INTRODUCTION

Three coal fired-power plants have recently been installed in Mexico, which use domestic and imported coal (mainly from the USA) to generate electricity. The control systems of these coal-fired power plants are primarily electrostatic precipitators (ESPs), which results in virtually all mercury being emitted into the atmosphere, leaving only a small amount in the bottom ash and slag, and in the fly ash (^{USEPA 2005}, ²⁰¹¹). Insufficient studies have been conducted about the mercury in the coal used by Mexican power plants, and mercury emissions from these power plants, because there is not a national standard maximum emission level.

The Minamata Convention on mercury, signed by 140 countries including Mexico since 2013, agreed to reduce mercury emissions into the atmosphere, and to identify power plants that use coal to generate electricity (^{UNEP 2015}). Analyzing the mercury content in domestic and imported coal used for electricity generation at the three coal-fired power plants operating in Mexico and determining their mercury emissions is an important task.

In this study, mercury emissions for 2013 from the CETEPEC power plant were calculated using the USEPA methodology and information about fuel consumption per day and generating unit provided directly by the Federal Electricity Commission of Mexico. The obtained daily mercury emissions for 2013 will be useful in further studies of mercury emissions for this and other coal-fired power plants in Mexico (Carbón I and Carbón II). Considering the daily fuel consumption per unit of electricity generated, we can understand and propose control measures to increase the efficiency of each unit to reduce mercury emissions.

Since this study only uses information from the CETEPEC power plant for the year 2013, we think this information should be updated with additional data for the power plants Carbón I and Carbón II, which it will reflect a very important advance towards complying with the Minamata Convention.

MATERIALS AND METHODS

The method for calculating mercury emissions into the atmosphere considered the mercury concentration in coal and the modified emission factor (USEPA 1997), from which the following equation is obtained:

This equation accounts for mercury content in the coal. For this study, we used Eq. (1) because mercury concentration is different in the coal used by coal-fired power plants. To determine the mercury emission, we used a concentration of 0.335 mg Hg/kg of coal, determined by ^{Fuentes et al. (2017)} for Mexico.

Control systems used by coal-fired power plants installed in Mexico correspond to electrostatic precipitators with value 1, as recommended by the tables developed by the ^{USEPA (1997)}. In this case mercury emissions are not retained in the control system because they are released as gaseous elemental mercury (GEM). The modified emission factor with value 1 indicates that the GEM is emitted totally into the atmosphere because the ESP retains non-gaseous pollutant particles as GEM. This information corresponds to the USEPA methodology; however, in order to identify mercury emissions into the atmosphere, we determined the fractional GEM, GOM and PBM, as well as the mercury mass balance, in the following manner:

For GEM, GOM and PBM from coal-fired power plants that use electrostatic precipitators and consumption of coal by type (subbituminous or bituminous), we used the profile of GEM, GOM and PBM corresponding to 87.41, 12.52 and 0.06%, respectively (^{USEPA 2011}).

To consider the mercury mass balance in the CETEPEC power plant, we contemplated information about the total coal consumed in the process of generating electricity, of which 10 % corresponds to ash types (bottom, slag and fly); of these, 16 % corresponds to bottom ash and slag from the boiler, and 84 % to fly ash from the electrostatic precipitator and control system.

RESULTS AND DISCUSSION

Mercury emissions from the CETEPEC power plant calculated by Eq. (1) enable the identification of the temporal variability of mercury emissions per day and per generating unit.

Table V shows the results for mercury emissions from each generating unit of the CETEPEC power plant for 2013. We can note the working days for each generating unit. U7 shows the highest emissions (near 2 kg/day) of mercury into the atmosphere. The daily standard deviation of mercury emissions for each generating unit is low.

The temporal variability of mercury emissions per day is illustrated in figure 1. From April to July there was an increase in electricity generation, since in this time of the year (spring-summer) air conditioning systems operate at 100 % of their capacity. The highest mercury emissions (above 7.5 kg/day) from all units occurred in April 19, 24, and 25, with 7.53, 7.56, and 7.59 kg/day, respectively; May 2 and 3, with 7.52 and 7.57 kg/day, respectively. Total mercury emissions in 2013 were 1943 kg of mercury into the atmosphere from all units.

Fig. 1 Daily mercury emissions from the CETEPEC power plant in 2013 

Table VI presents emissions of gaseous elemental mercury (GEM, Hg⁰), gaseous oxidized mercury (GOM, Hg²⁺), and particle bound mercury (PBM, Hg_p) from the CETEPEC power plant in 2013. GEM is the major emission with respect to GOM and PBM, due to the control system installed in CETEPEC corresponding to the electrostatic precipitator. U7 presented the highest mercury emission as GEM, since it consumes about 6,000 t of coal per day due to its high capacity for electricity generation (about 700 MW).

To estimate mercury mass balance in CETEPEC it was necessary to consider the mercury concentration in coal (0.335 mg/kg), mercury concentration in bottom ash and slag (0.148 mg/kg), and mercury concentration in fly ash (0.017 mg/kg), all of them analyzed by ^{Fuentes et al. (2017)}. CETEPEC consumed 5 832 666 t of coal in 2013, 10 % (583 266 t) corresponding to bottom ash and slag, and fly ash. Table VII shows the mercury retained in the boiler and electrostatic precipitator, and the GEM emitted into the atmosphere, while figure 2 presents the mercury mass balance diagram considering the boiler and control system (electrostatic precipitator).

Fig. 2 Mercury mass balance diagram 

Figure 3 illustrates daily mercury emissions for each generating unit during May, as a case example. U7 emitted the highest amount of mercury, and all generating units were working during that month. Average mercury emitted into the atmosphere from U7 was 1.8 kg, due to its generating capacity of 700 MW; moreover, each generating unit from U1 to U6 have a generating capacity of 350 MW, and their average emission of mercury into the atmosphere was 0.9 kg. Similarly, some generating units were sometimes out of service for a few days or months because they were undergoing maintenance. A slightly higher amount of electricity was generated during April-July because most of the units were working and it was the period of greatest energy demand. Appendix 1 shows the daily mercury emissions for each month during 2013.

Fig. 3 Daily mercury emissions from each generating unit in May 2013 

Table VIII shows mercury emission rates for each generating unit during 2013. The values presented in this table are high as compared to mercury emissions from the USA (0.0059 to 0.054 kg Hg/GW h) (^{NRDC 2012}) due to the newer technologies employed in this country for control systems, such as fabric filters, selective catalytic reactors, and activated carbon injection.

The efficiency of each electricity-generating unit during 2013 is shown in figure 4. The efficiency of U7 was consistently higher than 90 %, whereas U2 was less efficient. This figure also indicates periods during which some generating units were not working.

Fig. 4 Percentage used of the load capacity for each generating unit during 2013 

The percentage of total electricity generation supplied by unit in the CETEPEC plant during 2013 is presented in figure 5. U7 had the highest performance (31 %), whereas U2 had the lowest (7 %) because it was out of service for several months. The other units accounted between 10 and 14 % of the capacity. CETEPEC administration should consider the performance of U7 during the period of major demand for electricity generation.

Fig. 5 Percentage of the plant’s total electricity generation supplied by unit of CETEPEC in 2013 

Figure 6 illustrates the amount of mercury emitted seasonally in 2013 by each generating unit of the CETEPEC plant. All units were working during the spring-summer season because of the high energy demand during this period. During autumn some generating units were working below their capacity, but mercury emissions from U7 were always above 90 % of efficiency for electricity generation.

Fig. 6 Mercury emitted seasonally by each generating unit during 2013 

Figure 7 presents the variability of mercury emissions of each generating unit. U2 presented the lowest mercury emissions since it was out of service for several months. The variability of mercury emissions from U1 to U6 was similar and fell within the 50th percentile (0.93 kg/day). The 25th percentile was about 0.83 kg/day, and the 75th percentile was about of 0.95 kg/day. Regarding U7, the 50th, 25th, and 75th percentiles were 1.74, 1.7, and 1.81 kg/day, respectively. The standard deviation is low with respect to the average, indicating that the variability of mercury emissions by each generating unit is low.

Fig. 7 Mercury emissions for each generating unit during 2013 

CONCLUSIONS

Since in Mexico there is no methodology to determine mercury emissions from its coal-fired power plants, we used the USEPA methodology to estimate mercury emissions from the CETEPEC power plant; however, the most important issue is to generate the emission factors for power plants in Mexico.

Using data information about coal consumption per day and per generating unit provided more reliable mercury emissions, and it also helps to find measures of prevention, minimization and control for each electricity-generating unit.

The period of major demand of electricity generation occurred from April to July, therefore mercury emissions were higher, which confirms that it is necessary to evaluate the load and efficiency of each generation unit during this period.

U7 had the highest mercury emissions into the atmosphere since the load of coal of this generating unit was 95 % of its capacity, and it worked for a longer time in electricity generation during 2013.

GEM emissions were larger than other forms of mercury, so it is necessary to develop new technologies to retain this gas according to the Minamanta Convention for the next years. Considering all the generation units, U7 released the larger amount of GEM into the atmosphere, thus it is important to evaluate this generating unit in the period corresponding to the electricity generation demand in Mexico.

The mercury emission rate for each generating unit was high with respect to the level reported by the USA, thus it is very important that the CETEPEC administration adopts new technologies to reduce the mercury emission rate.

Mexico should plan and implement energy generation alternatives, including reconfiguring or building next-generation power plants that operate at optimal efficiency and emit less mercury into the atmosphere.

The mercury emissions discussed in this study are very relevant because our country has a commitment to reduce them during the following years according to the Minamata Convention signed in 2013.