ARTICLE pubs.acs.org/EF
Nascent Biomass Tar Evolution Properties under Homogeneous/ Heterogeneous Decomposition Conditions in a Two-Stage Reactor Wen-guang Wu, Yong-hao Luo,* Yi Su, Yun-liang Zhang, Shan-hui Zhao, and Yun Wang Institute of Thermal Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: In order to study the mechanism of biomass tar formation and elimination in a two-stage downdraft gasifier, the nascent rice straw pyrolysis tar evolution properties under homogeneous/heterogeneous decomposition conditions have been investigated in a constructed lab-scale two-stage reactor by varying factors as temperature, concentration and reforming agents of CO2/H2O/O2, and char bed heights. The nascent tar was produced in the first stage reactor and then decomposed in the second stage with different reforming agents or char beds. In the first stage, the results showed that nascent pyrolysis tar yields increased with increasing pyrolysis temperature, tar was mainly produced during 200400 °C, and 400500 °C would be a proper pyrolysis temperature range in commercial operation due to little effect on tar yields in higher temperature. In the second stage, it can be observed that nascent biomass tar was converted into polycyclic aromatic hydrocarbons (PAHs) (even soot), thermally stable one ring aromatics, and noncondensable gases in homogeneous conditions with increasing temperature. Different effects were obtained in varying tar species under different homogeneous reforming agents. However, benzene, toluene, styrene, phenol, and naphthalene are the most typical compounds, accounting for 5075% in total tar concentration at 900 °C in all decomposition conditions. Char bed can selectively reduce PAH species remarkably and increase the toluene yields. As for the three reforming agents, steam showed the highest efficiency in tar elimination, while CO2 and O2 present will induce OH, H, and O radicals formation, which increases hydrocarbon conversion. The mechanism of tar destruction in a two-stage downdraft gasifier can be concluded as follows: nascent tar yields from the pyrolysis stage will be first reformed into PAHs, thermally stable one ring aromatics and noncondensable gases in the throat region, and then PAHs species are almost completely decomposed by the char bed, which are the main troublesome tar components in syngas, and finally the syngas with low tar was obtained.
1. INTRODUCTION With the increasing threat of environment pollution and oil import dependence, clean and renewable energy development should be of great importance for the sustainable development and energy supply security in China. Biomass is the fourth largest among the primary energy sources after coal, oil, and natural gas in the world,1 and biomass can produce a large amount of energy without deteriorating the environmental quality. Therefore, bioenergy has been attracting more and more attention in China. From 1995 to 2005, about 630 million tons of crop residues were produced annually in China. However, about 23% of the crop residues were used for foraging, 4% for industry materials, and 0.5% for biogas, and large parts were used with lower efficiency burning or wasted.2 Thus, advanced conversion technology development and utilization pattern adjustment are essential ways for effective utilization of residues in China. Gasification is a potential technology in biomass utilization since it is environmental friendly and flexible. However, unacceptable levels of tar contained in the producer gas can cause operation problems in downstream processes, such as blocking gas coolers, filter elements, and engine suction channels.3 Though researchers have made many efforts in recent years, tar elimination is still the main obstacle which limits the application of biomass gasification technology. Among different types of gasifier, a two-stage downdraft fixedbed gasifier is particularly favored because of its lower tar content in syngas and application flexibility in rural areas, especially villages which are far away from industrial centers and have large amounts r 2011 American Chemical Society
of biomass resources. A two-stage downdraft fixed-bed gasifier in the 100 kW-scale is suitable in energy self-supply with a distributed power system, and it might be a substitute solution of rural area energy utilization in China. As shown in Figure 1, a twostage downdraft gasifier is constructed by the pyrolysis stage, throat, and char bed, which are corresponding to the evolution processes of pyrolysis tar yields, partial oxidation conversion, and char bed decomposition, respectively.4 Biomass is first pyrolyzed in the pyrolysis stage of the gasifier, and then volatiles with raw tar flow through a throat where reforming agent (such as CO2/ H2O/O2) is injected for homogeneous reforming/oxidation. Afterward, reformed products pass through the fixed char bed for heterogeneous gasification reactions and tar decomposition reactions. Finally, the products are exported as syngas with low tar content. In these processes, reforming/oxidation and char bed decomposition are the two main processes in syngas tar concentration controlling, which induce tar conversion and destruction. The char bed is the final step for tar elimination in the twostage gasifier, so its tar removal capability directly determines tar concentration in syngas. Nevertheless, partial oxidation can directly affects the char bed reactions by varying primary gasification conditions such as heat flux, reformed tar species, and gas components, etc. This decoupling method isolating concept is based on reforming/cracking of tar and pyrolysis gas through catalysis Received: March 27, 2011 Revised: September 19, 2011 Published: September 20, 2011 5394
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Table 1. Proximate and Ultimate Analysis of Rice Straw Proximate Analysis (wt %, As Received) moisture
13.46
volatiles matter
62.79
fixed carbon
15.92
ash
7.84 Ultimate Analysis (wt %, Dry Ash Free)
carbon hydrogen
a
Figure 1. Two-stage gasifier concept.
of char to produce syngas with little tar.5 However, the heterogeneous char bed tar decomposition properties and the evolution mechanism of raw pyrolysis tar in the gasification condition are still not clearly investigated in the literature. Few scholars paid attention to the oxygen present partial oxidation properties in tar destruction. For example, Brandt et al.6 discussed tar elimination properties under partial oxidation and char bed gasification conditions, and the results illustrated that partial oxidation can achieve a reduction factor of 75500 and the char bed will lower the tar content gas to 15 mg/(N m3) emanating from the gasifier. Hosokai et al.7 investigated the effects of O2 in tar partial oxidation conversion, and the results indicated that O2 was consumed at 700 °C mainly by the oxidation of tar and light oxygenates, and the oxidation at 800 °C consumed H2 and lower hydrocarbons selectively, leaving the residual tar yield nearly unchanged. However, the tar in this study mainly consists of larger molecular oxygenate tar yielded in rapid pyrolysis. This is different from the two-stage gasifier, which was operated with an ordinary heating rate. Houben et al.8 studied the effect of partial oxidation effects on tar elimination by using naphthalene as a model tar compound. The results showed that over 90% of tars were cracked with an oxygen equivalence ratio of 0.2 injected into the burner. Onozaki et al.9 confirmed that over 90% of hydrocarbon gases and tar from a hot coke oven are converted to hydrogen and CO in the presence of steam or O2 at 900 °C. Some other literature1012 also mentioned the H2O/CO2 reforming. However, most are concerned about the reforming capabilities of oxidative agents with a catalyst under present conditions. The pyrolysis tar in the agent as CO2/H2O/O2 presented homogeneous reforming properties without adding catalyst conditions and are rarely investigated. Dufour et al.13 investigated the aromatic tar and gases evolution composition in relation to CH4 and C2H4 under high temperature conditions (7001000 °C); several tar species were quantified by gas chromatography/mass spectrometry (GC/MS), due to the long residence time and large reactor diameter, and pyrolysis yields encounter complex secondary reactions between the homogeneous or heterogeneous, which affect the tar species formation. Benzene is not treated as a tar species in many studies due to its high evaporation property.1416 However, benzene is considered a
35.58 4.63
oxygena
37.36
nitrogen
0.94
sulfur
0.2
Qb (MJ/kg)22
14.16
By difference. b High heating calorific value.
considerable byproduct in aromatic compound destruction or polymerization processes, which is also studied as an important tar model compound in other literature.13,1719 In this study, benzene is classified as an important tar species and special attention is paid. Moreover, the quantitative method has been improved in analyzing yields of tar, especially one ring aromatics as benzene and toluene, which are not or partly detected in our previous work.14,20,21 In order to reduce these one ring aromatics mass loss effects in the gravimetric tar preparation process, the collected tar solvent is promptly analyzed by GC/MS before the evaporation procedure of gravimetric tar preparation. Nascent tar represents pyrolysis tar yields in the pyrolysis stage reactor with little secondary reactions in this work, and it is more accurate in simulating the raw pyrolysis tar compounds than model tar species. Nascent pyrolysis tar evolution properties under different homogeneous/heterogeneous decomposition conditions are of great importance for improving the gasifier operation and controlling the tar content in syngas. The aim of the present study is to find the nascent biomass tar evolution properties in the two-stage downdraft gasifier by using a lab-scale two-stage reactor. Pyrolysis char and tar properties were considered. Effects of the homogeneous thermal cracking temperature, reforming/ oxidation agents as CO2/H2O/O2, and heterogeneous char gasification on nascent tar evolution properties in the second stage reactor were discussed. Special attention was paid to the evolution characteristics of five typical tar species as benzene, toluene, styrene, phenol, and naphthalene by GC/MS quantitative analysis.
2. EXPERIMENTAL SECTION 2.1. Materials. Rice straw is one of the most abundant crop residues in southern China. The biomass material used in this work is rice straw (RS) received from Chongming Island, Shanghai. The sample was sieved into a small size with a diameter range of 100150 μm by a KER-1/ 100A sealed sample preparation mill from Zhenjiang Kerui Zhiyang Shebei Co. Ltd. and then dried at an oven temperature of 30 °C for 12 h before being used. Table 1 shows the proximate and ultimate analysis results of rice straw. Char samples used in the second stage were derived from 150 to 420 μm rice straw sample pyrolysis with a heating rate of 30 K/min from ambient to 500 °C and then held for 1 h under N2 atmosphere at this temperature. Finally, it was cooled down and stored in glass bottles. 2.2. Reactor and Procedures. The two-stage reactor that has been reviewed to be successive evolution results from an earlier singlestage reactor constructed by O’ Brien et al.23 The Kandiyoti research 5395
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Figure 2. Schematic diagram of the two-stage reactor. group in Imperial College London promoted the application of a twostage reactor successively.15,16,24,25 Our previous study14 shows that it is a good way to learn the pyrolysis and gasification properties in the twostage reactor. In order to accurately investigate the tar conversion, some improvements were made, such as extending the second stage, and quantitative analysis of the tar species, especially the one ring aromatic ones. Figure 2 is the schematic diagram of the two-stage fixed-bed reactor. As we intended to simulate the conditions of a two-stage downdraft gasifier,4 the two stages of the reactor, respectively, simulate the pyrolysis and gasification processes, which are both made of 316-grade stainless steel with diameters of 12 mm 16 mm. The first stage is 300 mm, and the second stage is 200 mm (short) or 400 mm (long); each condition will be operated in the longer reactor except thermal cracking in both the longer and shorter ones. The reactor of each stage is directly heated by electrical resistance and controlled independently by K-type/S-type thermocouples. The system operates at atmospheric pressure. Two stages are connected by a pair of flanges, which are sealed with a copper washer. The inject throat between the flange is beneficial for investigating the destruction properties of pyrolyzed nascent tar with steam or other oxidants present. Tar yields after the first or second stage will be collected by a U-shaped stainless steel tube, with a diameter of 6 mm 8 mm
and a length of 300 mm, which is placed in a liquid nitrogen bath so that the tar can be condensed and trapped. A piece of a preweighed wire mesh plug was placed inside the reactor tube and stuck at a fixed position at the bottom of the heated section in the first stage, where the temperature gradient has been proven to be low. A mesh number of 200 was chosen to make sure particles over 75 μm are above the base. A total mass of 1 ( 0.01 g of rice straw was weighed and then charged into the reactor on top of the plug. The two stages were bolted together, and then the thermocouples and electrodes were assembled before heating. In order to inhibit the effect of secondary reactions in the first stage reactor, a carrier gas flow rate of 350 mL/min N2 was injected into the top of the T piece. In order to be consistent with the pyrolysis process in a downdraft two-stage gasifier and keep temperature uniformity along the radius direction, the operation program of the first stage reactor was set with 30 K/min from ambient to intended temperature and held for 15 min. The second stage will be first heated to the intended temperature before adding the oxidant agents. Different flow rates of CO2 or diluted oxygen (1:99 = O2/N2) controlled by MFC were injected from the throat position. While in the steam reforming situation, deionized water should be quantified by a peristaltic pump and heated to 400 °C by the preheater before being injected. The biomass char was weighed and 5396
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Energy & Fuels added into the second stage for char gasification. The flange, throat, and connecting pipe were heated and held at 400 °C, and the second stage reactor should be heated to the required temperature under different decomposition conditions. However, in the second stage, residual volatiles in the pyrolyzed char (pyrolyzed at 500 °C) may release and be collected in the trap. Thus, carrier gas should be operated before the second stage is heated from ambient to the intended temperature. Moreover, when all these temperatures reached the desired value, the tar trap was attached to the outlet of the reactor for the liquid products trapping. Finally, the first stage operation program was started. After the run, the reactor system was allowed to cool down. However, the flow of carrier gas will not be turned off until the reactor cooled to room temperature in order to prevent oxidation of the products. Each pyrolysis char in the first stage at 500 °C was collected so as to evaluate the pyrolysis process repeatability. The results show that the repeatability are within (1% and reliable. 2.3. Tar Sampling and Preparation. Two methods have been tested in this study. Method 1 is based on liquid nitrogen cold trapping as previously described.14 However, steam reforming will block the sampling pipe because of high water content in the flue gas, so we used another sampling method for steam reforming tar trapping only, which is described clearly in the other literature.26,27 In method 1, the mixture solvent (chloromethane and methanol) with a volume ratio of CHCl3/CH3OH = 4:1 is required for tar sample preparation. The trap equipment was washed, and the solution was filtered through a weighed filter paper with a 42.5 mm diameter for tar and char yields separation. Trace solvent was sampled and analyzed by GC/MS, then the solvent was evaporated in a RE3000A type rotary evaporator for 30 min, and the rest was placed in a drying oven with a temperature of 35 °C for 1 h in an inert atmosphere for the final evaporation and then weighed as gravimetric tar and sealed in a container. In method 2, cold isopropanol instead of liquid nitrogen for tar trapping was used in the steam reforming procedure. Three impingers were put in cold alcohol with a 20 °C bath, and another three impingers were immersed in a 20 °C water bath; all impingers were filled with isopropanol solvent. Finally, the solution was also analyzed, evaporated, and dried to obtain the mass of gravimetric tar. The results between the two methods collected in the thermal cracking at 900 °C condition show that the tar species and the contents between the two methods are agreeable. Each gravimetric product was normalized to 1 g of rice straw. 2.4. Tar and Water Analysis. Before each evaporation procedure, the solvent, with water removed by anhydrous sodium sulfate, is analyzed in GC/MS instruments. In addition, water is analyzed by AKF2010 Karl Fischer, which is produced by Shanghai Hegong Scientific Instrument Co. Ltd., and safety pyridine free reagent 3-5 is used in the test, with the maximum possible error of the system as (1.5%. Analysis with the Agilent 6890N GC system and QP2010NC type gas chromatograph/mass spectrometer (GC/MS) was carried out in duplicate and equipped with a splitter injector. The split ratio is 10:1, and the oven temperature is 280 °C. The mass spectrum was acquired using a quadrupole instrument with an electron voltage of 70 eV. The GC column was a HP-5MS column, 30 m long, 0.25 mm diameter, and with a film thickness of 0.25 μm, with a flow rate of 2.4 mL/min. The column oven temperature was programmed from 45 °C (held for 5 min) to 180 at 5 °C/min, then linearly increased from 180 to 300 °C at 20 °C/min, and held for 10 min; each sample volume is 1 μL. The GC/MS was quantified by the external standard method with 1 ppm, 2 ppm, 5 ppm, 10 ppm, 20 ppm, 50 ppm, and 100 ppm solutions of five typical tar components including benzene, toluene, styrene, phenol, and naphthalene. 2.5. Char Analysis. The rice straw char from pyrolysis at different temperatures was analyzed by Bruker’s Vertex 70 Fourier transform infrared spectrometer (FTIR) instrument. The samples and KBr were first dried in a 105 °C oven for 24 h and mixed with each other with a
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Figure 3. Temperature effects on pyrolysis of rice straw. mass ratio of pyrolysis char to KBr powder in a 1:1501:200 ratio, and then the mixture was ground to a fine powder with a diameter less than 2 μm under an infared lamp environment; afterward, these powder was pelletted with 10 MPa pressure for 1 min, and finally the pellet was analyzed in the FTIR analyzer. The instrumental parameters settings were as follows: resolution 2 cm1, speed 10 kHZ, scans to coadd 32, and aperture source 6504000 cm1. Before each sample measurement, the background spectrum was measured in order to reduce interference. Thus, the FTIR analysis results were based on the pellet sample spectrum minus the the background spectrum. Mineral contents at different temperature conditions produced char that were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES).
3. RESULTS AND DISCUSSION 3.1. Nascent Tar Formation during Pyrolysis in the First Stage. 3.1.1. Temperature Effects on Pyrolysis Yields. Rice straw
pyrolysis experiments were operated in the first stage reactor with temperatures ranging from 200 to 700 °C and a heating rate of 30 K/min. A high nitrogen flow rate of 350 mL/min was selected as the carrier gas in order to minimize the secondary reactions between pyrolysis char and tars. The mass of nascent tar and char yields were weighed, and water was measured in the tests, while gas yields were achieved by mass balance of the original rice straw and other products. As shown in Figure 3, with pyrolysis temperature increasing, gas and nascent tar increased monotonically and char yield decreased but water increased from 7.4% at 200 °C to a maximum value of 27.2% at 400 °C and then decreased to 22% at 700 °C, and it is indicated that water is one of the main products in biomass pyrolysis yields. Water yields in pyrolysis may mainly be contributed by dehydration of hydroxyl and decomposition of hydrocarbons. There are low nascent tar yields and almost no noncondensable gases yields at 200 °C. Pyrolysis nascent tar reached a maximum yield value of 19.3% at 700 °C. Tar increased monotonically showed that nascent tar was soon swept from the pyrolysis unit reactor, and the destruction effects by secondary reactions were inhibited. The volatiles were mainly produced within 200400 °C, and extra volatiles obtained would be decreased as the temperature exceeded 500 °C. It can be explained that the biomass mainly consists of cellulose, hemicellulose, and lignin. In these components, cellulose is decomposed around 360 °C, and hemicellulose will mainly decompose between 200 and 260 °C, while lignin has the highest 5397
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Figure 4. Effect of particle size on pyrolysis yields at 500 °C.
Figure 5. Rice straw char FTIR spectra under different temperature conditions.
temperature from 280 to 500 °C.28 A temperature of 400500 °C is considered to be an economic value for biomass pyrolysis in the two-stage gasifier. This result is also confirmed by Chen et al.,14 who suggested the economic temperature of 440 °C for the pyrolysis stage. 3.1.2. Effect of Particle Size. The effects of different particle sizes as 15001875 μm, 9401070 μm, 420500 μm, and 100150 μm have been conducted with a pyrolysis temperature of 500 °C. Figure 4 shows the particle size effects on pyrolysis products distribution. It can be observed that the char and gas yield increases and the tar decreases with the increasing of particle size. However, the effects of changing the particle size are not obvious in varying the products distribution. Similar tendencies were obtained in the fixed bed by pyrolysis of other biomass such as olive husk, corncob, and rice husk.29,30 The effects of particle size can be attributed to the variation of the heat and mass transfer capability, and the temperature gradient will increase along the radius of particles as the particle diameter is increased. Small diameter particles have a bigger area/volume ratio, and the larger surface area can interact with the volatiles and form final products that are separated out from the char matrix without secondary cracking reactions.31 However, it is considered that heat and mass transfer limitations are more significant effects in the case of larger diameter particles than the small ones. Different
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from Gaston et al.’s conditions,32 in this study, high carrier gas flow rate (with a residence time of 0.120.24 s corresponding to a pyrolysis temperature of 700200 °C) inhibits the secondary reactions obviously. The results show that the particle size parameter exerts a less important influence than temperature, and the pyrolysis reactions are still controlled by chemical kinetics. 3.1.3. Chemical Structure of Rice Straw Char. The chemical structure of rice straw char produced under different temperature conditions were analyzed by using the FTIR spectrometer. Figure 5 shows the char FTIR spectra under temperatures from 200 to 1000 °C, and Table 2 illustrates some of the typical corresponding functional groups and the possible compounds in the IR assignments. By comparison of the possible functional groups or structures of different IR bands in Table 2, it can be observed from Figure 5 that the IR bands of 2927 cm1, 3431 cm1, 1129 cm1, and 1054 cm1 in rice straw char likely consisted of typical functional groups or structures such as alkyl aromatics (30002850, 900700 cm1), hydroxyl group OH (37003000 cm1), aldehyde group CdO (17151700 cm1), aliphatic ether/alcohol group CO (11601030 cm1), etc. As shown in Table 2, the absorption at 30002850 cm1 corresponds to the CHx stretching vibration of aliphatic carbon, and the transmittance intensity with a band at 2927 cm1 shows CH2 stretching, which was significantly decreased with increasing the char pyrolysis temperature, and this structure completely disappeared when the rice straw char pyrolysis temperature exceeds 500 °C. However, with increased temperature, CdC bond in the aromatic rings stretching represented by the transmittance peak of 16001570 cm1 has been strengthened, which is explained as enhancement of aromatization effects. The appearance of the bands in the range of 17001715 cm1 indicates that CdO vibration, which is also decreased with temperature increasing, and this phenomenon can be illustrated as carbonyl group cracking into CO2. In all, aliphatic hydrocarbon and oxygen-containing functional groups in biomass char decreased with the temperature increasing; in contrary, the aromatization process (900800 cm1 represents the stretching of aromatic CH groups) increased gradually. 3.1.4. Minerals. Minerals in rice straw char have a capability of catalyzing on tar decomposition, above all, the alkali and alkaline earth metals (AAEM) species, and the catalytic effect was enhanced by the AAEM concentration when char was consumed on the stream. However, some AAEM will migrate with temperature increasing, and the migration property is one of the key factors in AAEM catalytic destruction of tar in char bed conditions. Table 3 shows seven minerals content in rice straw char, which were produced in the pyrolysis (300700 °C) and gasification (8001000 °C) stages of the two-stage reactor, respectively. In pyrolysis char, the results illustrated that with the temperature increasing, the concentration of AAEM species as Ca, K, Na, and Mg content in the pyrolysis char yield increased first and reached a maximum content value around 500600 °C and then decreased with temperature continuing to increasing, but the concentration of other species such as Al, Fe, and Si increased monotonically. Moreover, K is one of the most important minerals in rice straw char, which contains up to 6.522 wt %, while others are mainly below 1 wt % except Ca of 1.106 wt % at 500 °C. Some alkali and alkali earth metals (especially organic AAEM compounds species) will be volatilized when the pyrolysis temperature increases and even formed alkali aerosol with soot and tar.36,37 Thus, devolatilization of the AAEM species and 5398
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Table 2. Typical Corresponding Functional Groups and the Possible Compounds in IR Assignments bands (cm1)
functional groups
compounds
ref
36503200
OH stretching
acid, methanol, phenol
Yang,33,34 Ascough35
30002850
CHx stretching
alkyl, aliphatic
Yang,33,34 Ascough35
2930
-CH2 stretching
alkyl, aliphatic
Yang33
17151700
CdO stretching
aromatic carbonyl/carboxyl(COOH)
Yang,33,34 Ascough35
1632
CdC
benzene stretching ring
Yang33,34
16001570
CdC stretching
aromatic ring stretching
Ascough35
1416
OH bending
acid
Yang33,34
1405 11601030
CH bending CO/CO stretching
aliphatic ether/alcohol
Yang33,34 Yang,33,34 Ascough35
900800
CH
aromatic CH deformation
Ascough35
Table 3. Minerals Content in Pyrolysis Char and Gasification Char of Rice Straw pyrolysis char content (wt %)
gasification char content (wt %)
minerals
300 °C
400 °C
500 °C
600 °C
700 °C
800 °C
900 °C
1000 °C
Al Ca
0.08 0.608
0.103 0.835
0.169 1.106
0.168 1.152
0.171 0.983
0.141 0.969
0.14 1.067
0.127 0.906
Fe
0.552
0.737
0.964
1.098
1.125
1.087
1.06
1.152
K
3.01
4.544
6.522
5.694
5.166
5.25
5.285
5.237
Mg
0.262
0.391
0.553
0.562
0.497
0.378
0.373
0.35
Na
0.331
0.457
0.584
0.523
0.461
0.574
0.513
0.524
Si
0.128
0.159
0.169
0.215
0.219
0.21
0.203
0.174
Figure 6. Gravimetric tar yields versus different decomposition conditions.
volatiles (tar and noncondensable gas) are the two main mass losses in varying mineral content in pyrolysis char. However, the mass percentage of AAEM species decreased little from 800 to 1000 °C in the gasification char (gasified in the second stage) content, and the gasification reactions induced the char conversion efficiency range of 515 wt % based on the mass of pyrolysis char at 500 °C. This phenomenon is likely due to the AAEM migration rate being close to the char consumed rate on the stream as temperature increased from 800 to 1000 °C. 3.2. Nascent Tar Destruction in the Second Stage. 3.2.1. Effects of Temperature. Properties of nascent pyrolysis tar destruction in different decomposition conditions were carried out in the second stage of the fixed bed reactor. Figure 6 shows the
effects of each condition in detail. With the inclusion of thermal cracking in different lengths of the second stage reactor with 40 cm (long) or 20 cm (short), the diluted oxygen (1% O2 was used) with 1 L/min in oxidation, the water feeding rate of 0.35 mL/min and with a preheated temperature of 400 °C in steam reforming, and the 0.5 g of char (rice straw material with a diameter of 150 400 μm pyrolyzed at 500 °C) were investigated with a second stage temperature of 7001000 °C for tar destruction. As illustrated in Figure 6, the char beds have the best gravimetric tar destruction capability in all of the homogeneous/ heterogeneous conditions tested. With the temperature increasing, the mass of gravimetric tar decreases but varies in the conversion rate. In a shorter thermal cracking reactor, the tar conversion efficiency is about 23.4% at 700 °C. However, the effects of different conditions as steam reforming, diluted oxygen, and longer thermal cracking time are all with a conversion rate around 60%, while the char bed obtained a conversion rate of 78% at 700 °C. As the temperature increased, the difference between tar yields in each method reduced and the tar destruction efficiency increased. The char bed even reached 96% at 1000 °C, and other methods also exceeded 90% at a temperature of 1000 °C. It can be explained that few gravimetric tar species can exist in such high-temperature conditions. An increasing concentration of reforming agents such as H2O and O2 or increasing residence time can enhance the tar destruction. The presence of H2O or diluted O2 will increase the reaction of tar steam reforming or tar oxidation, while decreases in the residence time will reduce the opportunity for tar molecules to be reformed or oxidized in the high temperature. 3.2.2. Effects of Concentration. Compared to the temperature, the concentration of reforming agent is also an important effect 5399
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Table 4. Tar Yields in CO2/H2O Reforming and Char Bed conversion types
second stage temperature
throat nozzles
second stage
nascent tar remains
gravimetric tar yields
total tar yields
conversion
(°C)
(mL/mina)
char (g)
(mg/gb)
(mg/gb)
(mg/gb)
efficiency (%c)
TCd
900
0
0
7.5
19.1
26.6
88.0
CO2
815
10
0
4.8
21.6
26.4
86.7
CO2
815
30
0
5.9
7.8
13.7
95.2
CO2
815
100
0
8.2
4.5
12.7
97.2
H2O
900
0.21
0
2.4
7.4
9.8
95.5
H2O
900
0.35
0
2.5
6.8
9.3
95.9
H2O H2O
900 900
0.7 1.05
0 0
2.3 2.5
6.6 6.7
8.9 9.2
96 95.9
char bede
900
0
0.5
6.9
6.3
13.2
96.1
char bed
900
0
1
8.5
5.9
14.4
96.3
char bed
900
0
2
11.8
4.7
16.5
97.0
H2O + char
900
0.35
0.5
5.7
5.6
11.3
96.5
a
H2O reforming throat nozzles feeding rate is scaled in liquid water. b Unit mean gravimetric tar yields per gram of rice straw. c On the basis of consistent tar yields of rice straw pyrolysis at 500 °C. d Thermal cracking in a shorter second stage reactor. e Rice straw char produced at 500 °C. First stage: rice straw pyrolysis at 500 °C.
in tar elimination. Tar yields in different CO2, H2O concentrations and char mass have been conducted in the second stage reactor. The CO2 feeding rates of 10, 30, and 100 mL/min, deionized water feeding rates of 0.21, 0.35, 0.7, and 1.05 mL/min, while char masses of 0.5, 1, and 2 g were studied, respectively. The tar conversion efficiency is described as ηtar ¼
morigin mremains mout 100% morigin mremains
ð1Þ
where morigin represents of total nascent pyrolysis tar yields at 500 °C, mremains represents tar left on the pyrolysis char in the first stage reactor, while mout is the gravimetric tar collected after the second stage outlet. It can be observed that the nascent tar remains increased in small amounts with increasing of the CO2 flow rates or the char bed heights. Addition of gas or char bed blockage increases the reactor pressure and induces more nascent tar adsorbed on the pyrolysis char. The results indicated that thermal cracking has the lowest tar conversion efficiency of 88% in all 900 °C conditions, and with the CO2 feeding rate increasing from 10 to 100 mL/min at 815 °C, the gravimetric tar yields decreased noticeably from 21.6 to 4.5 mg/g rice straw. Nascent tar remains in the first stage increased a little, and the total tar yields decreased with increasing CO2 concentration. Steam reforming has good performance on tar destruction. All tar conversion efficiency are high up to 95%, and tar yields vary little as the steam feeding rate continues increasing in the experiments. It is considered that excessive steam was injected in the reforming condition, and the reaction rate of the hydrocarbon is independent of the steam partial pressure when the steam/ hydrocarbon ratio (S/C) is higher than the stoichiometric ratio. Simell et al.38 obtained similar conclusions as he studied steam reforming of benzene. As shown in Table 4, tar yields after the second stage decreased from 6.3 to 4.7 mg/g rice straw with char increasing from 0.5 to 2 g. When H2O and char bed were both present, the tar conversion efficiency increased to 96.5% from 96.1% with the char bed only or 95.9% with H2O only. This result could be concluded that integrating H2O with the char bed is a more effective method in tar destruction than independently. The effects of tar eliminated
on rice straw pyrolysis char can be concluded into two main parts: One is high alkali and alkali earth metals (AAEM) content in rice straw char as illustrated in Table 3, which are the effective catalysts on tar destruction;39 the other is a large amount of micropore and mesopore in the char particles, and this porous characteristic can promote aromatics adsorption capability.40,41 The reactions in the second stage mainly include (a) tar destruction nascenttar þ CO2 =H2 O=char f tertiarytar þ CO þ H2 þ CO2 þ H2 O þ Cn Hm þ CH4
ð2Þ
(b) gas reforming CH4 þ H2 O T CO þ 3H2 ΔH θ273K ¼ 206 kJ=mol
ð3Þ
CO þ H2 O T CO2 þ H2 ΔH θ273K ¼ 41 kJ=mol
ð4Þ
(c) Char gasification (char present): C þ CO2 f 2CO
θ ΔH273K ¼ 172 kJ=mol
ð5Þ
C þ H2 O f CO þ H2 ΔH θ273K ¼ 131 kJ=mol
ð6Þ
The tertiary tar in reaction 2 implies the total tar yields after different decomposition conditions. As in reaction 2, besides tertiary tar, noncondensable gases such as CO, CO2, H2O, CH4, CnHm, and H2 are also yielded. These products are the important syngas components, and its formation will increase the conversion efficiency of the gasifier. Reforming effects of CO2 on tar evolution is likely due to the OH radicals produced from CO2 during the thermal conversion of a syngas. A higher concentration of OH radicals enhances the oxidation process of tar.42 H2O can enhance nascent pyrolysis tar reforming and inhibit the polymerization reactions due to more hydrogen produced by reactions 3, 4, and 5. We can observe that the presence of CO2 or 5400
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Figure 7. GC/MS chromatogram of rice straw tar samples versus different conditions: (A) sample 1, pyrolysis at 500 °C; (B) sample 2, thermal cracking (with 40 cm) at 900 °C; (C) sample 3, CO2 reforming of 30 mL/min at 815 °C; (D) sample 4, H2O reforming of 0.7 mL/min at 900 °C; (E) sample 5, diluted 1% O2 oxidation at 900 °C; (F) sample 6, char gasification at 900 °C.
H2O agents also effects the formation of other noncondensable gases. However, gases in the outlet of gasifier are in a relatively
stable composition, which can be attributed to these reversible reactions 3 and 4. In a fixed bed gasification zone, char heterogeneous 5401
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Table 5. Identified Tar Components in Five Samples relative concentration(%) RT (min)
MW
formula
compound names
2
2.54
78
C6H6
benzene
3.77
79
C5H5N
pyridine
4.19
92
C7H8
toluene
15.733
12.158
7.21
106
C8H10
ethylbenzene
0.636
1.964
7.50
106
C8H10
p-xylene
2.222
2.798
7.78
102
C8H6
phenylethyne
1.780
8.28
104
C8H8
styrene
7.657
7.258
8.35 11.70
106 94
C8H10 C6H6O
o-xylene phenol
6.373
1.221 8.205
12.06
120
C9H12
benzene, 1-ethenyl-3-methyl-
0.906
3.489
12.12
118
C8H6O
benzofuran
1.706
3.966
12.19
118
C9H10
benzene, 2-propenyl-
0.414
13.76
116
C9H8
indene
7.870
5.214
14.20
108
C7H8O
phenol, 2-methyl-/ 4-methyl-
0.386
2.378
3.155
14.40
136
C8H8O2
acetic acid, phenyl ester
1.207
17.17 17.82
130 122
C10H10 C8H10O
naphthalene,1,2-dihydrophenol, 3-ethyl-
2.052
1.682
1.291
18.23
128
C10H8
naphthalene
12.702
19.84
129
C9H7N
quinoline
4
5
6
24.090
12.541
36.996
9.841
3.308
1.563
1.334
21.44
142
C11H10
naphthalene, 1-methyl-
4.777
25.03
154
C12H10
biphenyl
0.662
25.53
152
C12H8
biphenylene
1.737 16.353
22.113
48.020
10.570
11.286
3.215
6.968
9.582
7.381
5.794
3.209
3.038 8.262
4.148
1.552
3.167
0.993
0.665
1.039 3.527
3.890
6.090
1.758
13.585
14.180
7.438
8.561
2.779
5.862
1.787
0.669
1.601
3.304
2.761
3.672
1.772
11.190
0.656
27.09
206
C14H22O
phenol, 2,4-bis(1,1-dimethylethyl)-
0.783
2.897
28.77 32.94
166 178
C13H10 C14H10
fluorene anthracene/phenanthrene
1.452 2.177
1.632 4.170
1.403 5.829
34.41
192
C15H12
anthracene, 2-methyl-
0.153
0.300
35.76
202
C16H10
fluoranthene
0.317
0.241
36.13
202
C16H10
pyrene
0.625
0.946
0.434
66.56
53.75
74.91
percentage of five typical tar in total tar contenta a
3
0.934
53.45
65.73
Percentage of five typical tar contents including benzene, toluene, phenol, styrene, and naphthalene in the total tar yields.
gasification is the dominant endothermic reaction, which are of benefit to the char conversion efficiency. However, these endothermic reactions may also induce the temperature of char bed dropping to below 700 °C in the gasifier.43 Without sufficient energy taking along from the throat, the proceeding of gasification reactions will be inhibited. 3.3. GC/MS Analysis of Tar Components. GC/MS have been used to characterize the tar components yield after the first or second stage reactor. Tar evolution properties are investigated in terms of quantitative analysis of several typical tar species which are in relatively high concentrations or important conversion byproducts in the tar evolution process. Figure 7 shows the GC/MS chromatogram of tar samples 16: (sample 1) nascent pyrolysis tar yields at 500 °C in the first stage; (sample 2) tar yields from outlet of empty second stage (with 40 cm at 900 °C); (sample 3) tar from outlet of second stage with CO2 inlet through throat (at 815 °C and CO2 feeding rate of 30 mL/min); (sample 4) tar from outlet of second stage with H2O inlet through throat (at 900 °C and water feeding rate of 0.7 mL/min); (sample 5) tar from outlet of second stage with diluted oxygen inlet through throat (at 900 °C and with 1% O2 feeding rate of 1000 mL/min); (sample 6) tar from outlet of second stage with char bed (at 900 °C and 0.5 g of char pyrolyzed from 150 to 400 μm rice straw
at 500 °C). Figure 7 is the total ion chromatograms, and Table 5 is the identified tar components at the outlet of the second stage versus different conditions. As shown in Figure 7, in the total ion chromatograms of the six samples, the abundance value on the vertical axis represents the relative concentration of tar components. It is indicated that the number of peaks declined and the relative compounds concentration varied in the chromatograms with the presence of different agents in the second stage. As observed in Figure 7A, nascent rice straw pyrolysis tar consists of hundreds of species, which are separated and detected by gas chromatography/mass spectrometry (GC/MS), separately. The main species of nascent pyrolysis tar are detected before a residence time of 25 min, which shows stronger polarity and lower molecular weight characteristics in these compounds. The analytical results indicate that oxygenates or substituted one ring aromatics are the majority components in nascent rice straw pyrolysis tar yields at 500 °C, such as phenols, alcohols, furfural, benzofuran, etc. These tars are in high activity and easy to break down into small molecular gases in severe thermal conditions. In other chromatograms, the tar species are relatively consistent but the concentration varied obviously. Large amounts of benzene, toluene, and naphthalene were generated in thermal cracking at 900 °C (Figure 7B). However, fewer 5402
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Energy & Fuels components were detected in H2O reforming and char gasification. Moreover, PAH species except naphthalene completely disappeared after the char heterogeneous gasification process. This phenomenon is also confirmed by Brandt et al.6 Other decomposition methods such as CO2 dry reforming and diluted O2 partial oxidation can reduce parts of nascent tar and convert them into thermally stable components as toluene, naphthalene, and styrene. Because of the corrosion, condensation, and deposition effects of PAHs in syngas utilization, it is of great importance to reduce the PAHs content in the syngas, and especially these troublesome tars were decomposed inside the gasifier by pyrolyzed biomass char. In the two-stage gasifier, homogeneous partial oxidation and heterogeneous char bed conversion are the two key factors to ensure that low tar syngas is obtained. The selectively properties of rice straw char in eliminating PAHs will greatly increase the application prospects of the gasifier. Table 5 shows the relative concentration of the main identified tar components in samples 26. Because of the complexity of tar components, not all the compounds are listed in the table, especially those which are at a low concentration. Meanwhile, because the component is quite different from the decomposed tars, the nascent pyrolysis tar species are not listed in this table. Samples 26 are the tar species yielded from nascent pyrolysis tar under different decomposition conditions. The tar in samples 26 mainly consist of tertiary tar and relatively thermally stable species, which are obviously different from nascent pyrolysis tar. It can be considered that most nascent tars are converted into tertiary tar species as the temperature increases and reforming agent/oxidant is present. As shown in Table 5, although different decomposition condition induce different tar species yield, five representative tar compounds such as benzene, toluene, phenol, styrene, and naphthalene, account for 5075% of total tar yields in samples 26. Indene, ethylbenzene, 1-methyl-naphthalene, p-xylene, biphenylene, and anthracene are the relatively important components besides the five typical tar species, such as ethylbenzene at concentrations of 10.57% and 11.29% in diluted O2 and char bed conversion conditions, respectively. Benzene is the most important products in H2O present with a relative concentration of 37% but only 3.31% in char bed condition. Thermal cracking, CO2/H2O reforming are more likely to enhance the PAHs as naphthalene and anthracene formation, and the presence of these two and three ring aromatics implies that polymerization reactions take place. The char bed can completely convert high-molecular weight PAHs except naphthalene. However, relatively high toluene concentration (about 48.02%) still remains in the tar yields. It can be considered that the toluene may be induced by PAHs destruction or byproducts of char contained tar evolution reactions. S. M. Nunes’s15 investigation results also confirmed that the char beds have the least peak number in GC/MS chromatograms. 3.4. Typical Tar Quantitative Analysis. The sensitivity of the GC/MS to different compounds varies, and GC peak area (responses) percentages cannot exactly reflect the relative concentration of tars in solvent. As shown in Figure.8, benzene and naphthalene can induce higher responses in solvent with lower concentrations. However, phenol with ahigher concentration in solvent has a lower response in the GC chromatogram. In order to more accurately evaluate the tar evolution properties, quantitative analysis of these typical tar species is essential. Figures 913 shows the quantitative analysis of five typical tar evolution profiles in different decomposition conditions.
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Figure 8. Five typical tar calibration curves in GC/MS.
Figure 9. Tar evolution profiles versus pyrolysis temperature.
Figure 10. Tar evolution profiles versus thermal cracking temperature.
As shown in Figure 9, nascent tar yields increased with pyrolysis temperature increasing. Benzene, toluene, and styrene obtained a maximum value at 500 °C, while phenol increased until 600 °C and then decreased. Naphthalene started to generate at 600 °C and increased with the temperature increasing. Phenol is the dominant compound in nascent pyrolysis tar, while benzene and naphthalene are in low concentration in nascent 5403
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Figure 11. Tar evolution profiles versus different CO2 feeding rates at 815 °C.
Figure 12. Tar evolution profiles versus different H2O(l) feeding rates.
pyrolysis tar. However, the five typical tar only consists of 914% in relative concentration of total nascent tar with pyrolysis temperatures of 300700 °C. The thermal cracking temperature in the second stage (short) affects tar conversion remarkably. As shown in Figure 10, with temperature increasing, all these typical tars increased and reached a maximum value at 1000 °C except phenol, which yields a maximum value of 3.86 mg/g rice straw at 800 °C and then decreased and was completely destructed at 1100 °C. Benzene is one of the main components under thermal cracking of nascent tar. When temperature increased from 1000 to 1100 °C, all yields decreased significantly which may indicate soot formation in this period. These results are confirmed by Jess,44 and he considered that soot should be produced when naphthalene polymerized in high thermal cracking temperatures. Furthermore, soot and other organic cracking yields primarily react with H2O in consecutive reactions as H2O is present. Homogeneous reforming with CO2 feeding rates of 10, 30, and 100 mL/min have been investigated in the second stage reactor at 815 °C. Tar evolution properties in Figure 11 show that with increasing the CO2 feeding rate, all five typical tar yields decreased linearly. Benzene and toluene were almost completely destructed with 100 mL/min CO2 injection. It shows that a high
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Figure 13. Tar evolution profiles versus different char bed temperatures.
CO2 concentration can remarkably decompose the hydrocarbons. The reforming mechanism is due to more OH radicals being produced as increasing the CO2 concentration, and OH radicals can give rise to a faster oxidation of tars.13 Figure 12 illustrates the effects of steam feeding on typical tar formation. Each tar yields content decreased slowly with increasing steam concentrations. The effect on tar conversion properties of H2O is similar as CO2, which can accelerate tar destruction and yield of noncondensable gases. Taralas et al.45 investigated the steam reforming of n-heptane as a model tar. They observed that H2 and CO2 increases and C2C7 hydrocarbons decrease selectivity with increasing the H2O content (up to 34.2 kPa). The following are some typical reactions of model tar under homogeneous CO2/H2O reforming conditions published in the literature, with benzene, toluene, phenol, and naphthalene as the model tar and are illustrated in Table 6. We can observe from reaction eqs 717 that different reaction paths were obtained in the same tar species. Such as benzene in dry reforming condition, excess CO2 content in the environment will cause H2O formation rather than H2. However, toluene will produce more H2 as CO2 concentration increasing in reactions 9 and 10. In the steam reforming conditions, toluene conversion properties are more complicated in eqs 1315 with yields of CO, CO2, or C6H6, respectively. The results in eqs 16 and 17 showed that different products were obtained in phenol steam reforming under the same reactants and different temperature conditions. It can be explained that besides reactants, temperature is also an important factor that affects the tar conversion paths. A higher temperature will induce more free radicals as O, OH, and H yields, which can promote the tar decomposition dramatically. Effects of the char bed temperature in 0.5 g of rice straw char is presented in Figure 13. The results show that yields of four typical tar species decreased with temperature increasing except naphthalene, which increases monotonically and then decreases at temperatures higher than 900 °C. Toluene and phenol are the two main components. Compared to thermal cracking, benzene, styrene, phenol, and naphthalene decrease dramatically in the char bed. Benzene almost completely decomposed, and high benzene conversion efficiency was obtained with rice straw char. However, more toluene is produced when the char bed is present under the same temperature as thermal cracking conditions. This quantitative analysis result suggested that toluene is a byproduct of tar evolution under char bed reactions. 5404
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Table 6. Possible Reactions of Model Tar in Different Reforming Conditions reaction equations
agent
temperature (°C)
ref
C6 H6 þ 6CO2 ¼ 12CO þ 3H2
ð7Þ
CO2
750900
Simell38
C6 H6 þ 9CO2 ¼ 15CO þ 3H2 O
ð8Þ
CO2
750900
Simell38
C7 H8 þ 7CO2 ¼ 14CO þ 4H2
ð9Þ
CO2
900
Simell46
C7 H8 þ CO2 ¼ 18CO þ 4H2 O
ð10Þ
CO2
900
Simell46
C10 H8 þ 10H2 O ¼ 10CO þ 14H2
ð11Þ
H2O
750900
El-Rub47
C6 H6 þ 2H2 O ¼ 1:5CðsÞ þ 2CO þ 2:5CH4
ð12Þ
H2O
7001400
Jess44
C7 H8 þ 7H2 O ¼ 7CO þ 11H2
ð13Þ
H2O
800
Swierczynski48
C7 H8 þ 14H2 O ¼ 7CO2 þ 18H2
ð14Þ
H2O
800
Swierczynski48
2C7 H8 þ 3H2 O ¼ 2C6 H6 þ CO2 þ 5H2 þ CO
ð15Þ
H2O
650950
Taralas49
C6 H6 O þ 3H2 O ¼ 4CO þ 2CH4 þ 2H2
ð16Þ
H2O
7001400
Jess44
C6 H6 O þ 3H2 O ¼ 2CO þ CO2 þ 2:95CH4 þ 0:05CðsÞ þ 0:1H2 ð17Þ
H2O
5001000
Morf50
Figure 14. Tar evolution profiles versus different conversion conditions.
Figure 14 presents typical tar evolution profiles versus different decomposition conditions. The conditions for samples 16 are the same as given in Figure 7. It can be observed that these tars are the main tertiary tar yields in different homogeneous/ heterogeneous decompositions except pyrolysis. Benzene is more likely to be produced in large quantities under thermal cracking and H2O reforming conditions. Toluene is preferred with the char bed present. However, little benzene is produced in char bed conditions at 900 °C. A large quantity of phenol decreased in all conversion styles due to high temperature in the secondary stage reactor, and phenol was completed converted in H2O present. Styrene and naphthalene were yielded in relatively stable concentrations in these conversions.
4. CONCLUSIONS Nascent pyrolysis tar evolution properties are of great importance in improving tar elimination efficiency. A two-stage fixedbed reactor has been constructed to investigate the nascent tar formation during rice straw pyrolysis in the first stage and its destruction characteristics in homogeneous/heterogeneous decomposition conditions in the second stage. The results in the first stage reactor show that the pyrolysis temperature greatly
affected nascent tar formation. The mass of gravimetric tar will increase with the temperature increasing, and 200400 °C is the main devolatilization period. Increasing the particle size will decrease the nascent tar yields and increase the char percentage. Phenol is an important nascent tar component and other tar such as benzene, toluene, and styrene will decrease when the temperature is higher 500 °C. Little benzene is yielded in pyrolysis tar, and naphthalene is only yielded with temperatures higher than 500 °C. Nascent tar is converted into PAHs, thermally stable one ring aromatics, and noncondensable gases with increasing thermal decomposition temperatures. Toluene, benzene, styrene, phenol, and naphthalene are the most typical tar species accounting for 5075% of the total tar concentration with different decomposition conditions at 900 °C. More toluene is yielded as rice straw char is present, and the char bed can dramatically destruct the polycyclic aromatic hydrocarbons as anthracene and pyrene. It is extremely important for biomass gasification due to the high content of the PAHs and are the main obstacles in syngas utilization. Although homogeneous CO2/H2O reforming reactions are endothermic, increasing the concentration of CO2/ H2O agents will lead to a higher content of free radicals such as OH, H formation, which will accelerate the oxidation of tars. A high tar conversion efficiency of steam reforming shows its great potential in tar destruction. In all, it can be concluded that the mechanism of tar evolution in the two-stage downdraft gasifier as nascent pyrolysis tar is first yielded from the pyrolysis stage and then was homogeneous reformed/oxidized into PAHs, thermally stable one ring aromatics, and noncondensable gases at the throat region by different reforming agents/oxidants. Afterward, the char bed almost completely decomposed the PAHs (except naphthalene), which were the main troublesome tar species in syngas. The char bed is affected less by thermally stable one ring aromatics such as styrene and toluene, and these are the more acceptable species in syngas utilization.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 5405
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