Full Length ArticleElement speciation in UK biomass power plant residues based on composition, mineralogy, microstructure and leaching
Introduction
The combustion of biomass for heat and power generation is rising continuously because of limited availability of fossil fuels and evidence of global warming caused by CO2 emissions from fossil fuel combustion [1]. In the UK, 29% of the current 25 GWe coal-fired generating capacity will be lost after 2016 due to the stringent emission limits in the Large Combustion Plant Directive [2]. There are already 20 biomass power plants in the UK (∼1.1 GWe), and the urgent need to replace fossil fuels with renewable energy sources has led to proposals for a further 37 UK biomass power plants, which will generate ∼6 GWe and result in >2 Mtpa of biomass ash [3], [4]. According to Vassillev et al. [5], about 480 Mtpa of biomass ash, compared to 780 Mtpa of coal ash, may be generated worldwide. Therefore, it is clear that biomass ash management is an escalating issue. In accordance with the waste hierarchy, it would be desirable to reuse or recycle these wastes. Comprehensive characterisation of biomass ashes is needed to consider their further possible application. Data published on the properties of biomass ashes are not as detailed as for ashes of coal [6], [7]. The wide variety of fuels for biomass power plants, including forestry and agricultural crops and residues, animal residues, industrial residues, food wastes, sewage sludge, etc., as well as their use in different energy-from-waste systems [8], influence ash elemental and mineralogical composition [5], [6], and therefore solubility, leachability and biological availability of its components.
The aim of this work was comprehensive characterization, including bulk and trace element composition, mineralogical and microstructural investigation and water-leaching of bottom and APC residues from combustion of three types of waste biomass: (i) poultry litter, (ii) meat and bone meal and (iii) straw, in UK biomass power plants. In a context of potential utilization of these residues for soil nutrition, the emphasis was on determining the speciation, and solubility of nutrients and pollutants. This knowledge is essential for development of appropriate, environmentally friendly and economically rational biomass ash management.
Section snippets
Residues from biomass combustion
An important body of work on biomass ash, including an excellent review of the information available in the literature on biomass ash properties, has been recently carried out by Vassilev and his co-authors, which obviates the need for a full review and only key points are summarised here. Vassilev et al. [5] defined biomass as “a complex heterogeneous mixture of organic matter and, to a lesser extent, inorganic matter, containing various intimately associated solid and fluid phases with
Materials
This research was conducted with eight UK biomass ashes (approximately 5 kg of each samples), including three bottom ashes and five APC residues, obtained from five UK power plants that combust waste biomass. Table 1 shows the sample identification codes, which are based on the fuel type (S indicates straw, PL indicates poultry litter, i.e., a mixture of wood shavings, straw and poultry droppings, and MBM indicates meat and bone meal) and ash type (bottom ash, BA and air pollution control
Total element composition
The biomass ash moisture contents varied from 0.2 to 4.2% wet mass (Table 2). The moisture contents of the bottom ashes were usually higher than the moisture contents of the APC residues. The biomass ash organic matter content varied from 0.04 to 5.4% (Table 2).
The total concentrations of the 24 elements measured in the biomass ashes by XRF are presented in Table 2. All biomass ashes contained high concentrations of Ca (3.4–37%), P (2.2–9.0%) and K (0.93–14%). Also, the Mg concentrations were
Conclusions
All the UK biomass ashes investigated, whether from combustion of poultry litter, meat and bone meal, and straw, were alkaline and contained high concentrations of P, K, and Ca. Mg concentrations were also considerable in the poultry litter bottom ashes. The biomass bottom ashes belong to the CK type with low acid tendency. All APC residues belonging to HAB and MB groups are K type with low acid tendency. The biomass APC residues were enriched in the more volatile elements, such as Cl, S, K and
Recommendations and further investigation
As all biomass ashes investigated contain significant quantities of valuable potassium and phosphorus, it is not sustainable or rational to deposit them in landfills. Application of the poultry litter bottom ashes as a PK fertiliser in agriculture is appropriate. However, direct application of APC residues to agricultural fields is not appropriate because of the significant concentrations of potentially harmful elements and water-soluble Cl-bearing phases (can cause soil salination), which
Acknowledgements
The authors gratefully acknowledge the valuable assistance of the following people:
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Rob Carroll at the UK Quality Ash Association, and Mark Westwood from Fibrophos, for assistance in obtaining biomass ash samples.
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Jim Davy in the UCL Earth Sciences, for support with the SEM/EDS analysis;
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Judith Zhou for support in the UCL CEGE laboratory;
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Janet Hope and Kevin Roe in the Department of Geography (UCL), for support with the XRF analysis.
This work was funded by a UCL “Outgoing” Postdoctoral Mobility
References (36)
- et al.
An overview of the composition and application of biomass ash. Part 1. Phase-mineral and chemical composition and classification
Fuel
(2013) - et al.
An overview of the chemical composition of biomass
Fuel
(2010) - et al.
Characteristics of wood ash and influence on soil properties and nutrient uptake: an overview
Bioresour Technol
(2001) - et al.
A new approach for the combined chemical and mineral classification of the inorganic matter in coal. 1. Chemical and mineral classification systems
Fuel
(2009) - et al.
An overview of the organic and inorganic phase composition of biomass
Fuel
(2012) - et al.
An overview of the composition and application of biomass ash. Part 2. Potential utilisation, technological and ecological advantages and challenges
Fuel
(2013) - et al.
Boiler deposits from firing biomass fuels
Biomass Bioenergy
(1996) - et al.
Wood ash composition as a function of furnace temperature
Biomass Bioenergy
(1993) - et al.
Concentrations of inorganic elements in biomass fuels and recovery in the different ash fractions
Biomass Bioenergy
(1997) - et al.
Quantification of the release of inorganic elements from biofuels
Fuel Process Technol
(2007)
Phosphorus recovery from the biomass ash: a review
Renew Sustain Energy Rev
An overview of the behaviour of biomass during combustion: Part I. Phase-mineral transformations of organic and inorganic matter
Fuel
An overview of the behaviour of biomass during combustion: Part II. Ash fusion and ash formation mechanisms of biomass types
Fuel
Trace element concentrations and associations in some biomass ashes
Fuel
Abundance of chemical elements in the continental crust: a new table
Geochim Cosmochim Acta
Estimations of Clarkes for carbonaceous biolithes: world averages for trace elements in black shales and coals
Int J Coal Geol
Nutrient release from combustion residues of two contrasting herbaceous vegetation types
Sci Total Environ
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