ARTICLE pubs.acs.org/EF
Avoiding Mercury Emissions by Combustion in a Spanish Circulating Fluidized-Bed Combustion (CFBC) Plant M. Antonia Lopez-Anton,† Mercedes Diaz-Somoano,† Luis Diaz,‡ and M. Rosa Martinez-Tarazona*,† Instituto Nacional del Carbon (INCAR), Consejo Superior de Investigaciones Cientificas (CSIC), C/Francisco Pintado Fe, 26, 33011 Oviedo, Spain ‡ Hulleras del Norte S.A. (HUNOSA), Avenida de Galicia, 44, 33005 Oviedo, Spain †
ABSTRACT: This work evaluates the behavior of mercury in an industrial circulating fluidized-bed combustion plant (CFBC) of 50 MW situated in Oviedo, Spain. The results showed that mercury was not emitted in the gas phase in significant amounts, thereby proving this technology as an effective means of preventing mercury emissions. Most of the mercury (8597%) was retained in the fly ashes. The proportion of mercury emitted in the gas phase was on the order of 0.20.3 μg m3, whereas only 0.02 ng m3 at most was emitted in the particulate matter. An evaluation of the fly ash fractions separated from the hoppers of the electrostatic precipitator showed the highest proportion of mercury to be present in the fly ashes with the lowest particle size and the highest carbon content. However, this mercury proved to be stable when subjected to leachability tests.
1. INTRODUCTION In the present situation where the production of energy from coal is still necessary and the common goal is to reach zero emissions of pollutants, the reduction of greenhouse emissions must be accomplished by the reduction of other toxic pollutants. Among these pollutants, mercury is one of the elements of main concern, given that coal combustion is the main anthropogenic source of this element to the environment. The data available thus far show that the combustion of coal accounts for the 35% of global anthropogenic emissions of mercury, while on a European level, the figure is estimated to be 53%.1 Fluidized-bed combustion (FBC) is nowadays a highly developed technology for the production of energy. The important advantages of this technology are its ability to co-combust different solid combustibles, including biomass and wastes, and its capacity to avoid emissions of NOx and SO2. Moreover, FBC is compatible with processes for capturing CO2, such as oxycombustion. Although the behavior of trace metal pollutants in FBC plants has not been studied in depth, this technology may be well-suited to the reduction of emissions of toxic metal species evaporated from combustible blends. The present study evaluates one of the most important trace pollutants, mercury, whose performance and level of emission from this type of plant is still largely unknown. Many papers have already been published on mercury behavior during pulverized coal combustion (PCC). From these works, it is estimated that mercury emissions to the environment can range from anywhere between 0 and 90%.27 Most recent studies, with the support of the previous established data, provide results on the behavior of mercury in PCC plants that can be generalized. Broadly speaking, during combustion in PCC plants, the mercury species in coal evaporate as Hg0 in the boiler. As the gases cool and come into contact with fly ashes, part of Hg0 is oxidized and can remain in the form of different species in the gas phase or be captured in the fly ashes that are finally retained in the particle control devices. The extent of the capture depends upon r 2011 American Chemical Society
not only the characteristics of the coal and the composition of fly ashes but also the performance of the power plant and the type of DeNOx systems and particle control devices. Moreover, if power plants are equipped with wet desulfuration systems, oxidized mercury can be partially retained in the sub-products of these devices. Although the amount of mercury finally emitted to the air from power stations may be modified by several variables, the most recent inventories in Europe (E-PRTR 2008) estimate that 19 tons of mercury/year is emitted. This represents 53% of the total emissions to the air. Most of the combustion installations evaluated in this inventory are PCC facilities. Less attention has been paid to the evaluation of mercury behavior811 in FBC plants, and the full-scale data available are not sufficient to allow for a thorough assessment of the mercury emissions from these units. As is relatively frequent in studies carried out at industrial scale, huge differences have been found when mercury behavior is compared between different plants, probably as a consequence of the variations in coal characteristics and power plant performance. These variables are similar to those involved in PCC, but in the case of FBC, the lack of data makes it impossible to draw general conclusions. Moreover, because in FBC systems a part of the ash is recycled, it is very difficult to conduct a proper mercury mass balance. This explains why there is a lack of knowledge about mercury behavior in FBC plants and on the capacity of the different types of ashes produced to retain mercury. The aim of the present work is to make up for this lack of knowledge focusing on the following objectives: (i) to evaluate the level of accuracy of sampling and mass balancing in a CFBC, (ii) to estimate the amount of mercury emissions to the air, (iii) to establish the relationships between certain fly ash characteristics and mercury capture, and
Received: April 14, 2011 Revised: June 8, 2011 Published: June 08, 2011 3002
dx.doi.org/10.1021/ef200572x | Energy Fuels 2011, 25, 3002–3008
Energy & Fuels
ARTICLE
Figure 1. Schematic diagram of the sampling points.
(iv) to estimate the possible lixiviation of the mercury retained in fly ashes.
Table 1. Temperature of Ashes at the Sampling Points day 2
day 3
343 °C 317 °C
320 °C 319 °C
328 °C 323 °C
Sampling 1
2. MATERIALS AND METHODS Two sampling campaigns were carried out in a 50 MWe industrial circulating fluidized-bed combustion (CFBC) plant at La Pereda, Spain. The first was performed in a period in which the plant was fed with a blend of between 36 and 40 wt % bituminous coal, 5156 wt % coal wastes obtained from old disposal sites (Villallana and Batan), and around 6 wt % limestone. During the second sampling campaign, the plant was fed with a blend of 32.2 wt % coal, 4.9% wood, 54.5 wt % coal wastes obtained from the old disposal sites, and 5.4 wt % limestone. After the sampling systems were retrofitted, representative samples of each product were obtained by following a specifically designed sampling method over a period of 3 days. Sampling was carried out over a period of 6 h every day, with around 2 kg of sample being taken every 30 min. Figure 1 presents a schematic diagram of the sampling points within the CFBC facility. Each sample was homogenized to form a unit representative of the sampling point and day. The individual combustibles, limestone (L), coal blends (CM), bottom ashes (BA), and fly ashes (FA), were sampled along with the airborne particulates (P) and flue gases (G). The labeling code for the samples collected were differentiated by the letters “a” (sampling 1) and “b” (sampling 2). Of the total quantity of ash collected over the sampling period, 56% was bed ash and 44% was fly ash. A total of 15% of the fly ash accumulated in the hoppers of the heat recovery area of the plant, while 20% accumulated in the hoppers of the air heater and 65% accumulated in the electrostatic precipitator (ESP). Inside the ESP unit, the primary precipitator fields accumulated the bulk of the fly ash, with 40% being collected in the first field, 22% being collected in the second field, 2% being collected in the third field, and 1% being collected in the fourth field. The temperatures of the ashes sampled over the three sampling days are presented in
day 1
BA1 and BA2 FA1FA3
strippers heat recovery area
FA4FA9
air heater hoppers
134 °C
132 °C
133 °C
FA10FA17
ESP
145 °C
144 °C
146 °C
BA1b
stripper (left)
295 °C
292 °C
295 °C
BA2b
stripper (right)
336 °C
333 °C
334 °C
Sampling 2
FA1bFA3b
heat recovery area
315 °C
319 °C
319 °C
FA4bFA9b FA10bFA17b
air heater hoppers ESP
135 °C 142 °C
138 °C 145 °C
136 °C 145 °C
Table 1. The temperatures of the gases at the sampling points are presented in Table 1. These temperatures may be considered similar to the temperatures of the ashes at the very instant that they were sampled. To prevent the ashes from cooling, the sampling was performed using an empty hopper. The mercury content of the combustibles, bottom ashes, fly ashes, particles, and flue gases were compared. Gas sampling was performed according to the Ontario Hydro method, and the solid and liquid samples were analyzed using an automatic mercury analyzer (AMA). To confirm the results of mercury emissions to the air, further analyses of the mercury content in the gases before the stack were carried out several days after the two sampling campaigns already described. These analyses were performed using a UT 3000 continuous elemental mercury analyzer preceded by a solution of KCl, in which oxidized mercury was retained. 3003
dx.doi.org/10.1021/ef200572x |Energy Fuels 2011, 25, 3002–3008
Energy & Fuels
ARTICLE
Table 2. HHVs, Proximate Analysis, and Mercury Content of the Components of the Combustible Blenda sampling
components of the combustible blend Villallana waste Batan waste
sampling 1
M (%)
HHV (kcal/kg)
A (% db)
VM (% db) 7.52
945
5.52
82.3
0.28
1987
4.22
12.8
70.8
0.12
3498
5.22
16.6
54.0
limestone (1)
0.35
bituminous coal wood
a
0.12 0.04
Villallana waste sampling 2
Hg (μg/g)
916
5.80
82.3
0.36
Batan waste
2291
6.12
13.9
7.35
65.9
0.18
limestone (2) bituminous coal
3327
5.82
15.0
55.7
1.11 0.16
HHV, higher heating value; M, moisture; VM, volatile matter; and A, ash.
Table 3. Ash in Coals and LOI in Ashes, BET Surface Area (SA), and Mercury Content of All Products Analyzed sampling 1 sample
ash1LOI2
CMAa
sampling 2
Hg (μg/g)
% RSD (Hg)
sample
ash1LOI2
64.71
0.17 ( 0.02
9.0
CMAb
62.51
0.22 ( 0.06
25
CMBa
1
65.0
0.17 ( 0.01
3.5
CMBb
62.11
0.20 ( 0.03
14
CMCa
65.51
0.17 ( 0.01
3.3
CMCb
62.81
0.23 ( 0.06
22
CMDa
65.31
0.17 ( 0.06
CMDb
64.11
0.21 ( 0.04
16
SA (m2/g)
BA1a
29
