of global
emissions from brick kilns. Based on a very limited number of
measurements and expert judgment, Bond et al. (2013) estimated that
industrial coal combustion provided about 9 % of global BC
emissions in 2000, although that figure includes brick production as
well as small boilers, process heating for lime kilns, and coke
production for the steel industry. In Mexico, the 2008 National
Emissions Inventory (2008-MNEI) suggests emissions of 2.9, 0.5, and
19.7
A limited number of studies exist on the emission characteristics of
brick kilns. Le and Oanh (2010) measured the emission rates (ERs) of CO,
Due to the intensity of the emission fluxes, the high temperatures involved, and the varied geometry of the kilns, there are considerable technical challenges associated with the measurement of emission factors from brick kilns. Recently, based on a review of the available studies, the Climate and Clean Air Coalition (CCAC) Brick Production Initiative has developed guidelines for the measurement of brick kilns' emissions and energy performance (Weyant et al., 2016). The guidelines include procedures for the isokinetic probe sampling of effluents in kiln stacks when they are available and the use of an array probe in the open plume above the kiln to apply the carbon mass balance method (Thomson et al., 2016).
As part of the pilot field measurement campaign to characterize the
emissions from key sources of Short-Lived Climate Forcers in
Mexico (SLCF-2013 Mexico), we measured the emissions factors for BC, OC, the
inorganic PM components, CO,
Table 1 lists the characteristics of the brick kilns sampled and Fig. 1 shows the kilns. A description of their operation processes as well as the sampling location for each kiln is presented in the Supplement. The MK2 kiln and the traditional-campaign kiln were measured in El Refugio, a community of brick producers located in the periphery of Leon, Guanajuato. The traditional-fixed kiln was measured in a separate community of brick producers in Abasolo, Guanajuato. Measurements took place during the dry season on 12–16 March 2013. Close collaboration with the local authorities and the brick producers' associations allowed us to establish an agreement that other kilns would not be fired during the measurement period to minimize the influence from nearby sources. The selected kilns were operated by experienced brick producers under real-world operating conditions, with fuels types and practices they commonly use.
Brick kilns sampled:
Summary of the characteristics of kilns sampled.
A random sample of 60 bricks were identified, measured, and weighed
before the firing took place for each kiln. At the end of the firing,
these same bricks were again measured, weighed, and sent to
a laboratory to test their mechanical resistance and water absorption
content following the corresponding NMX-C-404-ONNCCE-2012 Mexican
standard (ONNCCE, 2012). Samples of fuels and raw materials were
collected before the firing to determine carbon content and heating
value of combustion for the fuels. The determination was carried out
with an elemental analyzer PE-2400 Series 1 and a microbalance. An
acetanilide standard was used to calibrate the equipment and obtain
the sample's carbon content. The heating value of combustion was
determined using ASTM standards (ASTM 1995) with a Parr 1108
calorimetric pump operating with an excess of oxygen to assure complete
combustion of the sample. The results of these analyses are presented
in Tables S1–S3 in the Supplement. Four thermocouples were
installed at each of the lower, middle, and upper levels to obtain
three cross sections and to determine the temperature profile inside
the kilns during their operation. These three levels were defined as
follows: 0.4
Two sampling techniques were used to obtain the emission factors of pollutants generated from the brick kilns. In the sampling probe technique, a temporary scaffolding was built on the side of the kiln for equipment and technicians, and a probe was installed on top of the kiln and connected to a sensor sampling train containing real-time sensors and filters for PM collection. This sampling technique is possible due to the relatively low velocities of the exhaust so that an isokinetic flow train is not required (Weyant et al., 2016). During the few seconds right after exiting the kiln and before they are well-mixed downwind, the emission plumes on top of the kiln can vary substantially in intensity and composition. This implies that the location of the sampling probe on top of the kiln is of key importance to the representativeness of the filter measurement. To account for this effect, the sampling probe was mounted on a rotating crane that was continuously spinning slowly on top of the kiln (see Fig. 1).
An inertial mass separator with a cut-point of 2.5
Exhaust flow in the sampling train was measured using a piston
flowmeter and directed to a continuous emissions monitoring system
(CEMS) with a Fourier-transform infrared spectrometer (FTIR) to
measure
The second technique used to sample the kilns was based on the tracer
ratio method in which the emission rate of the targeted source is
obtained by simultaneously measuring in real-time the above-background
concentrations of the species of interest and of a selected gas tracer
with a known release rate that is co-located at the emission source
(Lamb et al., 1995). This method is based on the fundamental
assumption that a relatively unreactive mixture of gases emitted from
a common location experiences a quasi-perfect co-dispersion and
equivalent dilution through the atmosphere. The tracer ratio method
does not quantify the dispersion of air pollutants or the spatial
representation of the brick kilns' emission plumes, but is used to
quantify the emission rates of co-emitted pollutants from a single
source. The source's emission rate (
The AML incorporates real-time data acquisition and data display
capabilities so that in situ decisions by the investigators can be
made to move the laboratory in and out of the emission plumes that are
identified by tracer detection. This is a key element for the
successful application of this technique since the dilution and
advection of the kiln emissions are dictated by local meteorological
conditions that can vary in short timescales. The mobile laboratory
was typically positioned between 20 and 100
The instrumentation onboard the AML included a soot particle aerosol
mass spectrometer (SP-AMS) developed by Aerodyne Research Inc. (Onasch
et al., 2012), which measured BC and OC using laser-induced
incandescence of absorbing soot particles to vaporize both the
coatings and BC cores of exhaust soot particles within the ionization
region of the AMS (Dallman et al., 2014). The SP-AMS also measured
other inorganic PM components including nitrates, sulfates, ammonium,
and chlorides corresponding to a particle size range of
50–600
The AML measured
The average fuel-based emission factors (
Average modified combustion efficiency (MCE), fuel-based emission
factors EF (
As shown in Table 2, the relative variability of emission rates is much higher compared to the variability of fuel-based emission factors. Time-based emission rates are highly variable particularly during the burning stage because they strongly depend on the fuel-feeding practices including the amount and type of fuel used, as well as the operator's decision of when to add fuel. The lower variability of the fuel-based emission factors compared to emission rates indicates that the normalization of the emissions of combustion by-products effectively takes into account the variations in the thermal energy employed in the cooking process. In addition, since estimations of the integrated emissions burden using emission rates depend on the total brick production time, emission rates are not a good indicator to compare the environmental performance of the kilns. However, emission rates can be useful during the development of emissions inventories as inputs in air quality models to better understand the time-based chemical evolution of the emitted species at local and urban scales.
A comparison of temporal profiles of CO, BC, and OC fuel-based emission factors for the traditional-fixed kiln between the two techniques is shown in Fig. 2. Comparisons of the temporal profiles for all measured pollutants are shown in Figs. S1–S3 for the MK2, traditional-campaign, and traditional-fixed kilns, respectively, in the Supplement. The results show that in general both techniques capture comparable temporal profiles of the kiln emissions while the magnitudes of the emission factors are remarkably similar. As the fuels used in the three kilns were mostly wood, the resulting identities of VOCs emitted are similar to those from biomass burning. Furthermore, the temporal profiles shown in Figs. S1–S3 indicate that high levels of VOCs can be emitted not only during the burning stage of the brick-cooking process but also during the smoldering and cooling stages. Measurements in this pilot study focused primarily on the burning stages and only included partial periods of the smoldering and cooling stages. Therefore, a complete characterization of VOC emissions for brick kilns would require the measurement of the full brick-cooking period.
Temporal profiles of fuel-based CO, BC, and OC emission
factors (
The data from the tracer ratio technique show that there is large
short-term variability of the emission factors for both gaseous and
particulate pollutants during the burning stage of the cooking
process; this variability is only partially captured by the
filter-based sampling probe technique. On the other hand, whereas the
sampling probe technique continuously measures the kiln's emissions in
Major components of
The measured ionic contents are quite high for the MK2 and the
traditional-campaign kilns; the sum is greater than the BC
Fluorides, bromides and other halogens are not typically high in
ambient filter-based PM samples but they may be present in trace
amounts in clay. Previous work has shown that fluorides from brick
kilns can have adverse effects on vegetation and crops (Ahmand et al.,
2012). Emission factors of particulate fluorides in this study were
small (1.1–
It should be noted that during these measurements both methane and
ethane emissions were quantified aboard the AML. The ethane
measurement is an important complement to methane because it is
a marker for non-biogenic methane emissions. Interestingly, the mass
ratio of ethane to methane was consistently 0.06–0.075 between the
three brick kiln types despite the substantial variation of the
The physical and chemical changes occurring in the bricks during the cooking process are associated with the burning, smoldering, and cooling stages, which are in turn determined by changes in thermal energy transfer rates within the kiln, and are closely related to the final quality of the cooked bricks. In describing the brick-cooking process, we define the burning stage as the time passed since the firing starts until the feeding of fuel is stopped, the smoldering stage as the time when the maximum temperature at the top of the kiln is reached minus the burning time, and the cooling stage as the time when the temperature at the bottom of the kiln reaches a stable minimum minus smoldering time. The temporal profiles of temperature at the lower, middle, and upper levels of the kilns and the brick-cooking stages are shown in Fig. 3. The corresponding rates of heating and cooling are obtained as the time derivatives of the temperature profiles.
Temperature profiles (top panels) and temperature change
rates (bottom panels) for
The data show that the cooking of bricks results from vertical transfer of thermal energy inside the kiln starting from the beginning of the burning stage when temperatures at the bottom layers rise quickly with very high heating rates. In general, higher temperatures are reached inside the traditional-fixed kiln, followed by the traditional-campaign and the MK2 kilns. The bricks located in the middle and upper layers of the kiln start their cooking process only after sufficient thermal energy is transferred from the bottom layer. Interestingly, in the case of the MK2 and the traditional-campaign kilns this can occur during the burning stage, but for the traditional-fixed kiln the cooking of bricks at the middle and upper layers occur only during the smoldering and cooling stages.
During the burning stage at the bottom of the kiln, the heating rate is much higher and smoother in the case of the traditional-fixed kiln compared to the traditional-campaign kiln, whereas the MK2 kiln shows highly variable but overall decreasing heating rates. This critical difference in the heating process at the burning stage is likely due to the physical arrangement of bricks and the design of the kiln. The traditional-fixed kiln seems to be particularly efficient in its vertical thermal energy transfer inside the kiln as temperatures in the middle and upper levels reach similarly high values (and at comparable heating rates) as those at the bottom even after the burning stage has finished. The primary effect of the initial period of the burning stage is to remove all the remaining moisture from the bricks. At the beginning of the process this is done only at the bottom layers as temperatures do not reach high values in the middle and upper layers until much later. Once the moisture is removed and the temperatures continue rising, the carbonaceous organic material contained in the clay is removed by combustion. The raw materials for the three kilns are comparable in mass and type of clay used, but the traditional-campaign and the MK2 kilns use about 3.5 wt % of manure whereas the traditional-fixed kiln use 8 wt % of sawdust (see Table 1). These materials are additives that the brick producers use during the clay preparation process, mixing them with water and crushing them until the mixture is ready for molding. These organic additives effectively act as internal fuel during the brick-cooking process and affect the quality and mechanical condition of the bricks (Martínez and Jiménez, 2014).
As temperatures continue to rise, the hydroxyl groups that are
combined with the chemical compounds forming the clay begin the
process of dehydroxylation, which effectively releases water and other
volatile compounds at about 450
Above
The environmental performance of the brick kilns can be assessed in terms of the relative magnitude of the emission factors during the brick production process. The use of fuel-based emission factors to compare brick kilns' performance is adequate when similar fuels are used among different kilns and when bricks have similar physical characteristics. In contrast, energy-based emission factors are adequate comparison indicators when fuels types are substantially different because they take directly into account the effective heating value of the fuels employed. Brick-based emission factors are adequate comparison indicators between kilns when the mass and size of the bricks produced are substantially different. Nevertheless, regardless of the type of emission factor used, an integrated assessment of the brick kilns' performance should also incorporate other parameters, such as energy efficiency, fuel consumption, combustion efficiency, production time, and the quality of bricks produced, among others.
Figure 4 shows an inter-comparison of the relative performance of the three sampled kilns along with the specific energy consumption, fuel consumption, modified combustion efficiency, and measured brick's mechanical resistance as a surrogate for the bricks' quality. In order to compare the relative environmental performance of the three kilns, we have normalized the fuel-based emission factors with the corresponding average of the three kilns for each pollutant in Fig. 4. Regardless of the base (fuel mass, brick mass, or energy) employed, the normalization effectively allows the simultaneous comparison of emissions factors for multiple pollutants that differ by orders of magnitude while providing information on their relative magnitudes.
Inter-comparison of emission factors normalized to the
average of the three kilns by pollutant for
The results show that the traditional-fixed kiln had lower modified
combustion efficiency, lower fuel (wood) consumption, and slightly
higher specific energy consumption compared to the other two
kilns. The results of the measured mechanical resistance of the bricks
produced are shown in Table S8 of the Supplement. The
traditional-fixed kiln also produced bricks with an average mechanical
resistance almost half of that compared to the traditional-campaign
kiln, in agreement with the much higher time integral of the
temperature profile above
Low combustion efficiency is related to higher pollutant emissions
produced during incomplete combustion. The traditional-fixed kiln had
the highest emissions factors for CO, BC, as well as
Very few studies are available on the chemical characteristics of emission factors for brick kilns. Previous work by Christian et al. (2010) includes measurements of multiple gases and PM composition for three traditional-fixed kilns in Mexico that used wood waste products as fuel. Of the five types of kiln designs measured by Rajarathnam et al. (2014) and Weyant et al. (2014) in India and Vietnam, only the downdraft kiln type used wood as fuel while the rest used mostly coal. Both the zigzag and clamp kilns measured by Stockwell et al. (2016) in Nepal also used coal as fuel. Jayarathne et al. (2017) recently reported the particle-phase results of the same kilns measured by Stockwell et al. (2016). Of these studies, Stockwell et al. (2016) and Christian et al. (2010) report fuel-based energy factors whereas Rajarathnam et al. (2014) and Weyant et al. (2014) report energy-based emission factors, allowing a proper inter-comparison with our results. Tables 3 and 4 show a comparison of the energy-based and fuel-based emission factors, respectively, with those obtained in other studies.
Comparison of energy-based emission factors (
Comparison of fuel-based emission factors (
Table 3 shows that the specific energy consumption for brick kilns
using coal as fuel in the studies of Rajarathnam et al. (2014) and
Weyant et al. (2014) is much smaller than for those measured in this
study, due to the much higher energy density content of coal
vs. wood.
The emission factors in this study are closer to the values reported
for the downdraft kiln by Rajarathnam et al. (2014) and Weyant
et al. (2014) and to the results by Christian et al. (2010) due to
similarities in kiln designs and fuels (wood) employed. However, there
are differences in the emission factors that suggest substantial
inter-variability of emissions even when fuels and kilns designs are
similar. The average BC and OC emission factors obtained in this study
for the traditional-fixed kiln of 0.54 and
0.14
The MK2 and the traditional-campaign kilns presented similar average
MCE values (0.94–0.96) that were higher than for the
traditional-fixed kiln (0.91–0.92). This is reflected in the much
higher CO emission factors for the traditional-fixed kiln in
comparison with the other two kilns, indicating overall smaller
combustion efficiency. In our study the
Overall, the comparison of the results in this study with the available literature reports indicate that there is substantial variability among brick kiln designs and fuel types. The observed variability is also the result of the combination of materials, fuels, kiln types, and operational practices that brick producers use. However, due to the small sampling size, it is not possible to infer from the data the contribution of fuel types and kiln design to the overall variability of emissions during brick production. Therefore, although both the traditional-campaign and traditional-fixed kilns are widely used in Mexico, caution should be taken in generalizing the results to other brick production regions with different fuels and operation practices. The results from this study are not intended to provide definitive generalizations of the brick making process, but to help in understanding the effects of different kiln designs and fuels on gaseous- and particulate-phase emissions from brick kilns. Nevertheless, since the number of studies with chemical composition of brick kiln emissions is so small, the results of this study represent valuable additions to the current literature.
Despite the widespread use of brick kilns in Mexico and other Latin American countries, there have been very few studies on their emission characteristics. An important part of the brick production in Mexico is still done by using traditional brick kilns that are operated with artisanal methods and thus the individual kiln's performance depends on the producer's operation skills, kiln design, and available materials and fuels. This diversity in operating conditions can result in large intra-variability on the pollutant emissions characteristics from brick kilns even when using similar designs and fuels. Therefore, there is a need for additional emissions measurements from brick production to better constrain the uncertainties of emissions estimates and mitigate their environmental and human health impacts. Since the tracer ratio method is not limited by mass saturation constrains, the results from this pilot project suggest that the tracer technique can be an alternative option to the filter-based sampling probe technique in understanding the temporal profile of the chemical composition of brick kilns' emissions.
The results of this study showed that a well-designed and operated MK2
kiln produced lower
The pertinent data described in the paper are provided in the Supplement. Additional information not included within the Supplement is available upon request (ltmolina@mce2.org, ltmolina@mit.edu).
The authors declare that they have no conflict of interest.
The SLCF-2013 Mexico measurement field campaign was coordinated by the Molina Center for Energy and the Environment under UNEP contract GFL-4C58. MZ and LTM acknowledge additional support from NSF award 1560494. The authors would like to thank the local brick producers from the Asociación de Productores de Barro y Arcilla del Refugio A. C. and the Productores de Ladrillo de Abasolo for their participation in this study. Special thanks to the Instituto de Ecología del Estado de Guanajuato (IEEG), Francisco Guardado from the Instituto Nacional de Ecología y Cambio Climático (INECC), and Carlos Frias Mejía from the Asociación de Productores de Barro y Arcilla del Refugio A. C. for logistical support. Edited by: James Allan Reviewed by: Charles Bruce and one anonymous referee