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PCDF emissions from co-combustion of coal and PVC in a bubbling fluidised bed boiler
Fuel 84 (2005) 2149–2157 www.fuelfirst.com
PCDD/PCDF emissions from co-combustion of coal and PVC in a bubbling fluidised bed boiler J.M. Sa´nchez-Herva´sa,*, L. Armestoa, E. Ruiz-Martı´neza, J. Otero-Ruiza, M. Pandelovab, K.W. Schrammb,c a CIEMAT, Fossil Fuels Department, Avda. Complutense 22, 28040 Madrid, Spain ¨ kologische Chemie, Ingolsta¨dter Landstr. 1, D-85764 Neuherberg, Germany GSF-Forschungszentrum fu¨r Umwelt und Gesundheit, Institut fu¨r O c TUM, Wissenschaftszentrum Weihenstephan fu¨r Erna¨hrung und Landnutzung, Department fu¨r Biowissenschaften, Weihenstephaner Steig 23, 85350 Freising, Germany
b
Received 19 October 2004; received in revised form 7 July 2005; accepted 12 July 2005 Available online 8 August 2005
Abstract This paper reports on the results obtained in the study of the co-combustion of PVC with hard coal from South Africa in a 0,5 MWth Bubbling Fluidised Bed Boiler. The research has included the study of the effect of combustion temperature, fluidisation velocity and PVC content. The addition of urea to the raw fuel, as a dioxin-preventing compound has also been evaluated. Results have been analysed in terms of combustion efficiency, major pollutants emission (NOx, CO), and PCDD/Fs formation in the flue gas and in the fly ash. Under the experimental conditions tested, co-combustion of coal and PVC has proved to be feasible from the combustion efficiency and emission of PCDD/Fs points of view, whose levels remained below limits set by existing legislation on persistent organic pollutants. The addition of solid urea to the fuel blend reduces the amount of chlorinated compounds emitted. However, it has a negative impact on nitrogen pollutants formation q 2005 Elsevier Ltd. All rights reserved. Keywords: Co-combustion; Bubbling fluidised bed; Gaseous pollutants; PCDD; PCDF
1. Introduction Interest in organic compounds released from combustion sources has increased considerably in recent years due to growing concern over health risks. Among them, polychlorinated dibenzodioxins/dibenzofurans (PCDD/Fs) are most commonly studied. Data on emissions of PCDD/Fs from coal combustion are scarce as, in the past, there have been no requirements for monitoring of these species. However, the few data available suggest that the clean and efficient use of coal is not a significant source of these emissions [1]. It is interesting to note that, because of its efficient combustion properties, coal can help reduce organic emissions from the combustion of waste materials, even those containing a high * Corresponding author. Tel.: C34913466145; fax: C34913466269. E-mail address: [email protected] (J.M. Sa´nchez-Herva´s).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.07.009
concentration of chlorine. The elevated combustion temperatures induced by the addition of coal can have an ‘afterburner’ effect which may help to reduce the release of organic compounds from waste materials. Further, the sulphur present in coal can inhibit the formation of PCDD/F from waste combustion [2]. This means that coal has the potential to reduce emissions of organic compounds from waste incineration. Existing legislation on persistent organic pollutants, such as Decision No. 2179/98/EC, Directive 2000/76/EC, do not apply specifically to organic emissions from coal combustion. So, any reductions being made voluntarily are of help in the general reduction of air pollution. Since organic emissions are generally products of incomplete combustion, many of the measures used to optimise plant performance result in a reduction in organic emissions. Although fuel type can affect the formation and release of organic compounds, in general it is those factors which control combustion efficiency, such as fuel mixing, which are more important. Chlorine in the feed fuel can potentially promote the formation of PCDD/F but, again, it is
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combustion conditions which play a greater role in controlling the release of these compounds than any variation in the fuel characteristics. Therefore, emissions of organic species from industrial coal combustion, for instance, are higher than those from full-scale coal-fired power plants simply because the combustion in the former is less efficient, [3–9]. Alternatively, chemical inhibitors have been considered as one possible solution to control the formation and emission of PCDDs and PCDFs in combustion processes. Some compounds have indeed been shown to reduce PCDD/F concentrations in flue gases [10–12]. Several compounds containing either sulphur or nitrogen have been tested at laboratory scale, and some have been shown to prevent PCDD/F formation in pilot or full-scale plants [13]. Likewise, in the case of molecules containing amino nitrogen, although they have shown an especially clear inhibitory effect when sprayed into the flue gases [14], research has also been carried out to study the effect when added to the feedstock, [15,16]. Based on the above premises, this work aims at studying PCDD/Fs emissions during co-combustion of PVC with hard coal from South Africa. The research has been carried out under financial support by an ECSC Coal RTD Project, (Project 7220-PR-108, Emissions Minimisation in CoalSolid Waste Co-combustion by Primary Measures). Pilot scale co-combustion tests have been carried out in a 0.5 MWth Bubbling Fluidised Bed Combustor (BFBC). Rigid recycled PVC, in pellet shape, was intentionally added to South African coal just aiming to increase PCDD/F emissions in order to have a better evaluation of the extend of primary measures effectiveness (i.e. optimisation of combustion parameters and addition of inhibitor compounds). The content of PVC has ranged from 1 to 2% w/w. The research has included the study of the co-combustion of coal and PVC, analysing the effect of temperature, fluidisation velocity and PVC content. In a second stage the effect of dioxin-preventing compounds on the minimisation of chlorinated organic release has been studied, evaluating the addition of urea to the raw fuel, as an inhibition agent. Two levels of urea content, 5% and 10% have been studied.
2. Experimental 2.1. Fuels For the tests, South African hard coal was used. Chlorine was added to the fuel in the form of recycled PVC. In addition, in several tests, urea, a dioxin formation inhibitor, was added in solid phase to the raw fuel, prior to combustion. For every run, the fuel mixture was prepared in advance and fed into the burner as a single fuel. Fuels and additives burned in the bubbling fluidised bed combustor were characterised in the laboratory. Characterisation included proximate and ultimate analysis
Table 1 Proximate, ultimate and calorific value of fuels and additives
Proximate Analysis Volatile Matter (%) Ash, (%) Fixed Carbon (%) Ultimate Analysis Carbon (%) Hydrogen (%) Nitrogen (%) Sulphur (%) Oxygen (%) Chlorine (%) Heating Value LHV (MJ/kg)
South African Coal
PVC
Urea
38.1 13.3 48.6
80.1 19.9 0.0
68.0 4.3 1.9 0.6 11.9
66.8 12.1 0.4 0.6 11.7 40
19.8 6.9 45.7 !0.1 27.6
26.0
38.2
–
and calorific value. Results of the characterisation have been summarised in Table 1. 2.2. Experimental unit Pilot scale co-combustion tests were carried out in a 0.5 MWth Bubbling Fluidised Bed Combustor (BFBC). The plant has been described in detail elsewhere [17]. The pilot plant consists of the fuel feeding system, the combustor, the cyclones and the bag filter. The feeding system consists of two screw feeders. The first one is mounted on a weighing scale to provide an accurate record of the feed rate. The second one feeds the material, very quickly and directly, into the combustor. For the tests, the fuel was fed at the bottom of the combustor (0,15 m above the fluidisation plate) and the mixtures were prepared in advance and fed as one single fuel. Fluidisation air was supplied at the riser bottom through an air distribution plate. The fluidisation air is preheated electrically, and with a gas burner during the start-up operation. During the actual combustion runs the preheating systems are turned off. During the experimental work a steady state was maintained for 5 or 6 h. The inner diameter of the combustor is 0.2 m and the total height 3.5 m allowing bed depths up to 0.5 m. The bed material consists of ash from the combustion of coal. Combustion temperature is adjusted by means of a heat exchanger submerged in the bed. An electric heating system is installed at the top of the combustor to minimise heating loss in the freeboard. Gases leaving the combustor pass through two cyclones and a bag filter. A predictive-adaptive control system manages the plant and records the main process variables as follows: † Temperature along the length of the combustor cyclones and baghouse † Pressure differences in some points of the boiler † All input air flow rates, rate of feed fuel, etc
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Table 2 Experimental matrix Test
Coal (%)
PVC (%)
Urea (%)
Temperature (8C)
Fluidisation velocity (m/s)
O2 (% excess)
PCDD/Fs sampling
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
100 99 99 99 99 98 89 89 89 89 94 94
– 1 1 1 1 2 1 1 1 1 1 1
– – – – – – 10 10 10 10 5 5
850 800 830 850 880 850 830 850 880 850 850 850
0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 1 1 0.8
6 6 6 6 6 6 6 6 6 6 6 6
Yes
Yes Yes Yes Yes Yes
Co-combustion of South African coal and PVC.
The monitoring programme has taken into account gas composition and chlorinated organic collection for analysis of total dioxins and furans, as well as congener distribution. After sampling and conditioning, flue gases were analysed on-line continuously. O2 concentration was measured by a paramagnetic analyser. CO2, CO, and N2O were analysed by different NDIR spectrometers, NO and NO2 levels were determined by a chemiluminiscence analyser and hydrocarbons by using a flame ionisation analyser. Regarding PCDD/Fs collection, flue gases were sampled isokinetically according to VDI guidelines. Isokinetic sampling of PCDD/Fs emissions was carried out following the cooled probe method by means of an automatic sampling system. The sampling system has been described in detail elsewhere [18,19]. The sampling port was located in the 6 cm inner diameter flue gas duct, half way along a horizontal pipeline, downstream of the cyclones and upstream of the bag filter. Typical temperatures at the sampling point ranged from 90–130 8C. In addition, when isokinetic emission sampling of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) were carried out, samples of fly-ash were also collected in the cyclone and in the filter, and analysed for PCCD/Fs content. Cartridges and fly ashes were analysed for PCDD/Fs concentration. The method of analysis is the isotope dilution method with HRGC/HRMS. For the flue gas phase, results have been reported as the sum of polychlorinated compounds contained in the resin and the glass wool cartridges. In addition, in all combustion tests, samples from the combustor bed, from the cyclones and from the bag filter were collected. These samples were characterised using ASTM standard methods.
2.3. Experimental tests The experimental matrix is presented in Table 2. The effect of temperature between 800 8C and 880 8C and two levels of PVC, 1 and 2% w/w have been studied. In addition, combustion runs of fuel blends containing urea as dioxin prevention compound have been carried out (S7–S12). Toxic emission levels with and without inhibition agent addition are compared, especially as regards PCCD/Fs formation and NOx. Finally, the effect of inhibitor content, combustion temperature and fluidisation velocity is analysed.
3. Results and discussion 3.1. Combustion results Table 3 shows the operating conditions and flue gas analysis obtained during the bubbling fluidised bed combustion tests. The combustion efficiency was defined according to Eq. (1): ðEf KEa KEfg 100 (1) Ec Z Ef where Ef is the rate of energy input in the fuel fed, Ea is the rate of energy loss in the ashes from the cyclones and the bag filter and Efg is the rate of energy loss as carbon monoxide in the flue gas. As a consequence of the high moisture content in the flue gas, problems with the measurement of SO2 emissions arose. According to these results, good combustion efficiency, higher than 95% in all tests, has been obtained during the co-firing of South African coal and PVC. Two factors are the most important variables that affect CO emission,
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2152
Table 3 Operating conditions and flue gas analyses obtained during FBC tests using as fuel South African Coal and PVC As dry basis and 6% O2 excess (mg/Nm3) Test
PVC (%)
Urea (%)
T (8C)
Vf (m/s)
O2 (%)
H2O (%)
CO2 (%)
CO
NO
N2O
Ec (%)
S1 S2 S4 S5 S6 S8 S9 S10 S12
0 1 1 1 2 1 1 1 1
0 0 0 0 0 10 10 10 5
860 798 864 882 865 856 880 858 857
0.8 0.8 0.8 0.8 0.8 0.8 1.0 1.0 0.8
9.3 10.9 8.6 8.6 10.6 9.1 9.3 9.1 9.2
3.5 1.9
11.58 10.67 13.10 11.52 13.22 11.68 12.7 12.8 12.08
341 657 428 329 1104 1369 398 934 364
1353 1100 1350 1412
300 175 200 215 409 481 555 554 433
99.1 95.9 95.4 98.5 97.8 98.8 96.1 98.0 98.2
namely temperature and fuel characteristics. In this case, an increase in temperature results in a decrease in CO emissions (Fig. 1), and CO emissions increase when the amount of PVC in the mixture increases (Fig. 2), mainly because of the high volatile matter content of the PVC. After the volatilisation process, the volatile matter content is carried by the flue gas and burned in the freeboard where the temperature is lower than 800 8C. An examination of the temperature profile of the combustor confirms this phenomenon (Fig. 3). 3.1.1. Nitrogen oxide emissions During combustion, nitrogen oxides are formed mainly from the oxidation of molecular nitrogen present in the combustion air (thermal- NOx) and organic nitrogen present in the fuel (Fuel-NOx). There is no thermal-NOx formation in FBC because of the low combustion temperature, so NOx is derived almost entirely from fuel nitrogen. The low combustion temperature, however, enhances formation of N2O ranging from 20 to 300 ppm in comparison to levels observed in conventional boilers of 10 ppm or less. The emissions of NOx (NO, NO2) and N2O from FBC are very dependent on a number of operating conditions (temperature, oxygen, etc) and many homogeneous and
3.7
3.9 3.8 3.3
heterogeneous reactions are important for the formation and destruction of NOx and N2O. Combustion involves, as a primary step, devolatilisation where the organically bound fuel nitrogen is partitioned into volatile nitrogen and char nitrogen. In the volatile vapour, nitrogen is known to exist as NH3, HCN and tar nitrogen. In the char, nitrogen is bound in aromatic structures. The subsequent step of oxidation result in the conversion of volatile- and char-N into NO, N2O and N2. The presence of urea in the fuel mixture increases the fuel-N content, so, as might be expected, both NOx and N2O emissions increase when the share of urea increases (Fig. 4) Moreover, it is important to point out that in the tests with urea addition, lower quantities of HCN have been found in the flue gases (!2 mg/Nm3). 3.2. PCDD/F results Table 4 shows total PCDD/F concentrations for the combustion of South African coal. Usually, emissions of different congeners of PCDD/F are given in toxicity equivalents (TE) in comparison to 2,3,7,8-TCDD using the system proposed by the NATO Committee on the Challenges on Modern Society(NATO-CCMS) in 1988. 1200
700
1000 CO
500 400 300 200
CO emissions (mg/Nm3)
600 CO emissions (mg/Nm3)
1910 1886 1661 1753
CO 800
600
400
200
100
0
0 798
864
882
Bed Temperature (ºC)
Fig. 1. Effect of the bed temperature on the CO emissions.
0
1
2
PVC in the mixture (%) Fig. 2. Effect of the share of PVC in the mixture on the CO emissions.
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157 1200
S1South African Coal, 850ºC, 0,8 m/s, 6% O2 S4 South African Coal + 1% PVC, 850ºC, 0,8 m/s, 6% O2
Table 4 Toxicity equivalents from from co-combustion of South African coal
Temperature (ºC)
S6 South African Coal + 2% PVC, 850ºC, 0,8 m/s, 6% O2
800
400
0 0
100
200
300
Height (cm)
860
Temperature (ºC)
South African Coal (% w/w) Sampling Temperature (8C) TE (NATO/ CCMS) (pg/m3) Cyclone ash (g/h) TE (NATO/ CCMS) cyclone (pg/g)
S1
S1D
S1D
Blank1
Blank2
Blank3
100
100
100
100
100
100
106
108
2,4
Not anal.
Not anal.
Not anal.
Not anal.
Not anal.
889
159
511
190
189
225
301
3
2
1
2.3
3.1
Not anal.: Not analysed. Bubbling fluidised bed, 850 8C, 0.8 m/s, [O2] excessZ6%.
880
840 820 800 S1South African Coal, 850ºC, 0,8 m/s, 6% O2
780
S4 South African Coal + 1% PVC, 850ºC, 0,8 m/s, 6% O2 S6 South African Coal + 2% PVC, 850ºC, 0,8 m/s, 6% O2
760 0
20
40
60
80
Height (cm)
Fig. 3. Temperature profile in the combustor (a) and in the bed (b)
Then, toxicity equivalent concentration is calculated according to NATO/CCMS. This approach has been followed here to report PCDD/F emissions. It is noticeable how severe the ‘memory effect’ is. Thus, quantitatively, the relative emission levels varied by a factor of 102 in the first combustion run of South African coal in comparison with other South African combustion tests. However, when coal 2500
Concentration (mg/Nm3)
2153
NO N2O
2000
was burned for several days, low steady emissions have been achieved. Similarly, toxicity equivalent concentration (ITEQ) according to NATO/CCMS for co-combustion of South African coal with the addition of PVC and urea has been summarised in Table 5. The homologue distributions of PCDD/Fs in the flue gas during co-combustion of South African coal and PVC are presented in Fig. 7. The pattern shows a dominance of tetraand penta-chlorinated dioxins and furans. There is a significant dominance of dioxins over furans. The addition of urea to South African coal-PVC mixtures makes possible a reduction of the amount of chlorinated compounds and especially of lower homologues and thus the total PCDDs/PCDFs content. One noticeable result is that the first coal baseline run showed low yields of PCDD but unexpected high yields of PCDF formation. Another significant finding is that the combustion of coal with 2% PVC gave lower contents of PCDD/Fs in the flue gases. Since only one run with this content of PVC has been carried out, it cannot be concluded that an increase in PVC share results in a reduction of chlorinated compounds in the flue gas (Fig. 5). Table 5 Toxicity equivalents from co-combustion of South African coal and PVC (bubbling fluidised bed, 850 8C, [O2]excessZ6%)
1500
1000
500
0 0
Urea (% W/w)
Fig. 4. Effect of the share of urea in the mixtures on the Nitrogen oxides emissions (Temperature 860 8C, Vf 0.8 m/s, 1% PVC in the mixture).
South African Coal (% w/w) PVC (% w/w) Urea (% w/w) Fluidisation velocity (m/s) Sampling Temperature (8C) TE (NATO/CCMS) (pg/Nm3) Cyclone ash (g/h) TE (NATO/CCMS) cyclone (pg/g)
S4
S6
S8
S10
S12
99
98
89
89
94
1
2
0.8
0.8
1 10 0.8
1 10 1
1 5 0.8
86
90
81
95
103
11.6
1.4
0.67
0.72
2.43
196 19.2
214 0.93
192 1.3
200 0,43
250 3.1
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157
1.6 1.4
ng/Nm3
1.2 1.0
P7 South African Coal P9 South African Coal + 1% PVC P11 South African Col + 2% PVC P12 South African Coal + 1% PVC + 10% urea P13 South African Coal + 1% PVC + 10% urea. 1 m/s P14 South African Coal + 1% PVC + 5% urea
0.8 0.6 0.4 0.2 0.0 TCDD PeDD HeDD HpDD OCDD TCDF PeCDF HeCDFHpCDF OCDF
Fig. 5. Homologue pattern of dioxins and furans in flue gas during cocombustion of coal from South Africa and PVC.
ng/g
The sum of PCDDs and PCDFs in the flue gas shows that in the case of the co-combustion of coal with 1% PVC a huge amount of PCDDs is formed. Total emission level ranges from 2292 pg/Nm3 PCDDs and 308 pg/Nm3 PCDFs when coal plus 1% PVC was burned at 850 8C, with a fluidisation velocity of 0,8 m/s and 6% excess of oxygen, to 77 pg/Nm3 PCDDs and 41 pg/Nm3 PCDFs under the same conditions but adding 10% urea to the fuel blend. Fig. 6 shows the homologues distribution of PCDDs and of PCDFs in fly ash from the cyclones. As in the case of flue gas, main homologues in fly ash from the cyclones are tetra-, penta- and hexa-dibenzodioxins and dibenzofurans. Blank tests, in which South African coal was burned at 850 8C, show reproducible results. It is remarkable that the test with 1% of PVC, resulted in a significantly higher content of PCDD/Fs in the ash as happened in the flue gases. To analyse the effect of the different experimental variables on PCDD/Fs formation, one must bear in mind that, apart from fuel’s composition and the presence or absence of dioxin and furan prevention compounds, a number of parameters may play a role in their formation. 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Firstly, two types of reactions are involved in the PCDD/Fs formation during the combustion process: homogeneous reactions (in the gas phase) and heterogeneous reactions (on a solid surface). For both types of reaction there is a critical range of temperatures, 800 to 500 8C for the homogenous reactions and 400 to 200 8C for the heterogeneous reactions. In addition to temperature, other combustion variables affect PCDD/Fs formation. Taking into account the four principal pathways of the homogenous reactions formation, CO and SO2 emissions have a strong influence on the PCDD/Fs formation. As regards the heterogeneous formation, PCDD/Fs can form via two routes, either from precursor molecules that adsorb on the surface or from elemental carbon (the de novo reaction). Both routes are influenced by the ash carbon content, particle size distribution, chlorine content and by the presence of metals such as Cu, Pb, Zn, Cr and Fe, which can catalyse the PCDD/Fs formation. Bearing in mind all of the above, the effect of some of the experimental parameters involved in PCDD/Fs formation is discussed in the following section. 3.2.1. Effect of PVC content on PCDD/Fs values The effect of PVC content on PCDD/F emissions is shown in Fig. 7. It is noticeable how in the test with 1% PVC a huge amount of dioxin and furans formed, both in the gas phase and in the particle phase. An increase in PVC content resulted in a decrease of PCDD/F emission in the flue gas as well as in the particle phase. The only reasonable explanation for this apparently contradictory effect of PVC on the emissions is to be found in the combustion efficiency obtained in those tests, which must play a stronger part in the formation of dioxins and furans, as discussed below. 3.2.2. Effect of urea on PCDD/Fs values The effect of urea content on PCDD/F emissions is shown in Fig. 8. The addition of solid urea to the fuel blend 20 18
P8 South African Coal P9 South African Coal + 1% PVC P10 P10 South African Coal P11 South African Coal + 2% PVC Blank1 South African Coal P12 South African Coal + 1% PVC + 10% urea P13 South African Coal + 1% PVC + 10rea. 1m/s P14 South African Coal + 1% PVC + 5% urea
3
ITEQ flue gas (pg/Nm ) TEQflyash fly ash (pg/g) South African coal 850ºC 0,8 m/s BFBC 6% O2
16
Toxicity equivalents
2154
14 12 10 8 6 4 2 0
TCDD PeDD HeDD HpDD OCDD TCDF PeCDFHeCDFHpCDF OCDF
0
1
2
PVC content (% w/w) Fig. 6. Homologue pattern of dioxins and furans in fly ash from the cyclones during co-combustion of coal from South Africa and PVC.
Fig. 7. Effect of PVC content on PCDD/F emissions.
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157 1.4
20
1.3
18
1.2 3
14
South African coal 1% w/wPVC 850ºC, 0,8 m/s, 6% O2
12 10
BFBC
8 6
3
I-TEQ flue gas (pg/Nm ) I-TEQ fly ash (pg/g)
1.1
Toxicity equivalents
I-TEQ flue gas (pg/Nm ) I-TEQ fly ash (pg/g)
16
Toxicity equivalents
2155
1.0 0.9 0.8
South African coal 1% w/wPVC 10% w/w urea 850ºC. 6% O2
0.7 0.6 0.5 0.4
BFBC
0.3
4
0.2
2
0.1 0.0
0 0
5
0.7
10
0.8
0.9
1.0
1.1
1.2
1.3
Fluidisation velocity (m/s)
Urea content (% w/w)
Fig. 8. Effect of urea content on PCDD/F emissions.
Fig. 9. Effect of fluidisation velocity on PCDD/F emissions.
reduces PCDD/F concentration in the flue gases. Regarding PCDDs the greatest reduction was achieved with a urea concentration of 5%. No significant further improvement in PCDD/F reduction was obtained with 10% urea. Conversely, when a mixture of 1% PVC in coal was burned together with 10% urea, increasing the fluidisation velocity to 1 m/s resulted in a slight increase in PCDD emissions. With respect to PCDFs the increase in urea content in the fuel from 5 to 10% means further reduction in PCDF concentration in the flue gas. As also shown in Fig. 8 adding urea to the fuel leads to a reduction of PCDD concentration in the fly ash. This effect is slightly higher when 10% of urea is added as compared to 5%. However, as regards PCDF concentration the addition of 5% of urea has a negligible effect on its reduction. Compared with the run carried out under the same experimental conditions, (1% PVC with coal, 850 8C at 0.8 m/s of fluidisation velocity and 6% excess of oxygen), but without adding urea, similar concentration and profile patterns are obtained. It is easy to see that in both cases the trend is similar and that urea actively participates in the reduction of toxic emissions.
the PCDF/PCDD ratio increased when the fluidisation velocity increased. 3.2.4. Effect of CO on PCDD/Fs emissions CO concentration in the flue gas is one of the parameters reflecting the quality of the combustion process. Some studies have reported a correlation of CO emission to PCDD/Fs concentration during combustion processes. However, other studies have not found any significant correlation between CO values and PCDD/Fs levels in flue gas or fly ash. Studies carried out by Weber R et al. [9], showed a weak trend of increasing PCDD/Fs concentration in fly ash with increasing CO values but they also claimed that the variability in the correlation of CO to PCDD/PCDF levels showed that other factors should have a significant influence on PCDD/PCDF formation. In this case, Fig. 10 shows the effect of the CO emissions on PCDD/Fs concentration in flue gas and in the fly ash, expressed as TE (NATO). It can be seen that no correlation 20
1400
18 1200
16
1000
12 10
3
CO mg/Nm )
8
3
TE flue gas (pg/m ) TE fly ash (pg/gr)
6 4
800 600
CO (mg/Nm3)
3.2.3. Effect of the fluidisation velocity on PCDD/Fs values The fluidisation velocity is a variable related to the residence time of the solid and flue gas in the combustor. In order to study the effect of residence time on PCDD/Fs concentration in flue gas and fly ash, two tests using different fluidisation velocities were carried out. In Fig. 9, it is readily seen that when the fluidisation velocity increases the PCDD/Fs values in the fly ash decrease. Conversely, an increase in the fluidisation velocity means a reduction of the PCDD/Fs concentration in the flue gas. However, the addition of 10% urea has a beneficial effect to PCDF concentration reduction in both fluidisation velocities tested. Both in flue gas and fly ash,
TE (NATO)
14
400
2 0
A
l) oa (C
a C C rea ure PV PV %u 5% 10 + 1% 2% + + + C A A VC PV %P 1% +1 + A A
200
Fig. 10. Effect of CO emissions on PCDD/Fs values in the flue gas and in the fly ash from the cyclones expressed as TE (NATO) for different combustion tests. (T 860 8C, Vf 0.8 m/s).
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157
between CO emissions and the PCDD/F concentration in flue gas was found. The same result was obtained regarding the effect of CO emissions on the PCDD/F values in the ash from the cyclones.
40
20 18
35
16
30
3.2.5. Effect of combustion efficiency on PCDD/PCDFs emissions As is widely known, PCDD/F formation is closely related to combustion efficiency. Thus, when combustion efficiency increases, PCDD/F emissions decrease. The results obtained are in agreement with the relationship between combustion efficiency and PCDD/F formation. As expected, the study of toxic equivalents (TE) against combustion efficiency (Fig. 11) shows that the lowest TE values are obtained for the highest values of combustion efficiency. Likewise, when studying the effect of combustion efficiency on PCDD/F concentration in the fly ash collected from the cyclones, a similar conclusion is reached. The better the combustion efficiency, then the lower the PCDD/F concentration. Thus, even with a higher chlorine content in the fuel (2% PVC), lower PCDD/F concentration is possible as long as combustion efficiency is good. 3.2.6. Effect of the ash carbon content in the PCDD/Fs values The formation of PCDD/Fs in the combustion process may be either in the vapour phase (homogeneous reaction), or on solid surfaces such as soot or ash particles (heterogeneous reactions). Two are the most important routes to form PCDD/Fs by heterogeneous reaction: from precursors such as chlorophenols and chlorobenzenes or from elemental carbon (the ‘de novo’ reaction). The precursor molecules adsorb on
18 98
16
TE (NATO)
14 96
12 Combustion efficiency Ec (%)
10 8
94
3
TE flue gas (pg/m ) TE fly ash (pg/gr)
6
92
4
Combustion Efficiency Ec (%)
100
20
2 90
0
P 1%
l+ oa
C
%P
l+2
a Co
al+
U 0%
5
+1
C PV 1%
Co
rea %U
rea
VC
VC
C+
PV 1%
al+
Co
Fig. 11. Effect of the combustion efficiency on the PCDD/PCDFs values in the flue gas and in the fly ash from the cyclones expressed as TE (NATO) (T 860 8C, Vf 0.8 m/s)
TE (NATO)
14 25 12 20
10 Ctotal (%)
8
15
3
TE flue gas (pg/m ) TE fly ash (pg/gr)
6
10
4
Fly ash carbon content (%)
2156
5
2
0
0
%P
VC
l+1
a Co
%P
VC
0%
l+2
a Co
1%
al+
Co
C PV
U
rea
5%
+1
V %P
U
rea
C+
1
al+
Co
Fig. 12. Effect of total carbon content in the ash on the PCDD/Fs values in the flue gas and in the fly ash from the cyclones expressed as TE (NATO) (T 860 8C, Vf 0.8 m/s).
the surface and react under the influence of metal catalysts to form PCDD/Fs. The ‘de novo’ reaction is defined as the breakdown reaction of carbon matrix, which may be residual in the ash. Some authors reported that in the fluidised bed combustion processes the most important routes to PCDD/Fs formation by heterogeneous reactions is the ‘de novo mechanism’. The most important variables involved in the ‘de novo reaction’ are the temperature, ash concentration and size distribution, and carbon content. Fig. 12 shows the effect of carbon content in the fly ash on the PCDD/F values in the flue gas and fly ash expressed as TE (NATO). The results show that the PCDD/Fs values found in flue gas and fly ash increase when the total carbon content in the fly ash increases. This effect is most important in the tests without urea addition. This might indicate that urea addition mainly affects the ‘de novo’ formation mechanism [14].
4. Conclusions Pilot scale tests to study the co-combustion of hard coal from South Africa with different amounts of PVC have been carried out in a 0,5 MWth Bubbling Fluidised Bed Combustor (BFBC). The content of PVC in the fuel blends ranged from 1 to 2% w/w. The research has included the study of the co-combustion of coal and PVC, analysing the effect of temperature, fluidisation velocity and PVC content. In a second stage the effect of dioxin-preventing compounds on the minimisation of chlorinated organic release has been studied, evaluating the addition of urea to the raw fuel, as an inhibition agent. Two levels of urea content, 5% and 10% have been studied.
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157
The main conclusions to be drawn from the analysis of the experimental results are: – Co-combustion of coal and PVC has proved to be feasible from the combustion efficiency and emissions of PCDD/F standpoints. Under the experimental conditions tested, emission levels of polychlorinated-p-dibenzodioxins and dibenzofurans remained below limit values set by existing legislation on persistent organic pollutants. – The addition of PVC to coal results in an increase of CO emission levels and an increase of carbon content in fly ash. Consequently, combustion efficiencies obtained are slightly lower than what is usually obtained when burning coal in fluidised bed boilers. – The homologue distribution of PCDD/Fs in the flue gas during co-combustion of coal and PVC in the pilot BFBC shows a dominance of tetra- and pentachlorinated dioxins and furans. This pattern is typical of combustion in fluidised bed boilers – Likewise, typical FBC homologue distribution profiles have been obtained in the fly ash being tetra-, penta- and hexa- dibenzodioxins and dibenzofurans the most predominant species – Fuel composition (chlorine content), as well as operating conditions (combustion temperature, temperature profile in the combustor, fluidisation velocity, etc.), strongly affect PCDD/Fs formation and destruction. – Solid urea -added directly to the coal-PVC feedstock- has proved to be a promising inhibitor for the reduction of PCDD/Fs concentration both in flue gases and in fly ash. However, an increase of urea content in the fuel -from 5 to 10% w/w does not seem to yield further PCDD/F reduction. At the same time, it must be noticed that the addition of urea means higher emissions levels of nitrogen oxides. Therefore, it is worthy to investigate, alternative inhibitors, especially if they are cheap, easily available and they do not promote noxious emissions. – When analysing PCDD/F emission results it is crucial to take into account the memory effect of the facilities which can have a dramatic effect on PCDD/F formation and destruction processes. – High differences in the PCDD/F content between duplicate runs, or under very close starting operation conditions, reveal that very small
2157
changes in the experimental conditions may have a profound effect on toxic emissions. However, as shown for coal, it has been confirmed that duplicating runs when necessary makes possible to obtain accurate, representative and repeatable results. Moreover, such fluctuation is very unlikely to happen in demonstration and industrial size units since they operate under more stable operating conditions, with very little change in operating conditions.
Acknowledgements The authors wish to thank the European Commission for financial support within the ECSC Programme (Contract number 7220-PR-108).
References [1] Sloss L. Organic compounds from coal utilisation, CCC/51. ISBN 929029-364-0. [2] Addink R, Paules R, Olie K. Organohalogen Compd 1993;12:27. [3] Kicherer A, Gerhardt T, Spliethoff H, Hein HRG. Final report IVD, APAS-clean coal technology programme; 1995. [4] Van de Kamp WL, Morgan DJ. Final report IRF, APAS clean coal technology programme; 1995. [5] Quass U, Fermann MW, Broker G. Organohalogen Compd 1998;36:7. [6] Gast CH, Visser JG. Final report KEMA, APAS clean coal technology programme; 1995. [7] Frankenha¨user M, Manninen H, Kojo I, Ruuskanen J, Vartiainen T, Vesterinen R, et al. Chemosphere 1993;27:309. [8] Liu K, Pan P, Riley JT. Fuel 2000;79:1115. [9] Weber RS, Ueno S, Nishino J. Chemosphere 2002;49:127. [10] Addink R, Paules R, Olie K. Organohalogen Compd 1993;12:27. [11] Dickson LC, Lenoir D, Hutzinger O, Naikwadi KP, Karasek FW. Chemosphere 1989;19:1435. [12] Naikwadi KP, Karasek FW. Chemosphere 1989;19:299. [13] Buekens A, Huang H. J Hazard Mater 1998;62:1. [14] Ruokoja¨rvi P, Aatamila M, Tuppurainen K, Ruuskanen J. Chemosphere 2001;43:757. [15] Samaras P, Blumenstock M, Lenoir D, Schramm KW, Kettrup A. Chemosphere 2001;42:737. [16] Ruokoja¨rvi P, Tuppurainen K, Mueller C, Kilpinen P, Ruuskanen J. Chemosphere 2001;43:199. [17] Armesto L, Bahillo A, Cabanillas A, Veijonen K, Otero J, Plumed A, et al. Fuel 2003;82:993. [18] Lehnardt R, Kaune A, Schramm KW, Kettrup A. Organohalogen Compd 1998;36:77. [19] Schramm KW, Kaune A, Lehnardt R, Hofmaier A, Henkelmann B, Kettrup A. Organohalogen Compd 1998;36:289.
PCDD/PCDF emissions from co-combustion of coal and PVC in a bubbling fluidised bed boiler J.M. Sa´nchez-Herva´sa,*, L. Armestoa, E. Ruiz-Martı´neza, J. Otero-Ruiza, M. Pandelovab, K.W. Schrammb,c a CIEMAT, Fossil Fuels Department, Avda. Complutense 22, 28040 Madrid, Spain ¨ kologische Chemie, Ingolsta¨dter Landstr. 1, D-85764 Neuherberg, Germany GSF-Forschungszentrum fu¨r Umwelt und Gesundheit, Institut fu¨r O c TUM, Wissenschaftszentrum Weihenstephan fu¨r Erna¨hrung und Landnutzung, Department fu¨r Biowissenschaften, Weihenstephaner Steig 23, 85350 Freising, Germany
b
Received 19 October 2004; received in revised form 7 July 2005; accepted 12 July 2005 Available online 8 August 2005
Abstract This paper reports on the results obtained in the study of the co-combustion of PVC with hard coal from South Africa in a 0,5 MWth Bubbling Fluidised Bed Boiler. The research has included the study of the effect of combustion temperature, fluidisation velocity and PVC content. The addition of urea to the raw fuel, as a dioxin-preventing compound has also been evaluated. Results have been analysed in terms of combustion efficiency, major pollutants emission (NOx, CO), and PCDD/Fs formation in the flue gas and in the fly ash. Under the experimental conditions tested, co-combustion of coal and PVC has proved to be feasible from the combustion efficiency and emission of PCDD/Fs points of view, whose levels remained below limits set by existing legislation on persistent organic pollutants. The addition of solid urea to the fuel blend reduces the amount of chlorinated compounds emitted. However, it has a negative impact on nitrogen pollutants formation q 2005 Elsevier Ltd. All rights reserved. Keywords: Co-combustion; Bubbling fluidised bed; Gaseous pollutants; PCDD; PCDF
1. Introduction Interest in organic compounds released from combustion sources has increased considerably in recent years due to growing concern over health risks. Among them, polychlorinated dibenzodioxins/dibenzofurans (PCDD/Fs) are most commonly studied. Data on emissions of PCDD/Fs from coal combustion are scarce as, in the past, there have been no requirements for monitoring of these species. However, the few data available suggest that the clean and efficient use of coal is not a significant source of these emissions [1]. It is interesting to note that, because of its efficient combustion properties, coal can help reduce organic emissions from the combustion of waste materials, even those containing a high * Corresponding author. Tel.: C34913466145; fax: C34913466269. E-mail address: [email protected] (J.M. Sa´nchez-Herva´s).
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.07.009
concentration of chlorine. The elevated combustion temperatures induced by the addition of coal can have an ‘afterburner’ effect which may help to reduce the release of organic compounds from waste materials. Further, the sulphur present in coal can inhibit the formation of PCDD/F from waste combustion [2]. This means that coal has the potential to reduce emissions of organic compounds from waste incineration. Existing legislation on persistent organic pollutants, such as Decision No. 2179/98/EC, Directive 2000/76/EC, do not apply specifically to organic emissions from coal combustion. So, any reductions being made voluntarily are of help in the general reduction of air pollution. Since organic emissions are generally products of incomplete combustion, many of the measures used to optimise plant performance result in a reduction in organic emissions. Although fuel type can affect the formation and release of organic compounds, in general it is those factors which control combustion efficiency, such as fuel mixing, which are more important. Chlorine in the feed fuel can potentially promote the formation of PCDD/F but, again, it is
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combustion conditions which play a greater role in controlling the release of these compounds than any variation in the fuel characteristics. Therefore, emissions of organic species from industrial coal combustion, for instance, are higher than those from full-scale coal-fired power plants simply because the combustion in the former is less efficient, [3–9]. Alternatively, chemical inhibitors have been considered as one possible solution to control the formation and emission of PCDDs and PCDFs in combustion processes. Some compounds have indeed been shown to reduce PCDD/F concentrations in flue gases [10–12]. Several compounds containing either sulphur or nitrogen have been tested at laboratory scale, and some have been shown to prevent PCDD/F formation in pilot or full-scale plants [13]. Likewise, in the case of molecules containing amino nitrogen, although they have shown an especially clear inhibitory effect when sprayed into the flue gases [14], research has also been carried out to study the effect when added to the feedstock, [15,16]. Based on the above premises, this work aims at studying PCDD/Fs emissions during co-combustion of PVC with hard coal from South Africa. The research has been carried out under financial support by an ECSC Coal RTD Project, (Project 7220-PR-108, Emissions Minimisation in CoalSolid Waste Co-combustion by Primary Measures). Pilot scale co-combustion tests have been carried out in a 0.5 MWth Bubbling Fluidised Bed Combustor (BFBC). Rigid recycled PVC, in pellet shape, was intentionally added to South African coal just aiming to increase PCDD/F emissions in order to have a better evaluation of the extend of primary measures effectiveness (i.e. optimisation of combustion parameters and addition of inhibitor compounds). The content of PVC has ranged from 1 to 2% w/w. The research has included the study of the co-combustion of coal and PVC, analysing the effect of temperature, fluidisation velocity and PVC content. In a second stage the effect of dioxin-preventing compounds on the minimisation of chlorinated organic release has been studied, evaluating the addition of urea to the raw fuel, as an inhibition agent. Two levels of urea content, 5% and 10% have been studied.
2. Experimental 2.1. Fuels For the tests, South African hard coal was used. Chlorine was added to the fuel in the form of recycled PVC. In addition, in several tests, urea, a dioxin formation inhibitor, was added in solid phase to the raw fuel, prior to combustion. For every run, the fuel mixture was prepared in advance and fed into the burner as a single fuel. Fuels and additives burned in the bubbling fluidised bed combustor were characterised in the laboratory. Characterisation included proximate and ultimate analysis
Table 1 Proximate, ultimate and calorific value of fuels and additives
Proximate Analysis Volatile Matter (%) Ash, (%) Fixed Carbon (%) Ultimate Analysis Carbon (%) Hydrogen (%) Nitrogen (%) Sulphur (%) Oxygen (%) Chlorine (%) Heating Value LHV (MJ/kg)
South African Coal
PVC
Urea
38.1 13.3 48.6
80.1 19.9 0.0
68.0 4.3 1.9 0.6 11.9
66.8 12.1 0.4 0.6 11.7 40
19.8 6.9 45.7 !0.1 27.6
26.0
38.2
–
and calorific value. Results of the characterisation have been summarised in Table 1. 2.2. Experimental unit Pilot scale co-combustion tests were carried out in a 0.5 MWth Bubbling Fluidised Bed Combustor (BFBC). The plant has been described in detail elsewhere [17]. The pilot plant consists of the fuel feeding system, the combustor, the cyclones and the bag filter. The feeding system consists of two screw feeders. The first one is mounted on a weighing scale to provide an accurate record of the feed rate. The second one feeds the material, very quickly and directly, into the combustor. For the tests, the fuel was fed at the bottom of the combustor (0,15 m above the fluidisation plate) and the mixtures were prepared in advance and fed as one single fuel. Fluidisation air was supplied at the riser bottom through an air distribution plate. The fluidisation air is preheated electrically, and with a gas burner during the start-up operation. During the actual combustion runs the preheating systems are turned off. During the experimental work a steady state was maintained for 5 or 6 h. The inner diameter of the combustor is 0.2 m and the total height 3.5 m allowing bed depths up to 0.5 m. The bed material consists of ash from the combustion of coal. Combustion temperature is adjusted by means of a heat exchanger submerged in the bed. An electric heating system is installed at the top of the combustor to minimise heating loss in the freeboard. Gases leaving the combustor pass through two cyclones and a bag filter. A predictive-adaptive control system manages the plant and records the main process variables as follows: † Temperature along the length of the combustor cyclones and baghouse † Pressure differences in some points of the boiler † All input air flow rates, rate of feed fuel, etc
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2151
Table 2 Experimental matrix Test
Coal (%)
PVC (%)
Urea (%)
Temperature (8C)
Fluidisation velocity (m/s)
O2 (% excess)
PCDD/Fs sampling
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
100 99 99 99 99 98 89 89 89 89 94 94
– 1 1 1 1 2 1 1 1 1 1 1
– – – – – – 10 10 10 10 5 5
850 800 830 850 880 850 830 850 880 850 850 850
0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 1 1 0.8
6 6 6 6 6 6 6 6 6 6 6 6
Yes
Yes Yes Yes Yes Yes
Co-combustion of South African coal and PVC.
The monitoring programme has taken into account gas composition and chlorinated organic collection for analysis of total dioxins and furans, as well as congener distribution. After sampling and conditioning, flue gases were analysed on-line continuously. O2 concentration was measured by a paramagnetic analyser. CO2, CO, and N2O were analysed by different NDIR spectrometers, NO and NO2 levels were determined by a chemiluminiscence analyser and hydrocarbons by using a flame ionisation analyser. Regarding PCDD/Fs collection, flue gases were sampled isokinetically according to VDI guidelines. Isokinetic sampling of PCDD/Fs emissions was carried out following the cooled probe method by means of an automatic sampling system. The sampling system has been described in detail elsewhere [18,19]. The sampling port was located in the 6 cm inner diameter flue gas duct, half way along a horizontal pipeline, downstream of the cyclones and upstream of the bag filter. Typical temperatures at the sampling point ranged from 90–130 8C. In addition, when isokinetic emission sampling of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) were carried out, samples of fly-ash were also collected in the cyclone and in the filter, and analysed for PCCD/Fs content. Cartridges and fly ashes were analysed for PCDD/Fs concentration. The method of analysis is the isotope dilution method with HRGC/HRMS. For the flue gas phase, results have been reported as the sum of polychlorinated compounds contained in the resin and the glass wool cartridges. In addition, in all combustion tests, samples from the combustor bed, from the cyclones and from the bag filter were collected. These samples were characterised using ASTM standard methods.
2.3. Experimental tests The experimental matrix is presented in Table 2. The effect of temperature between 800 8C and 880 8C and two levels of PVC, 1 and 2% w/w have been studied. In addition, combustion runs of fuel blends containing urea as dioxin prevention compound have been carried out (S7–S12). Toxic emission levels with and without inhibition agent addition are compared, especially as regards PCCD/Fs formation and NOx. Finally, the effect of inhibitor content, combustion temperature and fluidisation velocity is analysed.
3. Results and discussion 3.1. Combustion results Table 3 shows the operating conditions and flue gas analysis obtained during the bubbling fluidised bed combustion tests. The combustion efficiency was defined according to Eq. (1): ðEf KEa KEfg 100 (1) Ec Z Ef where Ef is the rate of energy input in the fuel fed, Ea is the rate of energy loss in the ashes from the cyclones and the bag filter and Efg is the rate of energy loss as carbon monoxide in the flue gas. As a consequence of the high moisture content in the flue gas, problems with the measurement of SO2 emissions arose. According to these results, good combustion efficiency, higher than 95% in all tests, has been obtained during the co-firing of South African coal and PVC. Two factors are the most important variables that affect CO emission,
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2152
Table 3 Operating conditions and flue gas analyses obtained during FBC tests using as fuel South African Coal and PVC As dry basis and 6% O2 excess (mg/Nm3) Test
PVC (%)
Urea (%)
T (8C)
Vf (m/s)
O2 (%)
H2O (%)
CO2 (%)
CO
NO
N2O
Ec (%)
S1 S2 S4 S5 S6 S8 S9 S10 S12
0 1 1 1 2 1 1 1 1
0 0 0 0 0 10 10 10 5
860 798 864 882 865 856 880 858 857
0.8 0.8 0.8 0.8 0.8 0.8 1.0 1.0 0.8
9.3 10.9 8.6 8.6 10.6 9.1 9.3 9.1 9.2
3.5 1.9
11.58 10.67 13.10 11.52 13.22 11.68 12.7 12.8 12.08
341 657 428 329 1104 1369 398 934 364
1353 1100 1350 1412
300 175 200 215 409 481 555 554 433
99.1 95.9 95.4 98.5 97.8 98.8 96.1 98.0 98.2
namely temperature and fuel characteristics. In this case, an increase in temperature results in a decrease in CO emissions (Fig. 1), and CO emissions increase when the amount of PVC in the mixture increases (Fig. 2), mainly because of the high volatile matter content of the PVC. After the volatilisation process, the volatile matter content is carried by the flue gas and burned in the freeboard where the temperature is lower than 800 8C. An examination of the temperature profile of the combustor confirms this phenomenon (Fig. 3). 3.1.1. Nitrogen oxide emissions During combustion, nitrogen oxides are formed mainly from the oxidation of molecular nitrogen present in the combustion air (thermal- NOx) and organic nitrogen present in the fuel (Fuel-NOx). There is no thermal-NOx formation in FBC because of the low combustion temperature, so NOx is derived almost entirely from fuel nitrogen. The low combustion temperature, however, enhances formation of N2O ranging from 20 to 300 ppm in comparison to levels observed in conventional boilers of 10 ppm or less. The emissions of NOx (NO, NO2) and N2O from FBC are very dependent on a number of operating conditions (temperature, oxygen, etc) and many homogeneous and
3.7
3.9 3.8 3.3
heterogeneous reactions are important for the formation and destruction of NOx and N2O. Combustion involves, as a primary step, devolatilisation where the organically bound fuel nitrogen is partitioned into volatile nitrogen and char nitrogen. In the volatile vapour, nitrogen is known to exist as NH3, HCN and tar nitrogen. In the char, nitrogen is bound in aromatic structures. The subsequent step of oxidation result in the conversion of volatile- and char-N into NO, N2O and N2. The presence of urea in the fuel mixture increases the fuel-N content, so, as might be expected, both NOx and N2O emissions increase when the share of urea increases (Fig. 4) Moreover, it is important to point out that in the tests with urea addition, lower quantities of HCN have been found in the flue gases (!2 mg/Nm3). 3.2. PCDD/F results Table 4 shows total PCDD/F concentrations for the combustion of South African coal. Usually, emissions of different congeners of PCDD/F are given in toxicity equivalents (TE) in comparison to 2,3,7,8-TCDD using the system proposed by the NATO Committee on the Challenges on Modern Society(NATO-CCMS) in 1988. 1200
700
1000 CO
500 400 300 200
CO emissions (mg/Nm3)
600 CO emissions (mg/Nm3)
1910 1886 1661 1753
CO 800
600
400
200
100
0
0 798
864
882
Bed Temperature (ºC)
Fig. 1. Effect of the bed temperature on the CO emissions.
0
1
2
PVC in the mixture (%) Fig. 2. Effect of the share of PVC in the mixture on the CO emissions.
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157 1200
S1South African Coal, 850ºC, 0,8 m/s, 6% O2 S4 South African Coal + 1% PVC, 850ºC, 0,8 m/s, 6% O2
Table 4 Toxicity equivalents from from co-combustion of South African coal
Temperature (ºC)
S6 South African Coal + 2% PVC, 850ºC, 0,8 m/s, 6% O2
800
400
0 0
100
200
300
Height (cm)
860
Temperature (ºC)
South African Coal (% w/w) Sampling Temperature (8C) TE (NATO/ CCMS) (pg/m3) Cyclone ash (g/h) TE (NATO/ CCMS) cyclone (pg/g)
S1
S1D
S1D
Blank1
Blank2
Blank3
100
100
100
100
100
100
106
108
2,4
Not anal.
Not anal.
Not anal.
Not anal.
Not anal.
889
159
511
190
189
225
301
3
2
1
2.3
3.1
Not anal.: Not analysed. Bubbling fluidised bed, 850 8C, 0.8 m/s, [O2] excessZ6%.
880
840 820 800 S1South African Coal, 850ºC, 0,8 m/s, 6% O2
780
S4 South African Coal + 1% PVC, 850ºC, 0,8 m/s, 6% O2 S6 South African Coal + 2% PVC, 850ºC, 0,8 m/s, 6% O2
760 0
20
40
60
80
Height (cm)
Fig. 3. Temperature profile in the combustor (a) and in the bed (b)
Then, toxicity equivalent concentration is calculated according to NATO/CCMS. This approach has been followed here to report PCDD/F emissions. It is noticeable how severe the ‘memory effect’ is. Thus, quantitatively, the relative emission levels varied by a factor of 102 in the first combustion run of South African coal in comparison with other South African combustion tests. However, when coal 2500
Concentration (mg/Nm3)
2153
NO N2O
2000
was burned for several days, low steady emissions have been achieved. Similarly, toxicity equivalent concentration (ITEQ) according to NATO/CCMS for co-combustion of South African coal with the addition of PVC and urea has been summarised in Table 5. The homologue distributions of PCDD/Fs in the flue gas during co-combustion of South African coal and PVC are presented in Fig. 7. The pattern shows a dominance of tetraand penta-chlorinated dioxins and furans. There is a significant dominance of dioxins over furans. The addition of urea to South African coal-PVC mixtures makes possible a reduction of the amount of chlorinated compounds and especially of lower homologues and thus the total PCDDs/PCDFs content. One noticeable result is that the first coal baseline run showed low yields of PCDD but unexpected high yields of PCDF formation. Another significant finding is that the combustion of coal with 2% PVC gave lower contents of PCDD/Fs in the flue gases. Since only one run with this content of PVC has been carried out, it cannot be concluded that an increase in PVC share results in a reduction of chlorinated compounds in the flue gas (Fig. 5). Table 5 Toxicity equivalents from co-combustion of South African coal and PVC (bubbling fluidised bed, 850 8C, [O2]excessZ6%)
1500
1000
500
0 0
Urea (% W/w)
Fig. 4. Effect of the share of urea in the mixtures on the Nitrogen oxides emissions (Temperature 860 8C, Vf 0.8 m/s, 1% PVC in the mixture).
South African Coal (% w/w) PVC (% w/w) Urea (% w/w) Fluidisation velocity (m/s) Sampling Temperature (8C) TE (NATO/CCMS) (pg/Nm3) Cyclone ash (g/h) TE (NATO/CCMS) cyclone (pg/g)
S4
S6
S8
S10
S12
99
98
89
89
94
1
2
0.8
0.8
1 10 0.8
1 10 1
1 5 0.8
86
90
81
95
103
11.6
1.4
0.67
0.72
2.43
196 19.2
214 0.93
192 1.3
200 0,43
250 3.1
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157
1.6 1.4
ng/Nm3
1.2 1.0
P7 South African Coal P9 South African Coal + 1% PVC P11 South African Col + 2% PVC P12 South African Coal + 1% PVC + 10% urea P13 South African Coal + 1% PVC + 10% urea. 1 m/s P14 South African Coal + 1% PVC + 5% urea
0.8 0.6 0.4 0.2 0.0 TCDD PeDD HeDD HpDD OCDD TCDF PeCDF HeCDFHpCDF OCDF
Fig. 5. Homologue pattern of dioxins and furans in flue gas during cocombustion of coal from South Africa and PVC.
ng/g
The sum of PCDDs and PCDFs in the flue gas shows that in the case of the co-combustion of coal with 1% PVC a huge amount of PCDDs is formed. Total emission level ranges from 2292 pg/Nm3 PCDDs and 308 pg/Nm3 PCDFs when coal plus 1% PVC was burned at 850 8C, with a fluidisation velocity of 0,8 m/s and 6% excess of oxygen, to 77 pg/Nm3 PCDDs and 41 pg/Nm3 PCDFs under the same conditions but adding 10% urea to the fuel blend. Fig. 6 shows the homologues distribution of PCDDs and of PCDFs in fly ash from the cyclones. As in the case of flue gas, main homologues in fly ash from the cyclones are tetra-, penta- and hexa-dibenzodioxins and dibenzofurans. Blank tests, in which South African coal was burned at 850 8C, show reproducible results. It is remarkable that the test with 1% of PVC, resulted in a significantly higher content of PCDD/Fs in the ash as happened in the flue gases. To analyse the effect of the different experimental variables on PCDD/Fs formation, one must bear in mind that, apart from fuel’s composition and the presence or absence of dioxin and furan prevention compounds, a number of parameters may play a role in their formation. 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Firstly, two types of reactions are involved in the PCDD/Fs formation during the combustion process: homogeneous reactions (in the gas phase) and heterogeneous reactions (on a solid surface). For both types of reaction there is a critical range of temperatures, 800 to 500 8C for the homogenous reactions and 400 to 200 8C for the heterogeneous reactions. In addition to temperature, other combustion variables affect PCDD/Fs formation. Taking into account the four principal pathways of the homogenous reactions formation, CO and SO2 emissions have a strong influence on the PCDD/Fs formation. As regards the heterogeneous formation, PCDD/Fs can form via two routes, either from precursor molecules that adsorb on the surface or from elemental carbon (the de novo reaction). Both routes are influenced by the ash carbon content, particle size distribution, chlorine content and by the presence of metals such as Cu, Pb, Zn, Cr and Fe, which can catalyse the PCDD/Fs formation. Bearing in mind all of the above, the effect of some of the experimental parameters involved in PCDD/Fs formation is discussed in the following section. 3.2.1. Effect of PVC content on PCDD/Fs values The effect of PVC content on PCDD/F emissions is shown in Fig. 7. It is noticeable how in the test with 1% PVC a huge amount of dioxin and furans formed, both in the gas phase and in the particle phase. An increase in PVC content resulted in a decrease of PCDD/F emission in the flue gas as well as in the particle phase. The only reasonable explanation for this apparently contradictory effect of PVC on the emissions is to be found in the combustion efficiency obtained in those tests, which must play a stronger part in the formation of dioxins and furans, as discussed below. 3.2.2. Effect of urea on PCDD/Fs values The effect of urea content on PCDD/F emissions is shown in Fig. 8. The addition of solid urea to the fuel blend 20 18
P8 South African Coal P9 South African Coal + 1% PVC P10 P10 South African Coal P11 South African Coal + 2% PVC Blank1 South African Coal P12 South African Coal + 1% PVC + 10% urea P13 South African Coal + 1% PVC + 10rea. 1m/s P14 South African Coal + 1% PVC + 5% urea
3
ITEQ flue gas (pg/Nm ) TEQflyash fly ash (pg/g) South African coal 850ºC 0,8 m/s BFBC 6% O2
16
Toxicity equivalents
2154
14 12 10 8 6 4 2 0
TCDD PeDD HeDD HpDD OCDD TCDF PeCDFHeCDFHpCDF OCDF
0
1
2
PVC content (% w/w) Fig. 6. Homologue pattern of dioxins and furans in fly ash from the cyclones during co-combustion of coal from South Africa and PVC.
Fig. 7. Effect of PVC content on PCDD/F emissions.
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157 1.4
20
1.3
18
1.2 3
14
South African coal 1% w/wPVC 850ºC, 0,8 m/s, 6% O2
12 10
BFBC
8 6
3
I-TEQ flue gas (pg/Nm ) I-TEQ fly ash (pg/g)
1.1
Toxicity equivalents
I-TEQ flue gas (pg/Nm ) I-TEQ fly ash (pg/g)
16
Toxicity equivalents
2155
1.0 0.9 0.8
South African coal 1% w/wPVC 10% w/w urea 850ºC. 6% O2
0.7 0.6 0.5 0.4
BFBC
0.3
4
0.2
2
0.1 0.0
0 0
5
0.7
10
0.8
0.9
1.0
1.1
1.2
1.3
Fluidisation velocity (m/s)
Urea content (% w/w)
Fig. 8. Effect of urea content on PCDD/F emissions.
Fig. 9. Effect of fluidisation velocity on PCDD/F emissions.
reduces PCDD/F concentration in the flue gases. Regarding PCDDs the greatest reduction was achieved with a urea concentration of 5%. No significant further improvement in PCDD/F reduction was obtained with 10% urea. Conversely, when a mixture of 1% PVC in coal was burned together with 10% urea, increasing the fluidisation velocity to 1 m/s resulted in a slight increase in PCDD emissions. With respect to PCDFs the increase in urea content in the fuel from 5 to 10% means further reduction in PCDF concentration in the flue gas. As also shown in Fig. 8 adding urea to the fuel leads to a reduction of PCDD concentration in the fly ash. This effect is slightly higher when 10% of urea is added as compared to 5%. However, as regards PCDF concentration the addition of 5% of urea has a negligible effect on its reduction. Compared with the run carried out under the same experimental conditions, (1% PVC with coal, 850 8C at 0.8 m/s of fluidisation velocity and 6% excess of oxygen), but without adding urea, similar concentration and profile patterns are obtained. It is easy to see that in both cases the trend is similar and that urea actively participates in the reduction of toxic emissions.
the PCDF/PCDD ratio increased when the fluidisation velocity increased. 3.2.4. Effect of CO on PCDD/Fs emissions CO concentration in the flue gas is one of the parameters reflecting the quality of the combustion process. Some studies have reported a correlation of CO emission to PCDD/Fs concentration during combustion processes. However, other studies have not found any significant correlation between CO values and PCDD/Fs levels in flue gas or fly ash. Studies carried out by Weber R et al. [9], showed a weak trend of increasing PCDD/Fs concentration in fly ash with increasing CO values but they also claimed that the variability in the correlation of CO to PCDD/PCDF levels showed that other factors should have a significant influence on PCDD/PCDF formation. In this case, Fig. 10 shows the effect of the CO emissions on PCDD/Fs concentration in flue gas and in the fly ash, expressed as TE (NATO). It can be seen that no correlation 20
1400
18 1200
16
1000
12 10
3
CO mg/Nm )
8
3
TE flue gas (pg/m ) TE fly ash (pg/gr)
6 4
800 600
CO (mg/Nm3)
3.2.3. Effect of the fluidisation velocity on PCDD/Fs values The fluidisation velocity is a variable related to the residence time of the solid and flue gas in the combustor. In order to study the effect of residence time on PCDD/Fs concentration in flue gas and fly ash, two tests using different fluidisation velocities were carried out. In Fig. 9, it is readily seen that when the fluidisation velocity increases the PCDD/Fs values in the fly ash decrease. Conversely, an increase in the fluidisation velocity means a reduction of the PCDD/Fs concentration in the flue gas. However, the addition of 10% urea has a beneficial effect to PCDF concentration reduction in both fluidisation velocities tested. Both in flue gas and fly ash,
TE (NATO)
14
400
2 0
A
l) oa (C
a C C rea ure PV PV %u 5% 10 + 1% 2% + + + C A A VC PV %P 1% +1 + A A
200
Fig. 10. Effect of CO emissions on PCDD/Fs values in the flue gas and in the fly ash from the cyclones expressed as TE (NATO) for different combustion tests. (T 860 8C, Vf 0.8 m/s).
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157
between CO emissions and the PCDD/F concentration in flue gas was found. The same result was obtained regarding the effect of CO emissions on the PCDD/F values in the ash from the cyclones.
40
20 18
35
16
30
3.2.5. Effect of combustion efficiency on PCDD/PCDFs emissions As is widely known, PCDD/F formation is closely related to combustion efficiency. Thus, when combustion efficiency increases, PCDD/F emissions decrease. The results obtained are in agreement with the relationship between combustion efficiency and PCDD/F formation. As expected, the study of toxic equivalents (TE) against combustion efficiency (Fig. 11) shows that the lowest TE values are obtained for the highest values of combustion efficiency. Likewise, when studying the effect of combustion efficiency on PCDD/F concentration in the fly ash collected from the cyclones, a similar conclusion is reached. The better the combustion efficiency, then the lower the PCDD/F concentration. Thus, even with a higher chlorine content in the fuel (2% PVC), lower PCDD/F concentration is possible as long as combustion efficiency is good. 3.2.6. Effect of the ash carbon content in the PCDD/Fs values The formation of PCDD/Fs in the combustion process may be either in the vapour phase (homogeneous reaction), or on solid surfaces such as soot or ash particles (heterogeneous reactions). Two are the most important routes to form PCDD/Fs by heterogeneous reaction: from precursors such as chlorophenols and chlorobenzenes or from elemental carbon (the ‘de novo’ reaction). The precursor molecules adsorb on
18 98
16
TE (NATO)
14 96
12 Combustion efficiency Ec (%)
10 8
94
3
TE flue gas (pg/m ) TE fly ash (pg/gr)
6
92
4
Combustion Efficiency Ec (%)
100
20
2 90
0
P 1%
l+ oa
C
%P
l+2
a Co
al+
U 0%
5
+1
C PV 1%
Co
rea %U
rea
VC
VC
C+
PV 1%
al+
Co
Fig. 11. Effect of the combustion efficiency on the PCDD/PCDFs values in the flue gas and in the fly ash from the cyclones expressed as TE (NATO) (T 860 8C, Vf 0.8 m/s)
TE (NATO)
14 25 12 20
10 Ctotal (%)
8
15
3
TE flue gas (pg/m ) TE fly ash (pg/gr)
6
10
4
Fly ash carbon content (%)
2156
5
2
0
0
%P
VC
l+1
a Co
%P
VC
0%
l+2
a Co
1%
al+
Co
C PV
U
rea
5%
+1
V %P
U
rea
C+
1
al+
Co
Fig. 12. Effect of total carbon content in the ash on the PCDD/Fs values in the flue gas and in the fly ash from the cyclones expressed as TE (NATO) (T 860 8C, Vf 0.8 m/s).
the surface and react under the influence of metal catalysts to form PCDD/Fs. The ‘de novo’ reaction is defined as the breakdown reaction of carbon matrix, which may be residual in the ash. Some authors reported that in the fluidised bed combustion processes the most important routes to PCDD/Fs formation by heterogeneous reactions is the ‘de novo mechanism’. The most important variables involved in the ‘de novo reaction’ are the temperature, ash concentration and size distribution, and carbon content. Fig. 12 shows the effect of carbon content in the fly ash on the PCDD/F values in the flue gas and fly ash expressed as TE (NATO). The results show that the PCDD/Fs values found in flue gas and fly ash increase when the total carbon content in the fly ash increases. This effect is most important in the tests without urea addition. This might indicate that urea addition mainly affects the ‘de novo’ formation mechanism [14].
4. Conclusions Pilot scale tests to study the co-combustion of hard coal from South Africa with different amounts of PVC have been carried out in a 0,5 MWth Bubbling Fluidised Bed Combustor (BFBC). The content of PVC in the fuel blends ranged from 1 to 2% w/w. The research has included the study of the co-combustion of coal and PVC, analysing the effect of temperature, fluidisation velocity and PVC content. In a second stage the effect of dioxin-preventing compounds on the minimisation of chlorinated organic release has been studied, evaluating the addition of urea to the raw fuel, as an inhibition agent. Two levels of urea content, 5% and 10% have been studied.
J.M. Sa´nchez-Herva´s et al. / Fuel 84 (2005) 2149–2157
The main conclusions to be drawn from the analysis of the experimental results are: – Co-combustion of coal and PVC has proved to be feasible from the combustion efficiency and emissions of PCDD/F standpoints. Under the experimental conditions tested, emission levels of polychlorinated-p-dibenzodioxins and dibenzofurans remained below limit values set by existing legislation on persistent organic pollutants. – The addition of PVC to coal results in an increase of CO emission levels and an increase of carbon content in fly ash. Consequently, combustion efficiencies obtained are slightly lower than what is usually obtained when burning coal in fluidised bed boilers. – The homologue distribution of PCDD/Fs in the flue gas during co-combustion of coal and PVC in the pilot BFBC shows a dominance of tetra- and pentachlorinated dioxins and furans. This pattern is typical of combustion in fluidised bed boilers – Likewise, typical FBC homologue distribution profiles have been obtained in the fly ash being tetra-, penta- and hexa- dibenzodioxins and dibenzofurans the most predominant species – Fuel composition (chlorine content), as well as operating conditions (combustion temperature, temperature profile in the combustor, fluidisation velocity, etc.), strongly affect PCDD/Fs formation and destruction. – Solid urea -added directly to the coal-PVC feedstock- has proved to be a promising inhibitor for the reduction of PCDD/Fs concentration both in flue gases and in fly ash. However, an increase of urea content in the fuel -from 5 to 10% w/w does not seem to yield further PCDD/F reduction. At the same time, it must be noticed that the addition of urea means higher emissions levels of nitrogen oxides. Therefore, it is worthy to investigate, alternative inhibitors, especially if they are cheap, easily available and they do not promote noxious emissions. – When analysing PCDD/F emission results it is crucial to take into account the memory effect of the facilities which can have a dramatic effect on PCDD/F formation and destruction processes. – High differences in the PCDD/F content between duplicate runs, or under very close starting operation conditions, reveal that very small
2157
changes in the experimental conditions may have a profound effect on toxic emissions. However, as shown for coal, it has been confirmed that duplicating runs when necessary makes possible to obtain accurate, representative and repeatable results. Moreover, such fluctuation is very unlikely to happen in demonstration and industrial size units since they operate under more stable operating conditions, with very little change in operating conditions.
Acknowledgements The authors wish to thank the European Commission for financial support within the ECSC Programme (Contract number 7220-PR-108).
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