Article Cite This: Energy Fuels 2019, 33, 6463−6472
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Ignition and Kinetic Studies: The Influence of Lignin on Biomass Combustion Yuxin Yan,†,‡ Yang Meng,†,‡ Luyao Tang,†,‡ Emily Tsambika Kostas,§ Edward Lester,∥ Tao Wu,†,‡ and Cheng Heng Pang*,†,‡
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Department of Chemical and Environmental Engineering and ‡New Materials Institute, The University of Nottingham Ningbo China, Ningbo 315100, PR China § Department of Biochemical Engineering, The Advanced Centre for Biochemical Engineering, University College London, Bernard Katz Building, London WC1E 6BT, U.K. ∥ Department of Chemical and Environmental Engineering, The University of Nottingham, Nottingham NG7 2RD, U.K. S Supporting Information *
ABSTRACT: This study investigates the relationship between the thermal behavior of nine biomass samples during combustion and lignocellulosic composition. The lignocellulosic composition was determined via standard biological assays, and it was observed that in most samples, cellulose is the most abundant organic component. Combustion kinetics and thermal behavior were examined using thermogravimetric analysis, while an Ash Fusion Furnace equipped with camera was used to monitor ignition temperatures in real time. The kinetic studies were repeated at heating rates of 5, 10, and 17.5 °C/min. All derivative thermogravimetric graphs, for each biomass, show two distinct peaks, thus suggesting two major reaction stages. Linear correlations were found between lignocellulosic composition of biomass and its combustion kinetics (both first and second stage), DSC peak temperature, as well as the ignition point of biomass. This is associated with the chemical characteristics and biological arrangements of lignocellulose in plant cells. Such tests potentially provide a quick and straightforward indication for selecting highly reactive and efficient biomass for combustion processes, which is linked directly to biomass composition. Many research studies on biomass combustion12,13 have focused on the effects of impurities and ash-forming metals on biomass combustion,14,15 and the influence of lignocellulose on the char morphology16 as well as the link between mineral composition and slagging and fouling.17−19 However, studies on biomass composition and biomass ignition are rarely reported. Ignition is the early stage of combustion and it plays a crucial role in the overall combustion process. As such, poor ignition will affect boiler operation, generation efficiency, and plant emissions. In order to achieve optimal biomass combustion, it is essential to have knowledge on its combustion behavior and performance, particularly around ignition characteristics.20 However, the characterization of ignition is complicated as the ignition point is not a fundamental parameter that can be easily provided for a given fuel,21 that is, it heavily depends on particle size, sample mass, heating rate, and gas composition. Analogous to coal having three major maceral components of liptinite, inertinite, and vitrinite, biomass has three major organic components of lignin, cellulose, and hemicellulose,22,23 that is, lignocellulose. Biomass sources vary widely from virgin white wood to agrifuels and wastes from the food industry. Despite these variations, most biomass mainly consist of lignocellulose but in varying proportions. This study
1. INTRODUCTION It is now universally accepted that the use of fossil fuels to generate electricity is one of the main causes of pollution and climate change. Biomass, on the other hand, is generally viewed as a clean, renewable, and sustainable alternative to coal and gas. Using biomass to generate electricity offers many environmental benefits including reduced greenhouse gas emissions, acid rain, soil erosion, water pollution, and pressure on landfills1,2 while improving sustainability, energy security, and job opportunities.3,4 Although the utilization of biomass as a fuel dates back to early civilization (for conventional heating and cooking), direct combustion for power generation is less well understood and the industry has engaged in a steep learning curve over the last 20 years to make the transition away from coal.5 Such phenomena arose because industrial scale combustion of biomass is not without difficulties, particularly around handling, milling, ash slagging and fouling.6−9 Biomass combustion also faces issues like low thermal efficiency, heat load instability, ash deposition, and low temperature corrosion.10,11 These issues are more apparent when biomass is combusted in furnaces originally designed for coal. Although biomass is now commonly used to generate electricity in power stations as a replacement to coal, the two fuel types differ significantly in many ways, particularly during combustion. Compared to coal, biomass has more volatile matter and moisture content, with lower fixed carbon and ash content. © 2019 American Chemical Society
Received: April 8, 2019 Revised: June 14, 2019 Published: June 19, 2019 6463
DOI: 10.1021/acs.energyfuels.9b01089 Energy Fuels 2019, 33, 6463−6472
Article
Energy & Fuels
HPAEC-PAD. The monosaccharide concentrations were quantified using Dionex ICS-3000 Reagent-Free Ion Chromatography, electrochemical detection using ED 40 and computer controller following the method outlined elsewhere.28 A CarboPacTM PA 20 column (3 × 150 mm) was used, with a mobile phase of 10 mM NaOH at an isocratic flow rate of 0.5 mL/min. The injection volume was 10 μL and the column temperature was maintained at 30 °C. Authentic standards of monosaccharides (arabinose, galactose, glucose, and xylose) were used to generate calibration curves (0.0625−1 g/L) for monosaccharide quantification. Concentrations of monosaccharides (g/L) in the acid hydrolysates were adjusted by an anhydrous correction factor. This was performed to measure cellulose (glucose) and hemicellulose (summation of arabinose, galactose, and xylose) content in each biomass on a dry weight basis. 2.2.4. Quantification of Lignin. Acetyl bromide (AcBr)-soluble lignin was used as a rapid lignin quantification method using a modification of the method described by Fukushima and Hatfield.29 Samples (100 mg) were incubated with 4.0 mL of 25% AcBr solution (in glacial acetic acid) at 50 °C for 2 h. After cooling, the volume was made up to 16 mL with glacial acetic acid and left to settle for 30 min. An aliquot (0.5 mL) of this solution was further diluted with glacial acetic acid (2.5 mL) and 0.3 M NaOH (1.5 mL) and mixed by hand agitation. Then, 0.5 mL of 0.5 M hydroxylamine hydrochloride solution and a further 5.0 mL of glacial acetic acid were added and the absorbance at 280 nm was measured (7315 spectrophotometer; Jenway, Stone, UK). Lignin concentrations were calculated using the extinction coefficient generated from corresponding measurements using lignin standards. 2.2.5. Extinction Coefficients Were Calculated from Standard Curves as Follows. Isolated lignin (10 mg) (after corrections for carbohydrate and protein contaminants) was dissolved in 5.0 mL of dioxane, and aliquots of 0.2, 0.3, 0.4, 0.5, and 0.6 mL were pipetted into separate tubes. To each tube, 0.5 mL of 25% acetyl bromide in glacial acetic acid was added. Tubes were tightly capped (PTFE-lined caps) and put in a 50 °C water bath for 30 min. After cooling, all tubes received 2.5 mL of acetic acid, 1.5 mL of 0.3 M NaOH, and 0.5 mL of 0.5 M hydroxylamine hydrochloride solution. Tubes were shaken and acetic acid was added to give a final volume of 10.0 mL. Solutions were read in a spectrophotometer at 280 nm. All reagents were of AnalaR grade and obtained from SigmaAldrich (UK) and Fisher Scientific (UK) unless otherwise specified. All water used was subjected to deionized reverse osmosis and of ≥18 MΩ purity. 2.3. Thermal Characterization. The thermal properties of biomass samples were examined via a nonisothermal method using the same TGA as mentioned above (TGA−DSC, NETZSCH STA449F3, Germany). All samples were individually heated from room temperature to 900 °C at three different heating rates of 5, 10, and 17.5 °C/min, respectively. Air atmosphere (80 vol % nitrogen and 20 vol % oxygen) was used in order to study the combustion characteristics of each biomass. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves were obtained to determine the initiation temperature (Ti) and kinetic parameters for different combustion stages. The firststage initiation temperature (Ti‑1) is commonly used to indicate the start of the first stage of biomass combustion and is the temperature where mass loss rate reaches 1 wt % min−1 for the first time, post moisture release.26 The end temperature of the first stage of combustion refers to the local minimum point between the two peaks. The end temperature of the first stage is also the initiation temperature (Ti‑2) of the second stage. The end point of the second stage is the temperature where mass loss rate reaches 1 wt % min−1 for the last time. The experiments were repeated 3 times to ensure accuracy and repeatability within ±3%. 2.4. Kinetic Studies. The Coats−Redfern integral method was adopted in this study to determine the kinetic parameters of biomass combustion. It is acknowledged that several methods are available for calculating kinetic parameters.30,31 However, most kinetic models use the Arrhenius equation and the conversion rate equation as shown in eqs 1 and 2
investigates the significance of lignocellulose and its association with biomass combustion. More specifically, this work studies the influence of lignin on ignition characteristics and kinetics of various stages of biomass combustion. In addition, a newly developed optical method for identifying ignition point is introduced and employed.
2. MATERIALS AND METHODS 2.1. Samples and Sample Preparation. A total of nine different biomass feedstocks were used in this study as listed in Table 1. These samples cover a wide range of biomass species and represent inexpensive feedstocks that are readily and widely available.
Table 1. Biomass Samples Used in This Study types of biomass energy crop industrial wastes
agricultural biomass wastes
names of biomass
particle size as-received (mm)
particle size after milling (μm)
miscanthus peach wood spruce wood rice husk corn straw
>1000 10−20 10−20 10−20 >1000
