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Emissions during co-firing of two energy crops in a PF pilot plant: Cynara and poplar
Fuel Processing Technology 113 (2013) 75–83
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Emissions during co-firing of two energy crops in a PF pilot plant: Cynara and poplar C. Bartolomé ⁎, A. Gil CIRCE, University of Zaragoza, C/Mariano Esquillor Gómez 15, 50018 Zaragoza, Spain
a r t i c l e
i n f o
Article history: Received 22 October 2012 Received in revised form 11 March 2013 Accepted 13 March 2013 Available online 11 April 2013 Keywords: Biomass Cynara Poplar Coal Emissions Co-firing Pulverized fuel Swirl burner
a b s t r a c t Co-firing of coal and biomass appears as a promising technology to improve CO2 emission levels. Even though it has been extensively studied, there is a need of widening the range of biomass fuels that could be applied to the process. With this aim, two energy crops (cynara and poplar) were tested with coal in a 500 kWth co-firing pilot plant and compared from an emission viewpoint. Energy crops were co-fired with a bituminous coal at different shares (0–15%) in energy basis, and flue gas concentration (CO, CO2, SO2, O2 and NOx) was measured at stack. Combustion efficiency was evaluated by means of CO concentration, showing good performance in all cases and proving the feasibility of the process with low emissions. Small differences in particle size distribution are probably the main cause of different CO trends as cynara share is increased. SO2 levels decreased for both cases, although, as expected, the SO2 reduction was more pronounced for poplar co-firing than for cynara. NOx emissions were higher in poplar experiments than in cynara mainly due to volatile matter content and air distribution differences. This work also includes a comparison with similar experimental results from literature, where high data variability was found. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Coal is present in many countries as the main energy source in electricity generation and its demand is expected to grow in the coming years [1,2]. Biomass is considered as a suitable option to substitute a part of the coal by means of co-firing technologies. This strategy might mean a short-medium term palliative measure against the increase of CO2 emissions. Co-firing is again coming to scene due to the worldwide economic situation as it can be introduced in existing power plants without noteworthy difficulties [3,4] and without great investments. Although wood residues have been applied at low shares without important inconveniences, these biofuels are scarce in Southern European Countries, and hence, it is necessary to find alternative resources. Cynara (Cynara cardunculus L.) may be a suitable option because of its low-water growing requirements, high yield and relative high heating values compared to other herbaceous biomasses [5,6]. Poplar (Populus spp.) was chosen as a good example of a short rotation coppice crop, mainly for its capability of adapting to different climate conditions and its low nutrient requirements. The increasingly strict NOx and SO2 emission limits make important to study biomass influence on their formation. Sulfur dioxide generation depends both on combustion efficiency and fuel sulfur content. As the S-content in biomass is usually lower than in most ⁎ Corresponding author. Tel.: +34 976761863; fax: +34 976732078. E-mail addresses: [email protected], [email protected] (C. Bartolomé). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.03.011
coals, biomass co-firing usually results in lower SO2 emissions [3]. Moreover, alkaline elements' presence in biomass may contribute to SO2 reduction because they may react with SO2 forming sulfates which would be retained in ash [7]. Nitrogen oxide emissions present great interest because of their harmful characteristics and the complexity of their formation which leads to deep analyses of the different parameters influencing the mechanisms. Fossil fuel combustion is responsible for more than half of the global emissions of these components, and up to two-thirds of the emissions from human activities [8]. NOx formation has been extensively studied in the past (i.e. [9–12]). NOx from combustion processes mainly arise from three mechanisms: thermal NOx, prompt NOx and fuel NOx (i.e. [10,11,13]). Fuel nitrogen, volatile content, fuel-air mixing patterns as well as burner design can influence NO emissions. Many works have been published in last years including co-firing tests results [14,15]. Nevertheless, those where emissions can be analyzed in detail, because operational conditions are better controlled, correspond to small scales. Moreover, experimental data about emissions are rather controversial, as there is no general agreement about the actual influence of biomass [16,17]. Biomass introduction could reduce NOx and SO2 values due to its lower nitrogen and sulfur contents [3,18]. However, this trend is not always fulfilled in the same way for all biomass types, and further research is needed on this area. As an example, Kazagic et al. [19] did not clearly find in their study which the influence of biomass addition was on NOx emissions. This research is presented as an exhaustive comparative labor at semi-industrial scale and at stable and highly controlled conditions.
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It allows investigating the effect on emissions of cynara and poplar when being co-fired with a bituminous coal. It comprises several co-firing experiments and the subsequent analysis of all test data information and comparison with previous research in the same field. 2. Nomenclature PF pulverized fuel SAC South African coal CYN Cynara cardunculus POP Poplar ASTM American Society for Testing and Materials UNE Spanish standards PLC programmable logic controller d.b. dry basis HHV high heating value e.b. energy basis T temperature wt weight NDIR nondispersive infrared sensor PA primary air SA secondary air EA excess air DTG differential thermogravimetric analysis EFR entrained flow reactor
3. Experimental 3.1. Fuels Two different types of biomasses have been considered for this study: cynara (C. cardunculus L.) and poplar (Populus spp.). Both of them were co-fired with a South African bituminous coal (SAC) in the experimental setup described below. Cynara was composed of pre-crushed raw material (containing leaves, capitula and stem) which was stored indoor after harvesting and ground to a particle size smaller than 0.5 mm in a hammer mill. The same process was followed by poplar, which was previously chipped and then milled to a similar particle size. Coal was delivered pre-ground to a particle size lower than 0.3 mm, as in the majority of pulverized fuel power plants. Differences between biomass and coal milling were due to two main reasons: technical challenges in biomass milling, e.g. the energy requirement for cynara increased noticeably for lower particle sizes [20] and high volatile content in biomass which allows coarser particle sizes in pulverized fuel combustion [14,18]. Particle size is an important parameter in a fuel when analyzing combustion performance and species formation. Fig. 1 shows the particle size distribution experimental data and the Rosin–Rammler fit [21] for both biomasses and the coal. Although both biomasses were processed with the same milling equipment and to the same sieve opening (0.5 mm), some differences in particle size distribution can be observed. Thus, statistical parameters derived from particle size distribution were calculated [22]. As it can be derived from results in Table 1, differences in particle size distributions are small (d50 is 1% higher in cynara than in poplar). Nevertheless, cynara's distribution is wider and it is slightly deviated to coarser sizes. Proximate and ultimate analyses of the three fuels are shown in Table 2. Analyses were based on ASTM normative: ASTM 2016-65 and ASTM 871-872 for moisture; ASTM D-1102-84 and SS 187171 for ash (550 °C); ASTM E872-82 for volatiles; carbon, hydrogen, nitrogen and sulfur were measured with an elemental analyzer and chlorine was determined according to the standards ASTM D-2361-66 and UNE 32024. High heating value was measured according to the standard UNE 164001EX.
Fig. 1. Cumulative particle size distributions of the three test fuels. Solid lines indicate Rosin–Rammler fitting.
Important differences have been identified considering standard analyses. On one hand, biomass volatile matter content is much higher than that of coal, the ratio between volatile matter content and fixed carbon (VM/FC) is 0.43 for SAC, and 5.46 and 5.81 for CYN and POP, respectively. High volatile content results in high reactivity and influences thermal behavior and flame formation process [23,24]. Regarding the ash content, it is higher in cynara than in poplar, but lower in both biomasses compared to coal (13.40 versus 8.90 in CYN and 3.24 in POP). It is worth noting that the chlorine content in cynara is much higher than in the other biomasses; this fact means a potential operational problem such as corrosion in superheaters at metal temperatures higher than 450 °C [25]. The influence of ash content and composition will not be analyzed in the present paper but they will be studied in detail in a subsequent one. Finally, as the nitrogen content of biomass is lower than that of coal, the total nitrogen content is globally reduced when biomass is co-fired with coal. Nitrogen content is also lower in poplar than in cynara (a 22% lower). Therefore, although there are several parameters that affect NOx emissions [18], a decrease is supposed in the total amount of this pollutant. Sulfur is also significantly lower in biomass, hence a diminution in SO2 emissions is also expected. Table 3 depicts the ash analyses of the three fuels. They were analyzed by atomic emission spectroscopy and the results are expressed as oxides in percentage (on a weight dry basis). Lastly, in order to further compare both fuels, thermogravimetry has been considered. Tests were performed in a thermogravimetric analyzer CAHN, model TG-31. This technique allows studying the way fuels are burned and analyze the differences among them. This kind of analyses is
Table 1 Particle size analysis results: statistical parameters. Arithmetic parameters
South African
Cynara
Poplar
(μm)
Coal (SAC)
(CYN)
(POP)
61.07 204.6 406.5 6.656 345.4 2.711 196.2 223.1 125.0 0.334 2.263
70.44 201.0 364.7 5.178 294.3 2.244 160.0 215.8 110.9 0.317 2.594
d10 Median or d50 d90 (d90/d10) (d90–d10) (d75/d25) (d75–d25) Mean (X ) Standard deviation (σ) Skewness (Sk) Kurtosis (K)
6.359 44.64 71.72 11.28 65.36 4.557 46.97 43.34 21.86 0.613 3.214
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Table 2 Fuel chemical analyses.
Moisture (%) Proximate analysis (% d.b.)
Ultimate analysis (% d.b.)
Ash Volatile matter Fixed carbon C H N S Oa
HHV (MJ/kg d.b.) a
SAC
CYN
POP
3.60 13.40 26.00 60.60 69.60 4.00 2.05 0.50 10.45 27.80
10.09 8.90 77.00 14.10 46.25 4.94 1.05 0.12 38.74 19.32
8.20 3.24 82.56 14.20 45.73 5.80 0.23 0.00 45.00 17.19
Calculated by difference.
widely used as a simple method to predict pyrolysis and combustion behavior. Fig. 2 depicts differential thermogravimetric analyses (DTG) for coal, cynara and poplar [26]. Coal and cynara were analyzed using 21% O2, small amounts (b 10 mg samples) and a heating rate of 20 °C/min. No poplar analyses were available when writing this paper so literature results [26] were included in the plot to compare the curves shape. In this last experiment 10 °C/min and a poplar char sample mass of 20–25 mg had been used. Results from Fig. 2 clearly indicate that combustion behavior of cynara and poplar is different. The first one shows a bimodal DTG curve while the second one seems to decompose in a single reaction.
Fig. 2. DTG combustion profiles of SAC, CYN and POP char [26].
was designed within the framework of previous studies to analyze the fouling tendency of different fuels. All signals were collected by an automatic acquisition system and sent to a computer. At the same time, different processes in the facility were controlled by a Programmable Logic Controller (PLC) [30], such as refrigeration or fuel feeding.
3.2. Combustion facility
3.3. Test description
A 500 kWth pulverized fuel test rig was used for the research. This facility is mainly composed of a pulverized fuel swirl burner on top of a combustion chamber. The facility, sketched in Fig. 3, includes a loss-in-weight feeding system which allows a precise dosage of both fuels. Secondary air is preheated (to 250 °C approx.) using an auxiliary natural gas burner of 150 kWth. The combustion chamber is divided into six independent watercooled rings; the first three are refractory-lined in order to promote flame stability. Secondary air is swirled using movable tangential vanes before entering the combustion chamber coaxially to the primary air pipe. A complete facility description can be found elsewhere [27]. CO, CO2, SO2 and NOx emissions were monitored at stack using standard ABB analyzers. The measuring principle was NDIR absorption for all the components except for O2, which was measured by an electrochemical sensor and the repeatability of this analyzer is ≤ 0.5% of span. The setup also includes an air-cooled tube bank (see [28]) where fouling can be analyzed, as well as a complete set of instrumentation to study heat transfer, flame and combustion stability and deposition tendency [27,29]. Furthermore, a deposition probe
The main goal of these tests was to experimentally compare the differences in emissions produced when two energy crops are co-fired with a bituminous coal, a typical fuel in Spanish PF power plants. Three different cynara-coal shares, in energy basis, were selected for the study: 5, 10 and 15%. After proving the neglectable influence on the process of the 5% share, analog experiments were carried out with poplar with 10% and 15% shares. Operating variables were maintained at stable conditions in the trials to obtain reliable emission measurements and representative ash samples for a subsequent study. Table 4 summarizes the main operating conditions used in the experimental campaign. 100% coal test was repeated twice (test nos. 1 and 5) just before each biomass co-firing campaign in order to verify test reproducibility. With regard to air and temperature conditions, excess air values, which were almost constant and around 11%, are shown in Table 4. Secondary air temperature was maintained at 250 ± 3 °C. Primary and secondary air swirl numbers were maintained at 1.54 and 0.67, respectively. Regarding air distribution, primary–secondary air ratio varied between 0.32 and 0.37 (mass basis) depending on the biomass share and the biomass itself. This change is due to the fuel flowrate needed in each case to achieve 500 kWth of thermal power and the requirement of primary air for safe pneumatic transport of the coal–biomass mixture, because the primary air–fuel ratio was kept constant at 2.3 for all the experiments. Finally, flue gas temperature inside combustion chamber was measured just below the last refractory ring and average registered values are also shown in Table 4.
Table 3 Fuel ash chemical analyses. Composition (% d.b.)
SAC
CYN
POP
Al2O3 BaO CaO Fe2O3 K2O MgO Mn2O3 Na2O P2O5 SO3 SiO2 SrO TiO2 ZnO Cl
26.00 0.18 6.40 2.40 0.47 1.50 0.07 0.11 1.40 3.60 40.00 0.21 1.40 0.01 0.02
2.99 – 30.13 1.52 10.06 6.41 – 7.98 – – 20.64 – 0.25 – 1.15
4.50 0.03 34.00 2.10 8.60 7.00 0.11 1.20 2.50 2.70 23.00 0.27 0.32 0.10 0.05
4. Results and discussion 4.1. Gaseous emissions CO, CO2, O2, SO2 and NOx were measured and registered every 5 s during the experiments and their concentration values were all standardized to 6 vol.% O2. All results shown in this section graphs refer to mean concentration values during 10 min of stable combustion conditions taken as a representative sample. Reduction values of minor species (CO, SO2 and NOx) are represented in Figs. 4, 6, and 9 for all
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Fig. 3. Layout of the test rig.
the tests shown in Table 4. Error bars in the graphs correspond to standard deviation values. In addition, CO and NOx levels are also analyzed versus time and temperature in Figs. 5 and 13 respectively. 4.1.1. CO emissions Fig. 4 depicts the evolution of the reduction in CO concentration with biomass–coal share, expressed as percentage with regard to the base case (100% coal). CO concentration was kept below 120 mg/Nm3 for all tests (mean concentrations were 60–90 mg/Nm3 in cynara tests and 40–50 mg/Nm3 in poplar tests). CO mean levels increased when introducing higher quantities of cynara (28% for 15% CYN) but they remained approximately constant regardless of the poplar share up to 15% POP, where CO emissions decreased 15% approximately. Moreover, we can observe in Fig. 4 that error bars show a bigger dispersion (higher standard deviation) in cynara CO emissions which indicates higher instabilities in these experiments. This fact might be in accordance with the proportion of bigger cynara particles which is increased when increasing cynara share but more experiments would be necessary to clarify it. There are several parameters which control CO formation: temperature, particle size, residence time and mixing efficiency. Firstly, residence time and air mixing ratios were kept constant in all the experiments and did not substantially change from one to another
(Table 4). Temperature and particle size are two of the main affecting parameters in solid fuel combustion. Fig. 5 shows carbon monoxide trends at stack when co-firing coal with 10% cynara (Fig. 5a) and 10% poplar (Fig. 5b). At the same time, the temperature registered inside the combustion chamber is represented in the plot (right Y axes) in order to analyze its interaction with CO. Carbon monoxide concentrations below 90 mg/Nm3 were achieved during the interval for both biomasses so combustion efficiencies were within acceptable ranges. CO concentration was higher in cynara tests than in poplar, which is associated to a higher combustion efficiency in poplar co-firing. Temperature increased with time in all the tests because of the re-irradiation of refractory walls and this increase was, to a certain extent, related with a decrease in CO levels for both biomasses. Nevertheless, the
Table 4 Experimental operating conditions. TEST
1
2
3
4
5
6
7
%SAC %CYN %POP Coal flowrate (kg/h) Biomass flowrate (kg/h) PA/SA ratio (kg/kg) EA (%) Temperature (°C)
100 0 – 68.48 0
95 5 – 65.03 5.55
90 10 – 61.60 11.22
85 15 – 58.16 16.85
100 – 0 68.42 0
90 – 10 61.60 11.11
85 – 15 58.17 16.69
0.32 0.34 0.35 0.37 0.32 0.34 0.35 10.17 10.24 10.46 10.83 11.31 11.05 11.05 929.81 992.00 952.66 982.82 977.09 956.53 990.04
Fig. 4. CO concentration reduction at stack (with respect to 100% SAC test) as a function of biomass share in cynara–coal and poplar–coal tests.
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Fig. 6. SO2 concentration reduction at stack and theoretical SO2 reduction data (with respect to 100% SAC test, [SO2] = 870 mg/Nm3) as a function of biomass share in cynara–coal and poplar–coal tests.
Fig. 5. CO concentration and temperature inside combustion chamber as a function of operation time in a) 10% CYN test and b) 10% POP test.
temperature variation was similar in both cases (between 940 and 970 °C approximately) therefore, it should affect CO emissions in a similar way. The deviation between both biomasses in CO emissions (Fig. 5) could be explained by the slight particle size difference between cynara and poplar [31]. Poplar presented a particle size distribution less dispersed than cynara, (Table 1), which could be the reason to justify the lower CO emissions. Moreover, moisture and ash content are lower in poplar than in cynara, which also influences combustion development in a positive way. 4.1.2. Sulfur dioxide emissions (SO2) Fig. 6 shows SO2 emission reduction (calculated from averaged values, mg/Nm 3 at 6% O2) at different blending ratios. In general terms, a decrease in these emissions is observed when biomass share is augmented. Again, SO2 reduction is higher for poplar co-firing than for cynara, reaching 13% reduction when introducing 15% poplar in energy basis (707 versus 813 mg/Nm3). Several factors influence sulfur dioxide formation during the combustion process. SO2 formation takes place by an oxidation reaction which means that a complete combustion entails a high conversion of sulfur to SO2. Therefore, fuel-S content should be directly related to SO2 emissions as shown in Fig. 7. All these results agree with some others found in literature [18], where it was stated that 80– 100% of fuel sulfur reacts forming SO2 in a coal flame. Fig. 8 compares SO2 reduction results from present research to several previous studies [16,17,32–35]. SO2 reduction is represented versus sulfur reduction in the fuel mixture (with regard to sulfur injection in the base case 100% coal). The fuel characteristics and experimental
conditions are shown in Table 5. Volatile matter, nitrogen and sulfur content are expressed as kg/GJth for the secondary fuels included in the comparison study. Thermal size of the facility is also included, as well as the type of technology and the reference to the original paper. All the considered studies used bituminous coals for their experiments, except Chao et al. [33] that used a sub-bituminous one. Some scattering is observed in the graph, not all the experiments show the same trends when injected sulfur is augmented. Sulfur content values in the fuel go from zero (plastic [35] and poplar) to 0.27 kg/GJth (tire [35]). Molcan et al. [16] and Khodier et al. [17] did not obtain a positive effect on SO2 emissions, whereas experiments of [32–34] did. Considering that sulfur content in the fuel is the parameter with a higher influence in SO2 formation and in all cases it was lower in biomass than in coal, this negative trend should probably be associated with instabilities in the process.
4.1.3. Nitrogen oxide emissions (NOx) Nitrogen oxide emissions are measured in the facility as the sum of NO and NO2. NOx reduction was calculated from mean emissions in each co-firing test and represented in Fig. 9. This figure shows reduction values as a function of biomass share (% in energy basis) for
Fig. 7. Sulfur content in the fuel feeding in cynara–coal and poplar–coal tests.
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Fig. 8. SO2 reduction as a function of sulfur reduction in the fuel mixture in several studies.
cynara and poplar. An uncertainty band based on standard deviation was represented for all the reduction data. It is seen that NOx level decreased with increasing biomass share. This result corroborates previous works [11,18,19,34,36], where a positive effect of biomass introduction was also found. Moreover, it was observed that the decline is more pronounced in cynara than in poplar, achieving values more than 10% lower in the former case. As it was previously described, nitrogen oxide formation is a complex process, affected by numerous parameters such as fuel chemical properties, temperature, devolatilization conditions or fuel/oxidizer mixing patterns [11,37]. Thus, many of the aspects which rule NOx formation mechanisms are still presented as an open research topic. A deep description of these mechanisms is out of the scope of the present research. However, the influence of some of the main parameters on NOx formation has been experimentally analyzed in detail. The observed reduction on emissions may be caused by numerous effects. First, fluid-dynamics in pulverized fuel burners influence NO formation because the controlling parameter in volatile nitrogen conversion is the mixture degree between secondary air and fuel [10,38]. In the present rig, the difference among experiments can be evaluated depending on air distribution between primary and secondary air flows, expressed in Table 4 as PA/SA ratio. This ratio increases when biomass share is augmented and it is higher in cynara's tests than in poplar's. NOx emission results are lower as the PA/SA is increased,
Table 5 Characteristics of secondary fuels and facilities found in literature. Secondary fuel
VM/LHV (kg/GJth)
N/LHV (kg/GJth)
S/LHV Thermal Technology Reference (kg/GJth) size (MWth)
Straw Bamboo Rice husk Sawdust Solid refused fuel Miscanthus Tire Plastic Cynara
46.85 49.49 47.15 56.34 36.43
0.27 0.47 0.48 0.05 0.48
0.11 0.05 0.08 0.06 0.20
2.50 0.06 0.01 3.00 0.004
PF PF PF PF EFR
[32] [33] [33] [16] [34]
48.09 19.64 21.55 48.13
0.39 0.15 0.00 0.66
0.07 0.27 0.00 0.08
0.10 0.08 0.08 0.50
PF PF PF PF
Poplar
51.83
0.16
0.00
0.50
PF
[17] [35] [35] Present work Present work
Fig. 9. NOx concentration reduction at stack (with respect to 100% SAC test, [NOx] = 800 mg/Nm3) as a function of biomass share in cynara–coal and poplar–coal tests.
which may mean fluid-dynamics had an important effect on nitrogen oxide formation in our facility. With regard to the fuel own characteristics, several parameters were analyzed and shown in Fig. 10 (fuel nitrogen and total volatile matter). A lower nitrogen content in biomass compared to that of coal could justify this decrease. Nevertheless, when cynara and poplar results are compared, higher emissions are obtained with the latter. This is the reason why nitrogen content in the fuel was discarded as responsible of NOx formation process. On the other hand, volatile matter or temperature differences could explain the deviation in NOx emissions. When combustion begins, fuel nitrogen is distributed between volatiles and char. This distribution mainly depends on fuel structure and the system temperature [11,39]. Nitrogen in biomass volatile phase preferably forms NH 3 [11,18], whereas in coal other species are formed (mainly HCN). NH 3 formed may react in two different ways: either with O2 to form NO, or with NO to form N2 [11]. Fig. 11 shows a simplified scheme of the fuel–nitrogen reactions [13]. The total volatile content in POP–SAC blends is higher than in CYN– SAC blends, and it grows when the biomass share is increased in both cases. A higher volatile matter content could explain the results because some authors [38] affirm that at a furnace temperature around 1200 °C, 60–80% of the fuel NO was produced by oxidation of the nitrogen released with the volatiles to form volatile NO. N-fuel conversion ratio was calculated to study further the influence of the nitrogen in each of the fuels on the NOx emissions. It is defined in Eq. (1) as the relationship between NOx and carbonated compound emissions and nitrogen and carbon content in the fuel [40]. ηNfuel ¼
½NO=ð½CO þ ½CO2 Þ 100: Nfuel =Cfuel
ð1Þ
N-fuel conversion (ηN-fuel) is defined as the amount of fuel nitrogen that reacts to NOx. For its calculation, it was considered that all the NOx came from the N-fuel. It was determined from the mean concentration for each biomass share test, for both biomass fuels, and represented in Fig. 12. The effect of increasing biomass shares is again different for each fuel. The conversion of N-fuel to NOx in POP tests increases with biomass percentage whereas CYN experiments reveal an opposite tendency. The CYN N-fuel conversion trend was also found by others, i.e. [32,34], and the reason of this decreasing effect when increasing the biomass share was, according to the authors, the creation of a reductive zone in the near-burner region which
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Fig. 12. NOx conversion as a function of biomass share in cynara and poplar co-firing tests.
Fig. 10. a) Nitrogen content in the fuel feeding and b) total volatile matter content for different biomass shares.
inhibited NO formation. The difference between both biomasses might be due to the effect of volatiles, as nitrogen reacts differently in volatile phase and char [38,41] and to the particle size differences between both fuels. Moreover, the formation of thermal NOx could also mean an increase in ηN-Fuel in POP experiments because NOx emissions would increase with regard to nitrogen content in the fuel. As the influence of the fuel characteristics does not justify all the trends obtained for NOx emissions, temperature impact was also analyzed. Fig. 13 shows mean NOx concentration (mg/Nm3 at 6% O2) as a function of mean temperature, all of them calculated in 10 min intervals in the 10% biomass experiments. NOx emitted with poplar co-firing
increased slightly with increasing temperatures. On the other hand, NOx values associated to cynara fluctuated without steep changes and did not follow a clear trend. Even though it has been postulated that thermal NOx formation does not usually occur in pulverized fuel combustion systems [10,11], temperature does have an influence on the fraction of fuel nitrogen that is devolatilized and the fraction that remains in the char during combustion [10]. Thus, it has an indirect impact on the amount of NOx that is formed and is measured in the flue gases. Finally, present results are compared in Fig. 14 with others from previous recent investigations. They are represented as a function of nitrogen reduction in the feeding fuel, always referred to the base case for each study (100% coal). Most of the studies included in Fig. 14 found that NOx reduction decreased with increasing nitrogen injection. Nevertheless, Molcan et al. [16] did not find a clear trend when changing the amount of sawdust in the system due to the variability of the test conditions (fuel flow rate and stoichiometry ratio). Chao et al. [33] observed that the reduction in NOx emissions increased up to 20% biomass (above 10% Nfuel reduction) and then it decreased again, for both biomasses. Khodier et al. [17] did not observe any change up to biomass shares over 30% (20% Nfuel reduction) but from this point, they also achieved noticeable NOx reduction with biomass share increasing due to the difference in volatile content. Our results agree with those obtained by Pedersen et al. [32], Chao et al. [33] and Wu et al. [34] who arrived at a positive effect of co-firing, although results from [34] were obtained in a EFR which makes them less comparable because of the differences in fluid-dynamics and combustion performance. Several factors justify the variability of results found in literature. The biomass characteristics, see Table 5, and the operating conditions are postulated as the main ones.
Fig. 11. Nitrogen transformation from fuel–nitrogen [13].
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As not all the biomasses affect nitrogen oxide formation in the same way, further research could be addressed to ameliorate NOx reduction primary techniques using biomass introduction. Finally, stable feeding and operating conditions are postulated as crucial when analyzing emissions at laboratory and pilot scales. SO2 decreased in most systems when increasing the biomass share and scattering was probably due to instabilities in the processes. We observed that, depending on the biomass characteristics, only high shares produced a positive effect on NOx emissions. Considering the relatively large size of our facility, the results obtained in the present work may provide a valuable contribution to expand the current knowledge about parameters influencing NOx formation in energy crops co-firing.
Acknowledgments
Fig. 13. Mean NOx concentration at stack as a function of mean temperature at 10% biomass share.
This work was part of the project “Pulverized fuel combustion in swirl burners: Experimental and scaling methods and rigorous numerical modeling” SIMEXCALE (ENE2010-16011) funded by the Ministry of Education in Spain.
References
Fig. 14. NOx reduction as a function of nitrogen reduction in the fuel mixture in several studies.
5. Summary and conclusions Co-firing coal with two energy crops (cynara and poplar) was analyzed in a 500 kWth pulverized fuel pilot plant. Three different biomass shares, in energy basis, were tested to study its influence on emissions. Major and minor species (CO, CO2, SO2, O2 and NOx) were measured at stack. Test results indicated a good performance of combustion with reduced CO concentrations in both cases. Lower CO emissions were achieved when co-firing poplar compared to cynara mainly because of a slightly finer particle size distribution in this material. From the foregoing discussion, it can be concluded that both cynara and poplar have positive effects on SO2 and NOx emissions from co-firing. Sulfur content in biomass, which is lower than in coal, involves a reduction of SO2 in the exhaust gases. According to our experimental research, the effect of diminishing total sulfur content in the fuel prevailed over the retention in ashes due to alkali reactions. Volatile matter content has been identified as one of the most important parameters influencing nitrogen chemistry in pulverized fuel combustion facilities. It affects nitrogen oxide formation because most of the fuel NO is produced from nitrogen released in volatiles. Temperature was found to have an indirect impact on NOx formation. Primary and secondary air distribution also influences NOx emissions.
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Emissions during co-firing of two energy crops in a PF pilot plant: Cynara and poplar C. Bartolomé ⁎, A. Gil CIRCE, University of Zaragoza, C/Mariano Esquillor Gómez 15, 50018 Zaragoza, Spain
a r t i c l e
i n f o
Article history: Received 22 October 2012 Received in revised form 11 March 2013 Accepted 13 March 2013 Available online 11 April 2013 Keywords: Biomass Cynara Poplar Coal Emissions Co-firing Pulverized fuel Swirl burner
a b s t r a c t Co-firing of coal and biomass appears as a promising technology to improve CO2 emission levels. Even though it has been extensively studied, there is a need of widening the range of biomass fuels that could be applied to the process. With this aim, two energy crops (cynara and poplar) were tested with coal in a 500 kWth co-firing pilot plant and compared from an emission viewpoint. Energy crops were co-fired with a bituminous coal at different shares (0–15%) in energy basis, and flue gas concentration (CO, CO2, SO2, O2 and NOx) was measured at stack. Combustion efficiency was evaluated by means of CO concentration, showing good performance in all cases and proving the feasibility of the process with low emissions. Small differences in particle size distribution are probably the main cause of different CO trends as cynara share is increased. SO2 levels decreased for both cases, although, as expected, the SO2 reduction was more pronounced for poplar co-firing than for cynara. NOx emissions were higher in poplar experiments than in cynara mainly due to volatile matter content and air distribution differences. This work also includes a comparison with similar experimental results from literature, where high data variability was found. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Coal is present in many countries as the main energy source in electricity generation and its demand is expected to grow in the coming years [1,2]. Biomass is considered as a suitable option to substitute a part of the coal by means of co-firing technologies. This strategy might mean a short-medium term palliative measure against the increase of CO2 emissions. Co-firing is again coming to scene due to the worldwide economic situation as it can be introduced in existing power plants without noteworthy difficulties [3,4] and without great investments. Although wood residues have been applied at low shares without important inconveniences, these biofuels are scarce in Southern European Countries, and hence, it is necessary to find alternative resources. Cynara (Cynara cardunculus L.) may be a suitable option because of its low-water growing requirements, high yield and relative high heating values compared to other herbaceous biomasses [5,6]. Poplar (Populus spp.) was chosen as a good example of a short rotation coppice crop, mainly for its capability of adapting to different climate conditions and its low nutrient requirements. The increasingly strict NOx and SO2 emission limits make important to study biomass influence on their formation. Sulfur dioxide generation depends both on combustion efficiency and fuel sulfur content. As the S-content in biomass is usually lower than in most ⁎ Corresponding author. Tel.: +34 976761863; fax: +34 976732078. E-mail addresses: [email protected], [email protected] (C. Bartolomé). 0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2013.03.011
coals, biomass co-firing usually results in lower SO2 emissions [3]. Moreover, alkaline elements' presence in biomass may contribute to SO2 reduction because they may react with SO2 forming sulfates which would be retained in ash [7]. Nitrogen oxide emissions present great interest because of their harmful characteristics and the complexity of their formation which leads to deep analyses of the different parameters influencing the mechanisms. Fossil fuel combustion is responsible for more than half of the global emissions of these components, and up to two-thirds of the emissions from human activities [8]. NOx formation has been extensively studied in the past (i.e. [9–12]). NOx from combustion processes mainly arise from three mechanisms: thermal NOx, prompt NOx and fuel NOx (i.e. [10,11,13]). Fuel nitrogen, volatile content, fuel-air mixing patterns as well as burner design can influence NO emissions. Many works have been published in last years including co-firing tests results [14,15]. Nevertheless, those where emissions can be analyzed in detail, because operational conditions are better controlled, correspond to small scales. Moreover, experimental data about emissions are rather controversial, as there is no general agreement about the actual influence of biomass [16,17]. Biomass introduction could reduce NOx and SO2 values due to its lower nitrogen and sulfur contents [3,18]. However, this trend is not always fulfilled in the same way for all biomass types, and further research is needed on this area. As an example, Kazagic et al. [19] did not clearly find in their study which the influence of biomass addition was on NOx emissions. This research is presented as an exhaustive comparative labor at semi-industrial scale and at stable and highly controlled conditions.
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It allows investigating the effect on emissions of cynara and poplar when being co-fired with a bituminous coal. It comprises several co-firing experiments and the subsequent analysis of all test data information and comparison with previous research in the same field. 2. Nomenclature PF pulverized fuel SAC South African coal CYN Cynara cardunculus POP Poplar ASTM American Society for Testing and Materials UNE Spanish standards PLC programmable logic controller d.b. dry basis HHV high heating value e.b. energy basis T temperature wt weight NDIR nondispersive infrared sensor PA primary air SA secondary air EA excess air DTG differential thermogravimetric analysis EFR entrained flow reactor
3. Experimental 3.1. Fuels Two different types of biomasses have been considered for this study: cynara (C. cardunculus L.) and poplar (Populus spp.). Both of them were co-fired with a South African bituminous coal (SAC) in the experimental setup described below. Cynara was composed of pre-crushed raw material (containing leaves, capitula and stem) which was stored indoor after harvesting and ground to a particle size smaller than 0.5 mm in a hammer mill. The same process was followed by poplar, which was previously chipped and then milled to a similar particle size. Coal was delivered pre-ground to a particle size lower than 0.3 mm, as in the majority of pulverized fuel power plants. Differences between biomass and coal milling were due to two main reasons: technical challenges in biomass milling, e.g. the energy requirement for cynara increased noticeably for lower particle sizes [20] and high volatile content in biomass which allows coarser particle sizes in pulverized fuel combustion [14,18]. Particle size is an important parameter in a fuel when analyzing combustion performance and species formation. Fig. 1 shows the particle size distribution experimental data and the Rosin–Rammler fit [21] for both biomasses and the coal. Although both biomasses were processed with the same milling equipment and to the same sieve opening (0.5 mm), some differences in particle size distribution can be observed. Thus, statistical parameters derived from particle size distribution were calculated [22]. As it can be derived from results in Table 1, differences in particle size distributions are small (d50 is 1% higher in cynara than in poplar). Nevertheless, cynara's distribution is wider and it is slightly deviated to coarser sizes. Proximate and ultimate analyses of the three fuels are shown in Table 2. Analyses were based on ASTM normative: ASTM 2016-65 and ASTM 871-872 for moisture; ASTM D-1102-84 and SS 187171 for ash (550 °C); ASTM E872-82 for volatiles; carbon, hydrogen, nitrogen and sulfur were measured with an elemental analyzer and chlorine was determined according to the standards ASTM D-2361-66 and UNE 32024. High heating value was measured according to the standard UNE 164001EX.
Fig. 1. Cumulative particle size distributions of the three test fuels. Solid lines indicate Rosin–Rammler fitting.
Important differences have been identified considering standard analyses. On one hand, biomass volatile matter content is much higher than that of coal, the ratio between volatile matter content and fixed carbon (VM/FC) is 0.43 for SAC, and 5.46 and 5.81 for CYN and POP, respectively. High volatile content results in high reactivity and influences thermal behavior and flame formation process [23,24]. Regarding the ash content, it is higher in cynara than in poplar, but lower in both biomasses compared to coal (13.40 versus 8.90 in CYN and 3.24 in POP). It is worth noting that the chlorine content in cynara is much higher than in the other biomasses; this fact means a potential operational problem such as corrosion in superheaters at metal temperatures higher than 450 °C [25]. The influence of ash content and composition will not be analyzed in the present paper but they will be studied in detail in a subsequent one. Finally, as the nitrogen content of biomass is lower than that of coal, the total nitrogen content is globally reduced when biomass is co-fired with coal. Nitrogen content is also lower in poplar than in cynara (a 22% lower). Therefore, although there are several parameters that affect NOx emissions [18], a decrease is supposed in the total amount of this pollutant. Sulfur is also significantly lower in biomass, hence a diminution in SO2 emissions is also expected. Table 3 depicts the ash analyses of the three fuels. They were analyzed by atomic emission spectroscopy and the results are expressed as oxides in percentage (on a weight dry basis). Lastly, in order to further compare both fuels, thermogravimetry has been considered. Tests were performed in a thermogravimetric analyzer CAHN, model TG-31. This technique allows studying the way fuels are burned and analyze the differences among them. This kind of analyses is
Table 1 Particle size analysis results: statistical parameters. Arithmetic parameters
South African
Cynara
Poplar
(μm)
Coal (SAC)
(CYN)
(POP)
61.07 204.6 406.5 6.656 345.4 2.711 196.2 223.1 125.0 0.334 2.263
70.44 201.0 364.7 5.178 294.3 2.244 160.0 215.8 110.9 0.317 2.594
d10 Median or d50 d90 (d90/d10) (d90–d10) (d75/d25) (d75–d25) Mean (X ) Standard deviation (σ) Skewness (Sk) Kurtosis (K)
6.359 44.64 71.72 11.28 65.36 4.557 46.97 43.34 21.86 0.613 3.214
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Table 2 Fuel chemical analyses.
Moisture (%) Proximate analysis (% d.b.)
Ultimate analysis (% d.b.)
Ash Volatile matter Fixed carbon C H N S Oa
HHV (MJ/kg d.b.) a
SAC
CYN
POP
3.60 13.40 26.00 60.60 69.60 4.00 2.05 0.50 10.45 27.80
10.09 8.90 77.00 14.10 46.25 4.94 1.05 0.12 38.74 19.32
8.20 3.24 82.56 14.20 45.73 5.80 0.23 0.00 45.00 17.19
Calculated by difference.
widely used as a simple method to predict pyrolysis and combustion behavior. Fig. 2 depicts differential thermogravimetric analyses (DTG) for coal, cynara and poplar [26]. Coal and cynara were analyzed using 21% O2, small amounts (b 10 mg samples) and a heating rate of 20 °C/min. No poplar analyses were available when writing this paper so literature results [26] were included in the plot to compare the curves shape. In this last experiment 10 °C/min and a poplar char sample mass of 20–25 mg had been used. Results from Fig. 2 clearly indicate that combustion behavior of cynara and poplar is different. The first one shows a bimodal DTG curve while the second one seems to decompose in a single reaction.
Fig. 2. DTG combustion profiles of SAC, CYN and POP char [26].
was designed within the framework of previous studies to analyze the fouling tendency of different fuels. All signals were collected by an automatic acquisition system and sent to a computer. At the same time, different processes in the facility were controlled by a Programmable Logic Controller (PLC) [30], such as refrigeration or fuel feeding.
3.2. Combustion facility
3.3. Test description
A 500 kWth pulverized fuel test rig was used for the research. This facility is mainly composed of a pulverized fuel swirl burner on top of a combustion chamber. The facility, sketched in Fig. 3, includes a loss-in-weight feeding system which allows a precise dosage of both fuels. Secondary air is preheated (to 250 °C approx.) using an auxiliary natural gas burner of 150 kWth. The combustion chamber is divided into six independent watercooled rings; the first three are refractory-lined in order to promote flame stability. Secondary air is swirled using movable tangential vanes before entering the combustion chamber coaxially to the primary air pipe. A complete facility description can be found elsewhere [27]. CO, CO2, SO2 and NOx emissions were monitored at stack using standard ABB analyzers. The measuring principle was NDIR absorption for all the components except for O2, which was measured by an electrochemical sensor and the repeatability of this analyzer is ≤ 0.5% of span. The setup also includes an air-cooled tube bank (see [28]) where fouling can be analyzed, as well as a complete set of instrumentation to study heat transfer, flame and combustion stability and deposition tendency [27,29]. Furthermore, a deposition probe
The main goal of these tests was to experimentally compare the differences in emissions produced when two energy crops are co-fired with a bituminous coal, a typical fuel in Spanish PF power plants. Three different cynara-coal shares, in energy basis, were selected for the study: 5, 10 and 15%. After proving the neglectable influence on the process of the 5% share, analog experiments were carried out with poplar with 10% and 15% shares. Operating variables were maintained at stable conditions in the trials to obtain reliable emission measurements and representative ash samples for a subsequent study. Table 4 summarizes the main operating conditions used in the experimental campaign. 100% coal test was repeated twice (test nos. 1 and 5) just before each biomass co-firing campaign in order to verify test reproducibility. With regard to air and temperature conditions, excess air values, which were almost constant and around 11%, are shown in Table 4. Secondary air temperature was maintained at 250 ± 3 °C. Primary and secondary air swirl numbers were maintained at 1.54 and 0.67, respectively. Regarding air distribution, primary–secondary air ratio varied between 0.32 and 0.37 (mass basis) depending on the biomass share and the biomass itself. This change is due to the fuel flowrate needed in each case to achieve 500 kWth of thermal power and the requirement of primary air for safe pneumatic transport of the coal–biomass mixture, because the primary air–fuel ratio was kept constant at 2.3 for all the experiments. Finally, flue gas temperature inside combustion chamber was measured just below the last refractory ring and average registered values are also shown in Table 4.
Table 3 Fuel ash chemical analyses. Composition (% d.b.)
SAC
CYN
POP
Al2O3 BaO CaO Fe2O3 K2O MgO Mn2O3 Na2O P2O5 SO3 SiO2 SrO TiO2 ZnO Cl
26.00 0.18 6.40 2.40 0.47 1.50 0.07 0.11 1.40 3.60 40.00 0.21 1.40 0.01 0.02
2.99 – 30.13 1.52 10.06 6.41 – 7.98 – – 20.64 – 0.25 – 1.15
4.50 0.03 34.00 2.10 8.60 7.00 0.11 1.20 2.50 2.70 23.00 0.27 0.32 0.10 0.05
4. Results and discussion 4.1. Gaseous emissions CO, CO2, O2, SO2 and NOx were measured and registered every 5 s during the experiments and their concentration values were all standardized to 6 vol.% O2. All results shown in this section graphs refer to mean concentration values during 10 min of stable combustion conditions taken as a representative sample. Reduction values of minor species (CO, SO2 and NOx) are represented in Figs. 4, 6, and 9 for all
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Fig. 3. Layout of the test rig.
the tests shown in Table 4. Error bars in the graphs correspond to standard deviation values. In addition, CO and NOx levels are also analyzed versus time and temperature in Figs. 5 and 13 respectively. 4.1.1. CO emissions Fig. 4 depicts the evolution of the reduction in CO concentration with biomass–coal share, expressed as percentage with regard to the base case (100% coal). CO concentration was kept below 120 mg/Nm3 for all tests (mean concentrations were 60–90 mg/Nm3 in cynara tests and 40–50 mg/Nm3 in poplar tests). CO mean levels increased when introducing higher quantities of cynara (28% for 15% CYN) but they remained approximately constant regardless of the poplar share up to 15% POP, where CO emissions decreased 15% approximately. Moreover, we can observe in Fig. 4 that error bars show a bigger dispersion (higher standard deviation) in cynara CO emissions which indicates higher instabilities in these experiments. This fact might be in accordance with the proportion of bigger cynara particles which is increased when increasing cynara share but more experiments would be necessary to clarify it. There are several parameters which control CO formation: temperature, particle size, residence time and mixing efficiency. Firstly, residence time and air mixing ratios were kept constant in all the experiments and did not substantially change from one to another
(Table 4). Temperature and particle size are two of the main affecting parameters in solid fuel combustion. Fig. 5 shows carbon monoxide trends at stack when co-firing coal with 10% cynara (Fig. 5a) and 10% poplar (Fig. 5b). At the same time, the temperature registered inside the combustion chamber is represented in the plot (right Y axes) in order to analyze its interaction with CO. Carbon monoxide concentrations below 90 mg/Nm3 were achieved during the interval for both biomasses so combustion efficiencies were within acceptable ranges. CO concentration was higher in cynara tests than in poplar, which is associated to a higher combustion efficiency in poplar co-firing. Temperature increased with time in all the tests because of the re-irradiation of refractory walls and this increase was, to a certain extent, related with a decrease in CO levels for both biomasses. Nevertheless, the
Table 4 Experimental operating conditions. TEST
1
2
3
4
5
6
7
%SAC %CYN %POP Coal flowrate (kg/h) Biomass flowrate (kg/h) PA/SA ratio (kg/kg) EA (%) Temperature (°C)
100 0 – 68.48 0
95 5 – 65.03 5.55
90 10 – 61.60 11.22
85 15 – 58.16 16.85
100 – 0 68.42 0
90 – 10 61.60 11.11
85 – 15 58.17 16.69
0.32 0.34 0.35 0.37 0.32 0.34 0.35 10.17 10.24 10.46 10.83 11.31 11.05 11.05 929.81 992.00 952.66 982.82 977.09 956.53 990.04
Fig. 4. CO concentration reduction at stack (with respect to 100% SAC test) as a function of biomass share in cynara–coal and poplar–coal tests.
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Fig. 6. SO2 concentration reduction at stack and theoretical SO2 reduction data (with respect to 100% SAC test, [SO2] = 870 mg/Nm3) as a function of biomass share in cynara–coal and poplar–coal tests.
Fig. 5. CO concentration and temperature inside combustion chamber as a function of operation time in a) 10% CYN test and b) 10% POP test.
temperature variation was similar in both cases (between 940 and 970 °C approximately) therefore, it should affect CO emissions in a similar way. The deviation between both biomasses in CO emissions (Fig. 5) could be explained by the slight particle size difference between cynara and poplar [31]. Poplar presented a particle size distribution less dispersed than cynara, (Table 1), which could be the reason to justify the lower CO emissions. Moreover, moisture and ash content are lower in poplar than in cynara, which also influences combustion development in a positive way. 4.1.2. Sulfur dioxide emissions (SO2) Fig. 6 shows SO2 emission reduction (calculated from averaged values, mg/Nm 3 at 6% O2) at different blending ratios. In general terms, a decrease in these emissions is observed when biomass share is augmented. Again, SO2 reduction is higher for poplar co-firing than for cynara, reaching 13% reduction when introducing 15% poplar in energy basis (707 versus 813 mg/Nm3). Several factors influence sulfur dioxide formation during the combustion process. SO2 formation takes place by an oxidation reaction which means that a complete combustion entails a high conversion of sulfur to SO2. Therefore, fuel-S content should be directly related to SO2 emissions as shown in Fig. 7. All these results agree with some others found in literature [18], where it was stated that 80– 100% of fuel sulfur reacts forming SO2 in a coal flame. Fig. 8 compares SO2 reduction results from present research to several previous studies [16,17,32–35]. SO2 reduction is represented versus sulfur reduction in the fuel mixture (with regard to sulfur injection in the base case 100% coal). The fuel characteristics and experimental
conditions are shown in Table 5. Volatile matter, nitrogen and sulfur content are expressed as kg/GJth for the secondary fuels included in the comparison study. Thermal size of the facility is also included, as well as the type of technology and the reference to the original paper. All the considered studies used bituminous coals for their experiments, except Chao et al. [33] that used a sub-bituminous one. Some scattering is observed in the graph, not all the experiments show the same trends when injected sulfur is augmented. Sulfur content values in the fuel go from zero (plastic [35] and poplar) to 0.27 kg/GJth (tire [35]). Molcan et al. [16] and Khodier et al. [17] did not obtain a positive effect on SO2 emissions, whereas experiments of [32–34] did. Considering that sulfur content in the fuel is the parameter with a higher influence in SO2 formation and in all cases it was lower in biomass than in coal, this negative trend should probably be associated with instabilities in the process.
4.1.3. Nitrogen oxide emissions (NOx) Nitrogen oxide emissions are measured in the facility as the sum of NO and NO2. NOx reduction was calculated from mean emissions in each co-firing test and represented in Fig. 9. This figure shows reduction values as a function of biomass share (% in energy basis) for
Fig. 7. Sulfur content in the fuel feeding in cynara–coal and poplar–coal tests.
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Fig. 8. SO2 reduction as a function of sulfur reduction in the fuel mixture in several studies.
cynara and poplar. An uncertainty band based on standard deviation was represented for all the reduction data. It is seen that NOx level decreased with increasing biomass share. This result corroborates previous works [11,18,19,34,36], where a positive effect of biomass introduction was also found. Moreover, it was observed that the decline is more pronounced in cynara than in poplar, achieving values more than 10% lower in the former case. As it was previously described, nitrogen oxide formation is a complex process, affected by numerous parameters such as fuel chemical properties, temperature, devolatilization conditions or fuel/oxidizer mixing patterns [11,37]. Thus, many of the aspects which rule NOx formation mechanisms are still presented as an open research topic. A deep description of these mechanisms is out of the scope of the present research. However, the influence of some of the main parameters on NOx formation has been experimentally analyzed in detail. The observed reduction on emissions may be caused by numerous effects. First, fluid-dynamics in pulverized fuel burners influence NO formation because the controlling parameter in volatile nitrogen conversion is the mixture degree between secondary air and fuel [10,38]. In the present rig, the difference among experiments can be evaluated depending on air distribution between primary and secondary air flows, expressed in Table 4 as PA/SA ratio. This ratio increases when biomass share is augmented and it is higher in cynara's tests than in poplar's. NOx emission results are lower as the PA/SA is increased,
Table 5 Characteristics of secondary fuels and facilities found in literature. Secondary fuel
VM/LHV (kg/GJth)
N/LHV (kg/GJth)
S/LHV Thermal Technology Reference (kg/GJth) size (MWth)
Straw Bamboo Rice husk Sawdust Solid refused fuel Miscanthus Tire Plastic Cynara
46.85 49.49 47.15 56.34 36.43
0.27 0.47 0.48 0.05 0.48
0.11 0.05 0.08 0.06 0.20
2.50 0.06 0.01 3.00 0.004
PF PF PF PF EFR
[32] [33] [33] [16] [34]
48.09 19.64 21.55 48.13
0.39 0.15 0.00 0.66
0.07 0.27 0.00 0.08
0.10 0.08 0.08 0.50
PF PF PF PF
Poplar
51.83
0.16
0.00
0.50
PF
[17] [35] [35] Present work Present work
Fig. 9. NOx concentration reduction at stack (with respect to 100% SAC test, [NOx] = 800 mg/Nm3) as a function of biomass share in cynara–coal and poplar–coal tests.
which may mean fluid-dynamics had an important effect on nitrogen oxide formation in our facility. With regard to the fuel own characteristics, several parameters were analyzed and shown in Fig. 10 (fuel nitrogen and total volatile matter). A lower nitrogen content in biomass compared to that of coal could justify this decrease. Nevertheless, when cynara and poplar results are compared, higher emissions are obtained with the latter. This is the reason why nitrogen content in the fuel was discarded as responsible of NOx formation process. On the other hand, volatile matter or temperature differences could explain the deviation in NOx emissions. When combustion begins, fuel nitrogen is distributed between volatiles and char. This distribution mainly depends on fuel structure and the system temperature [11,39]. Nitrogen in biomass volatile phase preferably forms NH 3 [11,18], whereas in coal other species are formed (mainly HCN). NH 3 formed may react in two different ways: either with O2 to form NO, or with NO to form N2 [11]. Fig. 11 shows a simplified scheme of the fuel–nitrogen reactions [13]. The total volatile content in POP–SAC blends is higher than in CYN– SAC blends, and it grows when the biomass share is increased in both cases. A higher volatile matter content could explain the results because some authors [38] affirm that at a furnace temperature around 1200 °C, 60–80% of the fuel NO was produced by oxidation of the nitrogen released with the volatiles to form volatile NO. N-fuel conversion ratio was calculated to study further the influence of the nitrogen in each of the fuels on the NOx emissions. It is defined in Eq. (1) as the relationship between NOx and carbonated compound emissions and nitrogen and carbon content in the fuel [40]. ηNfuel ¼
½NO=ð½CO þ ½CO2 Þ 100: Nfuel =Cfuel
ð1Þ
N-fuel conversion (ηN-fuel) is defined as the amount of fuel nitrogen that reacts to NOx. For its calculation, it was considered that all the NOx came from the N-fuel. It was determined from the mean concentration for each biomass share test, for both biomass fuels, and represented in Fig. 12. The effect of increasing biomass shares is again different for each fuel. The conversion of N-fuel to NOx in POP tests increases with biomass percentage whereas CYN experiments reveal an opposite tendency. The CYN N-fuel conversion trend was also found by others, i.e. [32,34], and the reason of this decreasing effect when increasing the biomass share was, according to the authors, the creation of a reductive zone in the near-burner region which
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Fig. 12. NOx conversion as a function of biomass share in cynara and poplar co-firing tests.
Fig. 10. a) Nitrogen content in the fuel feeding and b) total volatile matter content for different biomass shares.
inhibited NO formation. The difference between both biomasses might be due to the effect of volatiles, as nitrogen reacts differently in volatile phase and char [38,41] and to the particle size differences between both fuels. Moreover, the formation of thermal NOx could also mean an increase in ηN-Fuel in POP experiments because NOx emissions would increase with regard to nitrogen content in the fuel. As the influence of the fuel characteristics does not justify all the trends obtained for NOx emissions, temperature impact was also analyzed. Fig. 13 shows mean NOx concentration (mg/Nm3 at 6% O2) as a function of mean temperature, all of them calculated in 10 min intervals in the 10% biomass experiments. NOx emitted with poplar co-firing
increased slightly with increasing temperatures. On the other hand, NOx values associated to cynara fluctuated without steep changes and did not follow a clear trend. Even though it has been postulated that thermal NOx formation does not usually occur in pulverized fuel combustion systems [10,11], temperature does have an influence on the fraction of fuel nitrogen that is devolatilized and the fraction that remains in the char during combustion [10]. Thus, it has an indirect impact on the amount of NOx that is formed and is measured in the flue gases. Finally, present results are compared in Fig. 14 with others from previous recent investigations. They are represented as a function of nitrogen reduction in the feeding fuel, always referred to the base case for each study (100% coal). Most of the studies included in Fig. 14 found that NOx reduction decreased with increasing nitrogen injection. Nevertheless, Molcan et al. [16] did not find a clear trend when changing the amount of sawdust in the system due to the variability of the test conditions (fuel flow rate and stoichiometry ratio). Chao et al. [33] observed that the reduction in NOx emissions increased up to 20% biomass (above 10% Nfuel reduction) and then it decreased again, for both biomasses. Khodier et al. [17] did not observe any change up to biomass shares over 30% (20% Nfuel reduction) but from this point, they also achieved noticeable NOx reduction with biomass share increasing due to the difference in volatile content. Our results agree with those obtained by Pedersen et al. [32], Chao et al. [33] and Wu et al. [34] who arrived at a positive effect of co-firing, although results from [34] were obtained in a EFR which makes them less comparable because of the differences in fluid-dynamics and combustion performance. Several factors justify the variability of results found in literature. The biomass characteristics, see Table 5, and the operating conditions are postulated as the main ones.
Fig. 11. Nitrogen transformation from fuel–nitrogen [13].
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As not all the biomasses affect nitrogen oxide formation in the same way, further research could be addressed to ameliorate NOx reduction primary techniques using biomass introduction. Finally, stable feeding and operating conditions are postulated as crucial when analyzing emissions at laboratory and pilot scales. SO2 decreased in most systems when increasing the biomass share and scattering was probably due to instabilities in the processes. We observed that, depending on the biomass characteristics, only high shares produced a positive effect on NOx emissions. Considering the relatively large size of our facility, the results obtained in the present work may provide a valuable contribution to expand the current knowledge about parameters influencing NOx formation in energy crops co-firing.
Acknowledgments
Fig. 13. Mean NOx concentration at stack as a function of mean temperature at 10% biomass share.
This work was part of the project “Pulverized fuel combustion in swirl burners: Experimental and scaling methods and rigorous numerical modeling” SIMEXCALE (ENE2010-16011) funded by the Ministry of Education in Spain.
References
Fig. 14. NOx reduction as a function of nitrogen reduction in the fuel mixture in several studies.
5. Summary and conclusions Co-firing coal with two energy crops (cynara and poplar) was analyzed in a 500 kWth pulverized fuel pilot plant. Three different biomass shares, in energy basis, were tested to study its influence on emissions. Major and minor species (CO, CO2, SO2, O2 and NOx) were measured at stack. Test results indicated a good performance of combustion with reduced CO concentrations in both cases. Lower CO emissions were achieved when co-firing poplar compared to cynara mainly because of a slightly finer particle size distribution in this material. From the foregoing discussion, it can be concluded that both cynara and poplar have positive effects on SO2 and NOx emissions from co-firing. Sulfur content in biomass, which is lower than in coal, involves a reduction of SO2 in the exhaust gases. According to our experimental research, the effect of diminishing total sulfur content in the fuel prevailed over the retention in ashes due to alkali reactions. Volatile matter content has been identified as one of the most important parameters influencing nitrogen chemistry in pulverized fuel combustion facilities. It affects nitrogen oxide formation because most of the fuel NO is produced from nitrogen released in volatiles. Temperature was found to have an indirect impact on NOx formation. Primary and secondary air distribution also influences NOx emissions.
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