Thermal Conversion of Cynara cardunculus L. and Mixtures with

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Thermal Conversion of Cynara cardunculus L. and Mixtures with Eucalyptus globulus by Fluidized-Bed Combustion and Gasification Pedro Abelha,*,† Carlos Franco,† Filomena Pinto,† Helena Lopes,† Ibrahim Gulyurtlu,† Jorge Gominho,‡ Ana Lourenço,‡ and Helena Pereira‡ †

Laboratório Nacional de Energia e Geologia (LNEG), Estrada do Paço do Lumiar, 1649-038 Lisbon, Portugal Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, Tapada da Ajuda, 1349-017 Lisbon, Portugal

‡

ABSTRACT: The behavior of Cynara cardunculus L. was studied during fluidized-bed (FB) combustion and gasification. The Cynara had a low moisture content and considerable lower heating value (LHV). Cynara presented significant quantities of S, Cl, and ash, which contained high levels of Na, K, P, Ca, and Si. The fuel N conversion to NOx was high because of the large release of NH3 and HCN during pyrolysis. The conversion of the fuel S to SO2 was low because of S retention mainly as alkali sulfates. HCl emissions were higher than the usual legal limits imposed in European Union (EU) countries, although retentions of 40− 55% fuel Cl could be estimated. The co-combustion of Cynara with eucalyptus was tested with benefits regarding process conditions, pollutant emissions, and ash behavior, but still, the HCl concentration surpassed the legal limit. The tendency for bed agglomeration was also observed during the gasification of cardoon. Two strategies were carried out to minimize this adverse effect: (1) co-gasification of cardoon with eucalyptus and (2) addition of natural minerals to the gasification bed. The results of the first strategy caused a decrease in H2 levels, while tar, hydrocarbon, and CO amounts were found to increase. On the other hand, the addition of natural minerals did not lead to any significant change in the major gas components, although some tar and hydrocarbon abatements were observed, with olivine being the most effective. Dolomite and ZnO gave rise to a greater reduction in HCl and sulfur compounds in the gas phase, respectively.

1. INTRODUCTION The use of biomass for fuels and heat production through thermal conversion processes is a subject of worldwide development. In several countries, tariffs applied for the use of renewable resources for electricity production benefits from national incentives. Furthermore, biomass feedstocks for energy have a large production potential, because significant amounts could be generated from forests, agriculture, and agro-industrial activities and in municipal solid-waste treatment plants. However, experience has shown that biomass availability is often a major barrier for its extensive use for bioenergy production. Seasonal variations in the availability, conflicting interests with other industrial processes using biomass as raw material, and complex chain logistics are some of the intervening factors. Therefore, policies involving the use of biomass resources should primarily aim at diversification through energy crop plantations to secure supply, particularly for small-decentralized installations. The choice of crops and their environmental suitability and productivity have been under close study, especially when soil and climatic conditions may dictate restrictions. This is the challenge in dry and hot regions, such as those prevailing around the Mediterranean basin, for which the thistle Cynara cardunculus L. proved to be one promising perennial crop that can be grown with high productivity.1−3 In the context of energy crops, C. cardunculus L. is generally known as Cynara or cardoon. It is an herbaceous species of the Asteraceae family (Compositae) that grows wild in the Mediterranean region and other parts of the world. This thistle has been the object of several European Union (EU)-supported © 2013 American Chemical Society

research and development programs to evaluate its aptitude for biomass production and potential use.4−6 The average annual production could be high, varying from 15 to 20 tonnes ha−1, depending upon soil and rainfall, partitioned as 40% stalks, 25% leaves, and 35% capitula and with a very low 11−15% moisture content at harvest.5,7,8 The Cynara biomass can be used for multiple purposes, e.g., energy from combustion,9,10 biodiesel from the seed oil,10−12 forage and enzymatic sources for milk coagulation in cheese making,13 or fiber for pulp and paper.2,14−17 The large-scale cultivation of this crop was studied in Portugal within the INTERREG European Project “Energy Crops in the Atlantic Space (ECAS)” in regions with hot and dry spring and summer periods.18 Approximately 140 ha was cultivated, using crop-adapted agricultural practices that allowed for testing mechanical harvest and transport. Albeit operational optimization that still has to be made, the results confirmed the potential of Cynara as an energy crop specially adapted to such areas. Fluidized-bed (FB) technology is one of the most accepted processes for biomass use as a fuel, because of its advantages regarding fuel flexibility, high conversion efficiencies and heattransfer rates, and lower operation temperatures and emissions. However, agriculture origin biomasses, such as cardoon, are identified as problematic regarding bed agglomeration phenomena, because of the high quantity of alkali compounds Received: July 2, 2013 Revised: October 3, 2013 Published: October 3, 2013 6725

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normally present in its ashes.19 The cardoon combustion may also have an additional problem, because of its chlorine content, which is higher when compared to forestry wastes, leading to higher HCl emissions and corrosion in the boiler tubes.20 Gasification is a promising energy conversion technology, because it allows for energy production from wastes with a reduced impact on the environment, because pollutant emissions may be more adequately controlled. Syngas produced by gasification may be used as a fuel or raw material in several chemical syntheses. However, raw syngas may contain undesirable contaminants, such as heavy hydrocarbons, tar, solid particles, and gaseous pollutants, which may compromise some of syngas end uses. For this reason, it is imperative to determine the gas composition and identify the most adequate methods for gas cleaning and further processing of syngas. One of the easiest ways to control the release of undesirable compounds into syngas is the addition of low-cost sorbent materials that could also act as catalysts during the gasification. The materials most commonly used are lime, dolomite, and olivine, because they promote tar cracking. These sorbent/ catalyst materials also favor the decomposition of hydrocarbons into CO2, CO, and H2, thus improving syngas composition for chemical synthesis. Some of these natural minerals may also react with H2S and HCl formed during gasification, leading to the retention of S and Cl in the fluidized bed. Several authors have studied catalytic gasification, and a large number of papers have been published.21−30 Dolomite is commonly used in gasification reactors, because it is inexpensive and easily available and has proven to be suitable for tar reduction. Corella et al.23 reported tar reductions of about 80% in the presence of dolomite and under controlled gasification conditions. Devi et al.25 reported tar reduction around 90% in the presence of dolomite when the syngas was produced through biomass gasification. Some other catalysts have also been tested, such as Ni-based and precious metals catalysts; they usually give better reduction yields but are more expensive and, thus, should only be used under syngas cleaning conditions that do not adversely affect the catalytic activity and durability. Garcia et al.26 and Pinto et al.27 stated that the performance of Ni- and Mg-based catalysts in tar destruction was higher than that of dolomite. The performance of Ni-based catalysts can be improved when they are used in monolith form.29 Pfeifer and Hofbauer30 tested several commercial catalysts and reported tar reduction around 98%. Tar conversion yields showed to be very promising when precious metals were employed as catalysts, although its industrial-scale application is not economically viable at this moment because of their high cost. The importance of biomass thermal conversion and the technological near-to-market status suggest the use of Cynara biomass for energy by combustion or gasification processes. Gasification gas may have different end uses depending upon their composition. Both the use as gaseous fuel and electricity generation will be considered, because other more demanding applications will demand expensive gas-cleaning and upgrading processes. This is the objective of the present work, where the behavior of Cynara biomass, the whole plant as harvested, including stalks, leaves, and capitula (with seeds), was studied during combustion and gasification, under FB conditions. A literature survey shows that information on Cynara thermal conversion is scarce, especially when considering FB technology.31,32 Cynara has lower bulk density and different mineral matter chemical composition when compared to forestry wastes more commonly investigated, and therefore, distinct behavior is

predicted during thermal conversion. Mixtures of Cynara and eucalyptus (Eucalyptus globulus) wastes, mainly branches from forest industry and cleaning operations, were also tested to evaluate eventual benefits regarding process conditions, pollutant emissions, and ash behavior that may contribute to the decrease of environmental and technical impacts of use of energy crops. Portugal has a relevant pulp and paper industry sector based mainly on the bleached eucalyptus Kraft pulp process (BEKP) using virgin fibers. Therefore, E. globulus forest wastes are available and are usually used for energy production. This was the main reason for choosing eucalyptus to mix with Cynara.

2. EXPERIMENTAL SECTION 2.1. Biomass Materials. The plant material used in this work came from a large field of C. cardunculus L. (cardoon) planted in Apariça [38° 07′ N; 07° 53′ W; 155 meters above sea level (m.a.s.l.)], near the city of Beja, Portugal. The region is characterized by very hot and dry summers. This field, included in the experimental cardoon field network of the INTERREG IIIB “Energy Crops in the Atlantic Space (ECAS)” Project,19 totalized a crop area of 77.4 ha. The field was installed using common agricultural practices and machinery. Cardoon is a perennial with an annual growth cycle. Installation by sowing was successful, despite the extreme drought that occurred during this first cycle (221 mm annual rainfall) and the plants developed well during the second cycle (with 556 mm rainfall). The field biomass yield was estimated at 7.5 tonnes ha−1, and the plants at harvest had on average 2.1 m height and 22 mm stalk diameter, with 5.3 capitula per plant and stalks representing 59.1% total dry biomass.3 The field was harvested at the end of August when the plant was dry. The harvested plants were chipped, piled in the field, and transported subsequently to the laboratory. The feedstock had chip dimensions of 5−30 mm and a bulk density of 125 kg m−3. Prior to testing and for convenience of the automatic feeding to the combustion or gasification process, the cardoon chips were densified by pelletizing, with dimensions of 20 × 6 mm in cylindrical form, using a pressing roll automatic pelletizer. The bulk density increased to about 500−600 kg m−3. Eucalyptus branch residues used in mixtures with Cynara were dried and chipped, acquiring a needle shape with a maximum size below 10 mm. 2.2. Material Characterization Methods. The fuels arrived in 20 kg batches and were disposed in a pile in the floor of the fuel preparation room. A composite sample (2 kg) of the fuel was collected by cutting up the pile in a grid shape and sampling for a representative result. Each fuel sample was milled to 0.25 mm and homogenized. Analysis was performed in duplicate, and the covariance was less than 10% of the average. Analyses were performed according CEN/TS standards for solid biofuels. Automatic analyzers were used for the determination of C, H, N, and S contents. For Cl quantification, combustion in a oxygen bomb was performed followed by analysis of Cl using capillary electrophoresis. Ash-forming elements were determined by atomic absorption spectroscopy (AAS) using 0.1 N HNO3 solutions of the lithium tetraborate fusion of biomass ash samples obtained at 550 °C. Phosphorus was determined by ultraviolet/visible (UV/vis) spectroscopy on ash samples subjected to microwave acid digestion (HNO3, HF, and H3BO3). Characterization results of cardoon and eucalyptus are presented in Table 1. The bed material is mainly composed of silica sand particles of 0.36 mm in average diameter, with a silica purity of above 99%. Several minerals, such as dolomite, olivine, and ZnO, were added to the bed to be investigated in the combustion and gasification assays and correlated with the gas composition and emissions of HCl, NOx, and SO2 and bed agglomeration behavior. Dolomite came from Portuguese mines around 100 km from Lisbon, and olivine came from northern Europe. The characteristics of those materials are presented in Table 2. Dolomite and olivine were sieved and used with a particle size between 0.5 and 1.5 mm. ZnO was a commercially available 6726

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integral−derivative (PID) control. Along the reactor, there are various points to measure the temperature and pressure. Dependent upon the bed height, the fuel is supplied to the combustor/gasifier above or in-bed by a continuous feeding system, composed of a set of two screw feeders and a variable speed motor. The dosing feeder, connected to the silo bottom, is previously calibrated for the mixture to be studied, and the mass of the fuel is weighted at the beginning and end of each run to confirm the fuel feed rate. The feedstock in the silo is permanently agitated by a mixer to prevent arching and voids, facilitating the flow. The fuel is discharged by gravity to the second screw feeder, which is located below and directly connected to the combustor/gasifier at 0.20 m from the fluidizing flow distributor. This screw is externally water-cooled to avoid pyrolysis during feeding and rotates at a fixed and fast speed with an auxiliary gas flow (nitrogen in gasification and air in combustion) to avoid a back flow of the reactor gases. A static bed height of 0.15 m was used (approximately 0.20 m when fluidized). In the combustor (1.5 kWth), the air is supplied by an oil-free compressor. Primary air is introduced at the bottom through a distributor plate located above the windbox, and secondary air is fed at 0.3 m above the top of the bed. The minimum fluidization velocity was 0.05 cm/s. The fluidizing velocity was maintained close to 0.35 ms−1 (gas residence time of approximately of 4 s). Because of the low fluidizing velocities used and to minimize bed agglomeration problems, which are normally related to the presence of alkalis (Na or K from fuel) and silica (from bed sand), forming silicates of low melting points, it was decided to operate with a maximum bed temperature of 750 °C. The freeboard temperature set point is 850 °C; however, the freeboard temperature profiles obtained here (Figure 2) were similar to those attained in previous tests burning similar biomass materials in a bubbling fluidized-bed pilot installation.33−35 At the exit of the combustor, there is a cyclone to remove coarse particles from the flue gases before they are sent to the exhaustion. The gases are sampled after the cyclone and pass through a heated box containing glass wool filters to retain the fine particles prior to gas feeding into the online O2, CO2, CO, N2O, NOx, and SO2 analyzers, which were previously calibrated before each run with zero and spancertificated gas bottles. The analysis methods used were paramagnetic for O2 and nondispersive infrared (NDIR) for the other gas species. Halogens (Cl and F) are sampled by standard discontinuous methods using cooled impinger trap solutions [Method 26 of the United States Environmental Protection Agency (U.S. EPA)]. In the gasification process, a mixture of steam or air is introduced through a gas distributor at the bottom of the reactor. Steam is produced in a generator, and its flow rate is controlled by a water pump. The presence of steam favors gasification reactions, namely, the production of H2 and the reduction of gaseous and liquid hydrocarbons because of steam-reforming reactions. Thus, in the presence of steam, lower char and tar contents are produced. The presence of air promotes partial combustion reactions and, thus, supplies the energy necessary for the gasification process. The minimum fluidization velocity of the gasification reactor was 4.5 cm/ s, and the average residence time was around 15 s under the gas velocity conditions used (10 cm/s). Gasification tests were performed in the presence of air and steam (Table 3). The gas produced leaves the reactor, passing through a cyclone to remove particulates. Tars and condensable liquids carried by the gas are removed in a quenching system, including a condenser, a liquid collector, and two glass wool filters. Then, the gas passes through a filter prior to be injected into the online CO and CO2 analyzers. The gas produced is also collected in bags to be analyzed on a gas chromatograph. The compositions of H2, CH4, and other heavier hydrocarbons with 2, 3, or 4 carbon atoms, whose total concentration is given as CnHm, are determined. Syngas is sampled to determine the contents of tar, H2S, NH3, and HCl. A different gas sample was taken for each of these determinations. The gas was sampled after cyclone for tar determination, while for H2S, NH3, and HCl, the gas was sampled after the condensation system, as shown in Figure 1B. Tar is analyzed according to “CEN/TS 15439”, using isopropanol (2-propanol or isopropyl alcohol). H2S is analyzed by Method 11 of the U.S. EPA. NH3 is determined according to

Table 1. Analysis of the Biomass Fuels Used in the Combustion and Gasification Studies wt %, ara

cardoon

moisture ash volatile matter fixed carbon C H N S Cl Al Ca Fe K Mg Na P Si LHV (MJ/kg) a

eucalyptus

Proximate Analysis 13.1 5.4 66.8 14.7 Ultimate Analysis 42.8 5.3 1.5 0.25 0.41 0.03 0.58 0.06 0.89 0.25 1.24 0.41 0.56 16.3

9.8 1.9 72.1 16.2 47.4 4.7 0.26 0.06 0.01 0.003 0.74 0.07 0.24 0.08 0.06 0.02 0.01 14.9

ar = as received.

Table 2. Characteristics of the Bed Materials bed material silica sand calcined dolomite

surface area, SBET (m2 g−1)a

pore volume, Vp (cm3 g−1)b

12.3

4.30 × 10−5

olivine

9.8

8.64 × 10−5

calcined olivine

4.7

9.51 × 10−5

27.5

5.32 × 10−4

ZnO (G-72-D)

main components SiO2 (>99%) magnesium oxide carbonate, Mg3O(CO3)2 lime, CaO portlandite, Ca(OH)2 forsterite, Mg2(SiO4) iron oxide hidroxide, FeO(OH) forsterite, (Mg,Fe)2SiO4 talc-2M, Mg3SiO4O10(OH)2 nacrite-1Md, Al2Si2O5(OH)4 forsterite, Mg2(SiO4) forsterite ferroan, (Mg1.44Fe0.56)(SiO4) enstatite, MgSiO3 zincite, ZnO nordstrandite, Al(OH)3

a

SBET = Brunauer−Emmett−Teller (BET) surface area determined at P/Po from 0.5 to 0.3. bVp = total pore volume determined at a P/Po of 1 × 10−5. catalyst with the identification of G-72-D. Usually sorbent particles, such as dolomite/olivine, suffer severe attrition under FB conditions. For this reason, an initial higher particle diameter was chosen to allow for convergence with the sand particle diameter and avoidance of excessive fine elutriation during the experimental run time. 2.3. Combustion and Gasification Pilot Units. Both combustion and gasification tests were carried out on bench-scale bubbling FB reactors under atmospheric pressure (Figure 1). Both reactors are made of a refractory steel tube, circular in cross-section, with an inside diameter of 0.08 and height of 1.5 m. The combustor/gasifier is located inside an electrical furnace, which provides the heat for reactions, with three independent heating zones under proportional− 6727

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Figure 1. Diagram of the bench-scale units used for biomass (A) combustion and (B) gasification studies. The time of each experiment, for both combustion and gasification, was around 60−90 min, from which the first 15−20 min was for stabilization. The operating conditions for the combustion and gasification studies are summarized in Table 3.

Method CTM-027 of the U.S. EPA, and for HCl analysis, capillary electrophoresis and a methodology based on Method 26 of the U.S. EPA are used. During one steady state, only one sample is collected for each determination, which means four samples for tar, H2S, NH3, and HCl, plus another gas sample collected in bags, all over the experiment, to be analyzed on a gas chromatograph. Because of the high volume of the collected gas, the experimental error should be low. However, to ensure the reproducibility of experimental results, at least two sets of runs were repeated under the same experimental conditions, when deviations higher than 5% that observed more tests were performed to maintain uncertainty at this level.

3. RESULTS AND DISCUSSION 3.1. Biomass Composition. The analysis results of the two tested biomass species, cardoon and eucalyptus, both used in the combustion and gasification studies, showed distinct properties, as observed in Table 1. Similar to other biomass feedstocks, Cynara has a high oxygen content (about 36% on a 6728

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compared to most biomasses, as is the case of the eucalyptus used (about 4 times higher). Special precautions should be taken regarding the use of Cynara as a fuel because of its high Cl content when compared to most wood biomass. Fuel Cl content levels, such as the one determined here for Cynara (0.41%, ar), may cause environmental problems because of HCl emissions and operational problems because of corrosion and fouling effects in the boilers. Cardoon ash is rich in alkaline elements, K and Na, followed by Ca, Si, and P, and minor quantities of Mg, with very low Fe and Al contents. The eucalyptus ash was found to contain mostly Ca and some K, followed by Mg, Fe, Na, and P, and very low Si and Al. Cardoon ash behavior during thermal conversions and the influence of using forest biomass and dolomite will be presented in a subsequent paper under preparation. 3.2. Combustion Results. The influence of the air/fuel equivalence ratio on the temperature profile and the gaseous emissions was investigated during the combustion of cardoon alone. The addition of eucalyptus wood wastes for cocombustion with cardoon and the use of dolomite in the FB, for sulfur and bed agglomeration control, were also investigated and correlated with the gas composition, namely, with the emissions of CO, N2O, NOx, SO2, HCl, and HF. N2O emissions were lower than 5 ppmv in all runs and were not affected significantly by the changes in the experimental parameters, except when dolomite was added to bed material because N2O decreased below the detection limit. The presence of Cl2 and HF in the gas phase was also monitored and found to be both below the detection limit. 3.2.1. Cardoon Monocombustion. The influence of different air/fuel equivalence ratio levels on the temperature profile during monocombustion of cardoon is presented in Figure 2. The temperature profile along the combustor height was quite similar among the different tests to the diverse air/fuel equivalence ratio levels tested. The temperature was nearly constant in the bed zone, which was maintained below 750 °C to prevent bed agglomeration. In fact, it was possible to perform the cardoon combustion during 2 h without bed defluidization. However, at the end of the cardoon combustion tests, there was evidence of several bed agglomerate formations with sizes up to 5 mm. In the freeboard, first there was a continuous temperature increase beginning just above the bed, near the feeding point, that was due to a higher combustion rate of the volatile fuel fraction, which was rapidly released, rising the temperature in that zone. Because of the high volatile content of the cardoon, the bed operates with less fuel (when compared to a typical coal combustion process) and a higher air staging ratio and a more intense mixing between fuel and the air in the freeboard are necessary for a complete combustion. The temperature values reached a maximum near 0.3 m above the air distributor plate, when most of the fuel is completely burned, and then decreases continuously through the installation height because of thermal losses. When the air/fuel equivalence ratio was increased from 1.25 to 1.35, there was a significant augment of the maximum temperature obtained in the freeboard (at 0.3 m), as seen in Figure 2, because the extra air supplied contributed to a more complete combustion and more char particle entrainment, releasing more heat in that zone. On the other hand, increasing the air/fuel equivalence ratio further to 1.5 contributed to reduce the freeboard maximum temperature, meaning that the combustion efficiency was not further incremented, which was

Figure 2. Temperature profiles obtained during cardoon combustion and co-combustion at different air/fuel equivalence ratio (λ) conditions.

Table 3. Operating Conditions Used for the Combustion and Gasification Studies variation range experimental parameter bed temperature (°C) freeboard temperature (°C) flow rate of biomass (g/min) particle size (mm) cardoon pellets (mm) eucalyptus chips (mm) flow rate of air (g/min) air/fuel equivalence ratio primary/secondary air (%) flow rate of steam (g/min) equivalence ratio (w/w) steam/biomass ratio (w/w) air/steam flow rate (L/min) thermal input power (kW)

combustion

gasification

720−745 see Figure 2 4.0−4.8

850 850 ≈5.0

20 × 6 20%), NOx should be decreased further by the enhancement of the DeNOx mechanism. About 90 and 50% S and Cl could be retained mainly as alkali sulfates or chlorides, respectively. However, the levels of HCl in the gases were much higher than the legal limits and most likely contributed to the high CO and NOx concentrations in the combustion gases. The co-combustion of cardoon with eucalyptus wastes was an efficient solution to decrease simultaneously the CO, SO2, NOx, and HCl emissions, although HCl still overtook the legal limit. Dolomite addition to the bed avoided agglomeration and decreased slightly the fuel Cl conversion to the gas phase. To solve the problem of bed agglomeration during the gasification of cardoon, blends with eucalyptus were performed. This also decreased the contents of H2S, NH3, and HCl in the gasification gas. On the other hand, the increase of the cardoon share in the fuel blend increased H2 levels while decreasing hydrocarbon and CO concentrations, which may be beneficial depending upon gas end uses. The presence of cardoon also led to a lower concentration of tar in the gases and char in the bed. The addition of dolomite, olivine, and ZnO did not influence the main gas components, although they led to some hydrocarbon abatement and tar reduction, with olivine being the most effective. Dolomite and ZnO were the most effective in reducing the release of HCl and sulfur compounds to the gas phase, respectively. The main advantage of cardoon cogasification is the possibility of producing a gasification gas that could be use afterward, as either a fuel or a raw material for synthesis of biochemicals or for biohydrogen production.

(1)

ZnCl2 was liquid at the gasification temperature, and it could have escaped from the sorbent, because of its significant vapor pressure. On the other hand, the use of steam in the gasification medium might also have decreased ZnCl2 formation and chloride retention. Some ZnCl2 formed might also have reacted with H2S that existed in the reaction medium, thus leading to the formation of ZnS (reaction 2). Gupta et al.65 reported that the formation of ZnCl2 was more favorable at a temperature lower than 650 °C, and therefore, reaction 2 would be more favorable at a temperature below 650 °C. 6735

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work presented in this paper was financed by the INTERREG III B Project “Energy Crops in the Atlantic Space (ECAS)”. The contract of Pedro Abelha was financed by the Portuguese Foundation for Science and Technology, through the CIÊ NCIA 2007 Program. The authors recognize the importance of this support.

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