Two-Step Pyrolysis Process for Producing High Quality Bio-oils

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Two-Step Pyrolysis Process for Producing High Quality Bio-Oils Nicole L Hammer, Rene A Garrido, John Starcevich, Charles Coe, and Justinus A. Satrio Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02365 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 11, 2015

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Industrial & Engineering Chemistry Research

Two-Step Pyrolysis Process for Producing High Quality Bio-Oils Nicole L. Hammer, ‡Rene, A. Garrido, John Starcevich, , Charles G. Coe,*Justinus A. Satrio

Department of Chemical Engineering, Villanova University, Villanova, Pennsylvania 19085, USA

* Corresponding author. Tel.: (610) 519-6658; Fax: 610-519-7354. E-mail address: [email protected] (J. Satrio)

Abstract A newly developed two-step pyrolysis process for the fractionation of lignocellulosic biomass components into sugar-rich and lignin-derived rich bio-oils by using pinewood as the feedstock has been studied. In the first step, biomass is pyrolyzed between 300°C and 350oC decomposing cellulose and hemicellulose fibers to produce bio-oil having significantly higher selectivities towards sugars and lower selectivities towards low molecular weight oxygenated compounds, such as organic acids, aldehydes, and ketones than those of bio-oil produced from the conventional one-step pyrolysis at 500oC.

In the second step, the lignin-rich biomass

remaining was pyrolyzed in the presence of HZSM-5 catalyst to produce aromatic-rich bio-oil with low selectivity towards oxygenated compounds. Comparison with the conventional one-step 500°C catalytic pyrolysis showed the advantage of biomass “heat pretreatment” in the first-step pyrolysis, which promoted the decomposition of lignin to monomeric phenolic compounds which were more easily converted to aromatics. Application of catalytic pyrolysis to both steps produced two bio-oil fractions, combined, having significantly higher selectivity towards aromatics and lower selectivity towards oxygenated compounds. Over the conventional one-step 1 ACS Paragon Plus Environment


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pyrolysis, by decomposing biomass components separately at different pyrolysis temperature levels, the developed two-step pyrolysis process allows more flexibility for producing bio-oils with specific composition of chemical moieties that meet the requirements for their desired end uses.

Keywords: fast pyrolysis, catalytic pyrolysis, 2-step pyrolysis, HZSM-5, lignin conversion, lignocellulosic biomass

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1. Introduction Biomass is the only renewable energy source that can be used directly to substitute fossil fuels as source of carbon for the production of carbon-based products, such as liquid transportation fuels. Among various biomass resources, lignocellulosic biomass is considered the most promising one due to its large availability. While there are several pathways to convert lignocellulosic biomass, the thermochemical process called fast pyrolysis is very attractive due to its ability to utilize almost any type of feedstock (i.e. “feedstock agnostic”). Fast pyrolysis is a rapid decomposition of organic materials at a high heating rate and a short reaction residence time in the absence of oxygen [1]. The products of fast pyrolysis are in the form of a liquid (biooil), a solid (biochar) and non-condensable gases mixtures which all have significant benefits in the agricultural and energy sectors.

The typical present technologies of fast pyrolysis systems simultaneously convert all three components of lignocellulosic biomass (i.e. cellulose, hemicellulose, and lignin) at ‘standard’ 500oC into a liquid mixture of bio-oil composed of a wide variety of chemical compounds having diverse functionality. The chemical groups contained in bio-oil primarily include carboxylic acids, aldehydes, furans, anhydrosugars, ketones and phenols. The presence of the wide variety of chemicals having different physicochemical properties makes the utilization of bio-oil as intermediate feedstock for producing end products difficult. Bio-oil as intermediate feedstock for transport liquid production requires a reduction in ketones, aldehydes, and carboxylic acids, since high presence of these oxygenated compounds cause bio-oil to be highly acidic, highly polar, chemically unstable, and low in heating value [2]. The high polarity of bio-oil makes it immiscible with crude oil, making it difficult for using bio-oil as a cofeedstock in petroleum refineries. Furthermore, while many chemical constituents in bio-oil are valuable, their concentrations are low, making their recovery technically difficult and costly. As 3 ACS Paragon Plus Environment


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feedstock for the production of value-added chemicals, specific bio-oils that have high contents of target chemical functionalities, need to be produced.

Since cellulose, hemicellulose, and lignin have different thermal decomposition behavior, the use of a single-step fast pyrolysis reaction temperature at 500oC may not be optimal. Specifically, cellulose and hemicellulose decompose at temperature ranges well below 500°C. Conversions of these biomass components at 500°C results in low formation of desired sugar components since the formed sugars are further degraded to non-condensable gases and undesirable low molecular weight components, many are highly oxygenated, which cause the bio-oil to be unstable and highly acidic [3,4] . The presence of these oxygenated chemical constituents can result in up to 40-50% oxygen in the bio-oil produced [4, 5]

Several approaches to improve the utilization of bio-oil have been studied and reported. One approach for producing bio-oil with lower amounts of oxygenated compounds is using HZSM-5, an aluminosilicate zeolite. Studies on catalytic pyrolysis using HZSM-5 have been widely reported in literature for aromatic production from biomass sources [6-10]. One approach to reduce the chemical complexity of bio-oil is fractionating bio-oil into fractions based on the similarities of the physicochemical properties of bio-oil components. Mixed with water, bio-oil can be separated into two separate phases, i.e. polar (water soluble) and non-polar (water insoluble) phases [11, 12]. Brown et al. patented a method of fractionating bio-oil vapors into separate fractions by passing the vapors through a combination of condensers and electrostatic precipitators (ESPs) by taking advantage of the differences of condensation temperature of the chemical compounds in the bio-oil vapors [13]. All these approaches focus on post-treatment of pyrolytic vapors produced from conventional fast pyrolysis of biomass, thus do not address the need to optimize the decomposition of each biomass component to liquid product. 4 ACS Paragon Plus Environment

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1.1.Two-step Pyrolysis Approach

This work reports an improved method for producing the bio-oil by optimizing the conversion of individual biomass components towards the production of liquids. In this approach the pyrolysis process is divided into two discrete stages to provide two liquid pyrolysis products. At the heart of the two-step process is the realization that lower temperature pyrolysis yields liquid products primarily from the conversion of the hemicellulose and cellulose components and leaves most of the lignin-derived intermediates available for the high temperature reaction in the second step.

In the two-step fast pyrolysis process, the bio-oil produced from pyrolysis at low temperature in the first step will consist of chemical compounds, primarily carbohydrates/sugars in addition to smaller molecular weight oxygenated compounds, such as alcohols, ketones, organic acids, and aldehydes, derived from cellulose and hemicellulose. Pyrolysis at lower temperature would be expected to minimize further decomposition of carbohydrates into smaller oxygenated compounds, thus produce bio-oil with higher sugar contents such as levoglucosan [14]. Carbohydrates, such as levoglucosan, are extremely valuable chemicals that are used in many applications [15]. Another advantage of the low temperature pyrolysis in the first step is the low presence of phenolic compounds since the conversion of lignin is minimized. The issue with standard pyrolysis at 500°C is phenolic compounds from lignin are present and are difficult to separate into oil fractions. When the production of bio-oil for transportation fuel production is desired, pyrolytic vapors produced in the first step can be passed over the external catalytic fixed bed to promote aromatic production using acidic catalysts, such as HZSM-5. Recent catalytic pyrolysis studies on individual biomass components by Wang et al. [9] using HZSM-5 has



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shown that cellulose and hemicellulose produce up to 30% and 20% of aromatic yields, respectively.

To produce bio-oil for transportation fuel, pyrolysis of the lignin-rich biomass remaining in the second step could also be done in a catalytic mode to convert phenolics into more desirable products, e.g. aromatics [16, 17]. It is known the lignin derived products are difficult to convert to aromatics under standard pyrolysis conditions with only 5% aromatics produced from switchgrass lignin [9]. What this two-step approach seeks to achieve is that the “pre-treatment” by exposing lignin to heat at lower temperature pyrolysis in the first step will help enabling the lignin fibers to more readily decompose into smaller monomeric phenols which are easier to convert over HZSM-5, hence increasing aromatic production.

Several studies on understanding pyrolysis reaction mechanisms using step-wise pyrolysis approach with cellulose and other model compounds have been reported [18-21]. Murwanshyaka et al. studied the evolution of phenols from woody biomass by performing onestep and step-wise pyrolysis in temperature range 25-550oC by using vacuum pyrolysis system [22]. A recent review on biomass pyrolysis highlighted recent work by several researchers on the staged thermochemical conversion of lignocellulosic biomass [23]. The first heat treatment typically reported is torrefaction, in which biomass is heated at a temperature between 150 and 300oC for at least 30 minutes resulting in evolution of small oxygenates, such as acetic acid and acetol, producing a solid biomass with lower oxygen content [24-27]. de Weld et al. proposed a staged degasification process where biomass was dried at 150oC and then torrefied between 150300oC before undergoing fast pyrolysis between 450 and 550oC to produce bio-oil fractions rich in sugars and phenols [24]. Westerhof et al. reported a study on step-wise pyrolysis in which pinewood was pyrolyzed at various temperatures between 260 and 360oC and 25 minute 6 ACS Paragon Plus Environment

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residence time in a fluidized-bed reactor system. The remaining biomass then was pyrolyzed at 530oC [28].

Srinivan et al. reported that catalytic pyrolysis of torrefied pinewood by using

HZSM-5 catalyst resulted in bio-oil having higher yield of aromatics compared to those from catalytic pyrolysis of untreated pinewood [26]. Reckamp and coworkers reported that pyrolysis of paper mill sludge which was pretreated by using a combined mild acid hydrolysis followed by torrefaction at 220oC resulted in bio-oil with very high selectivity of sugars, particularly levoglucosan and levoglucosenone [27]. All these studies reported pyrolysis of torrefied biomass resulted in significantly lower yields of bio-oil and higher yields of bio-char compared to pyrolysis of untreated biomass.

It was reported that, due to the long exposure to heat,

torrefication of biomass resulted in a change from mainly hydrophilic to hydrophobic solid enhancing storage life. Torrefaction improves the water repelling characteristics of biomass through the elimination of hydroxyl groups responsible for hydrogen bonding with water molecules, and the generation of a non-polar, hydrophobic compound [29, 30]. However, it was reported that torrefaction of biomass resulted in the low yield of bio-oil during subsequent pyrolysis [27, 31, 32].

The proposed two-step pyrolysis aims to overcome the shortcomings of the prior art by producing high yields of high quality bio-oil fractions. How the variation of the pyrolysis temperature in the first stage would affect the yields and chemical product distribution of bio-oil and how they are compared with those from the single-step pyrolysis process were evaluated. Finally, the fast pyrolysis temperature combination that gives the optimum yields of desired components in bio-oil, e.g. sugars and aromatics, was determined.



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2. Materials and Methodologies 2.1 Materials The pinewood material used in this research was obtained from American Wood Fibers, Columbia, Maryland. As received the pinewood particles were approximately less than 60 mesh (250 µm) in diameter. For the pyro-probe/GCMS experiments reported here, the wood material was ground further and then sifted to obtain pinewood samples with particle size less than 80 mesh (177 µm). Prior to use, the pinewood samples were dried in a 105°C oven for several hours to remove moisture completely.

2.2 Analytical Fast Pyrolysis Fast pyrolysis experiments were performed using the CDS 5200 micropyrolyzer with micro reactor system (CDS Analytical, Inc. Oxford PA). The CDS micro-pyrolysis unit involved a quartz pyrolysis tube that is purged with helium and can be heated to a desired temperature with a furnace, an interface and a column adapter tube with needle (inserted into the gas chromatograph (GC) injector). The micropyrolyzer was equipped with a CDS 5200 high pressure tubular reactor module, which was an independently temperature controlled reactor equipped with a back-pressure regulator. The heated tubular fixed bed micro-reactor (159 mm long x 6.35 mm ID) was located downstream of the pyroprobe reactor unit. The pyrolysis vapors from the micropyrolyzer were directly injected into the GC (HP 5890 Series II plus) with helium as the carrier gas. The components were separated in the GC column and identified using a mass spectrometer (MS HP 5972 series). The chromatographic separation of pyrolysis products was performed using an alloy capillary column (RTX – 1701) having high thermal resistance (60 m x 0.25 mm x 0.25 µm film thickness with stationary phase consisting of 14% cyanopropylphenyl and 86% dimethyl polysiloxan). In the method, the injector temperature program of 270 °C and a 8 ACS Paragon Plus Environment

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gas split ratio 1:100 were used. The GC oven temperature began with a 3 min. hold at 45 °C followed by heating to 130 °C at 3 °C/min and then 270 °C at 7 °C/min. The accurately weighed biomass samples, ~0.5mg, were heated to the pyrolysis temperature, ensuring rapid pyrolysis (20 °C/ms) The samples were heated for 10 seconds at the pyrolysis temperature. The first step was performed at either 300°C, 325°C or 350°C and the vapors were analyzed by GC/MS. The remaining biomass sample was pyrolyzed in the micropyrolyzer in the second stage at 500°C again under a helium purge. For one-step pyrolysis, biomass samples were heated to 500oC at a rapid heating rate (20oC/ms). To evaluate the mass balance of fast pyrolysis process, samples were weighed before and after pyrolysis using a Sartorius MC5 microbalance (5.1 g capacity and readability of 1 µg) to determine the initial sample weight and the weight of the final residual char.

For all catalytic pyrolysis tests fast pyrolysis vapors from the micropyrolyzer were passed through the external catalytic fixed bed micro-reactor and then analyzed by the GC/MS. The catalyst used was HZSM-5 (Si/Al=25) (Zeolyst, Conshohocken, PA) prepared by heating the ammonium form to 550°C. Approximately 2.5mg of HZSM-5 (particle size 60-100 mesh (150250 µm))was added to ~100mg of silicon carbide (200-400 mesh (27 – 74 µm)) and after mixing to form a homogeneous diluted catalyst was added to the external fixed bed reactor. The reactor temperature was set at 400°C. The catalyst bed was changed after 6 runs due to deactivation studies showing stable catalyst performance for up to 3mg of biomass being passed over HZSM5 (Si/Al=25) at 2.5mg The weight hourly space velocity (WHSV) and the gas hourly space velocity (GHSV) at 400oC in the fixed bed reactor were estimated to be approximately 160 hr-1 and 10,200 hr-1, respectively.

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The catalytic pyrolysis compounds are defined as benzene, toluene, and o, p- xylenes (BTX). In addition to the BTX compounds, the remaining aromatics can be categorized as C9 aromatics, which include indane, indene and alkylbenzenes, and C10+ aromatics, which include naphthalene and methylnaphthalenes. The remaining non-aromatics compounds are classified as oxygenates, which included acids, aldehydes, furans, and phenols. Each run was repeated three times and the standard errors are included in the tables and figures. The selectivity values of the compounds were calculated based on the integration areas of the chromatogram peaks of the respective compounds divided by total integration areas of all identified peaks in the chromatogram. The relative yields of the compounds were calculated based on the integration areas of the peaks divided by the amount of biomass being pyrolyzed. .

2.3 Thermogravimetric Analysis (TGA) The one-step and two-step non catalytic fast pyrolysis experiments were also performed by using a thermogravimetric analyzer (TGA) (Model TA IR5000). The TGA was equipped with an infrared-type heater which allowed fast heating rate. In a typical pyrolysis experiment the TGA heating program method was set to “jump” to the reaction rate to achieve the fastest rate capable by the apparatus. During this time, the mass of the sample, as a function of temperature and time, was continuously measured. Purified nitrogen (5.0 grade) at a flow rate of 80 ml/min was used as the carrier gas to provide an inert atmosphere for pyrolysis and to remove the gaseous and condensable products. For the two-step pyrolysis experiments, the TGA was set to jump to the first step temperature (250°C, 300°C, 325°C and 350°C) at the fastest rate capable by the apparatus and kept isothermal for 1 min. The TGA was set to cool down to equilibrate at 50°C then to jump to the second step temperature of 500°C at the fastest rate capable. Each run was repeated three times.


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The TGA was also used to determine the proximate analysis composition of the biomass materials by following the ASTM standard protocol. For each experiment sample material was heated to 110°C and then to 500°C at a heating rate of 10°C/min under inert atmosphere to determine the moisture and volatile matter content respectively. After the weight equilibrium was reached, air was introduced to the reactor and temperature was continued raised to 800°C at 10°C/min to burn off the remaining carbon material to determine the amount of fixed carbon and inorganic matters. The TGA method for determining the proximate analysis composition of biomass materials has been reported in literatures. [33, 34]

3. Results and Discussion Two schemes of the proposed two-step fast pyrolysis process system as outlined in Figure 1 were evaluated. In the first scheme, biomass is pyrolyzed under non-catalytic condition to produce sugar-rich bio-oil in the first step and then pyrolyzed under a catalytic condition to produce aromatic-rich bio-oil in the second step (Figure 1(a)). In the second scheme (Figure 1(b)), biomass is pyrolyzed in both steps under catalytic conditions to produce two aromatic-rich bio-oil products.



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Figure 1. Schematic diagram of a two-step fast pyrolysis reactor system for processing biomass to produce fractionated bio-oils based on biomass components. (a) Scheme 1 shows the first step at low temperature is conducted under non-catalytic conditions then the remaining solid is heated to a higher temperature and the vapors are passed over an external catalytic fixed bed reactor. (b) Scheme 2 shows in both the low temperature first step and second high temperature step the vapors are passed over an external catalytic fixed bed reactor. The pinewood sample used in this study was analyzed for its fiber composition and proximate analysis composition. The results of these analyses are shown in Table 1. Table 1. Fiber Composition and Proximate Analysis Compositions of Pinewood. Weight% Weight% Biomass Fiber Proximate (dry basis) (dry basis) Composition Analysis 86.0 28.5 Volatile Matter Hemicellulose 13.9 39.5 Fixed Carbon Cellulose 0.1 24.2 Inorganic Matter Lignin 7.8 Extractives


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3.1 Determination of Optimal Temperatures in Two-Step Pyrolysis of Pinewood Effects of temperature combinations in non-catalytic two-step pyrolysis on pinewood conversion level were studied using the TGA method. Table 2 shows the conversions of pinewood from undergoing two-step pyrolysis at four different temperature combinations. These results were compared with those from one-step pyrolysis at 500oC. Pyrolysis of pinewood at 500°C resulted in the decomposition of approximately 89% of pinewood to produce bio-oil and gases, leaving 11% of char remain. All temperature combinations resulted in the same overall conversions (88%), which were also the same with that of the one-step pyrolysis. Based on the fiber composition of pinewood, the amount of cellulose and hemicellulose combined is approximately 68 wt% (dry basis) of the pinewood. These pyrolysis results indicate that to obtain optimal conversion of cellulose and hemicellulose, while minimizing the conversion of lignin, will require pyrolysis between 325 and 350oC. Consistent with these results others have reported that the majority of cellulose decomposed between 300 and 350oC [21] and hemicellulose has a lower decomposition temperature than cellulose [35].

Table 2. Two Step Pyrolysis of Pinewood at Different Pyrolysis Temperature Combinations (Results in Weight %). Temperature T=250-500°C T=300-500°C T=325-500°C T=350-500°C 1 Step 500°C

1st Step (%)

2nd Step (%)

23 ± 2.2 44 ± 1.2 56 ± 0.2 74 ± 1.4

84 ± 3.0 78 ± 1.9 73 ± 3.5 53 ± 3.4

Overall Conversion (%) 87 ± 2.0 87 ± 1.0 88 ± 1.4 88 ± 1.1 89 ± 1.1

Char Remaining (%) 13 ± 2.0 12 ± 1.0 12 ± 1.4 12 ± 1.1 11 ± 1.1

3.2 Analysis of Chemical Product Distribution of Fast Pyrolytic Vapors Effects of temperature combinations on the chemical product distribution of bio-oil products from fast pyrolysis of pinewood using the two-step pyrolysis approach were studied. 13 ACS Paragon Plus Environment


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Experiments were performed for the non-catalytic and catalytic pyrolysis scenarios. Results from the two-step pyrolysis schemes in Figure 1 were compared with those from the conventional onestep pyrolysis approach.

3.2.1

Conventional One-Step Pyrolysis Approach Pinewood pyrolysis at 500oC was performed as a basis for evaluating the performance of

the two-step pyrolysis approach. Figure 2 shows the relative chemical product distribution by chemical groups for one-step non-catalytic and catalytic pyrolysis of pinewood over HZSM-5 at 500°C. For non-catalytic pyrolysis the significant components produced were ketones and acids with 34% and 24%, respectively which were derived from hemicellulose and cellulose [36, 37]. The other primary components were carbohydrates and phenolics (6% and 22%, respectively). One-step catalytic pyrolysis of pinewood at 500oC, summarized in Figure 2b, converted a significant portion of oxygenates produced into aromatics. The overall selectivity towards aromatics was 68%, the remaining being phenolics and other oxygenates (i.e. acids, aldehydes, ketones) at 5% and 25%, respectively.


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20 15 10 5

5 0

0

Figure 2 Pyrolysis product distribution of one-step non-catalytic (a) and catalytic pyrolysis of pinewood by using HZSM-5 catalyst at 500°C (b). 3.2.2

Two-Step Pyrolysis Approach, Scheme 1: Non-catalytic in Step 1 and Catalytic in Step 2

Figure 3 shows the chemical product distribution from the two-step pyrolysis of pinewood, in which pinewood underwent non-catalytic pyrolysis at 300, 325 and 350oC in the first step followed by catalytic pyrolysis of the remaining solids at 500oC in the second step. As seen in Figure 3, increasing the first-step pyrolysis temperature increased the overall yields of bio-oil vapors produced from the first step and reduced bio-oil overall yields in the second step. These results were in agreement with those from the TGA study discussed earlier. The first-step pyrolysis temperatures also significantly affected the relative selectivity of the chemical components in bio-oil products from both steps. Among various chemical groups in bio-oil produced from the first-step, carbohydrates (sugars) had the most significant increase in yield and selectivity with increasing of the first-step temperature. Increasing the temperature


Integrated Area/mg Biomass (x 10^8)

st

1 Step: Non-Catalytic

2.5 2.0 1.5 1.0 0.5 0.0

325 350 1st Step Temperature

(a) 2.5 2.0 1.5 1.0 0.5 0.0 300-500C

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Acid

Aldehyde

Carbohydrates

Furan

Ketone

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Benzene C9 Aromatics Other Oxygenates

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Toluene C10+ Aromatics

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Aldehyde

300-500C 325-500C 350-500C 2nd Step Temperature

325C 350C 1st Step Temperature Carbohydrates

Furan

Ketone


Phenol


Xylenes Phenolics

Figure 3. Product distribution of non-catalytic first step pyrolysis of pinewood for 300°C, 325°C and 350°C and catalytic pyrolysis of the remaining solids in the second step at 500°C using HZSM-5 catalyst. (a) Relative yields of products using integrated area/mg biomass (b) Product selectivity using % of integrated area. from 300oC to 325oC increased the relative selectivity towards carbohydrates from 18% to 24%, and then almost doubled to 46% when the temperature was increased to 350oC. The increase of 16 ACS Paragon Plus Environment

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carbohydrates was balanced by decreasing selectivities of the other oxygenates, particularly ketones and phenolics (36% to 12% and 17% to 9%, respectively).

Comparison of the relative chemical product distributions of bio-oil fractions from pyrolysis at lower temperatures in the first step to that of bio-oil from the conventional one-step pyrolysis showed a clear advantage of the low temperature pyrolysis for producing carbohydraterich bio-oil. As shown in Figure 2, the one-step pyrolysis of pinewood produced bio-oil with 6%, 35% and 22% selectivities towards carbohydrates, ketones, and phenols, respectively. Compared to the one-step pyrolysis, the first-step pyrolysis at 350°C in the two-step pyrolysis process produced bio-oil with selectivities 6.5 times higher towards carbohydrates and 3.5 and 2.5 times lower towards ketones and phenols, respectively. These results suggest that lower temperatures promote the decomposition of the cellulose and hemicellulose structure towards carbohydrates rather than further decomposing these sugars into lower molecular weight oxygenated compounds. Furthermore, at lower temperatures the decomposition of lignin is limited, which reduces the formation of phenolic compounds in the bio-oil product. The relative selectivities towards phenolics from pyrolysis of pinewood at 325oC and 350oC were approximately 5 and 9%, which was significantly lower than the 21% selectivity of phenolics from pyrolysis at 500oC.

In the two-step pyrolysis process of pinewood, the remaining solids from the first step would be composed primarily of lignin and fractions of cellulose and hemicellulose at different compositions depending on the first-step pyrolysis temperature levels. These solids were catalytically pyrolyzed at 500oC in the second step of the process with the goal of increasing the selectivity towards aromatics from lignin. Figure 3 shows that for pinewood, in general, increasing pyrolysis temperature in the first step decreases the overall product yields in the second step. In addition, the selectivities of the chemical moieties were also affected at different 17 ACS Paragon Plus Environment


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levels. Increasing the first step pyrolysis temperature from 300oC to 350oC increased the selectivities towards aromatics from 69% to 81% and reduced oxygenates from 16% to less than 3% for the second step. The conventional one-step 500oC pyrolysis produced vapors with 68%, 26% and 5% selectivities towards aromatics, oxygenates and phenolics, respectively. The high aromatics and low oxygenates selectivities of the two-step pyrolysis demonstrated the advantage of the two-step pyrolysis over the one-step pyrolysis for producing aromatics-rich bio-oil.

It could be explained that the variation of the chemical selectivities of different chemical groups of the bio-oil products in the second step was largely due to the proportions of cellulose, hemicellulose, and lignin remaining from the first-step-pyrolysis which are the “feedstock” for the second-step. As described previously from the TGA studies, increasing the first-step pyrolysis temperature resulted in solid remains having reduced cellulose and hemicellulose fractions and increased lignin fraction. As shown in figure 3(a), the overall yield of bio-oil from the second-step catalytic pyrolysis of solid remaining from the first-step pyrolysis at 350oC was approximately three times lower compared to that from the solid remaining for the first step pyrolysis at 300oC. This was in agreement with the fraction of biomass that was converted in the second step pyrolysis (i.e. 14% vs. 44% of the original biomass amount from first-step pyrolysis at 350oC and 300oC, respectively). Further comparison (Figure 3(b)) showed that the overall phenolic and other oxygenate fractions in bio-oil from catalytic pyrolysis of solid remaining from the 350oC first-step pyrolysis increased by 70% and reduced by almost 600%, respectively, compared to those from pyrolysis using solid remaining from the 300oC first-step pyrolysis. The decrease of the oxygenates composition in bio-oil from pyrolysis of solid remaining from the higher temperature first-step pyrolysis was likely due to the reduced cellulose and hemicellulose fraction. The decrease of the oxygenates was advantageous since it also resulted in the increase of selectivity toward the desired aromatic products. From the point of few of aromatic 18 ACS Paragon Plus Environment

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production, others have found that compared to catalytic pyrolysis of lignin, catalytic pyrolysis of cellulose and hemicellulose produce higher yields and significantly higher selectivity towards aromatics and oxygenates [9, 38]. Based on this prior art, it is expected that feedstock having lower proportions of cellulose and hemicellulose and higher proportion of lignin would produce bio-oil having lower selectivity towards aromatics and oxygenates, with increasing selectivity towards phenolic components. The observed results from the second-step catalytic pyrolysis as shown in Figure 3 do not agree with this expected trend. It was observed that catalytic pyrolysis of solid remaining from first-step pyrolysis at higher temperature (having lower proportions of cellulose and hemicellulose and higher proportion of lignin) resulted in products having higher selectivity towards aromatics and lower selectivity towards oxygenates. The higher selectivity towards aromatics could only be explained by the higher conversions of lignin towards aromatics. We proposed that the low temperature pyrolysis in the first step may have changed the lignin morphology making the lignin more accessible for conversions to produce smaller phenolic molecules which are more easily passed over the catalyst promoting the formation of aromatic in the second-step pyrolysis

3.2.3

Two-Step Pyrolysis Approach, Scheme 2: Catalytic Pyrolysis in Both Steps 1 and 2

In the two-step pyrolysis process, when the production of aromatic-rich bio-oil is desired, both first and second steps could be set up as catalytic pyrolysis mode (Scheme 2 in Figure 1). Figure 4 shows the interesting results of the two-step catalytic process using pinewood in which pyrolytic vapors were passed through the H ZSM-5 catalyst bed reactor at 400oC. As shown in Figure 4(a), the overall vapor yields increases from 300°C to 350°C and decreases in the second step, agreeing with TGA results for non-catalytic pyrolysis. At 300oC only approximately 40% of pinewood was converted, primarily the hemicellulose and cellulose components; however, as 19 ACS Paragon Plus Environment


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seen in Figure 4(b) its selectivity to aromatics was more than 95% while the selectivities towards oxygenates and phenolics were less than 2% and 1%, respectively. At lower temperature most of the pyrolytic vapors in the first step, primarily derived from cellulose and hemicellulose, were converted to aromatics. The low conversion of lignin at lower temperature prevented the formation of phenolics, thus preserving the high aromatic selectivity.

Increasing pyrolysis

temperature to 350oC, increased the overall yields, but reduced the selectivity towards aromatics to approximately 83% and increased selectivities towards oxygenates and phenolics to 5.4% and 16%, respectively, due to the higher conversion of lignin. This is likely due to selective decomposition of hemicellulose components at the lower temperature and the unsaturated compounds produced which were transformed to aromatics over HZSM-5. However, in the second step pyrolysis at 500oC, the production of oxygenates increase to almost 20% due to the decomposition of hemicellulose and cellulose to light oxygenated compounds, such as acids, aldehydes and ketones, instead of the more desired sugars. When the first step pyrolysis temperature was increased to 325°C and 350°C there was a decrease in oxygenates due to lower availability of cellulose and hemicellulose for the second-step conversion. On the other hand, higher first-step pyrolysis temperature resulted in the increase in concentration of phenolic compounds, which was expected due to the higher relative concentration of lignin in biomass remain from the first step pyrolyzed in the second step. .


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1.8

2.0

1st Step: Catalytic HZSM-5

1.8 Integrated Area/mg Biomass (x 10^8)

Integrated Area/mg Biomass (x10^8)

2.0

1.6 1.4 1.2 1.0 0.8 0.6 0.4

(a)

2nd Step: Catalytic HZSM-5

1.6 1.4 1.2 1.0 0.8 0.6 0.4

0.2

0.2 0.0

0.0 300C

325C

350C

300-500C

1st Step Temperature Benzene C9 Aromatics Other Oxygenates


Xylenes Phenolics


35

35

30

30 % Total Integrated Area

% Total Integrated Area

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 20 15 10

325-500C 350-500C 2nd Step Temperature Toluene C10+ Aromatics

Xylenes Phenolics

(b)

25 20 15 10

5

5

0

0 300C

325C

350C

300-500C

1st Step Temperature Benzene C9 Aromatics Other Oxygenates


Xylenes Phenolics

325-500C

350-500C

2nd Step Temperature Benzene C9 Aromatics Other Oxygenates


Xylenes Phenolics

Figure 4. Product distribution of two-step catalytic pyrolysis using HZSM-5 catalyst. (a) Relative yields of products using integrated area/mg biomass (b) Product selectivity using % of integrated area.



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Table 3 shows the overall selectivity to specific chemical products from combining biooils from both the first and second catalytic pyrolysis steps at different temperature combinations compared to those from the conventional one-step 500oC catalytic pyrolysis. Thus summarizing the key results from all the studies and clearly shows the advantage of two-step pyrolysis for the production bio-oil with higher relative selectivity towards aromatics. Investigation of the twostep catalytic pyrolysis using pinewood revealed the advantage of separating pyrolytic vapors derived from cellulose and hemicellulose from those derived primarily for lignin into two fractions prior to passing the vapors to HZSM-5 catalyst for producing high-quality aromaticrich bio-oil. The bio-oil produced from the two-step catalytic pyrolysis has significantly higher relative amount of aromatics and phenolics compounds and has significantly lower

other

oxygenates compared to those the one-step 500oC catalytic pyrolysis More production of aromatics and phenolics takes advantage of the ‘heat pretreatment’ possibly loosening the fibers facilitating the conversion of lignin-rich biomass to smaller phenolic components which in turn are more readily converted to aromatics.

The decomposition of pinewood at lower temperature in the first-step enabled the production of carbohydrate-rich vapors which were easily catalytically converted to aromatics. It is well known that catalytic pyrolysis of cellulose produces high yield of aromatics from woody biomass [9]. Aromatics were formed due to the dehydration, decarbonylation and dehydrogenation of levoglucosan and other light oxygenates, such as furfural and furan, derived from the depolymerization of cellulose in the presence of HZSM-5 catalyst [32]. The low relative selectivity of oxygenates due to the low further decomposition of carbohydrates and of phenolics due the low decomposition of lignin found in bio-oil produced from pyrolysis at lower temperature also contribute towards the production of higher quality bio-oil in the first step.


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Table 3 Overall Product Selectivity of Two-step Catalytic Pyrolysis Approach of Pinewood versus Conventional 500°C One-step Pyrolysis. Benzene

Toluene

Xylenes 16.5±0.4

C9 Aromatics 19.3±1.2

C10+ Aromatics

Other Oxygenates

Phenolics

25.6±0.3

5.1±0.2

1-Step 500°C

4.3±0.1

16.0±1.0

13.2±0.3

300-500°C

2.5±0.3

21.9 ±4.5

19.0±1.1

29.4±4.0

9.4±2.5

14.2±3.0

6.2±3.0

325-500°C

4.4±1.1

17.6±3.0

21.6±3.0

30.8±2.2

9.0±1.0

5.5±1.2

12.2±2.5

350-500°C

3.8±1.5

19.7±1.5

18.8±0.3

31.9±2.0

8.7±1.0

4.7±1.3

16.2±3.5

Overall Selectivity 2-step catalytic pyrolysis

Comparison of the two-step catalytic pyrolysis with the conventional one-step pyrolysis showed that, within the studied first-step temperature range, bio-oil produced from the first-step had a better quality compared to that from the conventional one-step pyrolysis, which had 68%, and 26% selectivities towards aromatics and oxygenates, respectively. Combining both steps’ selectivities gave 82% overall selectivity towards aromatics for the two-step pyrolysis at all temperature combinations, which was still higher compared to the 68% from one-step pyrolysis. The overall selectivities towards oxygenates decreased from 14% to 4.7% when the temperature increased from 300°C to 350°C. At all three temperatures the overall selectivity of oxygenates was at least half of the 26% selectivity found in the one-step pyrolysis. As shown in Table 3, the aromatic chemical components from two-step catalytic pyrolysis showed that there is a higher selectivity towards monocyclic aromatics such as toluene and xylene. As the temperature increases from 300°C to 350°C there is a decrease in toluene and xylene from 27% to 20% and 25% to 20%, respectively. The selectivities towards the C9 and C10+ aromatics did not seem to be affected by the change of the temperature within the studied range. Combining both steps, the overall selectivities of toluene and xylenes for the two-step pyrolysis were each 20% for all temperature combinations within the studied range. In all cases, the selectivities towards these monocyclic aromatics were higher than those of the one-step 23 ACS Paragon Plus Environment


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pyrolysis approach, which further demonstrated the advantage of the two-step pyrolysis approach in producing high quality aromatics-rich bio-oil products.

4. Conclusions A new two-step pyrolysis process for the fractionation of lignocellulosic biomass components into sugar-rich and lignin rich bio-oils has been demonstrated by using TGA and a micropyrolyzer-GC/MS system. Results from studies with pinewood have shown pyrolyzing woody biomass between 300°C and 350oC decomposed cellulose and hemicellulose components to produce sugar-rich bio-oil in the first step. Within the studied temperature range, the optimum yield and selectivity to sugars was obtained from pyrolysis at 350oC. For the production of biooil for transport fuels, the sugar-rich pyrolytic vapors could be passed over HZSM-5 catalyst to produce aromatic-rich bio-oil low in phenolics and oxygenates. Higher pyrolysis temperature increased phenolics and oxygenates content due to higher decomposition of lignin component. In the second step of the process, pyrolyzing the remaining solid at 500oC produced lignin-rich derived vapors that when passed over HZSM-5 catalyst to produce a more aromaticrich bio-oil than the conventional one-step 500°C approach. This two-step process showed the advantage of biomass “heat pretreatment” in the first-step pyrolysis, which increased the conversion of the lignin-derived components towards the production of aromatics. Application of catalytic pyrolysis to both steps produced two bio-oil fractions, combined, having significantly higher selectivity towards aromatics and lower selectivity towards oxygenated compounds. Thus the developed two-step pyrolysis process allows more flexibility in tailor making the process for producing bio-oils with specific composition of chemical moieties that meet the requirements for their end uses.


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5. AUTHOR INFORMATION Corresponding Authors: * Tel./Fax: +1-610-519-6658 / +1-610-519-7354. E-mail: [email protected]. Present Address: ‡

Department of Geographical Engineering, University of Santiago of Chile (USACH). E-mail:

[email protected]. Notes The authors declare no competing financial interest.

6. Acknowledgments Funding for this project partially was provided by DOE/USDA through BRDI program (Award # 2012-10008-20271, CRIS # 0231089), by the State of Pennsylvania through Keystone Innovation Starter Kit Program (KISK), and by the Villanova University Center of Advancement in Sustainable Engineering (VCASE) seed research program. Special thank you to Dr. Charles Mullen of Agricultural Research Services, United States Department of Agriculture, Eastern Regional Research Center, Wyndmoor, PA 19038, USA, for his valuable technical inputs in improving the quality of this manuscript.

.

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Two-Step Pyrolysis Process for Producing High Quality Bio-oils

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