Evaluation of bottom ash slagging risk during combustion of herbaceous and woody biomass fuels in a small-scale boiler by principal component analysis

  • Thomas ZengEmail author
  • Agata Mlonka-Mędrala
  • Volker Lenz
  • Michael Nelles
Original Article


In the short- and mid-term perspective, drastic measures for the reduction of anthropogenic emissions including extensive decarbonization of the residential and industrial heating sector have to be implemented. To replace fossil fuels, solid biogenic residues and wastes will have to be increasingly utilized. Compared to clean woody biomass, these biomass assortments are commonly characterized by higher Si and alkaline metal contents recognized as major driver for low ash melting temperatures resulting in elevated risk of bottom ash slagging. To facilitate the prediction of bottom ash slagging during combustion, several fuel indices have been proposed. Based on empirical correlations with parameters relevant for slagging behavior, e.g., ash melting temperatures or slag fraction of the bottom ash, these fuel indices were subsequently enhanced and adapted for an increasing range of biomass fuel characteristics. In this study, analysis data of 26 woody and non-woody fuels and experimental data derived from two combustion test campaigns with an automatically stoked small-scale boiler were investigated through principal component analysis. Thus, the complex interdependencies between the fuel composition and the resulting bottom ash characteristics and the applicability of existing fuel indices were evaluated. The chemometric analysis highlighted that Si, Ca, K, Mg, and also the remaining Al and S in the bottom ash are crucial fuel components in the context of bottom ash melting. On this basis, the molar ratio (Si + P + K)/(Ca + Mg) was adapted and correlated with the susceptibility to slag formation which is a new parameter derived from ash content, slag fraction > 16 mm in the bottom ash, and slag category. Thus, the applicability of a newly developed fuel index was evaluated with respect to the bottom ash slagging risk during real-scale combustion. Three ranges were distinguished for the fuel index corresponding to a specific susceptibility to slag formation (i.e., low < 20 mol/g for woody biomass, elevated between 20 and 75 mol/g, and serious > 75 mol/g for straw-like fuels and blends with wood). The linear regression of the fuel index with susceptibility to slag formation exhibits a high coefficient of determination (i.e., 0.99 for woody biomass and 0.84 for straw-like fuels and their blends with wood).


Fuel index Biomass Combustion Principal component analysis Ash Slagging Chemometric analysis 



Squared loadings for ith variable (−)


Contributions of the variables to the principal components (%)



Ash content of fuel i (wt%)


Maximal ash content of all investigated fuels (wt%)


Normalized ash content (−)


Base-to-acid ratio


Bottom ash fraction that forms slag


Bed agglomeration index


Dry basis


Deutsches Institut für Normung e.V. (German Institute for Standardization)


Ash deformation temperature (°C)


Processed wood chips (i.e., end product)


European standard


ENplus is an international acknowledged wood pellet certification scheme which was established in 2011. ENplus introduced quality classes and stronger requirements to those set by the European and international product standards for solid biofuels


Ash flow temperature (°C)


Fouling Index


Hydrofluoric acid


Ash hemisphere temperature (°C)




Principal component


Principal component analysis


Unprocessed wood chips (i.e., raw material)


Babcock index


Wheat straw


Ratio of Si and Al oxides


Standard deviation


Sinter category of fuel i (−)


Maximal sinter category of all investigated fuels (−)


Normalized sinter category (−)


Bottom ash fraction that forms slag > 16 mm of fuel i (wt%)


Maximal bottom ash fraction that forms slag > 16 mm of all investigated fuels (wt%)


Normalized bottom ash fraction that forms slag > 16 mm (−)


Susceptibility (to slag formation)


Slag viscosity index


Ash shrinkage starting temperature (°C)


Wood sawdust



The data sets used in this publication were funded under grant agreement number 22031814, 22035714, 22035814, and 22005815 of the Agency for Renewable Resources (Fachagentur Nachwachsende Rohstoffe e.V., FNR) in the name of the German Federal Ministry of Food and Agriculture (BMEL) on the basis of a resolution of the German Federal Parliament and upon work supported by the German Federal Ministry of Education and Research (BMBF) under Grant No. 03SF0347B. A.M.M. was supported by DAAD for an internship in DBFZ Deutsches Biomasseforschungszentrum gemeinnützige GmbH (DBFZ) through Research Grants - Short-Term Grants 2018 Program (funding program no. 57378443). Furthermore, funds of the Federal Ministry of Food and Agriculture (BMEL) supported this work based on a decision of the Parliament of the Federal Republic of Germany via the Federal Office for Agriculture and Food (BLE) under the innovation support program.

Supplementary material

13399_2019_494_MOESM1_ESM.docx (37 kb)
ESM 1 (DOCX 36 kb)


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.DBFZ Deutsches Biomasseforschungszentrum gemeinnützige GmbH (DBFZ)LeipzigGermany
  2. 2.Faculty of Energy and FuelsAGH University of Science and TechnologyKrakowPoland
  3. 3.Faculty of Agricultural and Environmental Sciences, Chair of Waste and Resource ManagementUniversity of RostockRostockGermany

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