Effect of Hot Vapor Filter Temperature on Mass Yield, Energy Balance

Oct 31, 2016 - The fast pyrolysis in a fluidized-bed reactor using pine sawdust has been studied at 500 °C with ceramic hot vapor filter (HVF). The e...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/EF

Effect of Hot Vapor Filter Temperature on Mass Yield, Energy Balance, and Properties of Products of the Fast Pyrolysis of Pine Sawdust Yuanfei Mei,†,‡,§ Ronghou Liu,*,†,‡,§ Weixuan Wu,†,§ and Le Zhang†,§ †

Biomass Energy Engineering Research Centre, School of Agriculture and Biology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China ‡ School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China § Key Laboratory of Urban Agriculture (South), Ministry of Agriculture, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China ABSTRACT: The fast pyrolysis in a fluidized-bed reactor using pine sawdust has been studied at 500 °C with ceramic hot vapor filter (HVF). The effects of HVF temperature (350, 400, 450, and 500 °C) on mass balance, energy balance, and physical properties of pyrolysis products were investigated. The results were reported that the maximum yields of bio-oil and organic liquid were 58.7 and 41.6 wt % at HVF temperatures of 350 and 400 °C, respectively. When HVF temperature increased from 350 °C to 500 °C, the maximum biochar yield was 25.3 wt % at 350 °C, while the minimum noncondensable gas (NCG) yield was 19.2 wt % at 400 °C. The energy consumption of pine sawdust pyrolysis was calculated as 2.38 MJ/kg. The optimized HVF temperature for energy balance was 450 °C. Under these conditions, the potentially recovered energy of bio-oil and total pyrolysis products reached maximum values of 7.49 MJ/kg and 17.91 MJ/kg, respectively. Maximum energy recovery ratios of total pyrolysis products and bio-oil were 0.92 and 0.39, respectively. The minimum energy consumption ratio and maximum theoretical energy efficiency were calculated as 0.64 and 67.62%, respectively. The results indicated that the HVF had a significant positive effect on improving the stability and quality of bio-oil. The molar content of NCG was affected by vapor secondary cracking reactions.

1. INTRODUCTION Biomass fast pyrolysis is a viable technology to transform mixed biomass feedstock into bio-oil for direct production of substituent liquid fuels.1 Bio-oil currently showed tremendous latent capacity to be used as substitute energy in the 21st century.2 Bio-oil is generally dark-colored, acidic, viscous, unstable, and has high moisture. Bio-oil can be derived from different raw material, including waste biomass (e.g., forestry residues, sawdust, bark, crop straw, and others). The increased fuel energy density, compared to raw biomass from pyrolysis, provides attractive options to the bio-oil producers. The physicochemical characterization of bio-oil is normally based on parameters such as raw material, reactor design, reaction conditions (including media size, temperature, pressure, and residence time), and separation unit.3 Bio-oil is thermal unstabile and contains hundreds of organic compounds, which cause the bio-oil to be unstable during storage. Biomass pyrolysis bio-oil has shown an increase in viscosity during storage under ambient conditions, especially when it is heated above ambient temperature.4 The instability of bio-oil is due to the high oxygen content, water content, and solid particles of bio-oil, which lowers the energy density.1 This result is a significant barrier to commercial-scale application of pyrolysis technologies. Several approaches to stabilize bio-oil have been investigated. The first method is the addition of solvent (such as methanol, ethanol, etc.) to the bio-oil. The addition of these solvents © 2016 American Chemical Society

dilutes the concentration of reactive compounds in bio-oil to slow the rate of aging.5,6 Furthermore, the physicochemical characterization of bio-oil could be likely achieved by upgrading techniques such as hot vapor filter (HVF). Recent literature has shown that the filtration of hot pyrolysis vapor can lower initial molecular weight of bio-oil and improve its stability.7,8 Recently, the majority of studies in HVF of the biomass fast pyrolysis have focused on upgrading bio-oils with different process to reduce the solid content of bio-oil by char and inorganic particles removal from the condensed liquid phase, since these particles were thought to have provided nucleation sites for polymerization or to catalyze polymerization or gasforming reactions.9 The HVF could influence the bio-oil yields10 and change the composition of bio-oil, which may contribute to bio-oil stabilization.11 HVF could play a large role in stabilizing the bio-oils. Thus, HVF has been developed to be a new and effective method to highly improve bio-oil quality. Some researchers have evaluated the effects of HVF on the pyrolysis process parameters (i.e., the type of reactor, pyrolysis temperature, biomass particle size, biomass feedstocks, filter elements, flow rate of feed and solid residence time).3,12 However, little research has focused on the effect of different HVF temperature gradients on mass yield, energy balance, and Received: July 29, 2016 Revised: October 27, 2016 Published: October 31, 2016 10458

DOI: 10.1021/acs.energyfuels.6b01877 Energy Fuels 2016, 30, 10458−10469

Article

Energy & Fuels physical properties of pyrolysis products by pine sawdust in a fluidized-bed reactor in the physical stabilization of bio-oil. The objectives of present research were to study the effect of HVF temperature from pine sawdust pyrolysis on the quality of bio-oil and to analyze the physical characterization of pyrolysis products such as bio-oil, noncondensable gas (NCG), and biochar. Meanwhile, the distribution of mass balance and energy balance such as potentially recovered energy (PRE), energy recovery ratio (ERR), energy consumption ratio (ECR), theoretical energy efficiency of pine sawdust pyrolysis in a fluidized-bed reactor with HVF temperatures increased from 350 °C to 500 °C were also studied to investigate the mass balnce and energy changes of the bio-oil under various HVF temperature conditions. Finally, the optimized HVF temperatures for mass balance and energy balance were discussed to prove the suitability of a fludized-bed reactor with integrated HVF for upgraded products of biomass pyrolysis in the energy field.

Table 1. Physicochemical Characterization of Pine Sawdust item

2. MATERIALS AND METHODS 2.1. Biomass Feedstock. Pine sawdust was used as biomass feedstock obtained from a wood-working factory in Minhang District of Shanghai, China. The biomass material was crushed by a withoutsifter forage grinder and then knife-milled and sieved from different mesh through a 40 screen mesh, according to ASTM E11. Particle size was 250−400 μm (D50:350 μm). The feedstock pine sawdust was used for fast pyrolysis. Elemental analysis containing C, H, N, and S of feedstock was measured by a Vario EL Cube Elemental Analyzer, and O content was investigated by the formula O (wt %) = 100 − (C + H + N + S + ash). Inductively coupled plasma (ICP) analysis was tested by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Thermo iCAP6300, using the JY/T015-1996 method. Composition analysis of hemicellulose, cellulose, and lignin was referenced to the biomass analytical protocol of the National Renewable Energy Laboratory (NREL). Proximate analysis including moisture, volatile matter content, and ash content was analyzed by standards ASTM E871, ASTM E872, and ASTM E1755, respectively. The moisture content was tested using a drying oven. Moreover, the volatile matter content and ash content were measured by a digital muffle furnace. The fixed carbon content was calculated by difference. The heating value was tested using an oxidation bomb calorimeter (Model XRY1B), according to ASTM E711. The thermal resistivity, thermal conductivity, thermal diffusivity, and specific heat capacity of pine sawdust were measured by a thermal properties analyzer (Model KD2 Pro, Decagon Devices, Pullman, WA, USA). Table 1 shows physicochemical characterization of pine sawdust. 2.2. Fast Pyrolysis System. A biomass fast pyrolysis apparatus developed by Shanghai Jiao Tong University (PRC) was used for biomass fast pyrolysis experiments.13 Figure 1 shows a schematic diagram of the 1−5 kg/h fast pyrolysis fluidized-bed reactor apparatus with HVF, fractional condenser, and electrostatic precipitator. The system consisted mainly of a main (Level 1) feedstock hopper, an assistant (Level 2) feedstock hopper, two twin screw stainless-steel feeders, a fluidized-bed reactor, a biochar pot, a cyclone separator, a fixed-bed ceramic HVF, four water-cooled condensers, five bio-oil pots, an electrostatic static precipitator (ESP), a gas sampling point, and a N2 control valve. The feedstock hopper was comprised of common steel plates. The biomass throughput was 1−5 kg/h. The biomass feedstock was stored in the Level 1 feedstock hopper (turbination; diameter, 0.55 m; height, 0.70 m), and then was conveyed to the Level 2 feedstock hopper by the first twin screw feeder, which was connected to the Level 1 feedstock hopper and the Level 2 hopper. The biomass was transported from the Level 2 feedstock hopper to the fluidized-bed reactor by the second twin screw stainless-steel feeder, which was connected to the reactor. The operating speed of the second twin

value

Elemental Analysis (dry ash-free basis, wt %) C H Ob S N H/C (dimensionless, molar ratio) O/C (dimensionless, molar ratio) molecular formula (dimensionless)

47.21 ± 0.31 6.25 ± 0.09 44.40 0.21 ± 0.03 0.05 ± 0.02 1.59 0.71 CH1.59O0.7

Proximate Analysis (received basis, wt %) moisture volatiles ash fixed carbona

7.84 ± 0.06 73.52 ± 0.31 1.88 ± 0.05 16.76

Chemical Analysis (received basis, wt %) cellulose hemicellulose lignin high heating value (received basis, MJ/kg)

42.17 15.52 19.36 19.41

ICP Analysis (dry ash-free basis, ppm) Na K Ca Mg Al Fe Mn P others

379 ± 11 435 ± 14 1236 ± 22 153 ± 14 93 ± 03 154 ± 14 28 ± 03 57 ± 04 43 ± 12

Thermal Properties thermal conductivity (W/(m K)) thermal resistivity (°C cm/W) thermal diffusivity (mm2/s) specific heat capacity (MJ/(m3 K))

0.102 979.4 0.156 0.653

a

± ± ± ±

± ± ± ±

0.45 0.19 0.26 0.23

0.005 39.5 0.018 0.024

Determined by difference.

screw feeder was faster than that of the first twin screw feeder that was connected to the Level 1 feedstock hopper. For this design, feedstock could not be blocked before the fluidized-bed reactor. The fluidizedbed reactor (diameter, 0.10 m; height, 0.70 m), which was composed of a 304 stainless steel tube, was located within a ceramic heating jacket. The fluidized-bed reactor was filled with 2.4 kg of 300−450 μm silica sand. Two auto nitrogen-gas flow meters were turned on, which were controlled by computer. The first nitrogen gas flow was preheated and used to fluidize sand. The pressure was controlled and the nitrogen flow rate was constant at 55 SLPM (standard liter per minute) in the fluidized reactor. The second nitrogen flow was in front of the side of the screw feeder, and the nitrogen flow rate, which was used as the inner sweep gas, was constant at 20 SLPM. The biomass had been pyrolyzed in a fluidized-bed reactor within 400 °C, the total bio-oil yield decreased to 52.1 wt % at 500 °C. The decreased bio-oil yield was due to a higher thermal decomposition of pine sawdust at elevated HVF temperature. The optimum HVF temperature for bio-oil production was 400 °C. In contrast, the increase of gas yield that was observed as the HVF temperature changed from 350 °C to 500 °C was mainly due to the bio-oil decomposition. It was shown that using the HVF had an apparent effect on biochar yield as the residence time and flow rate decreased. Biochar yield was sharply decreased from 25.3 wt % to 20.8 wt % as the HVF temperatures increase from 350 °C to 450 °C, but the tendency is not consistent, and then was slightly increased to 21.2 wt % at an HVF temperature of 500 °C. The biochar yield was essentially slowly decreased for each of the four pyrolysis conditions, except at an HVF temperature of 350 °C, indicating that the formation of gases is the major contributor to the lower liquid yield.18 The minimum char yield was 19.4 wt % without HVF, which could have occurred because of higher flow rates, causing shorter residence times of vapors before sufficient quenching. The maximum char yield was 25.3 wt % with an HVF temperature at 350 °C. The reason was that char could react with high-boiling-point tar and sticks to the surface of the HVF, where the recombination reaction of free radicals results in high biochar concentration and aggravation of char formation.19 Otherwise, low temperature could be beneficial for the pathway to biochar production.20 However, when the HVF was used, the downtrend in biochar yield is obvious, compared to non-HVF with increased HVF temperatures. Large biomass particles were usually not completely decomposed by short residence times, because dehydration and decarboxylation could occur in the inner part of the HVF. Short residence times and higher temperatures usually inhibit the formation of biochar. This was the reason why the yields of biochar with HVF were higher than that of the non-HVF and decreased as the HVF temperature increased. The NCG yield increased as the HVF temperature increased, while the composition of the NCG did not change very much. The NCG yield presented an increasing trend in the pyrolysis of biomass by using the HVF, because redundant NCG was generated by secondary cracking and the decomposition of pyrolysis vapors. As the HVF temperature was increased from 350 °C to 500 °C, the NCG yield increased from 18.2 wt % to 26.7 wt %, respectively, because the Boudouard gas reaction becomes active at high HVF temperatures, which leads to the formation of gases.19

3. RESULTS AND DISCUSSION 3.1. Fast Pyrolysis Product Distribution. 3.1.1. Yields of Bio-oil, Biochar, and NCG. The fast pyrolysis products of pine sawdust included bio-oil, biochar, and NCG. Table 2 shows the mass yield of products of the fast pyrolysis of pine sawdust. Bio-oil was obtained in four condensers and an ESP at 4 °C. Table 2 showed that the yield of L1 bio-oil decreased from 26.8 wt % to 24.5 wt % as the HVF temperature increased from 350 °C to 450 °C, and then increased to 27.5 wt % at 500 °C. The yields of L1 bio-oils with HVF were all lower than that of nonHVF L1 bio-oil, which was 30.7 wt %. The yield of L2 bio-oil was slightly influenced by the HVF temperature. The yield of L3 bio-oil was increased from 2.8 wt % to 4.6 wt % as the HVF temperature increased from 350 °C to 450 °C, but decreased to 3.1 wt % at 500 °C, compared to 3.7 wt % of the non-HVF L3 bio-oil. By contrast, the yield of L4 fraction was decreased from 15.1 wt % to 11.5 wt % as the HVF temperature increased from 350 °C to 500 °C, compared to 15.2 wt % of the non-HVF L4 bio-oil. The yield of ESP fraction increased from 8.6 wt % to 12.5 wt % as the HVF temperature increased from 350 °C to 400 °C, and then decreased to 9.2 wt % at 500 °C, compared to 10.5 wt % of the non-HVF ESP bio-oil. For L1, L2, and L3 bio-oils, each was divided into two fractions: (i) an aqueous phase including water and organic acids; and (ii) a heavy, smoky smelling, and viscous bio-oil that likely adhered to the condenser wall. L4 and ESP bio-oils contained much more organic liquid and less water, and showed only one fraction, compared to L1, L2, and L3 bio-oils. Yields of L1, L4, and ESP fraction were 27.9, 13.8, and 12.5 wt %, respectively, at an HVF temperature of 400 °C. Yields of the L2 and L3 fractions were only 2.1 and 2.4 wt % at an HVF temperature of 400 °C. The reason for this may be attributed to less residence time in the L2 and L3 condensation process, compared to L1, L4, and ESP. The total bio-oil yields at HVF temperatures from 350 °C to 500 °C were 52.8, 58.7, 53.5, and 52.5 wt %, respectively. The HVF was found to have influence on the yields of biomass pyrolysis products. The maximum bio-oil yield of 62.4 wt % was obtained without the HVF. Bio-oil yields are partially dependent on the heat transfer process of hot vapor in HVF. This was due to decreased limitations of mass transfer and heat without HVF, leading to maximum bio-oil yields.16 Comparison with the sample without the HVF shows that the HVF in the pyrolysis has an impact on the quality and quantity of bio-oils. The pyrolysis vapor flow was blocked in the HVF of a ceramic membrane with a honeycomb structure, which leads to higher pressure and longer residence times of pyrolysis vapors. For 10461

DOI: 10.1021/acs.energyfuels.6b01877 Energy Fuels 2016, 30, 10458−10469

Article

Energy & Fuels Table 3. Moisture Content of Bio-oils in Different Condensers (L1, L2, L3, L4) and ESP Moisture (wt %) L1a

product non-HVF HVF-350 HVF-400 HVF-450 HVF-500

60.13 38.59 58.51 50.17 63.04

± ± ± ± ±

L2 1.06 0.78 0.93 0.49 1.24

52.11 22.16 48.84 46.03 38.14

L3

± ± ± ± ±

1.12 0.51 0.74 1.12 1.02

50.03 19.28 50.26 42.34 33.19

± ± ± ± ±

L4 0.66 0.46 0.79 0.62 0.93

14.62 9.06 11.09 10.53 12.38

± ± ± ± ±

0.35 0.24 0.19 0.21 0.26

ESP

totalb,c

± ± ± ± ±

39.40 24.13 35.95 31.42 40.29

8.13 7.44 8.13 5.06 9.83

0.16 0.20 0.19 0.08 0.22

a L1, L2, L3, L4, and ESP represent the bio-oils collected from condenser 1, condenser 2, condenser 3, condenser 4, and ESP, respectively. bHere, moisturetotal is the moisture content of total bio-oils including bio-oil collected from L1, L2, L3, L4 and ESP: moisturetotal (%) =

yield pyrolytic water × massbiomass (yield organic liquid + yield pyrolytic water) × massbiomass

× 100. The data were based on Table 4. cReceived basis.

Table 4. Yields of Organic Liquids and Pyrolytic Water of Pine Sawdust Fast Pyrolysis in Different Condensers and ESP Yield (wt %)a b

Pyrolytic Waterc

Organic Liquid non-HVF HVF-350 HVF-400 HVF-450 HVF-500

L1d

L2

L3

L4

ESP

total

L1

L2

L3

L4

ESP

total

12.24 16.46 11.58 12.21 10.16

1.10 1.17 1.07 0.86 0.74

1.85 2.26 1.19 2.65 2.07

12.98 13.73 12.27 11.09 10.08

9.65 7.96 11.48 9.87 8.30

37.81 41.58 37.60 36.69 31.35

18.46 10.34 16.32 12.29 17.34

1.20 0.33 1.03 0.74 0.46

1.85 0.54 1.21 1.95 1.03

2.22 1.37 1.53 1.31 1.42

0.85 0.64 1.02 0.53 0.90

24.59 13.22 21.10 16.81 21.15

Received basis. bYieldorganic liquid = yieldCi × (1 − MCi/100). cYieldpyrolytic water = yieldCi × MCi/100. Here, YieldCi is the bio-oil yield of condenser i (i = 1, 2, 3, and 4, or ESP). The values are derived from Table 2. In addition, MCi is the bio-oil moisture content of condenser i (i = 1, 2, 3, and 4, or ESP). The values are derived from Table 3. dL1, L2, L3, L4, and ESP represent the bio-oils collected from condenser 1, condenser 2, condenser 3, condenser 4, and ESP, respectively. a

3.1.2. Moisture Distribution of Bio-oils. Bio-oils were condensed in condensers 1−4 and collected in ESP. Because of the influence of collection temperature, there was a great difference in the moisture content of bio-oils with various condensers and ESP. Table 3 shows the moisture content of bio-oils in different condensers (L1, L2, L3, L4) and ESP. The results given in Table 3 illustrated that the moisture content of bio-oils in different condensers at the same HVF temperature had a decreasing tendency. The moisture contents of condenser 1, condenser 2, and condenser 3 were much higher than that of condenser 4 and ESP. This is due to the higher-boiling-point fraction of bio-oil and different dew points. Thus, water was easy to be collected using preceding condensers at higher temperature. This is attributed to the higher separating efficiency of water and bio-oil for the longer residence times of condensers 1 and 2.21 Moreover, the higher temperature of the pyrolysis vapor in the HVF could be related to the increase of bio-oil moisture. This is due to the secondary cracking of pyrolysis vapor and the secondary decomposition of the biochar.22 The moisturetotal value was between 24.13 wt % and 40.29 wt %. The HVF has a significant impact on reducing the value of moisturetotal of bio-oil at HVF temperatures of 350, 400, and 450 °C, which decreased by 38.76%, 8.76%, and 20.25%, respectively, compared to non-HVF. The minimum moisture of total bio-oil was obtained at an HVF temperature of 350 °C. 3.1.3. Effect of HVF Temperature on Bio-oil Yields of Organic Liquids and Pyrolytic Water. The liquid product biooil of pyrolysis is a complicated liquid mixture that contains an organic liquid fraction and a pyrolytic water fraction.23 It is essential to investigate the effect of the HVF on the yield of organic liquids and pyrolytic water separately at different HVF temperatures.24

Table 4 shows the yields of organic liquids and pyrolytic water from the fast pyrolysis of pine sawdust in different condensers and ESP. The results shown in Table 4 showed that most of the organic liquids were concentrated in L1, L4, and ESP. The proportions are 30.7%−39.6% (L1), 30.2%−34.3% (L4), and 19.1%−30.5% (ESP) of total organic liquid. The HVF organic liquid in L1 and L4 exhibited a downward trend as the HVF temperature was elevated. The maximum yields of organic liquid were 16.46 wt % (L1) and 13.73 wt % (L4) at an HVF temperature of 350 °C, compared to 12.24 wt % (L1) and 12.98 wt % (non-HVF). The values were fluctuant in ESP. The yield was increased from 7.96 to 11.48 wt %, while the HVF temperature was increased from 350 °C to 400 °C, compared to 9.65 wt % for non-HVF, and then decreased continuously to 8.30 wt % at an HVF temperature of 500 °C. The greatest proportion of pyrolytic water was condensed in L1. The value was 73.1%−82.0% of the total pyrolytic water. Pyrolytic water in L1 decreased with the HVF. The maximum yield of pyrolytic water was 16.32 wt % at an HVF temperature of 400 °C. Figure 2 shows the yield distribution of bio-oil, organic liquids, and pyrolytic water obtained from pine sawdust fast pyrolysis at non-HVF and different HVF temperatures. Figure 2 elucidates the association between organic liquid, pyrolytic water, and bio-oil yield. Generally, the yields of organic liquid, pyrolytic water, and bio-oil decreased because of the use of HVF. From Figure 2, it was apparent that the change of bio-oil yield was dependent on the variation in pyrolytic water with non-HVF and HVF temperatures of 350−400 °C. At HVF temperatures of >450 °C, the variation of bio-oil yield was based on organic liquid. The maximum yield of organic liquid was 41.6 wt % at an HVF temperature of 350 °C. From Tables 4 and 5, the yield of organic liquid increased as the amount of solids particle decreased by using HVF. It is due to the removal of alkali metals.25 At an HVF temperature of 350 °C, the yields 10462

DOI: 10.1021/acs.energyfuels.6b01877 Energy Fuels 2016, 30, 10458−10469

Article

Energy & Fuels

from 400 °C to 500 °C, decreases in the bio-oil yield were dependent on the decrease in the amount of organic liquid. 3.2. Physical Properties of the Pyrolysis Products. Biooils were produced by pine sawdust fast pyrolysis without HVF and with HVF, operating at HVF temperatures of 350, 400, 450, and 500 °C. The biochar was collected using cyclone and HVF. Table 5 shows physical properties of pyrolysis products. From Tables 3 and 5, the bio-oil collected in condenser 4 had a higher pH value, a higher heating value, and a lower water content.13 Bio-oils and biochar contained lesser amounts of oxygen than pine sawdust. The carbon content of bio-oils was higher than that of pine sawdust, but the oxygen and hydrogen contents were lower. The metal content in the bio-oil produced by fast pyrolysis using the HVF fluctuated slightly. Table 5 shows that the metal content was concentrated in the biochar after pyrolysis. The main variation of metallic elements was attributed to the sodium in the biochar. From Table 5, the H/C and O/C ratios of biochar decreased, compared to the pine sawdust. The H/C ratio of bio-oils declined smoothly as the HVF temperature increased. The H/ C ratios of bio-oils obtained at HVF temperatures of 350, 400, 450, and 500 °C were 1.49, 1.48, 1.42, and 1.41, respectively. The main reason for decreasing H/C ratios was derived from

Figure 2. Distribution of bio-oil, organic liquids, and pyrolytic water obtained from the fast pyrolysis of pine sawdust at non-HVF and different HVF temperatures.

of organic liquids decreased and biochar and NCG yields had a tendency to increase, because of the use of the HVF. This is because an increase in the severity of the HVF pyrolysis could reduce the yield of the organic liquid, resulting from the decomposition of the vapors and the formation of NCG. The maximum yield of bio-oil appeared at an HVF temperature of 400 °C. The increased bio-oil yield was attributed to the increased yield of pyrolytic water. By contrast, the yield of organic liquid decreased. As the HVF temperature increased Table 5. Physical Properties of Pyrolysis Products

Bio-oil non-HVF C H N Oa H/C (dimensionless, molar ratio) O/C (dimensionless, molar ratio) N/C molar ratio (× 10−4) empirical formula

a

HVF-350

HVF-400

HVF-450

Elemental Composition (Received basis, wt %) 57.82 ± 0.64 54.27 ± 0.57 55.89 ± 0.49 54.61 ± 0.73 7.13 ± 0.16 6.74 ± 0.20 6.90 ± 0.14 6.45 ± 0.17 0.04 ± 0.02 0.03 ± 0.01 0.04 ± 0.02 0.03 ± 0.00 32.33 38.21 36.71 38.33 1.48 1.49 1.48 1.42 0.42 0.53 0.50 0.53 6±2 5±1 6±2 7±3 CH1. 48O0.42 CH1.49O0.53 CH1.48O0.50 CH1.42O0.53

Na (ppm) K (ppm) Ca (ppm) Mg (ppm) Al (ppm) Fe (ppm) Mn (ppm) P (ppm) ash content (wt %) volatile matter content (wt %) fixed carbon (wt %) solids content (wt %) HHV (MJ/kg) density (g/mL) pH value

473 ± 13 217 ± 05 287 ± 07 17 ± 01 3±0 47 ± 03 8±2 62 ± 04

Metal Analysisb 397 ± 12 312 ± 09 243 ± 06 238 ± 05 234 ± 06 292 ± 09 24 ± 02 12 ± 01 2±0 7±1 39 ± 03 48 ± 04 7±1 5±2 37 ± 02 48 ± 03

0.69 ± 0.02 23.83 ± 0.54 1.235 ± 0.002 3.57 ± 0.04

0.23 ± 0.01 23.03 ± 0.61 1.173 ± 0.003 3.31 ± 0.03

fresh bio-oil aged bio-oil stability index

52.5 ± 0.3 189.5 ± 0.9 2.61

0.17 ± 0.01 23.29 ± 0.56 1.159 ± 0.002 3.38 ± 0.02

Kinematic Viscosity @ 40 °C (mm2/s) 28.6 ± 0.2 26.9 ± 0.6 58.6 ± 0.4 52.2 ± 0.3 1.05 0.94

HVF-500

biochar

52.26 ± 0.63 6.14 ± 0.13 0.04 ± 0.01 40.82 1.41 0.59 7±2 CH1.41O0.59

78.47 ± 1.16 2.13 ± 0.07 0.03 ± 0.01 18.03 0.33 0.17 3±0 CH0.33O0.17

2348 ± 52 345 ± 09 356 ± 10 17 ± 01 9±2 36 ± 01 7±2 85 ± 08 11.38 ± 0.27 18.35 ± 0.38 70.26 ± 1.54

523 ± 18 265 ± 08 198 ± 04 13 ± 0 4±1 67 ± 05 6±2 34 ± 03

289 ± 06 251 ± 06 173 ± 03 16 ± 01 13 ± 3 52 ± 02 11 ± 3 72 ± 05

0.26 ± 0.02 24.46 ± 0.29 1.123 ± 0.003 3.40 ± 0.02

0.29 ± 0.03 25.88 ± 0.44 1.118 ± 0.002 3.47 ± 0.03

35.8 ± 0.5 61.9 ± 0.5 0.73

37.6 ± 0.4 72.7 ± 0.6 0.93

29.28 ± 0.62

By difference. bMetal analysis content of bio-oil was measured by ash of bio-oil. 10463

DOI: 10.1021/acs.energyfuels.6b01877 Energy Fuels 2016, 30, 10458−10469

Article

Energy & Fuels Table 6. HHVs of Pyrolysis Products Bio-oil (MJ/kg) L1a non-HVF HVF-350 HVF-400 HVF-450 HVF-500

5.34 2.56 2.11 4.56 3.36

± ± ± ± ±

L2a 0.12 0.07 0.08 0.09 0.04

14.34 9.59 7.81 8.46 9.41

± ± ± ± ±

L3a 0.31 0.25 0.21 0.18 0.21

16.17 10.08 9.08 10.84 11.40

± ± ± ± ±

L4a 0.39 0.25 0.23 0.27 0.32

23.83 23.03 23.29 24.46 25.88

± ± ± ± ±

ESPa 0.54 0.61 0.56 0.29 0.44

25.17 26.39 25.24 26.03 26.24

± ± ± ± ±

biocharb (MJ/kg)

0.68 0.52 0.46 0.53 0.41

28.47 29.29 29.55 28.47 29.16

± ± ± ± ±

0.72 0.79 0.63 0.55 0.59

NCG (MJ/Nm3) 8.23 9.24 9.06 8.75 9.46

± ± ± ± ±

0.18 0.21 0.19 0.17 0.20

a L1, L2, L3, L4 and ESP represent the bio-oils collected from condenser 1, condenser 2, condenser 3, condenser 4, and ESP, respectively. bHHV of biochar was calculated by biochar obtained from the cyclone and HVF mixtures.

The PRE is defined as the maximum energy potentially recovered from pyrolysis products including bio-oil, biochar, and NCG product, based on the HHV of bio-oils, biochar, and NCG products.26 The recoverable energy was expressed as the total recoverable energy of product distribution:

more NCG being produced at increased HVF temperatures, and the NCG normally has a higher H/C ratio than bio-oil.16 The H/C of bio-oil without the HVF was 1.48. The values of the HVF bio-oils showed that the application of the HVF does not cause the H/C ratio to change very much. The O/C ratio of HVF bio-oils has a range between 0.50 and 0.59. Compared to 0.42 for non-HVF, the HVF application has increased the O/ C ratio of bio-oils. Because biochar was removed from the HVF, the O/C ratio increased at increasing HVF temperatures. The solids content, heating values, density, pH value, viscosity, and stability index of bio-oils were in the ranges of 0.17−0.69 wt %, 23.83−25.88 MJ/kg, 1.118−1.235 g/mL, 3.31−3.57, 26.9−52.5 mm2/s, and 0.73−2.91, respectively. A decreased trend in the solids content of bio-oils using the HVF, compared to non-HVF, was observed. The solids content decreased from 0.69 wt % (non-HVF) to 0.24 wt % (average value of HVF bio-oils). This is because the HVF could clean and remove inorganic contaminants of bio-oil. In addition, the removal of inorganic substances would reduce the extent of secondary reactions in the long-term storage of bio-oil and the secondary reaction is related to solids by inorganic deposits. The density of the HVF bio-oils presents the same trend as moisture. The maximum density of bio-oil was 1.235 g/mL of non-HVF. With increased HVF temperature from 350 °C to 500 °C, the value decreased from 1.173 g/mL to 1.118 g/mL. The pH value of bio-oil decreased sharply to 3.31 at an HVF temperature of 350 °C, compared to pH 3.57 for non-HVF, but then increased to pH 3.47 at an HVF temperature of 500 °C. The heating value increased from 23.03 MJ/kg to 25.88 MJ/kg as the HVF temperature increased from 350 °C to 500 °C, compared to 23.83 MJ/kg for non-HVF. An increased viscosity of HVF bio-oils from 28.6 mm2/s at an HVF temperature of 350 °C to 37.6 mm2/s at an HVF temperature of 500 °C was observed, compared to 52.5 mm2/s without HVF. It was evident that significant differences in viscosity between nonHVF bio-oil and HVF bio-oils existed. It was because the HVF could remove inorganic particles, such as alkali metal. These particles were the main constituent of ash and solid in bio-oils, and these were thought to have provided nucleation sites for polymerization or to catalyze polymerization, with viscosity increased.10 For similar reasons, it was apparent that the stability index was decreased from 1.05 to 0.93 as the HVF temperature increased from 350 °C to 500 °C, compared to 2.61 for non-HVF. As discussed previously, high HVF temperatures have been associated with increased bio-oil stability and decreased liquid yields, because of secondary cracking reactions. 3.3. Energy Balance. 3.3.1. Potentially Recovered Energy (PRE). Table 6 shows the high heating values (HHVs) of the pyrolysis products.

Q recovery (MJ/kg) = Q bio‐oil + Q biochar + Q NCG

(1)

where Qrecovery is the PRE per 1 kg pine sawdust. Qbio‑oil, Qbiochar, and QNCG are the maximum energy potentials of bio-oils, biochar, and NCG per 1 kg of pine sawdust. 4

Q bio‐oil (MJ/kg) =

∑ (mass %Ci × HHVCi) + mass %ESP × HHVESP i=1

(2)

where mass%Ci is the result of mass of bio-oil from condenser i multiplied by its yields from 1 kg of pine sawdust. HHVCi is the high heating value of bio-oil from condenser i, mass%ESP is the result of mass of bio-oil from ESP multiplied with its yield, and HHVESP is the high heating value of bio-oil from ESP. Figure 3 shows the distribution of potential energy values in L1, L2, L3, L4, and ESP bio-oils obtained from the fast pyrolysis

Figure 3. Distribution of potential energy values in L1, L2, L3, L4, and ESP bio-oils obtained from the fast pyrolysis of pine sawdust at HVF temperatures from 350 °C to 500 °C and for non-HVF.

of pine sawdust at HVF temperatures from 350 °C to 500 °C and for non-HVF. From Figure 3, the potential energy values of HVF-350, HVF-400, HVF-450, HVF-500, and non-HVF were calculated as 6.86, 7.34, 7.49, 6.78, and 8.83 MJ/kg, respectively, and they have been decreased by 22.31%, 16.87%, 15.16%, and 23.22% when the HVF temperature increased from 350 °C to 500 °C, compared to non-HVF. The suitable HVF temperatures are 400 and 450 °C, for balance of bio-oils maximized yield and good quality, as well as a higher energy recovery. The non-HVF 10464

DOI: 10.1021/acs.energyfuels.6b01877 Energy Fuels 2016, 30, 10458−10469

Article

Energy & Fuels

Figure 4. Potential energy values of NCG, bio-oil, and biochar products per kilogram pine sawdust obtained from pine sawdust fast pyrolysis at different HVF temperatures, from 350 °C to 500 °C and for non-HVF.

has the best Qbio‑oil value. This was due to the fact that the yield of non-HVF was higher than that of HVF bio-oils, and the nonHVF bio-oil could contain much more carbohydrates than HVF bio-oils. Moreover, carbohydrates could contribute to the energy recovery. Q biochar (MJ/kg) = mass % biochar × HHVbiochar

VNCG (Nm 3/kg) ⎡⎛ mass massCH4,mol mass H2,mol CO,mol = yield NCG × ⎢⎜⎜ + + ⎢⎣⎝ MCO M H2 MCH4 +

(3)

(6)

where VNCG is the volume of NCG per 1 kg, MCO the molar mass of CO (kg/mol), MH2 the molar mass of H2 (kg/mol), MCH4 the molar mass of CH4 (kg/mol), and MC2−C4 the molar mass of C2−C4 (kg/mol). The potential energy values Qrecovery, including NCG, bio-oil, and biochar products per kilogram pine sawdust at different HVF temperatures from 350 °C to 500 °C and for non-HVF, are illustrated in Figure 4. It is evident that NCG using the HVF had a higher potential energy value of 3.41−4.91 MJ/kg (reference values of 3.15− 9.10 MJ/kg (ref 28)) than that of the non-HVF sample (2.27 MJ/kg). Potential energy values of NCG were 50.22%, 41.85%, 97.80%, and 115.86% higher with HVF temperatures from 350 °C to 500 °C than that of the non-HVF sample. The potential energy values of biochar were increased by 34.24%, 18.30%, 7.24%, and 9.78%, respectively, with HVF temperatures from 350 °C to 500 °C, compared to the non-HVF sample. Higher HVF temperatures also lead to higher biochar recovery energy and lower bio-oil recovery energy. The values of Qrecovery with the HVF process were higher than those of the non-HVF sample. The HVF process of pyrolysis produces more energy in the form of a total recovery energy than it consumes. The optimized HVF temperature for PRE was 450 °C, at which the maximum PRE values of total pyrolysis products and bio-oil were 17.91 MJ/kg and 7.49 MJ/kg, respectively. 3.3.2. Energy Recovery Ratio (ERR). The energy recovery ratio (ERR) and the energy consumption ratio (ECR) of biomass pyrolysis are measured for the energy balance of biomass pyrolysis process. ERR was investigated from the yield

where Qbiochar is the maximum energy potential of biochar, mass %biochar is defined as the product of the mass of biochar multiplied by its yield and HHVbiochar is the high heating value of biochar. The energy content of NCG is defined as QNCG, which is the sum of the energy of the NCG combustion. It can be quantified by the calculation of HHV. Q NCG (MJ/kg) = HHVNCG × VNCG

⎤ massC2 − C4,mol ⎞ ⎟⎟ × 22.4⎥ /100 ⎥⎦ MC2 − C4 ⎠

(4)

The HHV of NCG was calculated as described in ref 27: HHVNCG (MJ/Nm 3) = (12.63 × massCO,mol + 12.75 × mass H2,mol + 39.82 × massCH4,mol + 63.43 × massC2 − C4,mol)/100 (5)

where massCO,mol, massH2,mol, massCH4,mol, and massC2−C4,mol are given in the corresponding molar fractions (molar ratio) and their heats of combustion in the NCG. The molecular weight of ethylene was used for assuming ideal-gas behavior for the C2− C4 species. The HHV values of NCG are summarized in Table 6. The weight per unit volume of NCG correlated with the NCG yield, molar mass of gas, molecular weight of gas, and molar volume of gas. The weight per unit volume of NCG is defined as VNCG and is calculated as follows: 10465

DOI: 10.1021/acs.energyfuels.6b01877 Energy Fuels 2016, 30, 10458−10469

Article

Energy & Fuels

0.86 without HVF. It revealed that the pyrolysis process existed in thermodynamic nonequilibrium, from the energy point of view. An ERR value of >1 signifies that energy should be obtained from the outside, and the energy balance is subtractive. If the value is