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Energy & Fuels 2008, 22, 686–692
Straw Pellets Pyrolysis: Effect of Nonthermal Plasma on the Devolatilized Products Roger A. Khalil,*,† Morten Seljeskog,‡ and Johan E. Hustad† Norwegian UniVersity of Science and Technology, NO-7491, Trondheim, Norway, and Sintef Energy Research, NO-7465, Trondheim, Norway ReceiVed September 5, 2007. ReVised Manuscript ReceiVed NoVember 5, 2007
A batch reactor that also works as a macro-thermogravimetric analyzer (TGA) was used to study the thermal decomposition of pellets made of straw. The weight loss during pyrolysis was recorded continuously. The experimental work has focused on giving a detailed insight into the identification and quantification of the released volatile products at different pyrolysis temperatures. In addition, the sampled gas flow was processed in a nonthermal plasma reactor. The nonthermal plasma reactor processed the produced gas both before and after liquid removal from the gas fraction. Furthermore, CO2 was mixed with the N2 carrier gas to achieve a more complex gas composition for the nonthermal plasma. The purpose of using the nonthermal plasma reactor was to study its ability to improve the quality of the produced gas. This should be looked at in connection with the overall purpose of the nonthermal plasma, which is the abatement of pollutants. The effect of nonthermal plasma on the devolatilized products was most obvious at a pyrolysis temperature of 400 °C, where the gas fraction increased by 14 wt %. At higher pyrolysis temperatures, less gas was produced by the nonthermal plasma treatment. Because of the nonthermal cracking of the condensable fraction at low temperatures and the dissociation of the CO2 at all temperatures, the higher heating value of the produced gas increased as a result of the use of nonthermal plasma. Although no analysis of the liquid fraction was made, it is believed that the heavier hydrocarbon liquid fraction produced at low pyrolysis temperatures was decomposed into lighter fractions. At a pyrolysis temperature of 400 °C, differences in the condensed liquid viscosity compared to the experiment without plasma could easily be observed.
Introduction Energy production from biomass has been the focus of many research activities during the past decade. Biomass has the advantage over fossil fuels because of the recycling of the produced carbon through photosynthesis that occurs during the growth of the plant materials. The European Union (EU) has set as a target to increase the energy production from alternative energy sources. For the whole EU, this target is 21% of electricity and 12% of total energy by the year 2010.1 Straw can be regarded as an important fuel resource that, with its about 800 megatons in the EU and North America, can help reach the above-mentioned targets.2 Because pyrolysis is the first step in any technology that tends to convert solid fuels into energy through thermal conversion, the objective of this paper is to study the major products released from the pyrolysis of straw pellets. The effect of nonthermal plasma (NTP) on the devolatilized products will also be investigated. The idea is to investigate whether it is possible to gain back some of the energy used for the gas cleaning by improving the produced gas. The main application that can benefit from the NTP is hightemperature cleaning of pollutants, such as H2S, in a gasification process, where the produced gas is used for electricity production using a solid oxide fuel cell. * To whom correspondence should be addresssed. E-mail: roger.a.khalil@ sintef.no. † Norwegian University of Science and Technology. ‡ Sintef Energy Research. (1) Renewable Energy Policy Network for the 21st Century. http:// www.ren21.net/default.asp, Global Status Report, 2005. (2) Wolf, K. J.; Smeda, A.; Muller, M.; Hilpert, K. Energy Fuels 2005, 19, 820–824.
Thermal conversion processes for solid fuels, such as gasification, produce tars and other unwanted compounds that must be removed before the product gas can be used for electricity generation. For tar cleaning, scrubbers in combination with electrostatic precipitators can be used. This will lower the overall efficiency as the gas is cooled and needs to be heated up again prior to use. Maintaining a high efficiency in such processes would mean finding a solution for gas cleaning at high temperatures. Solutions are under development, but their commercial success remains to be documented.3 A hightemperature gas cleaning technology that looks promising for the pollution abatement is NTP. In the literature, NTP is being investigated for the cleaning of NOx, SOx, and volatile organic compounds (VOC).4–8 NTP can also be used for the reduction of odorous compounds, such as H2S, NH3, and (acetaldehyde) CH3COH, in areas where odor is considered hazardeous.9–12 In (3) Stanghelle, D.; Slungaard, T.; Sønju, O. K. J. Hazard. Mater. 2007, 144, 668–672. (4) Yankelevich, Y.; Pokryvailo, A. IEEE Trans. Plasma Sci. 2002, 30, 1975–1981. (5) Okubo, M.; Inoue, M.; Kuroki, T.; Yamamoto, T. IEEE Trans. Ind. Appl. 2005, 41, 891–899. (6) Yamamoto, T.; Okubo, M.; Nagaoka, T.; Hayakawa, K. IEEE Trans. Ind. Appl. 2002, 38, 1168–1173. (7) Jiang, C.; Mohamed, A. H.; Stark, R. H.; Yuan, J. H.; Schoenbach, K. H. IEEE Trans. Plasma Sci. 2005, 33, 1416–1425. (8) Kim, H. H.; Wu, C.; Kinoshita, Y.; Takashima, K.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 2001, 37, 480–487. (9) Ma, H.; Chen, P.; Ruan, R. Plasma Chem. Plasma Process. 2001, 21, 611–624. (10) Okubo, M.; Kuroki, T.; Kametaka, H.; Yamamoto, T. IEEE Trans. Ind. Appl. 2001, 37, 1447–1455. (11) Helfritch, D. J. IEEE Trans. Ind. Appl. 1993, 29, 882–886.
10.1021/ef700532y CCC: $40.75 2008 American Chemical Society Published on Web 12/19/2007
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addition to gas conditioning, NTP can be used for the production of high-value products, such as polymers, synthetic gas, or methanol.13–15 Another use of NTP is the improvement of gas combustion through plasma treatment of either the fuel or the oxidative agent (air) prior to combustion.16–18 The wide user range of this technology has resulted in a large parametric investigation. Among these are the gas composition of the treated fuel, the background gas used (inert or oxidative), reactor type and geometry, type of electric power supply (DC, AC, DC plus AC, or pulse), gas temperature, etc. NTP produces basically ions and electrons with high average kinetic energy. These electrons target mainly the background gas but also the rest of the compounds and produce free radicals and ions that in return activate chemical reactions. The molecular dissociation of the different compounds does not only occur through the intermediate radicals but may also happen through direct collision with the migrated electrons. Most of the studies thus far have been concerned with investigating the chemical reaction of a single compound in a background gas. Others have studied the inhibition effect of introducing different compounds on the removal efficiency. However, few papers have included NTP treatment of “real” gaseous products produced through direct devolatization of biomass. The aim of this work is to study the composition of the devolatilized gas products of straw pellets at different pyrolysis temperatures. In addition, a NTP reactor was used to enhance the quality of the gaseous products. This was done by performing experiments both with and without the use of the NTP reactor and by quantifying the major compounds in the produced gas. In an effort to study the NTP impact on the condensable fraction of the devolatilized products, experiments have been performed with the NTP reactor placed both before and after liquid condensation. In addition, CO2 was mixed with the N2 carrier gas to obtain a gas composition for the NTP that was less dominated by the high concentration of the background gas (N2). Another objective for increasing the CO2 concentration was to provide an oxygen source for the plasma. This oxygen source is normally present in the gasification products as CO2, CO, and H2O. Experimental Section Sample Preparation. The samples were prepared at the Technical University of Denmark (DTU), Department of Mechanical Engineering, in a laboratory pellet mill used for testing and optimizing the pellets production of different solid fuels.19 The straw with its smooth and shiny surface has been shown difficult to pelletize without the addition of a binding material. Suggestions of mixing a 10% calcium phosphorus solution (CAP) with the straw were considered. However, for this study, it was decided not to use CAP because further contamination of the elemental composition of the straw was not desired. Only 10% of water was added to (12) Shi, Y.; Ruan, J.; Wang, X.; Li, W.; Tan, T. Plasma Chem. Plasma Process. 2006, 26, 187–196. (13) Jensen, R. J.; Bell, A. T.; Soong, D. S. Plasma Chem. Plasma Process. 1983, 3, 139–161. (14) Jensen, R. J.; Bell, A. T.; Soong, D. S. Plasma Chem. Plasma Process. 1983, 3, 163–192. (15) Okubo, M.; Su, Z.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Applicat. 1999, 35, 1205–1210. (16) Lee, S. M.; Park, C. S.; Cha, M. S.; Chung, S. H. IEEE Trans. Plasma Sci. 2005, 33, 1703–1709. (17) Rodrigues, J. M.; Agneray, A.; Jaffrezic, X.; Bellenoue, M.; Labuda, S.; Leys, C.; Chernukho, A. P.; Migoun, A. N.; Cenian, A.; Savelev, A. M. Plasma Sources Sci. Technol. 2007, 16, 161–172. (18) Rosocha, L. A.; Coates, D. M.; Platts, D.; Stange, S. Phys. Plasmas 2004, 11, 2950–2956. (19) Holm, J. K.; Henriksen, U. B.; Hustad, J. E.; Sørensen, L. H. Energy Fuels 2006, 20, 2686–2694.
Energy & Fuels, Vol. 22, No. 1, 2008 687 Table 1. Ultimate and Proximate Analysis of the Straw Pellet (Dry Basis) proximate analysis volatiles
fixed carbon
ash
76.47
18.07
5.46
ultimate analysis (oxygen calculated by difference) C
H
N
O
S
Cl
46
6.2
0.63
46.09
0.11
0.45
the straw. The water cools the die temperature as it evaporates, while the pellets are being pressed. The ultimate analysis of the straw pellets is performed at the “Elsam Kraft A/S” laboratory in Denmark and is shown in Table 1. For the quantifications of C, H, and N, the American Society for Testing and Materials (ASTM) standard D 5373 was used. For S and Cl an inductively coupled plasma-optical emission spectroscopy (ICP-OES) technique was used. The proximate analysis was performed in our laboratories by following the ASTM standards D 1102 (ash) and E 872 (volatiles) and is also shown in Table 1. Macro-thermogravimetric Analyzer (TGA). The reactor is basically a vertical tube with an Al2O3 ceramic coating with an inner diameter of 100 mm and a length of 1000 mm. The core is insulated with five heating elements that can be temperaturecontrolled separately. The gas entering the reactor is preheated in a heating section situated at the entrance of the main reactor. During the experiment, the sample is weighted continuously with a “Sartorius CP 153” scale, with a precision of 1 mg. A chrome-alumina wire is used to attach the sample basket to the scale. When the top lock along with the scale is lowered into a closed position, the bottom of the sample basket is approximately at the start of the first heating zone. The setup of the reactor has been described in more details previously20 and is shown in the schematic drawing in Figure 1. NTP Reactor. The NTP reactor is a coaxial dielectric barrier discharge (DBD)-type reactor. A Powertron 1000S was used to supply an alternating current (AC) to a transformer, where both the main voltage and the frequency could be varied. A transformer was used to increase the voltage by 100 times on its secondary output. The inner electrode of the DBD is made of a brass tube with an outer diameter of 9 mm and length of 400 mm and was attached to the high-voltage source. The high-voltage electrode was mounted inside a quartz tube with an inner diameter of 14.6 mm and a thickness of 1.5 mm. The sample gas flows through the annular gap between the inner electrode and the quartz tube. The residence time of the producer gas in the plasma region was calculated to 0.12 s at standard conditions. The discharges take place in this gap and only over the length of the outer electrode. The outer electrode was coated on the outer wall of the quartz reactor with a fluid containing silver. The length of the outer electrode was 100 mm. The primary voltage/input (rms) voltage was determined through the high-voltage source, while for the secondary/output voltage, a Tektronix P6015A probe (1:1000) was used. From the outer electrode, a BNC cable was used for the measurement of electric discharges. Both signals were connected to a Tektronix TDS 684A oscilloscope and used for the calculation of the electrical power. The frequency for all of the experiments was set to 1500 Hz. The electrical power deposited through the plasma varied between 100 and 150 W. In the literature, the temperature increase through the NTP has also been proven to have some effect on the chemical reactivity.21 In our setup, we did not measure the gas temperature inside the NTP reactor. However, temperature measurements were performed prior to the experiments by heating up the macro-TGA to the (20) Becidan, M.; Skreiberg, Ø.; Hustad, J. E. J. Anal. Appl. Pyrolysis 2007, 78, 207–213. (21) Nair, S. A.; Yan, K.; Pemen, A. J. M.; Winands, G. J. J.; Gompel, F. M.; Leuken, H. E. M.; Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. J. Electrost. 2004, 61, 117–127.
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Figure 2. Devolatization rate as a function of temperature.
Figure 1. Schematic diagram of the reactor and the gas sampling line. Table 2. Temperature Measurements of Carrier Gas at the Inlet and Outlet of the NTP Reactor at Different Macro-TGA Temperatures macro-TGA (°C) NTP inlet (°C) NTP outlet (°C)
400 56 22
500 66 23
600 79 25
700 86 27
800 101 34
temperatures of 400-800 °C, with steps of 100 °C. At each step, the temperature was kept constant for 30 min. The macro-TGA was purged with air at 40 nL/min, and a flow of 5 nL/min was passed through the NTP. Two thermocouples were placed at the inlet and outlet of the plasma reactor. The results for the temperature measurements are presented in Table 2. The large temperature drop in the NTP reactor resulted in condensation of a small portion of the devolatilized products on its inner walls. Because of the short residence time of the devolatilized products, most of the condensable products were found in the ice-cooled trap downstream the NTP reactor. Gas Analysis. The major compounds from the pyrolysis gas are identified and quantified using a Varian CP-4900 micro-gas chromatography (GC) equipped with two injectors each connected to two different columns. The first column is a 10 m Pora-plot type, with an internal diameter of 0.25 mm, and uses helium as a carrier. This column is used for the separation of CO2, CH4, C2H2 plus C2H4, and C2H6. The second column is a 20 m long Molsieve, with an inner diameter of 0.25 mm, and uses argon as a carrier gas. Argon was used to be able to detect hydrogen. This column is able to quantify H2, O2, N2, CH4, and CO. The GC has two thermal conductivity detectors (TCDs), one for each column. In addition to the GC, a Bomem 9100 Fourier transform infrared spectroscopy (FTIR) analyzer equipped with a deuterated triglycine sulfate (DTGS) detector was used to quantify CO2, CO, CH4, C2H2, and
C2H4. The analyzer was kept at a constant temperature of 176 °C. The cell has an inner volume of 5 L and an optical path length of 6.4 m. Experimental Procedure. Only isothermal experiments were performed in this work. The reactor was preheated to the desired temperature and, at the same time, purged with nitrogen (25 nL/ min) for about 90 min. The pellets were kept at 105 °C for at least 24 h prior to the experiment. A total of 80 g of sample material was put inside the basket and attached to the scale. Once the oxygen concentration inside the reactor was reduced to a satisfactory level, the sample basket was rapidly lowered into the reactor. The experiment was stopped when no more major gas release was detected. The sample for gas analysis was taken through a hole on the side of the top lock that is drilled with an angle of 30° from a horizontal line. A quartz tube with an outside diameter of 6 mm and an inside diameter of 1 mm is used for the gas sampling. The sample gas passes first through an ice-cooled trap made of three glass bottles filled with glass wool. The sample gas is then led to another ice trap made of one steel condenser and another container filled with glass wool with a glass fiber paper filter on the top. The gas flows through a heated particle filter and then splits into two streams where one stream goes to the FTIR and the other to the GC. The sampling line is also shown, together with the reactor in Figure 1. In total, four sets of experiments were performed at the different pyrolysis temperatures: (1) for identification of main gas components from the pyrolysis of straw pellets, (2) with the NTP reactor placed upstream of the liquid condensation, macro-TGA flushed with nitrogen, (3) with the NTP reactor located downstream of the liquid condensation, macro-TGA flushed with nitrogen, and (4) with the NTP reactor placed upstream relative to liquid condensation, macro-TGA flushed with nitrogen and carbon dioxide (CO2/N2 molar ratio of 17:8). Having a high concentration of CO2 in the background gas might produce CO and oxygen radicals by the dissociation of CO2 according to CO2 + e- f CO + O-
(1)
The increased availability of oxygen radicals will change the chemical reaction path of some of the devolatilized products. As was previously explained, the aim of the last set of experiments is to investigate the role of the background gas on the product distribution and to bring the gas composition closer to the ones produced in gasification processes. It is important to point out that the temperature in the NTP reactor is much lower than the one in a typical gasification process. This will most certainly affect the reaction rate and efficiency of the NTP reactor. The investigation of the temperature effects are beyond the scope of this study and will be addressed in a later paper.
Results Weight Loss. Pyrolysis experiments with the pellets were run at temperatures of 400–800 °C. Figure 2 shows the weight loss rate in g/min as a function of time. For a temperature of
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Table 3. Distribution of the Main Products for the Straw Pellets temperature (°C) solid (weight %) liquid (weight %) gas (weight %)
400
500
600
700
800
34.8 44.6 20.6
29.6 43.0 27.4
27.8 38.2 34.0
26.7 23.8 49.5
25.7 15.5 58.8
distribution of the gaseous products in weight percent relative to the dry gas fraction CH4 CO2 C2H2 C2H4 C2H6 H2 CO
1.7 73.8
5.2 67.1
7.1 54.5
0.3 0.8 0.02 23.4
1.2 1.6 0.2 24.7
4.1 2.1 1.0 31.2
9.2 44.9 0.4 6.5 1.7 1.6 35.8
9.9 42.5 0.5 7.9 0.8 1.7 36.7
800 °C, the devolatization stage takes approximately 6 min, with a maximum rate of volatile release of 22 g/min occurring after 2 min from the start of the experiment. As expected, the dm/dt curve becomes flatter at decreasing temperatures, while the devolatization duration increases. For a temperature of 400 °C, the devolatization period is approximately 16 min. The gas concentration profile of the major compounds in the pyrolysis products can be integrated so that the total amounts can be calculated. Because the weight of the sample is also monitored, the same can be performed for the solid fraction. The liquid fraction is calculated by difference. The weight distribution of solid, liquid, and gas from pyrolysis of straw pellets is shown in Table 3. From this table, we can see that increasing the temperature favors the gas production at the cost of both the liquid and solid fractions. This result is in agreement with the literature.22–24 Table 3 also includes the distribution of the gas products, presented in percent relative to the gas fraction. Effect of NTP. In the following results, the devolatilized products have been subjected to NTP and Figure 3 shows the total amount of the produced gases in weight percent relative to the dry sample. At a pyrolysis temperature of 400 °C, the NTP placed upstream of liquid condensation is promoting an increase in the gas production (14 wt %). At this low pyrolysis temperature, the liquid fraction contains mostly heavy hydrocarbon compounds that are easily cracked by the plasma to produce gases and lighter liquid fractions. Although the liquid products were not analyzed, it was observed that the condensed liquids were less viscous when plasma was applied. It should also be mentioned that carbon residues were deposited on the inner wall of the NTP reactor, where the electron flux from the inner electrode to the outer one was present. For pyrolysis temperatures higher than 400 °C, less gaseous products are produced compared to experiments with no plasma treatment. At higher temperatures, the condensable fraction is composed of lighter compounds, which seem to be harder to chemically convert by NTP. This fact is confirmed by the literature,25 where field tests on tar removal with NTP have shown better efficiency on the removal of the heavier compounds. The loss of the total gaseous fraction at temperatures above 400 °C when NTP was used can be attributed to two sources: carbon deposits inside the DBD reactor and an increase of the liquid fraction. (22) Encinar, J. M.; Gonzalez, J. F.; Gonzalez, J. Fuel. Process. Technol. 2000, 68, 209–222. (23) González, J. F.; Encinar, J. M.; Canito, J. L.; Sabio, E.; Chacon, M. J. Anal. Appl. Pyrolysis 2003, 67, 165–190. (24) Figueiredo, J. L.; Valenzuela, C.; Bernalte, A.; Encinar, J. M. Fuel 1989, 68, 1012–1016. (25) Nair, S. A.; Pemen, A. J. M.; Yan, K.; Gompel, F. M.; Leuken, H. E. M.; Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. Fuel Process. Technol. 2003, 84, 161–173.
Figure 3. Total gas production in weight percent relative to the dry straw pellets sample.
Figure 4. Total methane production relative to the dry straw pellets sample.
Detailed Presentation of the Different Gas Fractions. Figures for the individual gas compounds are presented in Figures 4-11. The x axis represents the pyrolysis temperature, while the y axis gives the integrated values for the different compounds in g/kg dry straw pellets. The reproducibility of the gas measurements have been investigated by performing three repetitions of the experiment at 500 °C, with the NTP reactor placed upstream relative to liquid removal. The relative standard deviation (RSD) for the different gas products are presented in Table 4. It is important to point out that these figures do not reflect on the accuracy of the measurements; however, great care has been taken in performing the calibrations of both the GC and FTIR. Methane. Dependent upon the background gas, CH4 decomposes to give C2H4, C2H6, H2, CO, CO2, H2O, methanol, and formaldehyde.26–29 According to the literature26 methanol and formaldehyde are maximized at an O2 concentration of 15 vol % in N2. While in an inert atmosphere (N2), C2H6 and H2 are the major products.27–29 In the case of our experiments, CH4 can be destroyed by radicals but can also be produced as a result of the decomposition of the liquid fraction. From Figure 4, it can be noted that at 400 °C CH4 production increases compared to pyrolysis with no NTP treatment. For the rest of the temperatures, CH4 dissociation is dominant and reaches the highest conversion at higher pyrolysis temperatures. The introduction of CO2 to the background gas is increasing the rate of CH4 decomposition as well. (26) Okubo, M.; Kim, H.; Takashima, K.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 2001, 37, 1618–1624. (27) Fraser, M. E.; Fee, D. A.; Sheinson, R. S. Plasma Chem. Plasma Process. 1985, 5, 163–173. (28) Supat, K.; Kruapong, A.; Chavadej, S.; Lobban, L. L.; Mallison, R. G. Energy Fuels 2003, 17, 474–481. (29) Yang, Y. Plasma Chem. Plasma Process. 2003, 23, 283–296.
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Figure 5. Comparison of the hydrogen concentration profile (empty markers, no plasma; filled markers, NTP placed upstream relative to the removal of the condensable fraction).
Figure 6. Total hydrogen production relative to the dry straw pellets sample.
Figure 7. Total carbon monoxide production relative to the dry straw pellets sample.
Hydrogen. Hydrogen dissociation can occur by direct contact with the electrons generated by the NTP to produce hydrogen radicals. Hydrogen can also be produced because of the dissociation of other hydrogen-containing compounds. These can originate from the condensable fraction of the devolatilized products30 or from other gas compounds.31,32 The results for hydrogen are presented in Figures 5 and 6. Figure 5 shows the H2 concentration profile in volume percent as a function of time, (30) Prieto, G.; Okubo, M.; Shimano, K.; Takashima, K.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 2001, 37, 1464–1467. (31) Rosocha, L. A.; Kim, Y.; Anderson, G. K.; Lee, J. O.; Abbate, S. IEEE Trans. Plasma Sci. 2006, 34, 2526–2531. (32) Futamura, S.; Annadurai, G. IEEE Trans. Ind. Appl. 2005, 41, 1515– 1521.
Figure 8. Total carbon dioxide production relative to the dry straw pellets sample.
Figure 9. Total ethane production relative to the dry straw pellets sample.
where “zero” is defined as the start time when the pellet sample is lowered into the macro-TGA. For clarity, the profiles for the pyrolysis temperatures of 400 and 500 °C are delayed 15 min and appear on the right-hand side of the figure. At the temperatures of 400 and 500 °C, the hydrogen production using plasma increase, but at temperatures of 600 °C and above, the hydrogen concentration is lower when plasma is used. This seems to be the case both when the hydrogen production rate is at its highest as well as at the end of the production peak, after which the hydrogen concentration of the plasma-treated flow becomes slightly higher. For temperatures of 400 and 500 °C, the NTP significantly improves the rate of hydrogen production for the whole time period. At 400 °C, the total hydrogen production is approximately 33 times higher than for
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Figure 10. Total acetylene and ethylene production relative to the dry straw pellets sample.
Figure 11. Higher heating value relative to the weight of the dry straw pellets sample. Table 4. RSD of the Gas Products, Performed at 500 °C and with the NTP Placed Upstream of Liquid Condensation repetition 1 repetition 2 repetition 3 RSD (%)
CH4
CO2
C2H2 plus C2H4
C2H6
H2
CO
11.9 11.0 11.2 4.2
177.6 173.6 176.5 1.2
2.9 2.8 3.1 5.9
4.5 4.2 3.9 7.7
1.3 1.1 1.2 7.2
69.3 67.3 67.8 1.5
the experiment where no plasma was used (Figure 6). At 500 °C, the total hydrogen production for the plasma-treated flow is twice the amount of the experiment with no treatment. From Figure 6, we notice that adding CO2 to the nitrogen carrier gas increases the consumption of hydrogen. This is due to the produced oxygen radicals that significantly change the chemical reaction pathways. Carbon Monoxide. The production of CO increases with the use of plasma at temperatures of 400 and 500 °C, breaks even at 600 °C, and decreases at temperatures above 600 °C, as shown in Figure 7. As an additional proof that the dissociation of CO2 occur according to eq 1, we can clearly see from Figure 7 that the total production of CO increases by approximately a factor of 2 in the whole pyrolysis temperature range when CO2 is added to the background gas. It is important to point out that some of the increase of CO, in the case where CO2 is added, might be produced from active sites in the solid fraction through the Boudouard reaction (2).
Carbon Dioxide. At a low temperature (400 °C), up to 70 vol % of the gas product is released as CO2. As the temperature increases, more carbon is released from the char favoring the production of CH4, CO, and C2H4 compared to CO2. The CO2 is dissociated by NTP for all temperatures above 400 °C (Figure 8). This effect is advantageous because the heating value of the produced gas is increased. The NTP in such cases can reduce some of the compounds that are regarded as harmful for humans, nature, or even for the technological platform built to process the produced gas. An example of such an application that could benefit from NTP is the gasification of biomass, where carbon dioxide is generated to produce the necessary energy to drive this endothermic process. During biomass gasification, some of the sulfur contained in the biomass will most certainly end up as H2S and COS during gasification.33,34 The NTP can help at combating such compounds as well as increasing the heating value of the produced gas through carbon dioxide dissociation. Although NTP has proven to be quite effective in the abatement of H2S,9,11,35 its removal efficiency in gasification producer gas remains to be documented. Placing the NTP after liquid condensation has shown no effect on the CO2 concentration when compared to similar experiments with the NTP placed upstream relative to condensation. The later remark is true for all pyrolysis temperatures, except at 400 °C, where CO2 is increased through a chemical reaction in the condensable fraction. Ethane. Ethane production through pyrolysis of the straw pellets increases with an increasing temperature until 700 °C, after which the total production decreases. Ethane is more effectively produced when CO2 is mixed into the background gas in the macro-TGA. This is due to oxygen radicals changing the chemical reaction pathway in favor of ethane production. Acetylene and Ethylene. The acetylene concentration is below the detection limit at low temperatures and is only present in moderate quantities at 700–800 °C. As Figure 10 shows, the total production of these two compounds increases with increasing temperatures. Higher Heating Value (HHV). The HHV of the devolatilized gas fraction is calculated and presented in Figure 11. This value of HHV increases with the pyrolysis temperature from 4 MJ/ kg at 400 °C to 16 MJ/kg at 800 °C. The NTP has a slightly positive effect on the increase of the HHV. The largest increase is approximately 22%, at a pyrolysis temperature of 400 °C. At higher pyrolysis temperatures, the small increase of the heating value is probably due to the decomposition of CO2. Conclusion
(2)
Experiments on the pyrolysis of straw pellets have been performed in a macro-TGA. The experiments were performed at isothermal conditions in the temperature range from 400 to 800 °C. A detailed analysis of the main gas products has been presented along with the effect of a NTP reactor on the gaseous product distribution. The NTP reactor was used for treating a flow of 5 nL/min taken from the macro-TGA for the quantification of the gaseous products. The NTP treatment was performed in three different modes while keeping all of the geometric and electric properties of the DBD constant. The main findings are: (1) The use of NTP in general influences the total gas production as well as the product distribution and higher heating value of the pyrolysis gases.
The reaction rates for eq 2 are quite slow and are considered to be insignificant for temperatures below 800 °C. For the pyrolysis temperature of 800 °C, some of the CO shown in Figure 7 might be produced through eq 2.
(33) Kuramochi, H.; Wu, W.; Kawamoto, K. Fuel 2005, 84, 377–387. (34) Yrjas, P.; Hupa, M.; Iisa, K. Energy Fuels 1996, 10, 1189–1195. (35) Zhao, G.; John, S.; Zhang, J.; Hamann, J. C.; Muknahallipatana, S. S.; Legowski, S.; Ackerman, J. F.; Argyle, M. D. Chem. Eng. Sci. 2007, 62, 2216–2227.
C + CO2 T 2CO
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(2) The total gas production at a pyrolysis temperature of 400 °C was increased by 14% when the NTP reactor was placed upstream relative to liquid condensation. For temperatures above 400 °C, the total gas amount decreased because of carbon deposits in the NTP reactor and an increase of the liquid fraction. (3) For a pyrolysis temperature of 400 °C, placing the NTP upstream of the condensation unit has shown to have a positive effect upon increasing the production of all of the measured gaseous compounds. This also holds true at 500 °C for CO and H2. (4) The difference in changing the background gas in the NTP by purging the macro-TGA with CO2/N2 instead of pure N2 has shown to produce significant differences in the distribution of the gaseous products. The addition of CO2 significantly increased the total amount of CO and decreased the hydrogen content. Ethane was also slightly increased compared to NTP treatment with pure nitrogen. (5) The higher heating value of the produced gas increased with the use of NTP. This can be explained by the dissociation of some of the CO2 present in the devolatilized products. At a
Khalil et al.
pyrolysis temperature of 400 °C, the increase is also due to the conversion of some of the heavier hydrocarbons to combustible gaseous products. Future Work. The NTP efficiency in removing the sulfuric compounds H2S and COS will be addressed in a later paper. As mentioned earlier, gas cleaning at high temperatures is a major challenge. To be able to provide a gasification gas that is suitable for further use in gas turbines, engines, and solid oxide fuel cells, the sulfuric compounds must be removed. The possibility of using such a technology should be investigated in terms of both the economical costs and the removal efficiency. Acknowledgment. This work was supported by The Research Council of Norway through the program “Energy for the Future”. The authors thank Jens Holm at the Technical University of Denmark for his help during our work on the pellets production. The authors also thank the Master students Harris Utne and Monica Moen for their valuable help during the experimental work. EF700532Y
