ARTICLE pubs.acs.org/EF
Characterization of Biomass Ashes from Power Plants Firing Agricultural Residues Guoliang Wang, Laihong Shen, and Changdong Sheng* School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: The paper presents the results of systematically characterizing the bottom and fly ash from a typical water-cooled vibrating grate furnace and the fly ash from a circulating fluidized-bed (CFB) combustor of two power plants burning similar mixed agricultural residues. Multi-standard techniques were employed in the characterization in terms of basic properties, particle morphology, chemical composition, mineralogy, and leaching behaviors. It was found that, despite the similar elemental compositions, the basic properties, particle morphology, mineralogical composition, and leaching behaviors were quite different between the three ashes mainly because of the differences in combustion conditions and, consequently, ash formation behaviors. The characterization indicated that the grate furnace bottom ash and CFB fly ash have potential for agricultural use with proper handling, while the grate furnace fly ash is currently problematic for use on agricultural purposes as well as building material. The obtained data and implications of this study are valuable for evaluating ash quality and developing sustainable utilization and management of the ashes from power plants firing mixed agricultural biomass.
1. INTRODUCTION Biomass resources, particularly agricultural residues, are abundant in China. Apart from those returned to the field as fertilizer or used as fuels in rural households, forage, and industrial raw materials, etc., biomass materials available for energy production annually amounted to 365 million tons of coal equivalent (Mtce) in 2005.1 This amount is projected to be 500 Mtce in 2030. With potential energy crops and plantation included, it further goes up to 900 Mtce, equivalent to ∼30% of the current annual energy consumption.1 Therefore, the promotion of energy production from biomass resources is taken as one of the priorities in the development of renewable energy resources for both energy supply security and sustainable development. Being a proven effective and highly efficient technology for using biomass on a large scale, direct combustion for electricity generation is playing a crucial role in biomass energy conversion in China. In 2005, the total installed capacity of biomass-fired power plants was about 2.0 GW,2 in which conventional bagassefired facilities were dominant, while those firing other residues only took a small part. However, directly burning biomass to generate electricity is increasing substantially, and a number of new power plants have been recently built in agricultural areas.3,4 Moreover, the total installed capacity is planned to be 24 GW for 2020,2 of which most is expected to be based on the technology of direct combustion of agricultural residues. In China, agricultural residues commercially used in power plants have an average ash content of 5 10% by weight on a dry basis. As a result, increasing the burning of the residues has also led to a great increase in the amount of combustion residues, i.e., ashes. To ensure the economic, environmental, and social benefits of biomass electricity generation, use and management of biomass ash are becoming important issues associated with power plant operation and administration.3 Aiming at developing applications of biomass ash, extensive efforts have been made around the world.5 10 Biomass ash has r 2011 American Chemical Society
potential for bulk applications as fertilizers and building materials and products.5 7,11,12 Niche applications, such as uses for adsorbent and material recovery and production, also exist.6,8 10 Actually, ashes from clean biomass fuels, particularly from single species, are relatively easy to find value-added applications according to ash characteristics. For example, ash from rice husk can have many applications for producing silicon-based materials because of its very high silica content.9 Many kinds of biomass ashes, including those from rice husk,9 forest residues,13 wheat straw,14 sugar cane bagasse,15,16 and oil palm,10 may be added to concrete as mineral admixtures because of the pozzolanic properties. Co-firing ash is also not difficult for bulk use. Fly ash with the co-firing ratio within the interest of commercial operation at power plants was proven to be applicable in concrete.17,18 Naturally, the ashes from biomass combustion contain valuable plant nutrient elements, e.g., Ca, Mg, K, and P. It would be a sustainable way if the ashes could be used directly as fertilizer or as raw materials to manufacture fertilizers and returned back to soil.5,6,19 However, the ashes also contain toxic heavy metals, probably hindering these uses.5,11,19 Therefore, identification of a beneficial use relies on thorough characterization of biomass ashes. On the other hand, it is well-known that biomass ashes exhibit significant variations in composition, properties, and characteristics.20,21 The main reason is that ash-forming constituents in biomass fuels are very diverse, depending upon the species and part of biomass plants, soil conditions, and factors of growth and production.21 24 Additionally, ash properties are to some extent influenced by combustion technology and Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 31, 2011 Revised: October 8, 2011 Published: October 12, 2011 102
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Table 1. Basic Properties of Ash Samples GFFA
CFBFA
GFBA
GFFA
200 μm
CFBFA
200 μm
moisture (wt %, ar)
5.29
2.54
1.93
1.65
3.30
4.72
0.55
0.04
0.10
0.40
0.82
bulk density (kg/m3)
715
394
464
406
244
162
762
645
702
982
1135
TOC (wt %, db)
5.01
19.73
11.55
12.19
32.20
45.18
1.83
0.24
0.49
2.10
1.58
ash content (wt %, db)
94.99
80.27
88.45
87.81
67.80
54.82
98.17
99.76
99.51
97.90
98.42
pH value
11.3
10.2
12.1
10.4
8.9
8.6
12.5
12.6
12.5
9.5
9.1
conditions12,25 and even the location where the ashes are collected (e.g., bottom ash or fly ash).11 The nature of the significant variations also determines that characterization is essential for developing the uses of biomass ashes. Considering the situation in China, co-firing is hardly applied mainly because of the lack of policy stimulus for coal-fired power plants to commercially use biomass, as well as the technical difficulties.26 Instead, directly firing biomass fuels in dedicated power plants is favored on policy levels. Currently, the combustors employed in power plants are typically water-cooled vibrating grate furnaces and some circulating fluidized-bed (CFB) combustors with the units of 12 25 MW. To ensure fuel supply, the common practice is that various biomass fuels have to be used, although the plant may be designed to burn dedicated fuels, e.g., crop straws. To mitigate operation problems, such as corrosion of firing crop straws, different biomass residues are often mixed to burn. For these reasons, the variation of feedstock and, consequently, ash properties is among the reasons leading to the difficulty for ash use. Presently, less than 80% of biomass ashes produced from power plants is used, mostly as cheap supplemental material for producing K and P compound fertilizers.3 The rest is disposed of for landfill. Most times, this goes on without any form of control. While bottom ash is easy to find applications, fly ash is usually difficult to be accepted for use because of the lack of characterizing the quality. Although extensive work has been performed on basic characteristics of biomass ashes,27,28 little attention was paid on the characterization of the ashes from power plants considering the above practices. The purpose of the present study was to characterize biomass ashes originating from a grate-fired furnace and a CFB combustor burning the mixture of mainly agricultural residues. The characterization includes the analyses of basic properties, particle morphology, chemical composition, mineralogy, and leaching behaviors, with the emphasis on the pertinent usability of the ashes. The obtained data will provide valuable information for evaluating ash quality and developing sustainable use and management of the ashes from power plants firing mixed agricultural biomass.
The raw grate furnace bottom ash (GFBA) was quite coarse, containing sintering and melting blocks. For the sake of laboratory analysis, the GFBA was homogenized by grounding to pass through a 200 μm screen and used for the characterization, except for moisture determination using the raw material. The fly ash samples, i.e., the grate furnace fly ash (GFFA) and the CFB fly ash (CFBFA), were in the form of fine powder, which can be used directly for analysis. Even so, the samples were also sieved into four size fractions using a mechanical shaker with mesh screens of 50, 90, and 200 μm to investigate ash properties varying with particle size and ash formation behaviors. Consequently, particle size distributions of the two fly ashes were also obtained. The morphology of the fly ashes and their size fractions were observed with scanning electronic microscopy (SEM), using a FEI Sirion field-emission scanning electron microscope. The basic properties, including moisture content, bulk density, ash and total organic carbon (TOC) contents, and pH value, of the three ash samples and the fly ash size fractions were determined. The moisture content was measured by drying the ash sample at 105 °C to a constant weight. The bulk density was measured with a natural bulk method for powder material (Chinese standard GB-T 16913.2-1997). The TOC and ash contents were determined following the European standard for the determination of the organic matter content and ash of soil improvers and growing media (EN 13039:2000). A total of ∼5 g of sample was first dried and then ashed at 450 °C in air for 6 h using a muffle furnace. The organic matter was taken to be the loss on ignition, and the ash was taken as the residue on ignition. Both were expressed as a mass percent of the dried sample. Determination of the pH value was carried out using a pH meter according to European standard EN13037:2000 at a solid/liquid ratio of 1:5 (v/v). The chemical composition of the ashes was determined following European standard EN 15290:2006 for major elements and EN 15297:2006 for minor elements. The ash sample was digested with a H2O2/HNO3/HF mixture according to the standard methods. The resulting solutions were analyzed with a Perkin-Elmer OPTIMA 2000DV inductively coupled plasma optical emission spectrometer (ICP OES) to quantify the contents of major elements and with an Agilent 7700x inductively coupled plasma mass spectrometer (ICP MS) to determine the contents of minor and trace elements. To characterize mineralogical composition, the samples were subjected to X-ray diffractometry (XRD) analysis. The XRD patterns were determined using a Bruker D8 ADVANCE powder diffractometer with the characteristic Cu Kα radiation and operating conditions of 35 kV and 40 mA. The identification of the main crystalline phase was performed with the JADE 6.0 software package (MDI, Livermore, CA) and diffraction database of PDF2-2004. To evaluate the solubility of nutrient elements and the toxicity of the ashes, the three ash samples were leached with deionized water according to the horizontal vibration method of Chinese standard HJ 5572009. The sample was placed in a polyethylene bottle and extracted with deionized water at a liquid/solid ratio of 10:1 (L/kg). The ash water mixture in the bottle was agitated on a horizontal shaker for 8 h at room temperature and then allowed to settle for 16 h. The mixture was then
2. EXPERIMENTAL SECTION Ash samples were obtained from two power plants dedicated to directly burning agricultural residues for electricity generation. One is installed with a 15 MW water-cooled vibrating grate furnace. Representative fly ash was sampled under the baghouse filter, and bottom ash was collected under the grate. Another plant has a 12 MW CFB combustor, from which only fly ash was sampled from the baghouse filter. Although both plants were designed to use rice and wheat straws, the fuels actually burned were the mixture of crop residues of similar species, including rice straws, rice husk, groundnut shell, cotton stalk, etc. 103
dx.doi.org/10.1021/ef201134m |Energy Fuels 2012, 26, 102–111
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CFBFA, measured by sieving. As seen, the GFFA is coarser and has a wider size distribution, while the CFBFA is finer and has a narrower size distribution, with nearly 90% of particles smaller than 90 μm. Figure 2 shows SEM images of the two fly ashes and their size fractions. It can be seen in Figure 2a that the GFFA particles generally present as debris of irregular shapes with a wide size distribution and only a very small fraction of the particles has approximately spherical shapes, as observed also from the finer sizes, i.e.,
