ARTICLE pubs.acs.org/EF
Influence of Inorganic Additives on Pyrolysis of Pine Bark Chenguang Liu,† Xinliang Liu,† Xiaotao T. Bi,*,‡ Yunqi Liu,† and Congwei Wang‡,§ †
State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, CNPC, China University of Petroleum, Qingdao 266555, Shandong, China ‡ Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada § GuangZhou Institute Energy Conversion, Chinese Academy Sciences, Guangzhou 510640, Guangdong, China ABSTRACT: This paper investigated the effect of inorganic additives on pine bark pyrolysis using a thermogravimetry instrument. Both thermogravimetry (TG) and differential thermogravimetry (DTG) were performed to a final temperature of 600 C with heating rates of 10, 20, and 50 C/min, respectively, and a nitrogen flow rate of 50 mL/min. Six types of inorganic additives at different loading were tested. The pyrolysis kinetics data of the samples were fitted to the CoatsRedfern model. The results showed that the pyrolysis behavior and kinetics are significantly altered by the additives and are a strong function of the characteristics and concentrations of the additives.
1. INTRODUCTION Biomass has become increasingly important as one of many renewable energy sources. Thermal conversion of biomass (pyrolysis, gasification, and combustion) to secondary energy showed some advantages over fossil fuels. In recent years, pyrolysis has been successfully explored for the production of bio-oil from both forest and agricultural biomass residues, with biochar as a byproduct. In order to increase the economics of the biomass pyrolysis technology, upgrading biochars to high-value products such as activated carbon, carbon fibers, and carbon molecular sieves has received increasing attention in recent years due to their applications in gas purification and separation, gas storage, and transportation, and PEM fuel cells as catalyst supports and membrane materials, etc.1,2 Ash contained in raw biomass is mainly retained in the solid product after pyrolysis, and its content and composition can affect the biomass combustion and pyrolysis behavior, as well as product qualities as a catalyst support. Alkali and alkali earth metals have been reported to cause some adverse impacts such as fouling on reactor walls, furnaces, heat exchangers, turbines, emission control devices, and other downstream equipment during combustion and gasfication.3,4 In order to avoid the potential negative impacts of ash content in the biomass, biomass has to be pretreated to reduce its inorganic constituents for large-scale combustion applications.5,6 Metal ions contained in biomass can also modify biomass pyrolysis behavior substantially and enhance solid and gas yields. Carbon-supported catalyst has been prepared conventionally by impregnating the active catalyst metals to the activated carbon support produced from pyrolysis of biomass. Alternatively, one can preload the active metal precursor to the deashed biomass particles before pyrolysis and activation. Compared to the conventional method, the new method has several advantages. First, the catalytic effect of metal catalyst in the preparation process of activated carbon is utilized sufficiently to lower the pyrolysis temperature and/or to accelerate the pyrolysis reaction; second, the ash contained in biomass which may affect the catalyst characteristics and proformance5,7 can r 2011 American Chemical Society
be removed from the biomass before active metal loading; third, better catalyst distribution on the carbon support can be achieved;8 lastly, the oxidation treatment can introduce oxygen-containing groups to the support surface.9 In order to prepare carbon-support catalyst using this preloading method, it is necessary to investigate the influence of the added active metal components and the inorganic constituents contained in the biomass ash on the pyrolysis behavior. Previous studies on this topic mainly focused on the alkali metals such as potassium and sodium, the influence of other metals and their contents in biomass ash on biomass pyrolysis have not been well-studied. Furthermore, the majority of these studies conducted in the past have focused on cellulose, hemicellulose, and white biomass (woody and herbaceous plants), with little known about wood bark.5,6,10 In Canada, mountain pine bark (PB) is abundantly available as a residue from lumber mills and pulp mills. Because of its high ash content and hard and thick cell tissues,11 it has not been utilized effectively. As part of an effort to develop a carbon-supported base metal catalyst for hydrocarbon catalytic reduction of flue gas NOx and wastewater treatment using pine bark residues as the raw material, we have been exploring a new method in which ash was first removed from the bark sample before preloading the active metal solution, followed by pyrolysis and activation. In this paper, we report the pyrolysis performance of pine bark in the presence of six kinds of inorganic additives: (Fe(NO3)3, Al(NO3)3, Ca(NO3)2, K2CO3, Mg(NO3)2, Na2CO3). For the purpose of preparing a carbon-support Fe catalyst for catalytic reduction of NOx, five different Fe(NO3)3 concentrations were also tested to study the effect of metal content on the pine bark pyrolysis behavior. The influence of these six kinds of inorganic additives on pine bark pyrolysis was performed using a thermogravimetric (TG) analyzer. Received: January 28, 2011 Revised: April 11, 2011 Published: April 13, 2011 1996
dx.doi.org/10.1021/ef200152s | Energy Fuels 2011, 25, 1996–2003
Energy & Fuels
ARTICLE
2. EXPERIMENTAL SECTION 2.1. Material. Pine bark sample was collected from British Columbia of Canada and used in this study. All samples were ground to a particle size range of below 1 mm after being washed to have the soil or dirt removed and then oven-dried at 105 C to a constant weight. 2.2. Sample Preparation. 2.2.1. Demineralization. To avoid the interference of minerals in the raw biomass on the catalytic performance of inorganic additives, the pine bark sample 0.30.4 mm in diameter was leached by dilute nitric acid solution to remove the inorganic constituents contained in the raw biomass. Demineralization conditions were listed in Table 1. After soaking with dilute acid solution, samples were filtered and washed with deionized water until constant pH was obtained, followed by oven drying at 105 C to constant weight. 2.2.2. Impregnation. The six kinds of inorganic additives mentioned above were impregnated into the deashed pine bark samples using the incipient wet impregnation method, in which different aqueous solutions were added to demineralized pine bark and then stirred for 24 h at room temperature. The theoretical weight ratio of metal ions to pine bark is 0.01. In order to analyze the influence of different concentrations of inorganic additives on pine bark pyrolysis, samples at five different Fe(NO3)3 concentrations were prepared. 2.3. Ash Content and Composition. Ash content was determined following the ASTM standard method. The sample was first dried in the oven at 105 C over 24 h, followed by igniting the samples in a muffle furnace at 575 C for 3 h in the presence of air. The residue was weighed repeatedly until constant ash content was reached. The elemental compositions of the ash were determined by a local analytical lab, ALS Laboratory Group in Vancouver, Canada. The ash samples were digested with perchloric, nitric, hydrofluoric, and hydrochloric acids, the residues were topped up with dilute hydrochloric acid, and the resulting solutions were analyzed by inductively coupled plasma atomic emission spectrometry. 2.4. Thermogravimetric Analysis. All thermogravimetry experiments were performed on a TGA-50, made by Shimadzu Corporation,
Japan. About a 10 mg sample was heated to 600 C at three different heating rates (10, 20, and 50 C/min) with a nitrogen flow rate of 50 mL/min. To minimize the heat and mass transfer effects, fine biomass powders with a mean size of 0.30.4 mm were prepared by grinding and sieving and used in the experiment.
3. RESULTS AND DISCUSSION 3.1. Ash Content and Compositions. Table 2 presents the ash content and elemental distributions in untreated and nitric acid solution treated samples. After acid treatment, more than 80% ash was removed. In addition, most of the alkali and earth metals which exert a great catalytic effect on biomass pyrolysis
Table 2. Ash Content and Main Metal Element Concentrations (% dry mass) for Untreated and Nitric Acid Solution Treated Samples elemental distribution ash sample
content
Al
Ca
Fe
K
Mg
Na 0.025
untreated PB (U-PB)
2.42
0.0087 0.29 0.0079 0.36 0.24
acid treated PB (D-PB)
0.39
0.0032 0.20 0.0023 0.05 0.058 0.0043
Table 3. Ultimate Analysis and Metal Concentrations of Samples metal element sample impregnating salt
Table 1. Leaching Conditions for Pine Bark Samples leaching condition
C
H
N
O
concentration/%
41.23 43.03
0.49
D-PB Fe1-PB
No Fe(NO3)3
52.21 5.97
