Increasing Efficiency of Charcoal Production with Bio-Oil Recycling

Article pubs.acs.org/EF

Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Increasing Efficiency of Charcoal Production with Bio-Oil Recycling Aekjuthon Phounglamcheik,* Tobias Wretborn, and Kentaro Umeki Energy Engineering, Division of Energy Science, Luleå University of Technology, SE-971 87 Luleå, Sweden

Downloaded via KAOHSIUNG MEDICAL UNIV on August 30, 2018 at 14:20:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Charcoal from biomass is a promising alternative for fossil coal. Although its quality increases at high pyrolysis temperature, charcoal yield decreases, meaning lower economic performances of charcoal production processes. This work aims at demonstrating potential methods to increase charcoal yield while keeping its quality at satisfying levels. We suggested the recycling of bio-oil from the pyrolysis process as a primary measure. In addition, we also investigated in detail the consequence of utilizing CO2 instead of N2 as reaction media under practical conditions (i.e., thick particles). An experimental investigation was carried out in a macro-thermogravimetric (macro-TG) reactor. The sample (woodchips, bio-oil, and woodchips embedded with bio-oil) was exposed to the reaction temperature either instantaneously (isothermal condition) or by slow heating (slow pyrolysis) in controlled gas flows of N2 and CO2. The results showed that charcoal yield increases with the bio-oil recycling on woodchips at all pyrolysis temperatures (300−700 °C). By 20% of bio-oil embedding on woodchips, charcoal yield increased by 18.3% on average. The increase of charcoal yield was not only because of the increase in reactants but also due to the synergetic effect between bio-oil and woodchips upon physical contact. Bio-oil recycling had negligible effects on the property of charcoal, such as carbon content and heating value. Although CO2 did not affect primary pyrolysis, it had effects on mass transfer processes. As a result, significantly higher charcoal yield was obtained from pyrolysis in CO2 than in N2 by ensuring a good contact of volatiles and solid surface (i.e., usage of thick particles and slow heating). This study suggests that we can achieve high charcoal yield while maintaining the similar charcoal property by bio-oil recycling, CO2 purging, use of thick particles, and slow heating.

1. INTRODUCTION Use of biomass as an alternative fuel of coal has been one of the major development focuses for decades. It has been reported in 2017 by IEA Bioenergy that biomass has been cofired in over 150 coal-fired power stations around the world.1 Many recent studies focus on biomass upgrading to implement biomass fuels into industrial sectors with little impact on their operation. Charcoal, also called biochar or biocarbon, is a promising upgraded fuel. Charcoal is generally produced by slow pyrolysis, i.e., thermally driven degradation of biomass under an inert atmosphere at low heating rate. Carbon content, grindability, and heating value of charcoal are higher than those of biomass, making it an attractive substitute for fossil coal.2,3 Iron and steel industry could replace its usage of fossil coal by charcoal. For example, charcoal is possible to fully replace pulverized coal injection in blast furnaces as reducing agent.4,5 The metallurgical process, however, requires higher charcoal quality, i.e., high carbon content, low volatile matter content, low inorganic content (e.g., P, K, and S), high heating value, and good mechanical strengths.6 Consequently, charcoal produced at low pyrolysis temperature or torrefaction would not satisfy the requirement for the reducing agent of metallurgical processes. Therefore, high-temperature pyrolysis (>500 °C) is necessary to fulfill the requirement of the metallurgical application.4 In biomass pyrolysis, process parameters such as temperature, heating rate, residence time, pressure, and particle size affect process yield as well as chemical and physical qualities of charcoal.7−13 Among these parameters, temperature is the most influential parameter on charcoal properties and yield under slow pyrolysis conditions.4,13−15 Increase of temperature © XXXX American Chemical Society

generally has positive effects on thermochemical properties of charcoal, i.e., high carbon content, low volatiles, low retention of ash content, and high heating value.4,7,8,16−18 Meanwhile, charcoal yield becomes lower at high pyrolysis temperature, meaning lower economic performances of charcoal production. For example, charcoal yields from pyrolysis of pine chips19 decreased from 37% to 28% when the temperature increased from 300 to 500 °C. Such a trade-off between charcoal property and yield by pyrolysis temperature imposes a question: is there any means to increase charcoal yield while keeping its quality high? According to conversion pathways of biomass pyrolysis from the literature,14,20 secondary char formation during pyrolysis of biomass could be a key to answer this question. During pyrolysis reactions, volatile compounds from biomass that contains relatively large molecules can undergo recombination reactions at pore surface of particles and yield extra charcoal.14,20,21 This reaction will play a significant role in pyrolysis of thick biomass particles, such as woodchip, rather than pyrolysis of biomass powder due to longer residence time of volatiles in the pore structure.14,22−24 It is also reported that the longer residence time of volatiles can be achieved by other techniques such as pyrolysis in a closed system,12,25 high pressure,4,8,25−27 and low carrier gas flow rate.28−30 Type of carrier gas may also influence residence time of volatiles. As stated by Schonnenbeck et al.,31 using CO2 instead of N2 in pyrolysis may inhibit the volatiles release due to Received: July 5, 2018 Revised: August 16, 2018 Published: August 21, 2018 A

DOI: 10.1021/acs.energyfuels.8b02333 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels spreading of CO2 through the microstructure of solid particle, resulting in higher charcoal yield. On the other hand, CO2 may react with charcoal and decrease the yield at high temperature (ca. > 700 °C) due to gasification reactions.32 In addition, different carrier gases have different heat and mass transfer characteristics, and they might have consequences on other parameters such as heating rate and internal pressure. However, the interactions of these issues, which is important in industrial processes, are not well elaborated in the literature. An idea to promote the secondary char formation, by increasing volatiles concentration inside the pore structure of thick biomass, could be a promising method to increase charcoal yield. Hill and co-workers33 showed that charcoal yield from downstream of the fixed bed was higher than that from upstream during pyrolysis of Aspen woodchips due to the deposition of pyrolysis volatiles on charcoal. Moreover, the authors also showed that deposition of volatiles on biomass char during pyrolysis did not result in negative impact on microporosity and adsorption properties of charcoal.33 Another way to ensure the secondary char formation of volatiles is the recycling of bio-oil (large-molecule volatiles) into the pyrolysis processes. Secondary reactions of volatiles from bio-oil may progress even further at the surface of biomass structure by adsorbing bio-oil at the internal pore of biomass, as shown in Huang et al.34 Nevertheless, usage of thick particles, which are common in industrial processes, may hinder adsorption of heavy oil as well as sufficient contact between volatiles and pore surfaces. In addition, it is important to quantify the effect of various reaction parameters on synergetic effect in secondary char formation to give guidance for the design of pyrolysis processes with bio-oil recycling. The main objective of this paper is to demonstrate the potential of efficiency increase in charcoal production with high quality by recycling bio-oil from the pyrolysis process and other measures to ensure good contact of volatiles and solid surfaces. More specifically, we aim at elucidating the major causes behind the increase in charcoal yields by bio-oil recycling on thick wood particles. In addition, this paper aims at understanding how various reaction parameters, e.g., particle size and carrier gas species, i.e., N2 vs CO2, interact with each other in pyrolysis of thick wood particles. An experimental investigation was carried out in a macro-thermogravimeter (macro-TG), which allows pyrolysis of a large (∼20 mm) wood particle by exposing to reaction temperature instantaneously (isothermal condition) or by heating slowly (slow pyrolysis) with controlled gas flows. Properties of charcoal such as elemental composition (CHN/O) and higher heating value were measured in particular to represent the influence of pyrolysis conditions on mass and energy balances as well as charcoal properties.

Table 1. Characteristic of Raw Biomasses characteristics moisture content (original) moisture content (after drying) ultimate analysis carbon hydrogen nitrogen oxygen H/C molar ratio O/C molar ratio HHV

wt % wt %

spruce

birch

bio-oila

3.1 (±0.15) 0.4 (±0.08)

2.8 (±0.48) 0.1 (±0.01)

30.6 (±0.55)

49.5 6.1 0.25 42.8 1.48 0.65 19.7

47.1 6.2 0.27 43.6 1.58 0.69 18.3

55.4 6.6 0.14 37.9 1.43 0.51 23.4

wt %, dry

MJ/kg

a

Ultimate analyses and HHV of bio-oil are on wet basis due to its difficulty to separate water.

variety in its composition for each experiment. The composition of bio-oil from actual pyrolysis process is most likely to be different from that in the current study, but the general conclusions on the conversion behavior of bio-oil should be applicable for the actual sample. Water content of bio-oil was measured by using WT-KFV100 Karl Fischer water content tester. Properties of bio-oil35 are also shown in Table 1. Bio-oil was embedded on woodchips at room temperature by brushing bio-oil on the external surface of the woodchip. The embedded sample was placed into the reactor 3 min after embedding to ensure bio-oil to disperse into woodchips. In the base case, 20% (m/m) of bio-oil was embedded at the surface of woodchip, i.e., 20% of bio-oil and 80% of woodchip on mass basis. Meanwhile, 10% and 25% (m/m) of bio-oil on woodchip were also used for pyrolysis experiment at selected conditions. 2.2. Experimental Apparatus and Procedures. Pyrolysis experiments of woodchips were carried out in a macro-TG. This method gives the accessibility to measure the mass decay of a large particle during reactions. Figure 1 shows the schematic diagram of the

2. EXPERIMENTAL METHODS 2.1. Materials and Preparation. Debarked chips of Norway spruce and birch were selected as representatives of softwood and hardwood, respectively. Fiber structures of spruce and birch can be found in Figure S2 in the Supporting Information. Table 1 shows properties of woodchips. Moisture contents were measured with MJ33 by Mettler Toledo, elemental composition with EA3000 by Eurovector srl., and higher heating value with C200 by IKA. Dried woodchips were prepared in an oven at 105 °C for 24 h and kept in a desiccator before the experiment. Fast pyrolysis oil purchased from Fortum’s pyrolysis plant in Joensuu, Finland, was represented as condensed bio-oil from the pyrolysis process in this study. The bio-oil was selected due to its stability over the long period to avoid the

Figure 1. Macro-thermogravimetric analyzer.

macro-TG reactor. The reactor is an externally heated stainless-steel cylinder (grade 253 MA) with an internal diameter of 100 mm and 450 mm in length of heating zone. A wire mesh basket (or ceramic crucible for liquid sample) connected with a precision balance was hung from the top of the reactor chamber. The reactor temperature was measured by a type K thermocouple placed at the center of the reactor and 2 cm below the sample basket. The carrier gas entered the B


Article

Energy & Fuels reactor from the bottom and left at the top of the reactor with volatile gases generated during the experiment. The experiment was divided into two distinctive temperature histories of the reactor during the experiments: namely, isothermal condition and slow pyrolysis. Flow rate of the carrier gas, either N2 or CO2, was 7 L min−1 at standard state for both temperature conditions. The mass of sample and reactor temperature were recorded every 2 s with the precision of 1 mg and 3 °C, respectively. Prior to the experiments under isothermal conditions, the reactor was heated to the reaction temperature and purged with carrier gas. A single particle was manually lowered down into the heating zone typically in 2−3 s. Therefore, the sample was rapidly heated by surrounding gas flow inside the reactor. When the mass of the sample became stable, the sample was moved to the N2-purged cooling zone before being removed from the reactor. The reactor temperature ranged between 300 and 700 °C with the precision of ±3 °C. All of the experiments under isothermal condition had three repetitions. Mass yield of charcoal, yc, was calculated from the experimental data as yc =

mf × 100% m0

Figure 2. Effect of pyrolysis temperature on charcoal yield of spruce, birch, and bio-oil under isothermal conditions. The error bars are standard deviation obtained from three repetitions for each experimental point.

(1)

spruce resulted in higher charcoal yield than birch, which is generally mentioned in the literature.40,41 Furthermore, chemical structure of hardwood lignin (birch) has weaker thermal stability than softwood lignin (spruce), leading to lower charcoal yield in birch.42−44 Elemental composition of charcoals, shown in Figure 3, was mainly affected by pyrolysis temperature. Differences in

where m0 is the initial mass of sample and mf is the final mass of sample. The final mass of sample was defined as an interception point between major degradation line and post degradation line (details are shown in Figure S1 in the Supporting Information) to decrease random errors at the final stage of pyrolysis. Under the slow pyrolysis conditions, the sample was placed into the furnace at room temperature with carrier gas flowing through the reactor. Then, the reactor was heated to the reaction temperature at the heating rate of 3 °C min−1 with the precision of ±3 °C. Then, 10 s after the reactor temperature reached the desired temperature, the sample was moved to the N2-purged cooling zone and removed from the reactor. Additional experiments of wood powder were carried out in a PerkinElmer TGA 8000 Thermogravimetric Analyzer (TGA) to examine the pyrolysis behavior with negligible mass diffusion. Woodchips from the same origin were cut, and sawdust was collected as wood powder. TGA experiments of wood powder, 0.9−1.1 mg, were conducted under the same temperature profile as macro-TG experiment under slow pyrolysis condition (3 °C min−1) with a carrier gas flow rate of 20 mL min−1 purged around the sample crucible. Ultimate analysis of raw woodchips and charcoal samples were carried out with EA3000, a CHNS-O elemental analyzer from Eurovector srl. Higher heating value (HHV) of raw woodchips and charcoals were measured by an oxygen bomb calorimeter, model C 200 from IKA.

3. RESULTS AND DISCUSSION 3.1. Comparison between Spruce and Birch. Figure 2 compares the charcoal yields from pyrolysis of dried spruce, dried birch, and bio-oil under isothermal conditions. The carrier gas was N2, and the reaction temperature was varied at 300−700 °C. At a higher pyrolysis temperature, samples released more volatiles and charcoal yields decreased. Charcoal yield was more sensitive to pyrolysis temperature between 300 and 500 °C than between 500 and 700 °C. Charcoal yield from pyrolysis of bio-oil was the lowest while spruce gave higher charcoal yield than birch. Spruce showed higher charcoal yield than birch at all reactor temperature. One reason is the difference in chemical compositions of two biomass species, namely differences in cellulose, hemicellulose, lignin, and extractives. In general, spruce contains 45.6−47.2% of cellulose, 13.3−20.0% of hemicellulose, and 28.2−36.0% of lignin.36,37 While, birch contains 43.9−47.0% of cellulose, 25.9−28.9% of hemicellulose, and 20.2−22.0% of lignin.36,38,39 Higher lignin in

Figure 3. Elemental content of charcoal produced from pyrolysis under isothermal conditions in comparison with raw biomass and pulverized coal.5

elemental compositions of original spruce and birch did not have a clear impact on the elemental composition of charcoal. Carbon content in charcoal increased at higher pyrolysis temperature. The maximum carbon content of charcoals from spruce and birch were 90.3% and 90.2%, which are higher than the carbon content of pulverized coal (85%) reported by Wang et al.5 On the other hand, biomass charcoal contained a higher amount of oxygen than pulverized coal. 3.2. Bio-Oil Recycling for Enhancing Secondary Char Formation. Figure 4 shows the charcoal yields on mass basis with and without bio-oil recycling, which was prepared by embedding 20% (m/m) of bio-oil on the woodchip. Process charcoal yield, i.e., mass ratio of final charcoal to dried C


Article

Energy & Fuels

Figure 5. Van Krevelen diagram of charcoals from dried woodchip and bio-oil embedded woodchips under isothermal conditions.

Figure 4. Process charcoal yield of bio-oil embedded woodchips under isothermal conditions in comparison with those of woodchips and bio-oil (a) spruce and (b) birch. The error bars are standard deviation obtained from three repetitions for each experimental point.

woodchip, was used in Figure 4 to represent the expected process performance of pyrolysis with bio-oil recycling. With bio-oil recycling, charcoal yield increased for all of the reaction temperature. In pyrolysis of spruce, charcoal yield increased by 5.4% with bio-oil recycling on average. Likewise, the charcoal yield increased by 4.6% from dried birch with bio-oil recycling. Bio-oil recycling did not affect the elemental composition of charcoal significantly. Figure 5 represents van Krevelen diagram as a comparison of charcoal from original woodchips with bio-oil embedded woodchips. In agreement with the previous section, the temperature showed dominant effects on H/C and O/C ratios of charcoal, giving low values at high temperature. Elemental composition of charcoals was close to that of pulverized coal, but it was slightly rich in oxygen and lean in hydrogen than pulverized coal. Bio-oil recycling showed negligible influence on elemental composition. Figure 6 represents higher heating value (HHV) of charcoal. Bio-oil recycling had no significant effect on HHV of charcoal (pvalues by two-way ANOVA was 0.76 for spruce and 0.098 for birch), but it increased with reaction temperature. HHV was stable at the temperature above 500 °C, and the value was close to that of pulverized coal reported by Wang et al. (34.4 MJ kg−1).5

Figure 6. Higher heating value of charcoal with and without bio-oil embedding under isothermal conditions (a) spruce and (b) birch. The error bars are standard deviation obtained from three repetitions for each experimental point.

D


Article

Energy & Fuels

reactants (bio-oil + wood particle). Higher experimental values than the interpolation values mean the presence of synergetic effects. At the low-temperature range (300 and 340 °C), experimental results clearly showed higher charcoal yield than interpolation data. This synergetic effect was apparent regardless of the amount of bio-oil embedding. The synergetic effect between bio-oil and woodchip seems to have diminished at higher pyrolysis temperature (400 and 500 °C) as the experimental and interpolation data showed no significant differences (statistical analysis is shown in the Supporting Information). The most likely reason is due to the loss of physical contact between volatiles and the solid surface. At higher temperature in isothermal conditions, larger differences between reactor temperature and particle surface gives higher heating rate and causes more intensive devolatilization. The latter leads to accumulation of volatiles inside particle, increasing internal pressure and mechanical stress. This effect was observed by fragmentation and cracking in charcoal after pyrolysis at high temperature as shown in Figure 8. These

There are two possible reasons for the increases in process charcoal yield with bio-oil recycling. First, additional bio-oil in embedded samples simply added charcoal mass from pyrolysis of bio-oil origin. Another and important reason is the synergetic effect due to the physical contact between bio-oil and woodchips. Volatile compounds from bio-oil can be shifted toward charcoal by contact with the solid surface of woodchip/charcoal or by high concentration of volatiles inside the internal pore of the particle. To examine the degree of synergetic effect, the experimental data of bio-oil embedded woodchips was compared with the expected charcoal yield from interpolation between pure woodchips and bio-oil, as shown in Figure 7. The charcoal yield in the figure was calculated by dividing the mass of charcoal with total mass of

Figure 8. Differences in charcoal morphology at different pyrolysis temperature under isothermal conditions (a) spruce and (b) birch.

cavities or fragmentation in charcoal allowed volatiles generated during pyrolysis to be released from the particle easier, limiting the progress of secondary char formation. To isolate the effect of fragmentation and cavities caused by high heating rate, experiments were carried out at slow heating rate. Figure 9 shows the results from room temperature to 500 °C at the heating rate of 3 °C min−1 (reproducibility of the TG curves is illustrated in Figure S3 in the Supporting Information). The slow pyrolysis results showed well-known pyrolysis sequences. Up to about 150 °C, moisture in woodchip (and part of bio-oil) evaporated, and then major degradation took place. Mostly hemicellulose fraction

Figure 7. Examination of synergetic effects between woodchips and bio-oil by comparing charcoal yield of bio-oil embedded woodchips with interpolations between charcoal yields of bio-oil and woodchips (a) spruce and (b) birch. The error bars are standard deviation obtained from three repetitions for each experimental point. E


Article

Energy & Fuels

Figure 9. Thermogravimetric curve of woodchips in macro-TG at slow heating rate (3 °C min−1) under N2 flow (a) spruce and (b) birch.

Figure 10. Comparison of charcoal yield between pyrolysis in N2 and CO2 flows under isothermal conditions (a) spruce and (b) birch. The error bars are standard deviation obtained from three repetitions for each experimental point.

degraded at a temperature range between 150 to 280 °C, which can be observed as the shoulder of the derivative thermogravimetric (DTG) curves. Then, degradation of cellulose took place up to the temperature around 340 °C. The final region with slow degradation rate corresponds to the decomposition of lignin. Decomposition of hemicellulose showed visual overlap with cellulose decomposition in spruce (softwood) because spruce contains relatively lower hemicellulose compared with birch (hardwood). Furthermore, the difference in the type of hemicellulose in spruce and birch45 could influence the difference in decomposition path of hemicellulose.20,42 When considering the bio-oil embedded woodchips, the experimental results showed higher charcoal yield than interpolation results along with all the temperature range. Furthermore, charcoal obtained after slow pyrolysis did not appear cavities or breakage. The results revealed that the charcoal yield of bio-oil embedded woodchips was higher than that when bio-oil and woodchips were pyrolyzed independently. Moreover, keeping enough contact between woodchips and bio-oil is important to ensure profound effect of bio-oil recycling. 3.3. Influence of Reaction Atmosphere (N2 vs CO2) for Thick Particles. Figure 10 compares the charcoal yields from pyrolysis with the flow of N2 and CO2 under the isothermal

conditions. Bio-oil produced more charcoal in CO2 atmosphere than in N2 atmosphere, but the difference became narrower at higher temperature. In the case of spruce, there was no difference in charcoal yield between N2 and CO2 atmosphere except at 300 °C when charcoal yield was 42.5% in CO2 and 37.7% in N2. Similarly for birch pyrolysis, higher charcoal yield was obtained in CO2 than in N2 at low temperature (300 and 400 °C), while the difference diminished at higher temperatures. The difference between pyrolysis under N2 and CO2 might be explained by the differences in heat transfer to the particle and mass diffusion inside pores. The Nusselt number and Biot number, provided in the Supporting Information, can indicate the magnitude of heat transfer and the relative importance of external and internal heat transfer processes, respectively. According to the calculation results, the Nusselt number and Biot number of the system in CO2 atmosphere were higher than in N2 atmosphere. It means woodchips experienced higher heating rate and steeper internal temperature gradient in CO2 than in N2. Therefore, uniform heat transfer would be expected in pyrolysis under N2 than CO2. According to the heat transfer effect, lower charcoal yield would be expected in F


Article

Energy & Fuels

regions with respect to the similarity and difference between pyrolysis under CO2 and N2. At the temperature below 400 °C, which corresponds to the degradation of cellulose and hemicellulose, CO2 and N2 showed no visible difference. However, the pyrolysis of woodchip in CO2 showed higher charcoal yield than in N2 at the temperature between 400 and 700 °C, corresponding to the degradation of lignin. At T = 700 °C, the charcoal yield of spruce chip in CO2 pyrolysis was 27.6%, compared with 23.0% in N2 pyrolysis. Likewise, birch chips yielded 23.4% of charcoal from CO2 pyrolysis, while in N2 pyrolysis it was 18.4%. At the temperature higher than 700 °C, residual mass in CO2 atmosphere decreased sharply due to gasification reactions. In the pyrolysis of wood powder using TGA, the occurrence of secondary reactions was minimized so that only primary decomposition take place.46,47 Wood powder showed no significant difference in mass degradation path between CO2 and N2 at the temperature below 700 °C, confirming the absence of the effect of CO2 on intrinsic pyrolysis reaction. Charcoal yield of wood powder was much lower than woodchips due to the minimal occurrences of secondary char formation in pyrolysis of wood powder. For the pyrolysis under slow heating, heat transfer effect is negligible. Therefore, high yield of charcoal from woodchips under CO2 atmosphere at T = 400−700 °C is the effect of mass diffusion and subsequent change in reaction pathways. Absence of the CO2 effects in the TGA results with wood powder sample (minimal mass transfer limitation) indicates that it was low mass diffusivity in CO2 that have influenced the pyrolysis reactions (see the Supporting Information). Furthermore, the increase in mass yield can be explained either by the promotion of secondary reactions, the change in the reaction pathways, or merely adsorption of CO2 at the active sites of the charcoal matrix. If adsorption of CO2 is the case, the CO2 exist as a condense phase and could increase the charcoal yield. At the same time, CO2 also affects the elemental composition of charcoal due to its high oxygen content. However, adsorption of CO2 is unlikely to be the reason in this study since carbon content of charcoal was not affected. Promotion of secondary reactions is plausible because low diffusivity of volatiles through CO2 means high internal pressure or long residence time of volatiles. Both effects are favorable for the secondary char formation. On the other hand, it is still possible that the reaction pathways of primary pyrolysis were modified. A previous study showed that even short contact with steam can increase the size of the aromatic ring cluster in charcoal.48 Our results may be explained by a similar effect of CO2 on the charcoal matrix. Larger clusters of aromatic rings have lower vapor pressure. Hence, there are fewer chances for the mass loss of these substances as volatile gases during pyrolysis than smaller clusters and resulted in higher charcoal yield under CO2 atmosphere. 3.4. Carbon Recovery in Charcoal. Figure 12 shows a comparison of carbon yields among all of the pyrolysis conditions examined in this study (calculation can be found in the Supporting Information). As well discussed in the literature, a higher temperature reduced the carbon recovery although charcoal had a higher quality.4,7,8,16,18 However, other reaction parameters also affected carbon yields, where the magnitude of the combined effect can be equivalent to that of the reaction temperature by more than 300 °C. Bio-oil recycling and the use of CO2 as carrier gas instead of N2 are recommended when designing pyrolysis processes. However, it is important to apply these modifications with thick particles at

pyrolysis under CO2, which contradicts with the results shown in Figure 10. Mass transfer characteristic of N2 and CO2 atmospheres can be illustrated by binary gas diffusivity, provided in the Supporting Information. Diffusivity of the gases through CO2 was less than in N2, indicating that the release of volatiles from particles would be inhibited under CO2 in comparison with N2. Therefore, one can expect that either the residence time of the volatiles or internal pressure of particles would increase. Both changes may promote secondary char formation and give higher charcoal yield. This effect should diminish at higher temperature because of the fragmentation and particle cracking observed in Figure 8. In fact, Figure 10 shows that charcoal yields in two gas atmospheres showed no significant difference at high pyrolysis temperature. To isolate the transport effects from intrinsic chemical effects, thermogravimetric analyses was carried out at the heating rate of 3 °C min−1 in CO2 and N2 using both woodchip (with macro-TG) and powder (with TGA), as shown in Figure 11. The effect of heat transfer and particle breakage is absent for slow pyrolysis of woodchips, while heat and mass transfer as well as secondary pyrolysis had minimal effect on slow pyrolysis of wood powder. For slow pyrolysis of woodchip, the pyrolysis reactions can be divided into three

Figure 11. Thermogravimetric curve of woodchips in macro-TG and wood powder in TGA as a comparison between N2 and CO2 flows (a) spruce and (b) birch. G


Article

Energy & Fuels

the same pyrolysis temperature, on the other hand, charcoal quality remained similar while the yield increased from 16.2% to 26.7% for spruce and from 13.6% to 24.7% for birch.

4. CONCLUSIONS Bio-oil recycling in pyrolysis process was proposed to increase charcoal yield without a negative effect on charcoal quality. The results showed that bio-oil recycling on woodchips can increase charcoal yield as the results of increase in reactant as well as synergetic effect between bio-oil and woodchips. The synergetic effect was diminished by the physical disintegration of particles at high heating rate and high temperature while it remained for slow pyrolysis even at high temperature. Elemental composition and heating value of charcoal were mainly affected by the reaction temperature, and bio-oil recycling had negligible effects. Moreover, charcoal from this work had comparable properties to pulverized coal at the pyrolysis temperature above 500 °C. Pyrolysis under CO2 can result in higher charcoal yield than under N2 in some cases. CO2 seems to have no direct effects on primary pyrolysis reactions but via intraparticle heat and mass transfer. As a consequence of low mass diffusivity, volatiles and solid surface had better contact. This led to higher charcoal yield due to enhanced secondary char formation. This effect vanished at high temperature for isothermal conditions but remained for slow pyrolysis of thick particles. The findings from this work can be implemented in the development of a charcoal production process. Bio-oil recycling, CO2 atmosphere, and slow heating rate are key process parameters to achieve higher charcoal yield while maintaining similar charcoal property. With all of these measures applied, the optimized conditions in this study showed significantly higher efficiency of the charcoal production process (26.7 and 24.7% of carbon yield) than without these measures (16.2 and 13.6% of carbon yield).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b02333.

Figure 12. Carbon yield in char from various pyrolysis conditions (a) spruce and (b) birch.

slow heating rate in order to ensure the effect by taking advantage of good contact of volatile and particle surface. Slow pyrolysis of a thick particle, in contrast with isothermal conditions, produces charcoal without a major change in the fiber structure by cavities and breakages. This behavior is more favorable for secondary char formation and contact between internal solid surface and volatiles, resulting in higher charcoal yield. Moreover, particles under slow heating experience uniform heat transfer and long reaction time. Therefore, charcoal from slow pyrolysis contains higher carbon content (lower functional groups) than that from isothermal conditions. As a consequence, the benefit from bio-oil recycling and the use of CO2 as carrier gas is more apparent under slow pyrolysis. All of the measures combined, carbon yield from slow pyrolysis under CO2 flow with bio-oil recycling at 700 °C showed as high carbon yield as that at isothermal conditions under N2 flow at 300 °C. Meanwhile, the carbon content of charcoal was significantly higher; the carbon content in charcoal was ca. 90% instead of ca. 75%. When comparing

■

Identification of the final mass in the macro-TG experiment under isothermal conditions (Figure S1), X-ray microtomography of raw wood (Figure S2), statistical analysis of Figure 7, reproducibility of slow pyrolysis in the macro-TG (Figure S3), heat and mass transfer calculation (Figure S4), and calculation of carbon yield (PDF)

AUTHOR INFORMATION

Corresponding Author

* Phone: +46 920 49 3939. E-mail: aekjuthon. [email protected]; [email protected]. ORCID

Aekjuthon Phounglamcheik: 0000-0001-8372-4386 Kentaro Umeki: 0000-0001-6081-5736 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. H


Article

Energy & Fuels Notes

(28) Yorgun, S.; Yildiz, D. J. Anal. Appl. Pyrolysis 2015, 114, 68−78. (29) Demiral, I.;̇ Ş ensöz, S. Energy Sources, Part A 2006, 28 (12), 1149−1158. (30) Pütün, A. E.; Ö zbay, N.; Ö nal, E. P.; Pütün, E. Fuel Process. Technol. 2005, 86 (11), 1207−1219. (31) Zellagui, S.; Schö nnenbeck, C.; Zouaoui-Mahzoul, N.; Leyssens, G.; Authier, O.; Thunin, E.; Porcheron, L.; Brilhac, J. F. Fuel Process. Technol. 2016, 148, 99−109. (32) Guizani, C.; Escudero Sanz, F. J.; Salvador, S. Fuel 2014, 116, 310. (33) Veksha, A.; McLaughlin, H.; Layzell, D. B.; Hill, J. M. Bioresour. Technol. 2014, 153, 173−179. (34) Huang, Y.; Kudo, S.; Norinaga, K.; Amaike, M.; Hayashi, J. I. Energy Fuels 2012, 26 (1), 256−264. (35) Bach-Oller, A.; Kirtania, K.; Furusjö, E.; Umeki, K. Fuel 2017, 202, 46−55. (36) Taherzadeh, M. J.; Eklund, R.; Gustafsson, L.; Niklasson, C.; Lidén, G. Ind. Eng. Chem. Res. 1997, 36 (11), 4659−4665. (37) Lyu, G.; Wu, S.; Zhang, H. Front. Energy Res. 2015, 3, DOI: 10.3389/fenrg.2015.00028. (38) Sjöström, E. Wood chemistry, fundamentals and applications; Gulf Professional Publishing, 1993; Vol. 252. (39) Räisänen, T.; Athanassiadis, D. For. Refine; 2013; Vol. 4. (40) Gani, A.; Naruse, I. Renewable Energy 2007, 32 (4), 649−661. (41) Dorez, G.; Ferry, L.; Sonnier, R.; Taguet, A.; Lopez-Cuesta, J. M. J. Anal. Appl. Pyrolysis 2014, 107, 323−331. (42) Wang, S.; Dai, G.; Yang, H.; Luo, Z. Prog. Energy Combust. Sci. 2017, 62, 33−86. (43) Zhao, J.; Xiuwen, W.; Hu, J.; Liu, Q.; Shen, D.; Xiao, R. Polym. Degrad. Stab. 2014, 108, 133−138. (44) Wang, S.; Wang, K.; Liu, Q.; Gu, Y.; Luo, Z.; Cen, K.; Fransson, T. Biotechnol. Adv. 2009, 27 (5), 562−567. (45) Daniel Hayes. Wood composition http://www.carbolea.ul.ie/ wood.php (accessed Nov 18, 2017). (46) Di Blasi, C. AIChE J. 2002, 48 (10), 2386−2397. (47) Park, W. C.; Atreya, A.; Baum, H. R. Combust. Flame 2010, 157 (3), 481−494. (48) Keown, D. M.; Hayashi, J. I.; Li, C. Z. Fuel 2008, 87 (7), 1127− 1132.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support of Interreg Nord and Länsstyrelsen Norrbotten through RENEPRO project (20200224) and European regional development fund, Region Norrbotten, and Region Västerbotten through Bio4Metal project (20200585).

■

REFERENCES

(1) IEA Bioenergy. Biomass Combustion and Co-firing http:// task32.ieabioenergy.com/database-biomass-cofiring-initiatives/ (accessed Jun 20, 2018). (2) Koppejan, J.; Sokhansanj, S.; Melin, S.; Madrali, S. IEA Bioenergy Task 32 2012, 1−54. (3) Mousa, E.; Wang, C.; Riesbeck, J.; Larsson, M. Renewable Sustainable Energy Rev. 2016, 65, 1247−1266. (4) Suopajärvi, H.; Umeki, K.; Mousa, E.; Hedayati, A.; Romar, H.; Kemppainen, A.; Wang, C.; Phounglamcheik, A.; Tuomikoski, S.; Norberg, N.; Andefors, A.; Ö hman, M.; Lassi, U.; Fabritius, T. Appl. Energy 2018, 213, 384−407. (5) Wang, C.; Mellin, P.; Lövgren, J.; Nilsson, L.; Yang, W.; Salman, H.; Hultgren, A.; Larsson, M. Energy Convers. Manage. 2015, 102, 217−226. (6) Geerdes, M.; Toxopeus, H.; Vliet, C. v. d. Modern blast furnace ironmaking: an introduction, 2nd ed.; IOS Press BV, 2009. (7) Li, A.; Liu, H. L.; Wang, H.; Xu, H.; Jin, L. F.; Liu, J. L.; Hu, J. H. BioResources 2016, 11 (2), 3259−3274. (8) Recari, J.; Berrueco, C.; Abelló, S.; Montané, D.; Farriol, X. Bioresour. Technol. 2014, 170, 204−210. (9) Wang, L.; Skreiberg, Ø.; Van Wesenbeeck, S.; Gronli, M. G.; Antal, M. J. Energy Fuels 2016, 30, 7994. (10) Williams, P. T.; Besler, S. Renewable Energy 1996, 7 (3), 233− 250. (11) Fassinou, W. F.; Van de Steene, L.; Toure, S.; Volle, G.; Girard, P. Fuel Process. Technol. 2009, 90, 75−90. (12) Wang, L.; Trninic, M.; Skreiberg, Ø.; Gronli, M.; Considine, R.; Antal, M. J. Energy Fuels 2011, 25 (7), 3251−3265. (13) Kan, T.; Strezov, V.; Evans, T. J. Renewable Sustainable Energy Rev. 2016, 57, 1126−1140. (14) Neves, D.; Thunman, H.; Matos, A.; Tarelho, L.; Gómez-Barea, A. Prog. Energy Combust. Sci. 2011, 37 (5), 611−630. (15) Noumi, E. S.; Rousset, P.; De Cassia Oliveira Carneiro, A.; Blin, J. J. Anal. Appl. Pyrolysis 2016, 118, 278−285. (16) Antal, M. J.; Grønli, M. Ind. Eng. Chem. Res. 2003, 42 (8), 1619−1640. (17) Saleh, S. B.; Flensborg, J. P.; Shoulaifar, T. K.; Sárossy, Z.; Hansen, B. B.; Egsgaard, H.; Demartini, N.; Jensen, P. A.; Glarborg, P.; Dam-Johansen, K. Energy Fuels 2014, 28 (6), 3738−3746. (18) Yang, H.; Huang, L.; Liu, S.; Sun, K.; Sun, Y. BioResources 2015, 11 (1), 2438−2456. (19) Ş ensöz, S.; Can, M. Energy Sources 2002, 24 (4), 347−355. (20) Collard, F. X.; Blin, J. Renewable Sustainable Energy Rev. 2014, 38, 594−608. (21) Huang, Y.; Kudo, S.; Masek, O.; Norinaga, K.; Hayashi, J. I. Energy Fuels 2013, 27, 247. (22) Thunman, H.; Niklasson, F.; Johnsson, F.; Leckner, B. Energy Fuels 2001, 15 (6), 1488−1497. (23) Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1984, 62, 404−412. (24) Nik-Azar, M.; Hajaligol, M. R.; Sohrabi, M.; Dabir, B. Fuel Sci. Technol. Int. 1996, 14 (4), 479−502. (25) Basile, L.; Tugnoli, A.; Stramigioli, C.; Cozzani, V. Thermochim. Acta 2016, 636, 63−70. (26) Rousset, P.; Figueiredo, C.; De Souza, M.; Quirino, W. Fuel Process. Technol. 2011, 92 (10), 1890−1897. (27) Basile, L.; Tugnoli, A.; Stramigioli, C.; Cozzani, V. Fuel 2014, 137, 277−284. I


Increasing Efficiency of Charcoal Production with Bio-Oil Recycling

Recommend Documents