The characterization of biomass and their subsequent chars using microscopy and thermogravimetric analysis

Transcription

1 The characterization of biomass and their subsequent chars using microscopy and thermogravimetric analysis Claudio Avila and Edward Lester School of Chemical and Environmental Engineering, University of Nottingham. University Park, Nottingham (NG7 2JT). United Kingdom. +44 (0) Abstract Across Europe there has been an increasing interest in the use of biomass as a renewable energy source. However, published literature that uses microscopy to observe combustion performance. It is possible to assume that biomass is highly reactive and burns out more efficiently than coal, but this is not always the case. In this paper, ten different kinds of biomass were selected from different sources including directly grown energy crops, industrial waste material and specific wood types in order to characterize the intermediate combustion residues using microscopy and thermogravimetric (TGA) techniques. Each biomass was sieved into 6 different size fractions ranging from 1180um to 53um. Each size fraction was then pyrolysed using a fixed bed furnace operating at 1000ºC in order to produce biomass char samples. Following this, samples were analyzed using a non isothermal TGA method in order to measure the intrinsic reactivity of the resultant char material. Scanning electron microscopy and oil immersion microscopy was used alongside automated image analysis techniques to characterise the morphology of the biomass char products. Several new morphotypes groups were identified from the biomass char samples. In addition to this, a significant correlation was found to exist between char morphotypes and char reactivity (from TGA).

2 Introduction The growth in demand for energy and the spectre of global warming now drives the search for new sources of renewable energy, such as biomass and organic wastes. Both have a potential role to play in combustion and gasification processes [1]. Despite this, there is surprisingly little in the literature (relative to coal studies) in terms of burnout studies and detailed analysis of how the burnout process differs from coal. It is important to characterize char particles since these are the reactive intermediate that provide form the main component of the combustion and gasification process [2,3,4]. For biomass, it is necessary to study the thermal behaviour and the material morphology to generate predictive models. A number of techniques have been used to classify coal char morphology [4,5,6], and the same methods can be used for biomass particles. Some researchers have used scanning electron microscopy (SEM) analysis to observe char structures although this technique can be more qualitative than quantitative [7,8,9,10]. Oil immersion microscopy (potentially coupled with automated image analysis) could also be used for biomass char characterization. Thermogravimetric studies can also provide relevant information about the chemical reactivity (intrinsic reactivity) of the feed material and subsequent char products. Previous investigations have focussed on coal char reactivity using TGA analysis [11,12]. In this case, it is possible to find studies which relate physical factors with reactivity behaviour, such as heating rate effects [10,13], influence of atmosphere used [11], relevance of different pressures [9], and impact of particle size [9,10]. The existence of a link between shape and particle reactivity would be an interesting an important area for study, particularly since biomass particles are generally fired in much larger sizes (>500microns). Changes in reactivity and char morphology with increasing particle size, might also impact burnout performance. In this paper the characteristics of biomass char has been studied. The optical and thermal characteristics have been quantified by means of microscopy (SEM and Oil Immersion Microscope) and thermogravimetric analysis, in order to create a systematic method to classify biomass particles, which relates reactivity and morphologic parameters. Materials and methods Raw material is obtained from different sources suitable to be used in combustion processes. In this context, biomass have been taken from energy crops such as Miscanthus and Corn; from some typical agricultural feedstock such as wheat and short cereal; from industrial sources as sunflower and rapeseed (residues from processing of vegetable oils); DDG and DDGS (residues from distillation process); and some specific trees such as Olive and Swedish wood. Sample preparation Fresh biomass materials were divided and sieved to 6 different ranges of particle size: 53-75, , , , and m using the ASTM sieve standard as a guide. Also, a proximate analysis was carried out to standardize the original raw material (Table 1). After that, ceramic crucibles were filled with 10 g of fresh biomass, clogged with a

3 ceramic lid (to avoid air contact) and introduced in to a fixed bed furnace preheated at 1000 ºC in absence of oxygen. After 3 minutes, crucibles were removed from and left in a dessicator to avoid air contact and moisture capture. Finally, samples were checked using a TGA program to confirm the absence of volatiles. A fixed bed furnace was selected to create a high heating rate environment without air flow interactions. Sample analysis Thermal analysis Thermogravimetric analysis have been carried out using a Thermo gravimetric analyzer model TA Q500 (TA instruments Co.), with a gas flow of 100 ml min -1 and a sample range of mg. Three main programs have been used for sample analysis: a slow pyrolysis program (5 ºC min -1 in N 2 ), with the purpose to evaluate if samples are completely free of volatiles after furnace preparation; intrinsic reactivity in order to identify the propensity of samples to react with air (10 ºC min -1 in air). Finally, proximate analysis has been done with the aim to calculate volatile matter, carbon, ash and moisture contents. In this case biomass char has been exposed to an initial ramp of 10 ºC min -1 from room temperature to 105 o C maintaining the temperature constant for 15 minutes in nitrogen, to obtain the water content. After that, a ramp of 10 ºC min -1 from 105 to 920 o C in nitrogen was carried out to measure the amount of volatiles in the sample. Optical analysis Polished blocks were prepared using epoxy liquid resin for each sample in order to be characterized. Afterwards, a Zeiss Leitz Ortholux II POL-BK microscope with oil-immersion objective and 32X magnification were used to analyse particle morphology. Composite images (3090x3900 pixels) from mosaics of 15 x 15 were obtained from the digital camera Zeiss AxioCam attached to the microscope and operated with KS400 V3.1 software. A Quanta 600 Scanning Electron Microscope with energy dispersive X-ray was used to study surface and internal structure changes for biomass particles. Samples were mounted in adhesive carbon disks and exposed directly to light bean in order to analyse the external characteristic of particles. Simultaneously, polished blocks were exposed to the light bean, treated previously in a vacuum atmosphere to collect volatiles, in order to analyse the internal characteristic of particles. Results Thermal Characterisation Initially, proximate analysis has been set up to identify physical-chemical characteristics of char produced. This program is similar to proximate ASTM coal analysis. However, there is not an ASTM standard to analyze biomass char particles. Results are presented in Table 1. Subsequently, char particles were subjected to an intrinsic reactivity analysis by means of a ramp of temperature (10 o C min -1 ) in air. This analysis provides a standard method to compare between samples, in which initial temperature (at 1% conversion), peak temperature (maximum temperature) and final temperature (at 99% conversion) are contrasted. Finally, samples have been grouped into 5 main groups where similarities are apparent.

4 Table 1: Proximate analysis for raw and char biomass particles. Raw Material Char prepared Moisture Volatiles Carbon Ash Moisture Volatiles Carbon Ash Wheat Swedish Wood Miscanthus Sunflower Shorts Corn DDGS residue DDG residue Rapeseed Olive residue The first group is DDG and DDGS, both present similar behaviour and temperature range due to their same material origin (Figure 1). Also, several peaks are evidence of multiple reactions and organic compound added to carbon matrix. In this case, the profiles finish at an unusual high temperature of 800 o C. In addition, for small char particle size (less than 106 um) reaction happens at low temperature. However, in the case of DDGS it has a weight loss rate slightly higher. DDGS Figure 1: DTGA curves for DDGS.. In the second group, Miscanthus and Swedish wood have analogous DTGA curves; with similar weight lose rate profiles (Figure 2). About the temperature range, it is short and starts at ~350 finishing at 550 o C; additionally, char particle size does not have strong influence in DTGA profiles. The third group, Wheat and Shorts, has a comparable behaviour with DTGA curves. However, in this case the peak temperature and final temperature are elevated ( o C) and char particle size has an important influence in reactivity profiles (Figure 3). Miscanthus Figure 2: DTGA curves for Miscanthus. Same behaviour is shown by Swedish wood.

5 The fourth group is performed by Sunflower and Corn, which presents reactivity profiles with the lowest peak temperature compared to all samples studied. In this case the final temperature reaches ~450 o C for all particle size considered (Figure 4). For sunflower, the size has an influence in the peak temperature and in the final temperature too. Additionally, it has a high weight lose rate value being the most reactive sample studied. On the other side, Corn does not have particle size effects on reactivity profiles and the overall maximum weight lose rate value is reduced. Shorts Figure 3: DTGA curves for Shorts Cereal for different particle size. Sunflower Figure 4: DTGA curves for Sunflower. Corn presents a similar behaviour. Olive residue Figure 5: DTGA curves for Olive tree at different particle sizes. The fifth group is the unclassified samples of Olive and Rapeseed residue. Despite of the fact that in both samples the particle size has a strong influence over mass lose rate and the peak temperature value (Figure 5), in Olive sample a natural contamination of quartz was detected (identified by SEM microscopy), for this reason was eliminated. Moreover, Rapeseed does not fit in the described groups, even though it presents a number of similarities with

6 DDG and DDGS. For example the final temperature has a value high ~ , in which small particle size burnt at higher temperature. From different kinds of biomasses it is possible to compare TGA profiles using a particular particle size. The analysis of these diagrams indicates that there are clear reactive group of chars, according to the weight loss rate and peak temperature. In Figure 6, comparison among reactivity profiles of the lowest size fraction is shown (75-53 um), in which three main groups can be identified which relate to mass loss rate: Sunflower, Swedish wood and Miscanthus burning at high speed in a short time (17 % min -1 ); Corn, Wheat and Short reacting to medium rate with an increase in burnout time (9~11 % min -1 ); Rapeseed, DDG and DDGS with a low rate over a longer time period. Moreover, at least 5 peak temperature groups are detected: Sunflower (370 o C), Swedish wood and Corn (415~425 o C), Miscanthus, Rapeseed, DDG and DDGS (480~500 o C), Wheat (530 o C), and Shorts (575 o C). Figure 6: Comparison of DTGA profiles for char biomass samples (75-53 um). Following this procedure, each particle size was subjected to the same analysis. For all samples, there are five main potential groups to identify (Table 2). These correlated groups consider the nearest similarities in weight lose rate and in the maximum temperatures reached by samples. Table 2: Classification of biomass char by TGA analysis (reactivity in air). Microscopic approach Char morphology was studied using polished blocks and oil immersion microscopy. This approach produces cross-section pictures, useful for determining area, length, diameter, and macro porous calculation. Four main groups of particle morphology have been identified from samples. For these, micrographs of two different particle sizes are shown in Figure 7, in which one of these has been captured by oil immersion microscope and the

7 other with a SEM microscope. The main groups have been recognized according to their similarities in structure, length, thickness and roundness. Figure 7: Cross-sections and surface micrographs of biomass char particles using oil immersion microscopy and SEM analysis, in order to identify structural behaviour. A) Olive tree residue, B) Sunflower, C) Swedish wood, D) Miscanthus, E) Corn, F) Rapeseed residue, G) D.D.G. residue, H) D.D.G.S. residue, I) Wheat meal, J) Short cereal. It is clear that some chars maintain their original fibrous structure such as Swedish wood and Miscanthus (Figure 7, C and D). The same behaviour has been detected for Sunflower and Corn (Figure 7, B and E); nevertheless, these samples have a different shape distinctive from flowers, with cell structures and thick wall thickness (compare with softwoods). Olive residue is a combination of both, with a fibrous structure and also, char seems to have cells structures with a thick wall thickness (Figure 7, A). For both, Wheat meal and Shorts, the morphology is also similar (Figure 7, I and J). In this case, for big size of particle it is possible to identify long structures with big porous and thick wall thickness. When particle size becomes small, they are melted and agglomerated after passing through a plastic phase. In both cases, the process of volatile release produces a matrix with large pores. In the larger particle size range, there are a number of similarities between Rapeseed and DDG residues (Figure 7, F and G). Both have structures with thick walls and big cavities, probably formed due to

8 volatile release. However, differences exist between these biomass types with the smaller particle sizes. i.e. Rapeseed is similar to DDGS residue (Figure 7, H), which passes through a plastic phase having a solid matrix with big pores and thick walls (due to the softening effect of volatile release). Despite of the fact that DDG has the same biological origin than DDGS, DDGS has more similarities with Shorts cereal and Corn in the larger size ranges. It is possible to simplify these char morphologies into four main groups (Table 3). The most influential characteristic is the ability to plasticise and melt. Some biomass particles maintain their original cellulosic structure, in contrast with others altered due to the volatile release (supported by proximate data analysis). Table 3: Classification of biomass char particles in accord with their optical attributes. Combining Char Reactivity and Morphology A strong relationship between morphology (microscopy) and reactivity (thermal analysis) has been found, and is illustrated in Figure 8. In this diagram the results from Tables 2 and 3 have been integrated to produce groups or types that have a predictable reactive behaviour. Surprisingly, char particles that have a thick cell wall thickness were found to have a lower intrinsic reactivity compared with particles with a higher porosity and thinner cell walls. The combination of lower intrinsic reactivity and thicker char walls means that the burnout rate in an actual power station will be quite variable between biomass types. In addition, results confirmed the existence of several morphotypes unidentified and unclassified in the actual ICCP char classification system [14]. Figure 8: Link between reactive and structural groups for biomass char particles. Conclusions The results support the idea that there is a significant correlation between char morphotypes and char reactivity (from TGA). Also, several new morphotype groups were identified from biomass char samples, which are unidentified and unclassified in the actual ICCP char classification system.

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