Energy Fuels 2010, 24, 6548–6554 Published on Web 11/16/2010
: DOI:10.1021/ef100935r
Acidity of Biomass Fast Pyrolysis Bio-oils Anja Oasmaa,*,† Douglas C. Elliott,‡ and Jaana Korhonen† †
VTT Technical Research Centre of Finland, Vuorimiehentie 5, Espoo, Post Office Box 1000, FI-02044 VTT, Finland, and ‡ Pacific Northwest National Laboratory (PNNL), Post Office Box 999, Richland, Washington 99352, United States Received July 22, 2010. Revised Manuscript Received October 20, 2010
The use of the total acid number (TAN) method for measuring the acidity of biomass fast pyrolysis bio-oil was evaluated. Suggestions for carrying out the analysis have been made. The TAN method by American Society for Testing and Materials (ASTM) D664 or D3339 can be used for measuring the acidity of fast pyrolysis bio-oils and their hydrotreating products. The main difference between the methods is that ASTM D664 is specified for higher TAN values than ASTM D3339. Special focus should be placed on the interpretation of the TAN curves because they differ significantly from those of mineral oils. The curve for bio-oils is so gentle that the automatic detection may not observe the end point properly, and derivatization should be used. The acidity of fast pyrolysis bio-oils is mainly derived (60-70%) from volatile acids. Other groups of compounds in fast pyrolysis bio-oils that influence acidity include phenolics, fatty and resin acids, and hydroxy acids.
be broken into two titration categories: potentiometric and colorimetric. The potentiometric method uses a potentiometer to detect the acidic constituents and convert them into an electronic read out. The colorimetric method uses p-naphtholbenzein, which responds to a change in the pH indicator that has been added to the solution. Once the acidic constituents have been neutralized by potassium hydroxide (KOH), the sample will change from orange to blue-green, indicating the end point (EP).1 D3339 is more useful for bio-oil versus D974 because it uses a smaller, more dilute sample; thus, there is less interference with the colorimetric determination. The pH of fast pyrolysis bio-oils from untreated biomass is low (2-3). The acidity of a fast pyrolysis bio-oil is the sum of the acidity of its compounds (Table 2). The acids in biomass fast pyrolysis oils are mainly due to the degradation of hemicelluloses in wood. Fast pyrolysis bio-oils typically contain 3-6 wt % volatile acids, with the main compounds being acetic and formic acids. Carboxylic acids (Figure 1) are weak acids, which means that they are only partially dissociated in aqueous solutions. They are the most acidic compound group in fast pyrolysis bio-oil, with pKa values of 3-5 (Table 2). The pKa = -log([Hþ][A-]/[HA]), and for water, pKa = -log([Hþ][OH-]/[H2O]). Alcohols are amphoteric, which means that they are either weakly acidic or weakly basic. Alcohols can be protonated by a strong acid to form oxonium ions. In aqueous solutions, alcohols dissociate slightly to form alkoxides (Figure 2). Phenols are more acidic than alcohols and can also form phenoxide ions when reacted with hydroxide ion. In the recent U.S. Department of Energy Solicitation2 [Biomass Fast Pyrolysis Oil Stabilization, Funding Opportunity Announcement (FOA), DE-PS36-08GO98018], the TAN was specified for use as a quality indicator in pyrolysis oil stabilization. TAN was included as the definitive measure of acid in oil as used in the petroleum processing industry, with the belief that its reduction would be an important factor in facilitating the processing of upgraded bio-oil in existing petroleum refineries to reduce imports of petroleum. It was the
Introduction The acidity of fast pyrolysis bio-oils is typically determined and reported as pH. The pH is a representation of the corrosiveness of the oil, but it does not indicate the concentration of acidic constituents. The pH test method is useful for applications in which corrosive oil could cause considerable damage.1 The total acid number (TAN) is the preferred method for measuring the acidity of petroleum oils. Recently, interest has developed for its use with fast pyrolysis bio-oil. Currently, in North America, the term TAN as applied to bio-oil is being investigated and its replacement with acid number (AN) is being suggested. This change is based on the fact that AN tests do not detect the total acid concentration of the lubricant. The acid concentration of a petroleum lubricant contains both strong and weak components. Strong acidic components are referred to as the SAN. The weak and strong components are typically combined as the AN. Even though the AN comprises both acidic components, it does not represent all of the acidic components that might be found in the subject oil. AN tests are not affected by, for example, extremely weak acids that have a dissociation constant of less than 10-9. These weak acids are found in high concentrations in bio-oils yet may not have a significant impact on corrosion properties of the oil. This is the reason TAN is being replaced by AN when analyzing bio-oils.1 Table 1 lists the current American Society for Testing and Materials (ASTM) standard test methods for determining the TAN. Each test has been designed for specific purposes, with ASTM D664 and D974 being the two most commonly used tests. ASTM D1534 and D3339 are similar versions of D974 used for special cases. D3339 is especially intended when the sample size is too small for D664 or D974. The TAN tests can *To whom correspondence should be addressed. E-mail: anja.oasmaa@ vtt.fi. (1) Coverdell, A. A comprehensive look at the acid number test. Mach. Lubr. 2010, January; http://www.machinerylubrication.com/article_ detail.asp?articleid=1052. r 2010 American Chemical Society
6548
pubs.acs.org/EF
Energy Fuels 2010, 24, 6548–6554
: DOI:10.1021/ef100935r
Oasmaa et al.
Table 1. Common ASTM TAN Test Methods1 ASTM test
indication
reagent
notes
sample size TAN (0.05-1.0) (g)
D664 D974 D1534
potentiometric colorimetric colorimetric
potassium hydroxide potassium hydroxide potassium hydroxide
20.0 20.0 17.6
D3339
colorimetric
potassium hydroxide
slow, labor intensive dark oil obscure color change, labor intensive only viscosities less than 24 cSt, TAN range of 0.05-0.5 mg of KOH/g uses smaller oil samples
2.0
Table 2. Chemical Composition of a Pine Fast Pyrolysis Bio-oil fast pyrolysis bio-oil (pine)
wet
dry
water (wt %) acids (wt %) formic acid (wt %) acetic acid (wt %) propionic acid (wt %) glycolic acid (wt %) alcohols (wt %) ethylene glycol (wt %) methanol (wt %) aldehydes, ketones, furans, and pyrans (wt %) non-aromatic aldehydes (wt %) aromatic aldehydes (wt %) non-aromatic ketones (wt %) furans (wt %) pyrans (wt %) sugars (wt %) anhydro-β-D-arabino-furanose, 1,5- (wt %) anhydro-β-D-glucopyranose (levoglucosan) (wt %) dianhydro-R-D-glucopyranose, 1,4:3,6- (wt %) hydroxy, sugar acids (wt %) LMM lignin (wt %) catechols (wt %) lignin-derived phenols (wt %) guaiacols (methoxy phenols) (wt %) HMM lignin (wt %) extractives (wt %) fatty acids (wt %) triglyserides (wt %) resin acids (wt %)
23.9 4.3
0 5.6 1.5 3.4 0.2 0.6 2.9 0.3 2.6 20.3 9.72 0.009 5.36 3.37 1.10 45.3 0.27 4.01 0.17
2.2 15.4
34.4
13.4
1.95 4.35
17.7 0.06 0.09 3.82 2.6 5.7
C
H
N
O
pKa
40.0
6.7
0
53.3
37.5
12.5
0
50.0
40.0
6.7
0.0
53.3
48.6
8.11
0.0
43.2
44.1
6.6
0.1
49.2
3.8 15-16 15 16 16-36 17 17 20 32-36 32-34 3-16
68
6.7
0.1
25.2
3-5 9-10
15.7 3-5 3.8 4.7
10 10 63.5 75.4
5.9 9.0
0.3 0.2
30.3 15.4
7-16 9-10 7
determination of aliphatic, aromatic, and difunctional carboxylic acids. No interference by other compounds was observed. The EPs of the pyrolysis oil titration curves are not as sharp as those obtained for model compounds, but the EPs give reproducible carboxylic acid contents that are independent of the sample concentration. Moens et al.4 studied neutralization and stabilization of fast pyrolysis bio-oils and used the TAN as an indication of the change in acidity. They concluded that treating crude bio-oil with methanol in the presence of a solid acid catalyst is an inefficient process for neutralizing and stabilizing crude biooils. They are currently exploring other approaches to lowering the TAN of whole bio-oils and further investigating reactive distillation with methanol, because this may be useful in the development of alternative fractionation schemes for whole bio-oils. Agblevor et al.17 have suggested an alternative method for TAN determination of pyrolysis bio-oil. In the method, aqueous potassium hydroxide (0.1 M KOH) was used as a standard titrant instead of 0.1 M KOH in alcoholic solution and acetone was used as a titration solvent instead of mixtures of toluene and 2-propanol. The results showed that TAN of bio-oil samples obtained by the new method are in good agreement with the one obtained by the ASTM method. It was concluded that the new method was simpler, relatively cheaper,
Figure 1. Carboxylic acid ions in aqueous solutions.2
intention of this FOA to solicit applications for proposed methods to produce, treat, and modify pyrolysis oil, so that the method renders the pyrolysis oil stable for periods of at least 6 months. For the purposes of this FOA (on the basis of the understanding at the time), stability means that the following issues are addressed: (1) Molecular building reactions of various compounds present in the pyrolysis oil are reduced as measured by the rate of increase in the viscosity of the pyrolysis oil compared to the starting pyrolysis oil. It is desirable to reduce the rate of increase in viscosity by a magnitude of 10 compared to the original standard pyrolysis oil. (2) A reduction in the TAN to below 5 is reduced by the carboxylic functionality of compounds present in the pyrolysis oil. (3) The removal of residual char fines from the pyrolysis oil is measured by the pyrolysis oil ash content to less than 0.01 wt %. In his thesis, Nicolaides3 studied the determination of functional groups of pyrolysis oils by titration methods. He concluded that NaOH titration is a reliable method for the (2) http://www.federalgrants.com/Biomass-Fast-Pyrolysis-Oil-Bio-oilStabilization-12552.html. (3) Nicolaides, G. M. M.S. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 1984.
(4) Moens, L.; Black, S. K.; Myers, M. D.; Czernik, S. Study of the neutralization and stabilization of a mixed hardwood bio-oil. Energy Fuels 2009, 23 (5), 2695–2699.
6549
Energy Fuels 2010, 24, 6548–6554
: DOI:10.1021/ef100935r
Oasmaa et al.
Figure 2. Alkoxides in aqueous solutions.
and more suitable for TAN determination of the pyrolysis oils than the ASTM D664 method. The TAN methods have been used by the petroleum industries for many years and have some relevance in determining corrosiveness and acidity of the product. One of the main objectives in hydroprocessing of fast pyrolysis bio-oils is to enable a feed suitable for oil refineries. The properties of the upgraded bio-oils would be much closer to the typical hydrocarbon feed than original bio-oil, and hence, the same analytical protocol as for oil refinery feeds would be used. TAN is a basic tool for measuring the acidity of hydrocarbons. Its use for fast pyrolysis bio-oils or their upgraded products has not been verified. The purpose of this paper is to give more insight upon using TAN for these bio-oils. In addition, a discussion of the acidity of fast pyrolysis bio-oils is provided. The acidity is measured for fresh and aged oils, as well as for hydrotreated oils. Even though several TAN standards may be used for measuring the acidity of pyrolysis oil, the standard methods already available at VTT and PNNL were chosen. VTT has ASTM D664, which is meant for high TANs. PNNL uses ASTM D3339, which is developed for small TANs (0.01-3) and may be better suited for analyzing hydrotreated fast pyrolysis bio-oils. The main difference between these two methods is that the KOH used is 10 times more concentrated in ASTM D664 than in ASTM D3339.
Table 3. Sample Size for TAN Analysis expected acid number (mg of KOH/g of sample)
sample weight (g)
weighing accuracy (g)
0.05-0.9 1-4.9 5-19 20-99 100-250
10 ( 2 5 ( 0.5 1 ( 0.1 0.25 ( 0.02 0.1 ( 0.01
0.100 0.020 0.005 0.001 0.0005
anhydrosugars, anhydro-oligomers, and hydroxy and/or sugar acids). The WIS fraction consisted of dichloromethane-soluble [low-molecular-mass (LMM) lignin material, extractives] and dichloromethane-insoluble [high-molecular-mass (HMM) lignin material, solids] substances. Extractives are obtained when 3 g of pyrolysis oil is extracted with 40 mL of hexane in an ultrasonic bath for about 30 min. The hexane phase is decanted to a weighed bowl. This is repeated 5 times. The residual is evaporated in a fume cupboard and dried in a heating oven at 40 °C overnight. The residue is weighed. The capillary electrophoresis (CE) technique was developed for acid analysis at VTT. The analysis conditions are as follows: electrolyte, 25 mM sodium phophate/1 mM tetradecyltrimethyl ammonium hydroxide (OFM-OH); pH 8; capillary, 75 μm (inner diameter) 375 μm (outer diameter) 60 cm (length); temperature, 25 °C; voltage, 15 kV; current, 50 ( 5 μA; detection, direct UV at 185 nm; Hg lamp and 185 nm window; and time constant, 0.3 s. The method can detect anions and organic acids with the following elements/compounds/salts: Br, Cl, S2O3, NO2, CrO4, NO3, sulfide, oxalate, SO3, formate, citrate, tartrate, succinate, SCN, phthalate, acetate, glycolate, propionate, lactate, butyrate, glutamate, benzoate, and sorbate. At VTT, acetic, formic, glycolic, propionic, and lactic acids (about 95% of all detected compounds) were calibrated. In the method, fast pyrolysis bio-oil (2 g) is extracted with water (100 g) in an ultrasonic bath for about 45 min (temperature,
