Filterable and Condensable Fine Particulate Emissions from Stationary Sources

PM2.5 emissions from stationary sources contain a filterable and condensable portion. In this study, USEPA Method 201A/202 are used to measure filterable and condensable PM2.5 emissions from 5 stationary sources (power plants, boilers, brick manufacturing plant, incinerators and arc furnaces). The average filterable PM2.5 concentrations for power plant, boiler, brick manufacturing plant, incinerator and arc furnace are 0.75, 16.9, 8.67, 0.15 and 2.12 mg/Nm, respectively. The amount of PM2.5 residue on the exit tube of cyclone and front half of the filter holder is significantly higher when the filterable PM2.5 concentrations are low. It is necessary to collect both filter and the residue particulates to avoid underestimation of PM2.5 emissions. The condensable PM accounts for 61.2%, 73.5%, 44.2%, 52.8% and 51.2% of total PM2.5 for power plant, boiler, brick manufacturing plant, incinerator and arc furnace plant, respectively. The real PM2.5 contribution to the atmosphere would be underestimated if condensable PM is not included. The condensable PM fraction increases as the exhaust temperature rises. The inorganic fraction accounts for 89.0%, 69.4%, 72.3%, 89.8% and 72.8% of condensable PM, respectively, for power plant, boiler, brick manufacturing plant, incinerator and arc furnace. The inorganic fraction is dominant in the condensable PM, which might be due to the high content of SO4.


INTRODUCTION
Previous studies have shown that fine particulate (aerodynamic size less than 2.5 µm, PM 2.5 ) is harmful to human health (e.g., Pope et al., 2004;Chang et al., 2012).The level of atmospheric PM 2.5 is associated with cardiovascular disease and mortality (Pope et al., 2004;Chiu et al., 2013).It is essential to reduce PM 2.5 emission and control its formation in order to lower the level of atmospheric PM 2.5 .To realize the PM 2.5 characteristics and estimate the contributions from the emission sources is the first step of emission control.
The main sources of atmospheric PM 2.5 include volcanic eruption, sea salt spray, street dust, direct discharge of industrial processes and transport emissions (Chow et al., 1995;Gugamsetty et al., 2012;Subramoney et al., 2013).The combustion stationary sources are important PM 2.5 contributors (Sudheer and Rengarajan, 2012;Ward et al., 2012;Yu et al., 2013).Particulate matter emissions from stationary sources contain filterable and condensable portion.Filterable PM are particles directly emitted from a stack as a solid or liquid at stack conditions and captured on the filter of a sampling train.Condensable PM means material that is vapor phase at stack conditions, but condenses and/or reacts upon cooling and dilution in the ambient air to form solid or liquid PM immediately after discharge from the stack.Condensable PM is typically not measured because most countries do not require sources to measure it.PM 2.5 emission would be underestimated if condensable PM 2.5 is not included since condensable PM accounts for a significant portion for PM 2.5 .USEPA has announced Method 201A and Method 202 to measure filterable PM and Condensable PM, respectively.Numerous studies have investigated the measurement artifacts of Method 202.The measurement artifacts have been reduced by optimization of the new method (Corio and Sherwell, 2000;Richards et al., 2005).
In addition to USEPA Method 201A/202, various types of dilution systems for the measurement of PM 2.5 have been developed (e.g., Kong et al., 2013;Lee et al., 2013).Dilution sampling system collects filterable PM and condensable PM on the same filter and the chemical compositions can be analyzed.However, a few parameters would affect the measurement accuracy of the dilution sampling system and the standard specification is still developing.Before the standard or the guidelines of the dilution methods are established, USEPA Method 201A and 202 are the standard method for PM 2.5 measurement of stationary sources.Moreover, the extremely large bulky sampling equipments make it difficult to measure PM 2.5 emissions for many important sources.At this stage, USEPA Method 201A/202 is the most practical way to collect PM 2.5 samples from the stationary sources.Since CPM is seldom measured, very few literatures for condensable PM emission characteristics have been reported in the literature.In this study, USEPA Method 201A/202 are used to measure PM 2.5 emissions from 5 stationary sources (power plants, boilers, brick manufacturing plant, incinerators and arc furnaces).The emission characteristics of the filterable PM and condensable PM for these important PM 2.5 emission sources are discussed.

Sampling Campaign
Filterable and condensable PM 2.5 emitted from 5 stationary sources (power plants, boilers, brick manufacturing plants, incinerators and arc furnaces) were collected in this study.Power plants use coal and oil as fuels.The electricity capacities are 240-550 MW.Electrostatic precipitators are installed to remove PM emissions for all the power plants.Flue gas desulfurization is additionally used to remove SOx emissions for the coal-fired power plants.Boilers use coal as fuel.Most of these boilers are the textile industry.Cyclone and bag house are installed to remove PM emissions for the boilers.Brick manufacturing plants produce tile and brick.The main process is raw material (such as clay or slurries) drying and firing.Incinerators have capacities from 450-900 ton waste combustion per day.Bag houses are installed to remove PM emissions for all the incinerators.Semi-dryer absorption system is used to remove SO x emissions.Arc furnaces use scrap steel as feedstock.Bag houses are installed to remove PM emissions for all the arc furnaces.At leat 3 successful samples were collected for the 5 stationary emission sources.

Sampling Equipment and Methods
USEPA Method 201A (Determination of PM 10 and PM 2.5 emissions from stationary sources) and Method 202 (Dry impinger method for determining condensable particulate emissions from stationary sources) (USEPA, 2010) were performed for filterable and condensable PM measurements, respectively.APEX XC-5000 Automated Isokinetic Sampling Console sampling system which meet the requirement of USEPA Method 201A and Method 202 was used in this study.The main equipment of Method 201A includes front nozzle, PM 2.5 cyclone, filter holder, pitot tube and stainless steel (with glass liner) sampling tube, vacuum pump and the computer control console.The sampling rate is controlled within ± 20% isokinetically for the sampling in this study.Particulates with diameter less than 2.5 µm are sucked through the cyclone and are primarily collected on a 4.7 mm filter.The temperature of cyclone sampling head is maintained within ± 10°C of the stack temperature to ensure proper sizing and prevent condensation on the walls of the cyclones.
The equipment of Method 202 includes a condenser, water dropout impinger, modified Greenburg Smith impinger and condensable PM filter (Fig. 1).Condensable PM is mainly collected in the water dropout impinger and the (backup) modified Greenburg Smith impinger.Condensable PM filter placed between the second and the third impingers is used to improve the collection efficiency.Condensable PM is collected by condenser, dry impingers, pipelines and the backup Teflon filter after filterable PM is removed by a 4.7 mm filter.In addition, exhaust gas composition (N 2 , O 2 , CO 2 , CO) is measured by Orsat method.Leakage check is conducted before each sampling.To collect sufficient sample, the sampling volume is at least than 2 m 3 .Filterable PM 2.5 Pallflex 47 mm quartz filter is used for filterable PM collection.The filter is conditioned under temperature of 20-23°C and relative humidity of 30-40% for 24 hrs before and after sampling.After condition, the filter is weighed by the gravimetric technique (Sartorius balance, model Cubis 6.6S-DF).In addition to the filter sample, PM 2.5 on the exit tube of cyclone and front half of the filter holder is collected by acetone rinses.The rinses is quantitatively transferred to a tared 250 mL beaker, and evaporated to dryness at room temperature and pressure in a laboratory hood.Desiccate the sample for 24 hrs and weighed (USEPA, 2010).

Condensable PM 2.5
USEPA Method 202 is used to sample and analyze condensable PM.According to USEPA Method 202, it is necessary to measure condensable PM if the gas filtration temperature exceeds 30°C.Condensable PM is collected in the water dropout impinger, the modified Greenburg Smith impinger and the condensable PM filter of the sampling train.The impinger contents are purged with Ultra-High Purity compressed nitrogen immediately after sample collection to remove dissolved sulfur dioxide gases from the impinger.Condensable PM train is purged at a minimum of 14 L/min for at least 1 hr.Purified water and organic solvents (n-hexane and acetone) were used to rinse the whole sampling pipeline, condenser, water dropout impinger, modified Greenburg Smith impinger and condensable PM filter, respectively.The inorganic (water rinses) and organic (organic solvent rinses) fractions were dried and weighed in the laboratory.Condensable PM is the summation of the two fractions.Field blanks (e.g., Organic solvents and water field blanks) are measured for each sampling.Detailed procedures could be referred to USEPA Method 202.

Filterable PM 2.5 Concentrations and Its Composition
Filterable PM 2.5 emission concentrations for the investigated plants are shown in Fig. 2. The average filterable PM 2.5 concentrations for power plant, boiler, brick manufacturing plant, incinerator and arc furnace are 0.75, 16.9, 8.67, 0.15 and 2.12 mg/Nm 3 , respectively.Boilers have highest average emission concentrations.
A filter with 4.7 mm is installed after the PM 2.5 cyclone to collect the particulate less than 2.5 µm.However, residue might be left on the exit tube of cyclone and front half of the filter holder.According to USEPA Method 201A, the residue should be collected and be combined with the filter data for the emission estimate.The residue was recovered by acetone washing, evaporated, conditioned and weighed.The mass distributions for PM 2.5 are shown in Fig. 3.Most PM 2.5 was collected on filter, and only about 5% PM 2.5 was retained at the cyclone holder.For incinerator, significant amount PM 2.5 is retained at the cyclone holder.The reason is that filterable PM emission concentrations low.Less filterable PM 2.5 is collected on the filter and the particulate residue left on the exit tube of cyclone and front half of the filter holder is similar for all the plants, which results in the high percentage of the residue mass.The air pollution control devices can reduce PM (as well as PM 2.5 ) emissions effectively.PM 2.5 emissions would be even lower when the devices are used.The results show that it is important and necessary to collect both filter and cyclone holder residue particulates to avoid underestimation of PM 2.5 emission, especially for the stacks with low PM 2.5 concentrations.

Condensable PM 2.5 Concentrations and Its Composition
USEPA Method 202 was used to measure the emission of condensable PM in this study.plant, incinerator and arc furnace are 2.15, 29.3, 83.5, 0.17 and 2.53 mg/Nm 3 , respectively.The same as filterable PM, incinerator has the lowest emission concentrations.Condensable PM sample is the rinses of the sampling media by water and organic solvents (acetone and hexane) sequentially.Condensable PM thus consists of inorganic fraction (water rinse) and organic fraction (solvent rinse).The partitions of inorganic and organic fraction of condensable PM for the investigated plants are shown in Fig. 5.The inorganic fraction accounts for 89.0%, 69.4%, 72.3%, 89.8% and 72.8%, respectively, for power plant, boiler, brick manufacturing plant, incinerator and arc furnace.Inorganic fraction is dominated in condensable PM.The results agree with the previous studies (Corio and Sherwell, 2000;Richards et al., 2005).
Evaluations of Method 202 for coal-fired boiler emissions showed that the inorganic fraction accounted for higher than 95% of the condensable PM (DeWees et al., 1989).SO 4 compounds, primarily H 2 SO 4 , make up the largest category of inorganic condensable emissions.During combustion of fossil fuels, sulfur in fuel is oxidized primarily to SO 2 and a small fraction is converted to SO 3 .SO 3 reacts with water to form H 2 SO 4 in the stack gas.Besides, SO 4 may form if the plants use selective catalytic reduction which add ammonia (NH 3 ) to control NO x emission.H 2 SO 4 mist and particulate SO 4 may be emitted along with the particulate passing through the control device (Corio and Sherwell, 2000).The high content of SO 4 explains the predominance of inorganic fraction in condensable PM.

Percentages of Filterable and Condensable PM in PM 2.5
The percentages of filterable and condensable PM in PM 2.5 for the investigated plants are shown in Fig. 6.Condensable PM dominates for most plants.The condensable PM fraction accounts for 61.2%, 73.5%, 44.2%, 52.8% and 51.2% for power plant, boiler, brick manufacturing plant, incinerator and arc furnace plant, respectively.The results of Corio and Sherwell (2000) show that condensable PM, on average, comprises 76% of the total PM 10 stack emissions for coal-burning boilers.PM emissions were measured for power plants with three different fuels (bituminous, subbituminous coal and lignite).The results showed that condensable PM accounts for 59.7%, 49.1% and 73% of total PM emissions for fuels of bituminous, sub-bituminous coal and lignite, respectively (Farber and Sloat, 2005).The results of this study agree with the previous studies.Condensable PM concentrations are higher than filterable PM.This is partially due to better control of filterable particulate combined with emission controls for NO x that use catalysts that produce SO 3 or use ammonia injection.Condensable PM is not measured typically because regulations in most countries do not require sources to measure.However, the real PM 2.5 contribution to the atmosphere would be underestimated if condensable PM is not included.The percentages of condensable PM in PM 2.5 for different sources vary greatly.Condensable PM percentage depends on control devices, exhaust temperature and other sourcespecific conditions.Among the factors, stack temperature is the most significant since in-stack filter is used with Method 201A.As shown in Fig. 7, condensable PM percentage increases as the exhaust temperature increases.As mentioned in previous section, when the stack temperature is cooler, the SO 4 and other condensable materials are easier to condense on the filter, which would result in the higher percentage of filterable PM.The results again indicate the importance of including condensable PM in PM 2.5 emission measurement since stack temperature can affect filterable PM measurement result for in-stack measurement of Method 201A.No matter how the condensable and filterable PM distributions are affected by the stack temperatures, the emission of PM 2.5 to the atmosphere should be sum of filterable and condensable PM.

CONCLUSION
USEPA Method 201A and 202 are used to measure filterable and condensable PM 2.5 emissions, respectively, from 5 stationary sources.The average filterable PM 2.5 concentrations for power plant, boiler, brick manufacturing plant, incinerator and arc furnace are 0.75, 16.9, 8.67, 0.15 and 2.12 mg/Nm 3 , and the average condensable PM 2.5 concentrations are 2.15, 29.3, 83.5, 0.17 and 2.53 mg/Nm 3 , respectively.For filterable PM 2.5 collection, it is necessary to collect both filter and the residue particulates to avoid underestimation of PM 2.5 emission since the amount of PM 2.5 residue on the exit tube of cyclone and front half of the filter holder is significantly high when filterable PM 2.5 concentrations are low.The inorganic fraction accounts for 89.0%, 69.4%, 72.3%, 89.8% and 72.8% of condensable PM, respectively, for power plant, boiler, brick manufacturing plant, incinerator and arc furnace.The condensable PM accounts for 61.2%, 73.5%, 44.2%, 52.8% and 51.2% of total PM 2.5 for power plant, boiler, brick manufacturing plant, incinerator and arc furnace plant, respectively.The real PM 2.5 contribution to the atmosphere would be underestimated if condensable PM is not included.Stack temperature is an important factor affecting the distribution of filterable and condensable PM 2.5 .