Article pubs.acs.org/EF
Thermo-Catalytic Reforming of Woody Biomass Nils Jag̈ er,† Roberto Conti,† Johannes Neumann,† Andreas Apfelbacher,† Robert Daschner,† Samir Binder,† and Andreas Hornung*,†,‡,§,∥ †
Fraunhofer UMSICHT, An der Maxhütte 1, 92237 Sulzbach-Rosenberg, Germany University of Bologna, Via Selmi 3, 40126 Bologna, Italy § University of Birmingham, Edgbaston, Birmingham, West Midlands B15 2TT, United Kingdom ∥ Friedrich-Alexander University Erlangen-Nürnberg, Schlossplatz 4, 91054 Erlangen, Germany ‡
ABSTRACT: The objective of the present work is to determine the potentials of woody residues from agriculture (tree trimmings or vine shoots) for fuel applications. Fraunhofer UMSICHT is carrying out research for a new thermochemical conversion technology to convert biogenic residues into valuable storable products, such as carbonizates, hydrogen, and fuel. Thermo-catalytic reforming (TCR) is an intermediate pyrolysis combined with a unique integrated catalytic reforming step. In this experimental study, the TCR of three different woody biomasses has been investigated in a 2 kg/h laboratory-scale plant. The feedstocks were woody residues produced from the pruning of agricultural lands. This is a sustainable biomass solution with an up-to-date great unexploited potential. The results revealed that the composition of the feedstock had only a minor effect in the compositions and characterizations of the products, with the catalytic reforming step at 973 K. The gas had a higher heating value between 14.6 and 14.9 MJ/kg and a high H2 content at 33−36 vol %. The bio-oil obtained had a low water content and a high heating value. The high quality of the bio-oil is reflected in the low O/C ratio of 0.15 ± 0.04. The study revealed that the quality of pyrolysis bio-oils of other processes, such as fast pyrolysis, is only comparable to TCR bio-oils after additional intense hydrodeoxygenation treatment. The carbonizate generated had a high heating value and a high carbon content. As a result of the low O/C ratio, the carbonizate has a high stability and, consequently, good carbon storage in soil.
1. INTRODUCTION To meet the present energy challenges and to combat climate change, efficient and sustainable use of resources and energy is one building block. Biomass can add an important contribution to meet environmental and energy political targets;1,2 however, a major part of these resources already has alternative applications, energetically and materially, and the unused potentials compete with food production or are not sustainable.3 This leads to an increased competition for these resources, which has not only a big impact on the future competitiveness but also a negative effect on the political and social acceptance. The use of biomass has to be carefully considered, and a consistent use of existing sustainable biomass potentials is needed. There is great unexploited potential of woody biomass from pruning operations carried out in agricultural and forestry plantations.4 Pruning is a widespread technique that has its origin as a sustainable fuel production and is suspected to increase the productivity of the plants.5 In a study by Alejano et al.,5 this effect was not confirmed for the acorn production from oak trees. However, Martiń et al.6 investigated that light pruning had no negative effect on the tree growth and is a sustainable way for biomass production. In this experimental study, the thermo-catalytic reforming (TCR) of woody biomass from pruning olive trees (Olea europaea), evergreen oak trees (Quercus ilex), and vine (Vitis vinifera) has been investigated. In the Mediterranean Basin, these species have high economic importance with great unexploited potential of woody biomass.4,7−9 Residual biomass from olive pruning © XXXX American Chemical Society
reaches an average yield per year of 1.31 t/ha and, for older trees, up to 2.1 t/ha of dried wood.4 Spain alone has about 2 million ha of olive plantations, producing approximately 2.6 million tons of woody residues per year.8 Agriculture biomass wastes from the wine industry (vine shoots) create 1.4−2.0 t/ha, with over 8 million ha of cultivated vineyards in the world. This means an annual production of 11.2−16 million tons of residual woody biomass.9 Much of this potential is unused and destroyed by in-field burning or ploughing into the soil with only minimal environmental advantages.4,10,11 These practices have several disadvantages, such as uncontrolled spreading of fire or CO2 emissions.12 In addition, field biomass burnings have an impact on the levels and chemical composition of total particulate matter and may create a public health hazard as a result of fine dust.13 The objective of the present work is to determine the potentials of woody agriculture residues for fuel applications. Several biomass conversion technologies have been developed, including biochemical and thermochemical processes. Pyrolysis is a very promising thermochemical technology to convert solid biomass into liquid, gaseous, and solid products, which can be used as a fuel substitute.14 Pyrolysis of woody biomass for fuel applications has been studied extensively in the Special Issue: In Honor of Michael J. Antal Received: April 15, 2016 Revised: June 6, 2016
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DOI: 10.1021/acs.energyfuels.6b00911 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. Feedstock: (a) OL, (b) EO, (c) VS. Extremadura in Spain in 2015. The collected biomass was considered to be representative of woody residues produced from pruning of agriculture lands. As depicted in Figure 1, the feedstocks were pelletized with a diameter of 6 mm and a length of 10−25 mm. The evergreen oak pellets (Figure 1b) showed a dark brown color possibly as a result of the high content of bark. 2.2. Experimental Setup. The TCR test reactor,19 located at the site of Fraunhofer UMSICHT, Sulzbach-Rosenberg, Germany, consists of five main parts: a sealed feed hopper, the main horizontal TCR reactor, the vertical TCR post-reformer, a condensing unit with a container to collect the condensate, and a gas filtration system. The schematic diagram of the TCR plant has already been reported elsewhere.19,20 The TCR test reactor is a batch system, and the feed hopper has a total capacity of approximately 7 L. The horizontal TCR reactor is a three-step screw reactor with different temperature zones. The length of the reactor is 1000 mm with a diameter of 90 mm and is electrically heated. Dependent upon the physical and chemical characteristics of the feedstock, the temperature and heating rate of the feedstock can be varied. Throughout the experiments, the temperature in the first third of the reactor was 473 K and the temperature in the other two segments was 673 K. At the transition area between the feed hopper and the horizontal reactor, the temperature was approximately 400 K, and at the transition area between the horizontal reactor and the post-reformer, the temperature was approximately 873 K. The heating rates were in the range of 50− 300 K/min. In the horizontal reactor, the intermediate pyrolysis occurs. The average residence time of the feedstock in the horizontal reactor was 12 min. Through the patented process design, the carbonized feedstock is an in situ produced catalyst with excellent catalytic effects on the quality of the products.21 The feedstock is conveyed through the reactor by means of a screw and then enters a vertical post-reformer. The post-reformer has a height of 910 mm with a diameter of 100 mm and is electrically heated. During the experiment, the temperature profile in the reformer is between 900 and 973 K. In this unit, the carbonized catalytically activated feedstock and the pyrolysis vapors generated in the horizontal auger reactor are upgraded. The carbonized feedstock is converted into carbonizate, meaning the solid carbon residues after the pyrolysis process, with high carbon and low oxygen and hydrogen contents. The gaseous intermediate products are converted into hydrogen-rich synthesis gas and condensable vapors with significantly enhanced characteristics. The upgraded pyrolysis vapors are passed on to the condensing unit. In the countercurrent principle pipe cooler, the vapors are fully condensed and collected. The gas is led into a filtration system. The particles and aerosols still present in the gas are removed in a wash bottle filled with the aqueous phase from previous runs and with an upstream ice-cooled spiral condenser. Two additional fibrous filters ensure that the gas is free from particles that could influence the connected online gas measurement and gas analytics. 2.3. Analytical Methods and Measurements. Elemental analysis (CHNS) on dried raw materials and byproducts (bio-oil, carbonizate, and water phase) was carried out using an elemental
past 25 years and is still a current topic. In this respect, the emphasis in the last years has shifted considerably from studies that focused on the feasibility of the pyrolytic conversion of woody biomass8 toward a focus on the maximized oil yield15,16 and, more recently, to processes for product upgrading.17,18 Figueiredo et al.8 has explored pyrolysis of olive wood wastes in the temperature range from 573 to 1173 K with slow heating rates of 10 K/min. They found that the pyrolysis behavior of olive wood can be divided into four temperature steps. The results showed that the pyrolysis of wood at temperatures around 973 K is very promising. The carbonizate had a high carbon content, and the gas composed mainly of CO, CO2, CH4, and H2 but had a low heating value. Bridgwater et al.15 investigated the elemental and chemical composition of fast pyrolysis oils under different conditions. The maximum yield was obtained in the range of 723−823 K. Under these conditions, the compositions of the oil were very similar to the compositions of the feedstock. The results showed that the bio-oil yield decreased at higher temperatures and the oil deoxygenated. Finally, Bridgwater et al.15 pointed out that upgrading of the pyrolysis liquids is a challenge to be faced. A study on upgrading bio-oil from woody biomass was performed by Kim et al.17 They stated that the deoxygenation of the bio-oil is a mandatory requirement before commercial use of the oil is possible. In the study, they varied different process parameters and achieved a deoxygenation and reduced water content of the liquid product. This paper delivers a sustainable utilization pathway of woody residues generating high-quality storable energy carriers, such as syngas, bio-oil, and high-calorific carbonizate. Fraunhofer UMSICHT has developed the TCR process, which is based on an intermediate pyrolysis14 with an integrated reforming step. TCR was previously used successfully on waste streams, such as digestate,19,20 while, in this paper, the application of the patented technology for woody biomass is described. The aim of this study is to compare the TCR product distributions and qualities of three different wood residues from pruning operations, highlight possible applications, and compare the results to results from other technologies from the literature.
2. MATERIALS AND METHODS 2.1. Raw Materials. In this study, preconditioned agriculture residues [olive (O. europaea) (OL), evergreen oak (Q. ilex) (EO), and vine shoots (V. vinifera) (VS)] provided by Centro de Investigaciones ́ Cientificas y Tecnológicas de Extremadura (Cicitex) were used. The agriculture residues were obtained from the autonomous community B
DOI: 10.1021/acs.energyfuels.6b00911 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels analyzer vario MACRO cube from Elementar Analysensysteme GmbH. The ash content was calculated according to the ASTM method [ASTM E1534-93 (reapproved 2013)] as the mass percent of residue remaining after dry oxidation at 863 ± 10 K for a minimum of 3 h using a muffle oven. The oxygen content was calculated by difference [100 − (∑CHNS + ash)]. The high heating value (MJ/kg) was determined by a combustion bomb calorimeter IKA C2000 series from IKA. The moisture of feedstock and carbonizates was determined according to the ASTM method [ASTM E871-82 (reapproved 2013)] as the mass percent lost after drying in an oven at 376 ± 1 K overnight for approximately 16 h. The water content in bio-oil samples was determined by Karl Fischer titration using a 915 KF Ti-Touch titrator from Metrohm AG. In addition, the total acid number (TAN) of bio-oil samples was determined by 916 Oil Ti-Touch (Metrohm AG). For the online gas measurement, a gas detection system was used from Dr. Födisch Umweltmesstechnik AG. The system was calibrated for pyrolysis gas by Födisch. The system consists of a MGA 12, whose measurement principle is based on an infrared photometer (CO, CO2, CH4, and CxHy), an electrochemical cell (O2), and an additional thermal conductivity detector (H2). The calorific value of the gas and the density were measured with an online gas calorimeter CWD 2005 from Union Instruments GmbH. The gas analysers were calibrated with a calibration gas prior to the experiments.
in particular, taking into consideration the processing and origin of the biomass.9,22 Through the elemental composition testing, significant differences can mainly be observed for the carbon and ash contents. The carbon contents of OL and EO were nearly the same with the percentage value between 44.2 wt % (EO) and 44.9 wt % (OL), respectively. On the other hand, VS showed a slightly higher carbon content of 47.2 wt %. On the contrary, the ash content of VS was higher in comparison to OL and EO. VS showed an ash content of 4.1 wt %. OL had the lowest ash content of 2.2 wt %, followed by 3.6 wt % for EO. As expected, all biomass samples had a high oxygen content between 41 wt % (VS) and 46 wt % (OL). On a dry basis, OL, EO, and VS had similar HHVs of 19.3, 19.6, and 19.2 MJ/kg, respectively. As expected, all pelletized feedstocks had a similar moisture content. The moisture content of the OL pellets was 8.7 wt %, on average. The EO pellets had a moisture content of 10.0 wt %, and the VS feedstock had a moisture content of 7.6 wt %, on average. The analysis of the main chemical components of the woody biomass cellulose, hemicellulose, and lignin was performed by Cicitex. It is well-known that these components influence the product yields of the pyrolysis process. These three main components also decompose at different temperature ranges and, therefore, have a crucial influence on the pyrolysis process.23−26 With respect to OL, similar contents were published by Fengel et al.27 in an overview. 3.2. Carbonizate Characterization. Figure 2 shows the TCR carbonizates obtained after the TCR step at 973 K. The EO carbonizate had the same structure as the feedstock pellets. As a result of the shrinkage, the diameter and length of the carbonizate pellet were smaller. The carbonizate had a diameter of 4−5 mm and a length of about 8−22 mm, on average. The shape of the carbonizate from OL and VS was not similar to the original feedstock shape. The pellets were not stable, and most of them crumbled to a fine powder. For the feedstock OL, the carbonizate was completely decomposed throughout the TCR process. The powder had the same shape as the feedstock before the pelletization process. The elemental analysis, together with the calorific values, and moisture and ash contents of the carbonizate are presented in Table 2. The carbonizate from the feedstock OL had the higher carbon content (84.2 wt %), followed by EO (78.1 wt %) and VS (70.8 wt %). This is notably reflected in the HHVs of the carbonizates. The HHV of the carbonizate from the feedstock OL was found to be 30.3 MJ/kg. The heating values from EO
3. RESULTS AND DISCUSSION 3.1. Feedstock Characterization. The results of the analysis of the feedstocks are shown in Table 1. Elemental Table 1. Feedstock Characterization On a Dry Basis C (wt %) H (wt %) N (wt %) S (wt %) ash (wt %) Oa (wt %) HHV (MJ/kg) moisture (wt %) cellulose (wt %) hemicellulose (wt %) lignin (wt %) a
OL
EO
VS
44.9 6.5 0.4 0.06 2.16 46.0 19.3 8.7 31−33 23−27 18−21
44.2 6.3 0.5 0.04 3.60 45.4 19.6 10.0 41−45 29 20−30
47.8 6.2 0.6 0.05 4.08 41.2 19.2 7.6 24−36 37−40 22−24
Calculated by difference.
analysis and determination of the ash content, higher heating value (HHV), and water content were carried out. The elemental composition and proximate analysis of the woody biomass were similar to that published in the literature before,
Figure 2. Carbonizate: (a) OL, (b) EO, and (c) VS. C
DOI: 10.1021/acs.energyfuels.6b00911 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. Properties of Carbonizate C (wt %) H (wt %) N (wt %) S (wt %) ash (wt %) Oa (wt %) HHV (MJ/kg) H2O (wt %) a
Table 3. Properties of Oil
OL
EO
VS
84.2 1.2 0.8 0.1 12.2 1.5 30.3 0.4
78.1 1.2 0.9 0.1 18.1 1.6 27.3 0.4
70.8 1.0 0.9 0.1 23.1 0.7 25.5 0.6
C (wt %) H (wt %) N (wt %) S (wt %) ash (wt %) Oa (wt %) HHV (MJ/kg) H2O (wt %) TAN (mg of KOH/g)
Calculated by difference. a
and VS were lower, with values of 27.3 and 25.2 MJ/kg, respectively. As expected, the percentage of ash among the three carbonizates increased in the order of OL (12.2 wt %), EO (18.1 wt %), and VS (23.1 wt %). This is the same trend observed among the ash content of the feedstocks as the mineral content of the feedstock is transferred to the carbonizate. For the pyrolysis of olive wood sawdust, Figueiredo et al.8 published a carbon content in the carbonizate of 90 wt % at a pyrolysis temperature of 973 K. One explanation could be the different particle sizes in the experiment, which caused the higher carbon yield. In an experimental study on the effect of the particle size published by Manyà et al.,9 an opposite effect was described. The study revealed a positive relation between the particle size of the biomass and the fixed carbon yield.9 Therefore, it is more likely that a different composition of the OL biomass is a reason for the higher carbon content in the char. As a result of the missing feedstock characterization in the paper by Figueiredo et al.,8 a conclusive clarification is not possible. The main reason for the different carbon yields was the lower heating rate of the feedstock in the process. This is in accordance with the study published by Guerrero et al.28 that demonstrated the influence of the heating rate on the carbon content of char from the pyrolysis of eucalyptus wood. 3.3. Bio-oil Characterization. For the liquid phases, consisting of the aqueous phase and the organic bio-oil phase, a distinct phase separation was observed, resulting from their difference in density and polarity. The aqueous phase consisted out of the moisture of the feedstock and out of product water.29 Through this physical property, the bio-oil is easily to be separated from the aqueous phase after 14 h by gravity. The low carbon content of the aqueous phase (
