Elsevier

Fuel

Volume 211, 1 January 2018, Pages 712-725
Fuel

Full Length Article
Element speciation in UK biomass power plant residues based on composition, mineralogy, microstructure and leaching

https://doi.org/10.1016/j.fuel.2017.09.103Get rights and content

Abstract

Biomass ash management is an escalating issue in many countries because of increasing numbers of biomass power plants. Comprehensive characterization of biomass ashes with emphasis on element speciation, and solubility of nutrients (e.g., K and P) and pollutants is essential for potential utilization of these residues for soil nutrition. 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. The biomass air pollution control (APC) residues were enriched in K, Cl, S and Zn, and contained less lithophile elements, such as Al, Ca, P, Mg, Si, Ti, and Ba, compared to the bottom ashes. P appeared in: 1) bottom ashes as apatite and other phosphates (potassium hydrogen phosphate and potassium iron phosphate in the bottom ashes from combustion of poultry litter); 2) APC residues from combustion of poultry litter as potassium sodium calcium phosphate. K is present mainly in sylvite, arcanite, and some phosphates. Na, K, Cl, and S were easily leached by water from the biomass APC residues. However, water leaching of P, Ca, and Mg was very low, with leaching of P possibly controlled by hydroxyapatite. Aqueous Zn, Cu and Pb appear to prevail in the form of neutral and anionic hydroxide complexes, which are toxic and easily accessible chemical forms for live organisms. 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 but recovery of K and P from that material should be considered.

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:

  • Rob Carroll at the UK Quality Ash Association, and Mark Westwood from Fibrophos, for assistance in obtaining biomass ash samples.

  • Jim Davy in the UCL Earth Sciences, for support with the SEM/EDS analysis;

  • Judith Zhou for support in the UCL CEGE laboratory;

  • 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

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