Article pubs.acs.org/EF
Spruce Pretreatment for Thermal Application: Water, Alkaline, and Diluted Acid Hydrolysis Carlos Alberto Cuvilas*,†,‡ and Weihong Yang† †
Division of Energy and Furnace, Department of Materials Science and Engineering, Royal Institute of Technology (KTH), Brinellvägan 23, S-100 44 Stockholm, Sweden ‡ Department of Chemical Engineering, Faculty of Engineering, University Eduardo Mondlane (UEM), Avenue de Moçambique Km 1.5, Post Office Box 257, Maputo, Mozambique ABSTRACT: Hydrolysis a process that involves a separation of the main components of lignocellulosic material (LCM) primarily developed for ethanol production was applied in this work to upgrade biomass for thermal application. The purpose of the pretreatment was to remove hemicellulose and alkali metals and consequently increase the energy content of the biomass and improve the fuel properties. Freshly chopped (2−10 mm) spruce (Picea abies) samples were hydrolyzed (liquid/solid ratio of 800 mL/80 g), using water, diluted acid, and sodium hydroxide in a rotating autoclave at 180 ± 2 °C for 150 and 350 min. Several analyses, such as proximate and ultimate analyses, ash composition and fusibility characteristics, and thermogravimetric analysis under pure nitrogen, were performed. Despite the reduction of mass and energy yields with increment of the severity factor, a significant increment of the higher heating value and ash quality was achieved, revealing that hydrolysis using water or diluted acid is a promising method to upgrade biomass as fuel. For alkaline treatment, a huge degradation on the quality of the ash was observed.
1. INTRODUCTION
goal is the removal of lignin and hemicellulose and the reduction of the crystallinity of cellulose.7 Hydrolysis processing of lignocellulosic materials causes a variety of effects, including extractive removal, hemicellulose hydrolysis, and alteration of the properties of both cellulose and lignin. Dependent upon the operational conditions applied, hydrolysis can cause a selective removal of hemicellulose.2 Among the main LCM components, hemicelluloses are the most thermochemically sensitive, while lignin is believed to solubilize during the pretreatment and coalesces upon cooling, such that its properties are altered; it can subsequently be precipitated.8,6 The lower stability of hemicelluloses is mainly due to the fact that their side chains and pendant groups, such as glucuronic acid and acetic acid, inhibit the formation of hydrogen bonds. Hemicelluloses are therefore much more accessible to hydrolytic attack. Hydrothermal (autohydrolysis) pretreatment has the advantage of using no added chemicals and minimizing hemicellulose degradation. During the process, individual fibers and cell types are separated, hemicellulose is removed, and lignin is redeposited on the fiber surface.9 Dilute acid at moderate temperatures (140−190 °C) effectively removes and recovers most of the hemicellulose as dissolved sugars,10 while solubilized lignin precipitates onto biomass.11,6 Severe pretreatment conditions promote the condensation and precipitation of soluble lignin compounds, sometimes even with soluble hemicellulosic compounds, such as furfural and hydroxymethylfurfural (HMF).12−14
Lignocellulosic material (LCM) consists of mainly three different types of polymers with extensive chains, namely, cellulose, hemicellulose, and lignin, connected to each other.1,2 Its use is becoming increasingly important globally as a clean alternative source of energy to fossil fuel as a result of the rising energy demand, high cost of fossil fuels, and contribution of fossil fuel usage to global warming. Although it is a common renewable source of energy especially in developing countries, LCM handling and use cost can be very high because of several constraints,3 such as the presence of noncombustible inorganic constituents, which leads to different problems in energy conversion units, such as deposition, sintering, agglomeration, fouling, and corrosion.4 Other parameters negatively affecting the efficient use of biomass are high moisture content and low heating value. Thus, according to several authors,5,6 various pretreatment methods, such as (1) physical pretreatment (mechanical comminution and pyrolysis), (2) physicochemical pretreatment (steam explosion, ammonia fiber explosion, and carbon explosion), (3) chemical pretreatment (acid hydrolysis, alkaline hydrolysis, oxidative delignification, and organosolv process), (4) biological pretreatment, and (5) pulse-electric-field pretreatment, just to mention some, have been investigated and are today perhaps the most crucial step because they have a large impact on further steps of LCM use processes. Despite extensive scientific and commercial efforts, the theory and applicability of the majority of pretreatment methods remains, because of several reasons, insufficiently developed. Many of these methods were primarily devoted to panel production in the pulp and paper industry and lately to the production of bioethanol for fuel. In such processes, the main © 2012 American Chemical Society
Received: July 12, 2012 Revised: September 15, 2012 Published: September 18, 2012 6426
dx.doi.org/10.1021/ef301167v | Energy Fuels 2012, 26, 6426−6431
Energy & Fuels
Article
Table 1. Method, Standards, and Norms Used for the Determination of Wood Properties and Ash Analysis parameter high heating value volatile matter (VM) ash content fixed carbon (FC) C, H, and N O S and Cl
method
standard/norm
calculated heated at 900 °C out of contact with air for 7 min
SS 187182 CEN/TS 15148
incinerated at 550 ± 25 °C in air calculated as the difference between 100 and the sum of ash and volatile matter high-temperature combustion in oxygen (1050 °C), with an IR detection procedure calculated as the difference between 100 and the sum of C, H, N, S, and ash content high-temperature tube furnace combustion in oxygen (1350 °C) with an IR detection procedure (LECO SC 432), using an Eschka mixture, with titration by the Mohr procedure
SS 187171:1
ash fusion Si, K, Ca, Mg, Mn, melt with LiBO2, wet dissolving with HNO3, with final determination using ICP−AES Na, and P
Alkali pretreatments, well-described by Sun and Cheng15 and Agbor et al.,16 solubilize both lignin and hemicellulose. Its mechanism is based on saponification of intermolecular ester bonds cross-linking xylan hemicelluloses and other components, such as lignin. The aim of this work is to evaluate the use of hydrolysis (water, diluted acid, and alkaline) as a pretreatment method of biomass for thermal application, increasing the energy density and reducing the alkali metals.
The overall mass and energy yields on a dry and ash-free (daf) basis after pretreatment were calculated according to formulas below
⎛ mproduct ⎞ mass yield,Ymass (%) = ⎜ ⎟ × 100 ⎝ mfeed ⎠daf
⎛ HHVproduct ⎞ energy yield,Yenergy (%) = Ymass⎜ ⎟ ⎝ HHVfeed ⎠daf
3. RESULTS AND DISCUSSION 3.1. Effect of Pretreatment on Elemental Composition. The biomass composition was significantly affected; the ash content was reduced from 2.82 for untreated biomass to values between 1.29 and 2.4%, dry basis (db). The biggest ash reduction was observed on water treatment (55%), while alkaline treatment presented the lowest reduction, 39 and 15% for K1 and K2, respectively. On the other hand, fixed carbon was increased from 26.98 (S0) to 37.24 for A2, and the amount of volatiles were reduced in all treatments (Table 3). The concentrations of nitrogen (N), sulfur (S), and chlorine (Cl) in different biofuels are also of major importance because they can, upon combustion, be emitted as NOx, SO2, and HCl. 18 In this study, a general increment of the N concentration from 0.11% db for untreated biomass to 0.26% db for A2 was observed. However, it is important to refer to the fact that the formation of the gases mentioned above also depends upon other parameters, such as excess oxygen, CO concentration in the flue gas, furnace temperature, and geometry. According to Obernberger et al.,19 N is almost completely converted to the gaseous phase (N2 and NOx) during combustion. While Cl, which plays an important role in alkali transportation,20 can mainly form gaseous HCl and Cl2 or alkali chlorides, such as KCl and NaCl. Table 3 shows a very low and
Table 2. Pretreatment Conditions Used in the Present Study
W1 W2 K1 K2 A1 A2
water water NaOH NaOH H2SO4 H2SO4
(0.1 (0.1 (0.1 (0.1
mol/L) mol/L) mol/L) mol/L)
10 log RO
150 350 150 350 150 350
4.53 4.90 4.53 4.90 4.53 4.90
(4)
where HHVproduct and HHVfeed are higher heating values of the product and feed, respectively. The reported values are averages from two replicates from each sample. They were analyzed using the GLM procedure (SAS Institute, Inc., Cary, NC). The model included analysis of the effect of treatment (water, alkaline, and acid) and the effect of time (150 and 350 min) on the HHV and ash content.
In this work, six sets of pretreatment conditions are considered (Table 2) and untreated biomass is referred to as S0.
residence time (min)
(3)
where mproduct and mfeed are masses of the product and feed, respectively, on a daf basis and
(1)
treatment agent
CEN/TS 15370:2004/15404:2005 SS 028113-1
HHV = 0.3941C + 1.783H + 0.1005S − 0.0151N − 0.1034O (2) − 0.0211ash
Freshly chopped (2−10 mm) spruce (Picea abies) samples were placed in a 1000 mL autoclave, hydrolyzed using water, H2SO4 (0.1 mol/L), and NaOH (0.1 mol/L). The rotating autoclave, with 800 mL of solution and 80 g of biomass, was heated to 180 ± 2 °C (heating rate at 10 °C/min) and kept for 150 and 350 min. After treatment, the autoclave was cooled, the solid and liquid were separated through filtration, and the moisture content of the solid part was reduced at 80 °C. Several analyses, such as proximate and ultimate analyses, ash composition and fusibility characteristics, and thermogravimetric analysis (TGA) under pure nitrogen, were performed according to the respective standards and norms (Table 1). The effects of time and temperature of the treatment process can be represented by the severity parameter (RO) as defined by Overend and Chornet.17 The parameter combines time, t (min), and temperature, T (°C), in the form of
treatment designation
CEN/TS 15289:2006/15408:2007
the HHV of the biomass is also affected. In this study, the HHV was calculated as follows (eq 2):
2. MATERIALS AND METHODS
R O = t e[(T − 100)/14.75]
CEN/TS 15104:2006/15407:2007
The morphological structure of ash was analyzed by scanning electron microscopy (SEM, Hitachi, S-3700N) equipped with energydissipation X-ray spectroscopy (EDS). EDS was used to analyze the formed mineral content on the ash. Samples of ash were embedded in epoxy resin and then cut and polished for cross-section SEM observation and EDX analysis. 2.1. Higher Heating Value (HHV), Mass, and Energy Yield. There is a strong correlation between the chemical compositions of biomass with its energy content. Because of the composition change, 6427
dx.doi.org/10.1021/ef301167v | Energy Fuels 2012, 26, 6426−6431
Energy & Fuels
Article
Table 3. Proximate and Ultimate Analyses of Untreated and Hydrolyzed Biomass (%, db) S0
W1
fixed carbon volatile ash
26.98 70.2 2.82
29.49 69.2 1.31
carbon (C) hydrogen (H) nitrogen (N) oxygen (O) chlorine (Cl) sulfur (S)
50.2 6.2 0.11 40.67
