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Biofuels and Biomass
Deoxygenation of biomass pyrolysis vapors via in-situ and ex-situ thermal and bio-char promoted upgrading Lucas Raymundo, Charles A Mullen, Gary D Strahan, Akwasi A Boateng, and Jorge Otávio Trierweiler Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03281 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019
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Energy & Fuels
Deoxygenation of biomass pyrolysis vapors via in-situ and ex-situ thermal and bio-char promoted upgrading
Lucas M. Raymundo1, 2, Charles A. Mullen*1, Gary D. Strahan1, Akwasi A. Boateng1, Jorge O. Trierweiler2 1
USDA-ARS, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States 2
Federal University of Rio Grande do Sul (UFRGS), Department of Chemical Engineering, Engenheiro Luiz Englert st., Farroupilha, 90040040, Porto Alegre, RS, Brazil. E-mails:
[email protected],
[email protected] *To whom corresponding should be addressed. E-mail:
[email protected] (C.A. Mullen). Mention of trade names or commercial products in this publication is solely to provide specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
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Abstract Production of stable, partially deoxygenated biomass pyrolysis bio-oils is needed to increase the fungibility of bio-oil for refining into fuels and chemicals. While zeolite catalyzed upgrading is commonly used to produce such liquids, the associated catalyst deactivation is a significant hurdle to overcome. Our group has previously reported on the thermal deoxygenation of pyrolysis vapors that is carried out under an atmosphere partially consisting of recycled tail gas and without the use of externally added catalysts. In this study, thermal deoxygenation was further studied in a new and scaled-down (laboratory scale) pyrolysis system to allow for a systematic study and to better understand the factors affecting vapor deoxygenation. Temperature excursions from the fast pyrolysis temperatures near 500 ºC and/or the catalyzing effect of accumulated bio-char were hypothesized to be potential key parameters affecting vapor deoxygenation. Therefore, experiments were conducted by utilizing a recycled gas and inert atmosphere (N2) while varying process temperatures in the 500 - 750 °C range in both fluidized bed pyrolysis reactor (in-situ) and a static secondary chamber (ex-situ) positioned downstram, post removal of bio-char. Based on this arrangement, the oxygen content of bio-oils produced varied with temperature changes as follows: 31wt%, 30wt%, 19wt% for in-situ temperatures at 500 °C, 600 °C, 700 °C , and 31wt%, 27wt%, 23wt%, 19wt% for ex-situ temperatures of 500 °C, 600 °C, 700 °C and 750 °C, respectively. Compared with the use of nitrogen as carrier gas, utilization of the recycled gas atmosphere increased the yield of liquids and decreased production of gases. Organic bio-oil carbon yield was 18.5% from biomass under recycled gas at 700°C and only with 12.8% under N2; however, the oxygen contents of the bio-oils were similar. Concentrations of BTEX were where higher in bio-oils produced under the recycled gas atmosphere. 2
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Experiments were also conducted with bio-chars from switchgrass and rice hulls loaded in the ex-situ chamber and maintained at 500 °C and 600 °C fixed-bed. Bio-oils with oxygen contents as low as 19wt% were produced under promotion of rice hull bio-char at 600 ºC. This suggests that bio-char could have a catalytic deoxygenation effect particularly in a fixed or plugged bed, motivating further exploration of bio-char as a catalyst for bio-oil deoxygenation.
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1. Introduction Pyrolysis is a process where organic feedstocks, such as biomass, can be converted into liquid organics (bio-oil), combustible gases, water, and charcoal (bio-char).1 Research has focused on the application of bio-oils as a source for bio-fuels and chemicals.2–6 One key-characteristic of bio-oil is its high oxygen content. The presence of certain oxygenated compounds leads to thermal instability, affinity for water, corrosivity, high viscosity, and low heating values, characteristics that have limited the utility of biomass pyrolysis bio-oils. The key to mitigating these problems is to produce a more fungible bio-oil intermediate that can be further refined to reduce the oxygen content. This can be achieved by either utilizing post-production treatments, such as hydrodeoxygenation,7,8 or by changing the incipient pyrolysis process that will generate lower oxygenated bio-oils. The latter approach whereby low oxygen content bio-oils are produced by influencing the pyrolysis process has included a wide array of diverse techniques. For example, catalytic pyrolysis studies have explored the use of various zeolites9–15, alumina16, activated carbons17, calcium salts18,19 and bio-char20–24, in both in-situ (one pot) and ex-situ (decoupling of pyrolysis and catalysis reactions) process configurations. In the catalytic approach ZSM-5 zeolite is one heterogeneous catalyst that has been extensively explored and which has successfully reduced bio-oil oxygen content to as low as 4 wt%. However, this comes with a penalty of low yields and significant problems related to catalyst deactivation.9 Non-catalytic approaches have included the reforming of generated vapors at temperatures of around 750 °C, well above the typical optimum 500 °C pyrolysis temperature, in which bio-oils have been produced from biomass with oxygen content reduced from 23wt% to 8wt%. 22,23 Pre-treatment of biomass prior to pyrolysis has also been 4
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tested; a simple process such as torrefaction of hardwood pellets led to a decrease from 40 wt% to 24 wt% oxygen in the bio-oil product.25 In a previous work reported from our laboratory, deoxygenated bio-oils with as low as 12 wt% oxygen content were obtained from oak and switchgrass.26 Deoxygenation was achieved using a stream of recycled pyrolysis (tail) gases and volatile compounds in a bench-scale fluidized bed system, without the need for surplus reagents or catalysts. However, these results were obtained under very specific thermo-fluid dynamic conditions, which are not fully understood and have proven difficult to reproduce on other pyrolysis systems. Therefore, there has been a need for a systematic study to inform and understand the underlying specific conditions and mechanisms responsible for bio-oil deoxygenation. Therefore, a systematic experimental study on a smaller scale was undertaken for which the results are reported herein. The presence of unmeasured temperature spikes or high-temperature excursions, and a possible catalytic effect of accumulated bio-char in the reactor have been hypothesized to be among important key parameters responsible for the observed deoxygenation of vapors in addition to the presence of the reactive recycled gas. The hypotheses were supported by similar bio-oil deoxygenation reports in the literature where, thermal or bio-char promoted deoxygenation occurred without the use of other additives. 20–24 Therefore, the aim of this work was to study the influence of these two conditions on bio-oil composition and the associated product distribution. For this purpose, switchgrass was processed under various conditions in the aforementioned laboratory scale pyrolysis
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unit developed for this purpose. This unit, designed to process biomass at a rate ~560 g/h, comprises a fluidized bed reactor, a cyclone and hot vapor filter for bio-char separation and a secondary (ex-situ) reactor chamber. Experiments aimed at determining the influence of thermal excursions on deoxygenation were performed by passing pyrolysis vapors through heated zones with temperatures ranging from 500 to 750 °C. To test the influence of bio-char, the temperature was varied in two different zones: (1) inside the fluidized bed pyrolysis reactor, possibly influenced by the presence of generated bio-char, and (2) in the ex-situ chamber, downstream of the cyclone-filter system, therefore with no generated bio-char present. Experiments were carried out under both inert (N2) and recycled gas atmospheres. A third approach was explored whereby the ex-situ chamber was loaded with pre-made bio-char. 2. Materials and Methods 2.1.
Biomass Switchgrass was provided by the McDonnell farm in East Greenville, Pennsylvania. Prior to pyrolysis, loose switchgrass was
ground to 1 mm particle size and under using a Willey Mill. Elemental analysis of the feedstock is provided in Error! Reference source not found.. Dry basis analysis: 47.8 wt% C, 5.9 wt% H, 0.5 wt% N, 43.1 wt% O, 2.7 wt% ash. 2.2.
Experimental Apparatus
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The process flow diagram (Error! Reference source not found.) depicts the laboratory scale apparatus used. Like the original larger scale setup26 the apparatus comprises a fluidized-bed reactor with various auxiliary equipment. This includes a feeding system comprising of an enclosed biomass hopper, feed auger for feed rate control, a rotary airlock and a fast injection auger that quickly pushes biomass into the fluidized bed. The fluidized bed pyrolysis reactor is a stainless-steel tube with 35 mm internal diameter (ID) and 700 mm height. Two different sets of electric heaters are used to decouple the reactor bed temperature and the freeboard zone temperature. Bio-char is separated post freeboard using a Lapple type cyclone (Dc = 25mm) followed by a hot vapor filter with 20 µm mesh. Following the filter column is an ex-situ secondary reaction chamber equipped to explore the full range of temperature excursion and the catalytic effect of bio-char before vapor condensation. The ex-situ chamber has an internal diameter of 73 mm and a height of 150 mm; it was loaded with 600 g of 5 mm diameter x 5mm long quartz cylinders to provide flow resistance and ensure better flow distribution. In the experiments investigating the bio-char effect, the ex-situ chamber is fully loaded with pre-made biochars from switchgrass (SWG) and rice hulls (RH) created by slow pyrolysis at various temperatures (500°C or 600 °C) at a heating rate of 10°C/min. One advantage of the rice hull bio-char over that of the switchgrass is that it is denser thereby allowing more mass to be loaded within the same chamber. Rice hull derived bio-char was loaded at 40 g compared to 20 g for switchgrass, indicating weight hourly space velocities of around 8 h-1 and 16 h-1 respectively. A two-coil condenser in a series arrangement, each with an internal diameter of 25 mm and 400 mm height, was employed for vapor quenching and initial bio-oil separation by condensation before the electrostatic precipitation (ESP). The cooling water was kept 7
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at 4 °C. The ESP which is used to complete aerosol removal post-condensation has an internal diameter of 63 mm and 500 mm height, operating at 14kVDC. The flow stream is complete with a bleed line having a cold trap submerged in a dry-ice/isopropanol bath to collect any light volatile compounds that might have escaped the condenser-ESP arrangement. The entire process arrangement is completed with a gas recycle loop, enabled by a regenerative blower with its inlet section protected by a coalescence filter. To measure the recycled flow-rate an orifice plate is utilized, whereas a mass-flow controller controls nitrogen injection. Finally, a passage heater is used to pre-heat gases going into the reactor. 2.3.
Experimental method Before the experiments are started, the reactor was loaded with 230 g of sand, equivalent to 150 mm of fixed bed height. The sand
particle size was classified using 28 mesh Tyler and 35 mesh Tyler sieves, equivalent to 420 µm to 595 µm respectively. The system was preheated to operational temperatures before feeding. During the preheating process, the recycle blower was activated to keep a constant flow through the system, a small amount of nitrogen injection is utilized to purge the system of air and to keep it under positive pressure. When the set-point temperature was reached, the feeder was activated. Biomass (switchgrass) feed rate of 560 g/h was used in the current study. Gas flow into the reactor (recycle and nitrogen) is controlled at 6 NL/min during the run. During recycle runs a nitrogen flow of 0.5 NL/min is kept for reference stream in the gas composition. Observation of steady state during the experiments was based on product gas composition. After 20 to 30 minutes of biomass feeding, gas concentration (mol%) 8
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of measured species changed by less than 5% per sampling step (every 5 minutes).. No trends could be observed in gas composition or liquid composition over the course of the experiment. Reactor bed temperature, freeboard temperature, and ex-situ chamber temperature were manipulated according to the experimental plan shown in Error! Reference source not found.. Initially, control experiments at 500 °C were performed as a baseline (run code 5/5/5), before reactor temperatures were raised to 600 °C and 700 °C. Later, with the bed temperature kept at 500 °C, freeboard and the ex-situ chamber temperatures were varied from 500 up to 750 °C as per the conditions shown in Error! Reference source not found.. All experiments were performed in triplicate. Vapor residence times at the highest temperature point are presented in Error! Reference source not found. to give an indication of the secondary reaction times expected. These were estimated by assuming a plug flow of product vapor and gas at a rate of 9.6 NL/min, composed of 6 NL/min carrier gas flow and 3.6 NL/min product gas and vapor flow, within the space occupied by the sand bed and quartz in the ex-situ. Product quantification is carried out after the experiment is over and equipment allowed to cool down. Bio-char from the cyclone char-catch and filter were collected and weighed. Condenser liquid was separated between organic and aqueous fraction using a valve in the bottom of the collection flask. Both phases are weighed in jars. The bio-oil catch in the ESP does not typically phase-separate so it is collected in jars and weighed. Unlike ESP, cold trap samples do undergo phase separation and were decanted and weighed in 9
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vials. Gaseous product yield was estimated by the difference between the biomass fed into the reactor and products collected. After each run, the bed was burned with 12.0 NL/min of air at the temperature of 700 °C for 1 h. Negligible quantities (i.e.
