Characteristics of particulate matter from four coal-fired power plants with low-low temperature electrostatic precipitator in China

Enhancement of fine particle removal through flue gas cooling in a spray tower with packing materials

The efficient removal of fine particles from coal-fired flue gas poses challenges for conventional electrostatic precipitators and bag filters. Recently, a novel approach incorporating deep cooling of the flue gas has been proposed to enhance the removal of gaseous pollutants and particles. However, the achievable efficiency and underlying mechanisms of particle capture within the gas cooling system remain poorly understood. This study aims to elucidate the effectiveness of gas cooling in enhancing the removal of particles through a laboratory-scale spray tower equipped with packing materials. The results demonstrate a significant increase in particle removal efficiency, from 63.4 % to over 98 %, as the temperature of the spray liquid decreases from 20℃ to -20℃. Notably, this enhancement is particularly pronounced for particles sized 0.1-1 µm, with efficiency rising from approximately 40 % to 95 %, effectively eliminating the penetration window. Moreover, we find that the spray flow rate positively influences particle removal capability, while the height of the packing section exhibits an optimal value. Beyond this optimal height, particle removal performance may decline due to an inadequate liquid-to-packing ratio. To provide insight into the capture process, we introduce a single-droplet model demonstrating that particle capture is primarily enhanced through the augmented thermophoretic force.

Emission characteristics and removal of heavy metals in flue gas: a case study in waste incineration and coal-fired power plants

Heavy metal pollutants such as Hg, As, Pb, Cr, and Cd emitted from coal and waste combustion have received widespread attention. In this study, we systematically investigated the emission characteristics of heavy metals in waste incineration and coal-fired flue gases, focused on testing the removal effect of self-made cold electrode electrostatic precipitator (CE-ESP) on heavy metals in flue gas, and made a comparative analysis with the existing air pollution control devices (APCDs). Test results from waste incineration power plant showed that each APCD showed a certain effect on the removal of heavy metals in condensable particulate matter (CPM), with an average removal efficiency of bag filter was 86%, but its effect on Hg removal was slightly worse. Under the coupled field with electrified cold electrode plate operation mode, the average removal efficiency of CE-ESP on heavy metals in CPM was as high as 93%, including 76% for Hg. The removal efficiency of heavy metals (especially Hg) in CPM increased with the increase of flue gas temperature difference between inlet and outlet of CE-ESP. Test results from this coal-fired power plant showed that heavy metals were enriched in fly ash to a higher degree than in slag, the synergistic control of heavy metals in submicron particulate matter by the dust remover was not obvious, and there was a significant correlation between each heavy metal emission factor and its content in coal. Under the temperature field with non-electric cold electrode plate operation mode, the overall effect of CE-ESP on the removal of gaseous heavy metals was better than that of particulate heavy metals. Under the conventional electric field operation mode, CE-ESP was less effective in removing particulate Cr and gaseous Hg0. Under the coupled field with electrified cold electrode plate operation mode, the average removal efficiencies of CE-ESP for particulate and gaseous heavy metals were 82.37% and 76.16%, respectively.

Removal and emission characteristics of hazardous trace elements in total and graded particulate matters: A case study of a typical ultra-low emission coal-fired power plant

Particulate matters (PMs) and hazardous trace elements (HTEs) emitted from coal-fired power plants (CFPPs) have raised serious environmental and human issues. Herein, total PMs and graded PMs including PM<1, PM1-2.5 and PM2.5-10 at the inlet/outlet of air pollution control devices (APCDs) were collected from a representative ultra-low emission (ULE) CFPP in China. The removal efficiencies of total PMs by selective catalytic reduction (SCR), electrostatic precipitator (ESP), wet flue gas desulfurization (WFGD) and wet electrostatic precipitator (WESP) were 0.40 %, 99.9 %, 38.1 % and 85.3 %, respectively. PM2.5-10 was robustly removed by WFGD, while PM<1 and PM1-2.5 were readily removed by WESP. The removal efficiencies of As, Cd, Cr and Pb in total PMs by APCDs followed an order: ESP > WESP > WFGD > SCR. SCR significantly decreased Se concentration by 42.8 %, contrasting to the removal of As, Cd, Cr and Pb (10.8-20.8 %). As, Cd, Cr, Pb and Se concentrations in graded PMs at the outlets of ESP, WFGD and WESP decreased with particle size increasing. All As, Cd, Cr, and Pb contents in PM<1, PM1-2.5 and PM2.5-10 at WFGD outlet increased, surpassing their analogues at ESP and WESP outlets. However, the concentration of Se declined in PM<1 at WFGD outlet. The atmospheric emission factors (EFs) of As, Cd, Cr, Pb and Se in the studied ULE CFPP were respectively 7.32, 1.27, 6.05, 122.5 and 6.42 mg/t, in line with Monte Carlo simulations. This study would not only provide a basis for emission control of PMs and HTEs in CFPPs, but also promote the improvement of respective environmental policy.

Distribution of the existence forms of condensable particulate matter during condensation: The surface collection and the space suspension forms

Condensable particulate matter (CPM), as an air pollutant that has received wide attention in recent years, has a high emission concentration compared to filterable particulate matter (FPM), yet there is not a well-developed removal method. Air pollution control devices (APCDs) with a condensation process have a certain effect on CPM removal, which inspired us to study the condensation behavior of CPM. During the condensation process, the condensed CPM may exist in two final forms: one was collected by the cold surface that caused the condensation; the other was converted to fine particles and suspended in the space of the flue. In a sense, the surface collection form can reflect the removal of CPM, while the CPM in the space suspension form should be further separated with the aim of removal. In this work, we adopted a CPM sampling system based on EPA Method 202 to reveal the distribution of the condensation behavior of CPM. In this sampling system, the CPM collected by all the cooling surfaces, including the cooling coil and impingers, can be counted as the surface collection form, while those collected by the terminal CPM filter can be regarded as the space suspension form. It was found that about 75 % of CPM was collected by the cooling surfaces, which suggested that CPM preferred to be in the surface collection form than the space suspension form. This preference characteristic also could be observed in the inorganic (CPM_i) and organic components of the CPM (CPM_o). Among the CPM_i, almost all NH₄⁺ and SO₄^2- condensed in the form of surface collection. The preference characteristics in CPM's (and its components') condensation behavior are similar under every temperature reduction condition. In this work, the interference of CPM measurement error was resolved by the statistical method of ANOVA.

Emission characteristics of PM2.5 and components of condensable particulate matter from coal-fired industrial plants

Particulate matter (PM), including condensable particulate matter (CPM) and filterable particulate matter (FPM), emitted from coal combustion is one of the major contributors to air pollution. In this study, CPM and FPM were sampled from two coal-fired industrial boilers with air pollution control devices (APCDs). The emission concentration of total PM (CPM and FPM) and inorganic components of CPM were studied. The organic fractions in CPM and raw coal were analyzed using a gas chromatograph/mass spectrometer (GC/MS). The concentrations of total PM in the flue gas decreased from 1475.61 to 7.68 mg/Nm³ in unit 1, and from 2451.62 to 29.38 mg/Nm³ in unit 2 after the flue gas passed through the APCDs. CPM accounted for 51.42-91.93% of total PM emitted from stacks, of which organic components (73.87-96.30%) were one of the main constituents. Although aromatic hydrocarbons are one of the major components of raw coal, they were almost nonexistent in the CPM emitted from coal combustion. Saturated hydrocarbons accounted for the largest proportion of organic components in CPM, 49.19% in unit 1 and 61.16% in unit 2. The proportion of esters in the oxygen-containing derivatives of CPM emitted from two units was relatively high. SO₄^2- was the inorganic component with the largest concentration in CPM emitted from the boiler units. This study will improve the understanding of the emissions levels of PM_2.5 and the properties of CPM that originate from the coal-fired industrial processes.

Estimation of commercial cooking emissions in real-world operation: Particulate and gaseous emission factors, activity influencing and modelling

Measurements of real-world cooking emission factors (CEFs) were rarely reported in recent year's studies. However, the needs for accurately estimating CEFs to produce cooking emission inventories and further implement controlling measures are urgent. In this study, we collected cooking emission aerosols from real-world commercial location operations in Beijing, China. 2 particulate (PM_2.5, OC) and 2 gaseous (NMHC, OVOCs) CEF species were examined on influencing activity conditions of cuisine type, controlling technology, operation scales (represented by cook stove numbers), air exhausting volume, as well as location and operation period. Measured NMHC emission factors (Non-barbecue: 8.19 ± 9.06 g/h and Barbecue: 35.48 ± 11.98 g/h) were about 2 times higher than PM_2.5 emission factors (Non-barbecue: 4.88 ± 3.43 g/h and Barbecue: 15.48 ± 7.22 g/h). T-test analysis results showed a significantly higher barbecued type CEFs than non-barbecued cuisines for both particulate and gaseous emission factor species. The efficacy of controlling technology was showing an average of 50 % in decreasing PM_2.5 CEFs while a 50 % in increasing OC particulate CEFs. The effects of controlling equipment were not significant in removing NMHC and OVOCs exhaust concentrations. CEF variations within cook stove numbers and air exhausting volume also reflected a comprehensive effect of operation scale, cuisine type and control technology. The simulations among activity influencing factors and CEFs were further determined and estimated using hierarchical multiple regression model. The R square of this simulated model for PM_2.5 CEFs was 0.80 (6.17 × 10^-9) with standardized regression coefficient of cuisine type, location, sampling period, control technology, cook stove number (N) and N² of 5.18 (0.02), 5.33 (0.02), 1.93 (0.19), 9.29 (4.18 × 10^-6), 9.10 (1.71 × 10^-3) and -1.18 (2.43 × 10^-3), respectively. In perspective, our study provides ways of better estimating CEFs in real operation conditions and potentially highlighting much more importance of cooking emissions on air quality and human health.

Effects of air pollution control devices on volatile organic compounds reduction in coal-fired power plants

Air pollution control devices (APCDs) have been fitted to many coal-fired power plants to decrease the impacts of pollutants generated during coal combustion. APCDs remove conventional pollutants but also decrease volatile organic compound (VOC) emissions. In this study, flue gas samples were collected from different points in seven typical coal-fired power and two industrial boilers, and the VOC concentrations in the flue gas samples were determined by gas chromatography-mass spectrometry (GC-MS). Selective catalytic reduction (SCR) systems and electrostatic precipitators (ESP) can synergistically remove VOCs, the mean removal rate of VOCs by ESP was 42% ± 9%. This was caused by the catalyst in SCR systems and the condensation process in the ESP. Wet flue gas desulfurization (WFGD) affected different VOCs in different ways, increasing the halogenated hydrocarbons and aromatic hydrocarbons concentrations but decreasing the oxygenated VOCs concentrations by 12%. Wet electrostatic precipitators (WESP) increased VOC emissions. By calculating Ozone formation potential (OFP), aromatic hydrocarbons are important contributors to ozone production. The emission factor of the power plant was 0.69 g/GJ, and the Chinese annual emission was about 1.2 × 10⁴ t. VOCs emissions in different regions were affected by factors such as the economy and population. VOC emissions can be decreased by using the most appropriate unit load and improving the VOC removal efficiencies of the APCDs.

Distribution and emissions of trace elements in coal-fired power plants after ultra-low emission retrofitting

Various hazardous trace elements emitted from anthropogenic activities are attracting increasing public awareness. This study comprehensively explored the distribution and emissions of trace elements in coal-fired power plants (CFPPs) after ultra-low emission retrofitting by conducting field experiments, literature surveys, and model calculations. High levels of volatile Hg and semi-volatile As/Pb were mainly observed in fly ash and gypsum (96.6%-98.5%), while the proportion of non-volatile Cr in bottom ash was 9.23%. The Hg and As/Pb removal efficiencies were remarkably improved by ultra-low emission retrofitting, increasing by 5.67% and 2.08%/2.63%, respectively. However, ULE retrofitting only slightly affected (0.17%) non-volatile elements. These improvements were mainly attributed to the low-low-temperature electrostatic precipitator. Owing to the enhanced particle-capturing efficiencies, the concentrations of trace elements in the emitted gas of the tested CFPPs were low, ranging from 0.21-1.50 μg/m³, but accounted for a high proportion of the gas phase (61.8%-100%). Based on the national database of coal quality and their behaviour in CFPPs, we found that most of the concentrations of trace elements emitted from Chinese CFPPs were significantly lower than the internationally existing emission limits. However, owing to the skewed distribution characteristics of the emitted concentrations, we suggest issuing or revising the corresponding emission limits and improving the control of intense trace element pollution in China.

A Review on CO2 Capture Technologies with Focus on CO2-Enhanced Methane Recovery from Hydrates

Natural gas is considered a helpful transition fuel in order to reduce the greenhouse gas emissions of other conventional power plants burning coal or liquid fossil fuels. Natural Gas Hydrates (NGHs) constitute the largest reservoir of natural gas in the world. Methane contained within the crystalline structure can be replaced by carbon dioxide to enhance gas recovery from hydrates. This technical review presents a techno-economic analysis of the full pathway, which begins with the capture of CO2 from power and process industries and ends with its transportation to a geological sequestration site consisting of clathrate hydrates. Since extracted methane is still rich in CO2, on-site separation is required. Focus is thus placed on membrane-based gas separation technologies widely used for gas purification and CO2 removal from raw natural gas and exhaust gas. Nevertheless, the other carbon capture processes (i.e., oxy-fuel combustion, pre-combustion and post-combustion) are briefly discussed and their carbon capture costs are compared with membrane separation technology. Since a large-scale Carbon Capture and Storage (CCS) facility requires CO2 transportation and storage infrastructure, a technical, cost and safety assessment of CO2 transportation over long distances is carried out. Finally, this paper provides an overview of the storage solutions developed around the world, principally studying the geological NGH formation for CO2 sinks.

Solid Fuel Characteristics of Pellets Comprising Spent Coffee Grounds and Wood Powder

To help mitigate the effects of global warming and fossil fuel depletion caused by human use of fossil fuels, solid fuel pellets were developed from a mixture of spent coffee grounds (SCG) and pine sawdust (PS). The feasibility of SCG-PS pellets as biofuel was also verified by evaluating its fuel quality. An increase in the proportion of SCG in the pellet led to an increase in its calorific value, owing to the high C, H, and oil contents, and increases in the ash and S contents, owing to the high S content in SCG. Analysis of the feedstock particle size distribution revealed that SCG particles are smaller than PS particles; thus, the durability of the pellet decreases as the proportion of SCG increases. Accordingly, the samples with higher SCG proportions (70 and 90 wt.%) did not meet the moisture content standards for biomass solid refuse fuel (bio-SRF) set by the Korea Ministry of Environment, whereas the samples with lower SCG proportions did. In particular, CP10 (10 wt.% SCG + 90 wt.% PS) satisfied the quality standards of Grade 1 wood pellets, demonstrating the feasibility of using SCG as a raw material for biofuel pellet production.

Air pollutant emissions from coal-fired power plants in China over the past two decades

China is the largest coal producer and consumer in the world, and coal-fired power plants are among its major sources of air pollutants. The Chinese government has implemented various stringent measures to reduce air pollutant emissions over the past two decades. National statistical data, emission inventories, and satellite observations indicate that air pollutant emissions from coal-fired power plants have been effectively controlled. Field measurements at coal-fired power plants can provide valuable information about the long-term trend of air pollutant emissions and the driving factors. In this study, we evaluated air pollutant emissions from 401 units at 308 coal-fired power plants. An appreciable reduction in air pollutant concentrations and emission factors from coal-fired power plants in China is observed over the past two decades. The drivers for this trend from the perspective of policy making, application of removal technologies, tightening of emission standards, technological improvement, monitoring systems, and economic measures are discussed. Currently, concentrations of typical air pollutants from coal-fired power plants in China are lower than those in Japan, Germany, and the US. This can be attributed to the policies and lenient emission standards for power plants in these countries. The technological improvement of air pollution control devices is the key factor that has led to reductions in air pollutant emissions in China. China has built the largest system of clean coal-fired power plants in the world.

Variation characteristics of final size-segregated PM emissions from ultralow emission coal-fired power plants in China

The ultralow emission (ULE) retrofits for Chinese coal-fired power plants (CFPPs) are nearing completion. Large-scale and rapid retrofits have resulted in significant changes in the emission level and characteristics of particulate matter (PM). To investigate the variation characteristics of final three size fractions PM (PM_2.5, PM_10-2.5, PM_>10) emissions, we conducted field tests at the outlets of wet flue gas desulfurization (WFGD) and wet electrostatic precipitator (WESP) by a pair of two-stage virtual impactors in eight representative ULE CFPPs. Our results indicate that, after WESP installations, the mass concentrations of final PM are significantly reduced and those of the final total ions and elements decrease as most individual chemical compositions are reduced. WESP presents an excellent removal performance for large particle sizes and high PM concentrations. SO₄^2- is the major ionic component at both the outlets of WFGD and WESP, and its proportion in total ions is reduced to some extent through WESP. Furthermore, the average mass contents of SO₄^2- and most elements in PM_2.5 are significantly lower than those in PM_10-2.5 and PM_>10 whether at the WFGD-outlets or WESP-outlets. By comparison, chemical profiles of PM have substantially changed after ULE retrofits, and those after WFGD (e.g., sulfate, Zn, Pb, and Cu) have also changed relative to existing data. The end-tail emission factors (EFs) of PM_2.5, PM₁₀, and PM_total under the typical ULE technical routes of WESP are calculated in time, and the corresponding EFs are in the range of 2.82-8.97, 15.7-27.6, and 38.6-61.7 g t^-1, respectively. We believe the latest detailed PM EFs and the associated chemical profiles provided in this study are more representative of the current emission situations of Chinese CFPPs.

Refined assessment of size-fractioned particulate matter (PM2.5/PM10/PMtotal) emissions from coal-fired power plants in China

Chinese coal-fired power plants (CFPPs) are experiencing large-scale and rapid retrofitting of ultralow emission (ULE), causing significant changes in emission level of particulate matter (PM) from CFPPs. In this study, based on coal ash mass balance over the whole process, an integrated emission factors (EFs) database of three size-fractioned particulate matters (PM_2.5, PM₁₀, and PM_total) for CFPPs is constructed, which covers almost all typical ULE technical routes installed in CFPPs. To verify the reliability of PM EFs established in this study, we compare those with related results based on field tests. Overall, the gaps in the EFs of PM_2.5, PM₁₀, and PM_total obtained by the two methods are not outrageous within a reasonable range. By combined with the refined size-fractioned PM EFs and unit-based activity level database, a detailed high-resolution emission inventory of PM_2.5, PM₁₀, and PM_total from Chinese CFPPs in 2017 is established, with the corresponding total emissions of 143, 207, and 267 kt, respectively. Our estimation of PM_total emission is comparable to the official statistics announced by China Electricity Council (CEC), which further demonstrates the reliability of PM EFs constructed in this study. Moreover, potential reductions of PM from CFPPs at two stages before and after 2017 are assessed under three application scenarios of major ULE technical routes. We forecast the final annual emissions of PM_2.5, PM₁₀, and PM_total until 2020 will be reduced further, which fall within the range of 86-111 kt, 120-157 kt, and 142-184 kt, respectively, if all CFPPs achieve ULE requirements under the three scenarios. We believe our integrated database of PM EFs of CFPPs has good universality, and the forecast results will be helpful for policy guidance of ULE technologies, emissions inventory compilation, and regional air quality simulation and management.

Evaluating Airborne Condensable Particulate Matter Measurement Methods in Typical Stationary Sources in China

The dry impinger method, the indirect dilution method, and the direct dilution method can be used to measure the condensable particulate matter (CPM) emissions. We tested these methods in determining the CPM emissions from typical stationary sources in China and found that the CPM concentrations measured by the dry impinger method are much higher than those measured by the two dilution methods regardless of the type of stationary source. The soluble gases (e.g., SO₂, HCl, and NH₃) partially absorbed by the impinger solutions are the main reason for the overestimation of the CPM concentrations. This is supported by detecting more water-soluble ions (e.g., SO₄^2-, Cl^-, and NH₄⁺) from the CPM collected using the dry impinger method. The positive biases of the CPM concentration and its water-soluble ions collected by the dry impinger method are larger under the conditions with high concentrations of soluble gases such as at the flue gas desulfurization inlet in coal-fired power plants. Comparing to the direct dilution method, the indirect dilution method can better capture the rapid dilution, cooling, and condensation of condensable gas precursors in the presence of filterable particulate matter and is recommended as the appropriate method for the CPM measurement in stationary sources.

Migration and Emission Characteristics of Ammonia/Ammonium through Flue Gas Cleaning Devices in Coal-Fired Power Plants of China

To investigate the up-to-date migration and emission characteristics of NH₃/NH₄⁺ in coal-fired power plants (CFPPs) after implementing ultralow emission retrofitting, typical air pollution control devices (APCDs) in CFPPs, including flue gas denitrification, dust collectors, combined wet flue gas desulfurization (WFGD), and wet precipitators are involved in field measurements. The results show that most of the excessive injected and/or unreacted ammonia from the flue gas denitrification system, whether selective catalytic reduction (SCR) or selective noncatalytic reduction (SNCR), is converted into particle-bound NH₄⁺ (>91%), and the rest (less than 9%) is carried by flue gas in the form of gaseous NH₃, with a concentration value of 0.15-0.54 mg/(N m³) at the denitrification outlet. When passing through dust collectors, particle-bound NH₄⁺ concentration decreases substantially along with the removal of particle matter. In WFGD, the dissolution and volatilization effects affect the gaseous ammonia concentration, which decreases when using limestone slurry and a 20% solution of ammonia as a desulfurization agent, while liquid ammonia solution with a high concentration (99.8%) may cause the flue gas NH₃ concentration to increase considerably by 13 times. Particle-bound NH₄⁺ concentration is mainly influenced by the relative strength of desulfurization slurry scouring and flue gas carrying effects and increases 2.84-116 times through ammonia-based WFGD. Furthermore, emission factors of NH₃ for combinations of APCDs are discussed.

Emission characteristics of fine particulate matter from ultra-low emission power plants

As one of the highest energy consuming and polluting industries, the power generation industry is an important source of particulate matter emissions. Recently, implementation of ultra-low emission technology has changed the emission characteristic of fine particulate matter (PM_2.5). In this study, PM_2.5 emitted from four typical power plants in China was sampled using a dilution channel sampling system, and analyzed for elements, water-soluble ions and carbonaceous fractions. The results showed that PM_2.5 concentrations emitted from the four power plants were 0.78 ± 0.16, 0.63 ± 0.09, 0.29 ± 0.07 and 0.28 ± 0.01 mg m^-3, respectively. Emission factors were 0.004-0.005 g/kg coal, nearly 1-2 orders of magnitude lower than those reported in previous studies. The highest proportions of PM_2.5 consisted of organic carbon (OC), SO₄^2-, elemental carbon (EC), NH₄⁺, Al and Cl^-. Coefficients of divergence (CDs) were in the ranges 0.22-0.41 (for an individual plant), 0.43-0.69 (among different plants), and 0.60-0.99 (in previous studies). The results indicated that the source profiles of each tested power plant were relatively similar, but differed from those in previous studies. Enrichment factors showed elevated Se and Hg, in accordance with the source markers Se and As. Comparing source profiles with previous studies, the proportion of OC, EC and NH₄⁺ were higher, while the proportion of Al in PM_2.5 were relatively lower. The OC/EC ratio became concentrated at ∼5. Results from this study can be used for source apportionment and emission inventory calculations after implementation of ultra-low emission technologies.

Prospects of thermal power plants switching from traditional fuels to coal-water slurries containing petrochemicals

The amount of thermal and electric energy produced by coal combustion increases nonlinearly, because the production capacities and consumption of the corresponding energy are on the rise. The prospects of excluding coal from the picture are slim, because it has been traditionally considered one of the most attractive fuels in terms of cost and heat of combustion. What we need is major changes in the energy industry towards environmentally effective use of coals and their processing wastes. In this research, we show the possibility of coal-fired thermal power plants and steam shops switching to coal-water slurries containing petrochemicals (CWSP). Extra calculations are made for fuel oil and natural gas. The scientific novelty of the research consists in the comprehensive consideration of all the possible technological modifications in the fuel feeding, storage, and preparation system. We focus on potential benefits of thermal power plants and steam shops switching from coal, gas, and fuel oil to coal-water slurries containing petrochemicals, while taking into account all the main and most important environmental, economic, and energy performance indicators. Using CWSP instead of coal is much more environmentally friendly. By varying the content of water and additives in CWSP, we can lower the proportion of sulfur and nitrogen and slow down their oxidation. It is also possible to reduce temperature in the combustion zone and improve oxide retention in the ash without its release in the form of anthropogenic emissions. Throughout the world, tens of thousands of fuel oil and coal-fired TPPs with the annual gross electric output of 1.8 TW can switch to CWSP. The integrated performance indicators of CWSP fuels are only inferior to those of natural gas but these slurries are prepared from numerous industrial wastes.