Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article The effect of papermaking sludge as an additive to biomass pellets on the final quality of the fuel Miloš Matúš ⁎ , Peter Križan, Ľubomír Šooš, Juraj Beniak Slovak University of Technology in Bratislava, Faculty of Mechanical Engineering, Namestie Slobody 17, 812 31 Bratislava, Slovakia ARTICLE INFO Keywords: Biomass Straw Pellets Ash melting Papermaking sludge ABSTRACT The ever increasing demand on top quality solid biofuel in form of biopellets is limited by the sources and properties of raw materials. The best quality biopellets are suitable for burning in small scale boilers up to 100 kW t . The most limiting factor of this fuel is the ash melting temperature. Therefore the use of cheaper and readily available phytomass as a raw material is considerably restrictive. The research in this paper is based on the theory of the chemical process of ash creation and on the effect of additives on this process. The objective of the study is to find and apply an effective and available additive to produce the best quality phytomass-based fuel pellets, suitable for small scale boilers. A by-product of the paper recycling process, papermaking sludge was used as the most effective additive to wheat straw. This is due to the high content of calcite and kaolin in the sludge. It was also confirmed by experiments that the composition of sludge including cellulose fibres very positively affects the physical and mechanical properties of the composite pellets. Our research deals with an experiment concerning an appropriate production process of composite fuel based on wheat straw. Different weight proportions of papermaking sludge were added and the effect of the sludge on the fuel parameters, especially on the ash melting behaviour, was observed. The final quality of the composite fuel was thoroughly tested by physical, mechanical and thermal methods, and its properties were compared to European and International Standards for Biofuels. The results of the experiments demonstrate that the composite pellets produced by the above-mentioned process have a high level of fuel quality and, due to high ash melting tem- perature, they are suitable for small scale boilers. The results have shown that the appropriate range of pa- permaking sludge content for producing high quality wheat straw composite pellets ranges from 10% to 20%. 1. Introduction The ever-increasing interest in producing energy from local sources of agricultural residues poses new challenges for the development of technologies and processes in the production of biofuels suitable both for large and small combustion plants. Agricultural residues represent a cheaper energy source than wood, but their use has some technical limitations. The use of straw-based fuels, especially in small scale boi- lers up to 100 kW t, is troublesome due to the low softening and melting temperature of the ash [1]. Slagging and fouling problems frequently occur. There are two ways of eliminating the formation of slag. One is to reduce the temperature in the burner place. The other way is to alter the composition of the ash forming content. This can be achieved by using the right type of additive during the treatment of raw material before the pelletizing process [1–4]. The low ash-fusion temperatures of biomass are a serious dis- advantage [5]. It is widely accepted that most of the severe deposit formation, slagging and fouling problems during biomass thermochemical conversion result from the low ash-melting tempera- tures [5–8]. It was found that the high concentrations of K-, Si-, P-, S-, Fe-, Na- and Mg-containing minerals (excluding the highly enriched in Si biomass varieties) and low contents of Ca-, Al- and Ti-bearing mi- nerals are commonly responsible for decreased ash-fusion temperatures of biomass [8]. According to [5,8], the low ash-fusion biomass varieties normally have high slagging propensity due to formation of low-tem- perature melts and their subsequent intensive melt crystallization fol- lowed by abrupt glass generation during cooling at relatively low temperatures. The lower ash-fusion temperatures with short softening- melting range and high flow-dissolution rate seem to be the worst case for slagging and fouling [5,8]. A selection of optimal temperatures for thermochemical conversion is required to avoid the above problems. As the overviews in literature [5,8] describe, in additionally, a beneficial approach for problematic low ash-fusion biofuels or alternative bed materials for fluidized bed combustion is to use various additives, namely kaolinite, mullite, clinochlore, bentonite, K feldspar, plagio- clase, olivine, quartz, lime, bauxite, gibbsite, diaspore, corundum, https://doi.org/10.1016/j.fuel.2018.01.089 Received 30 November 2017; Received in revised form 16 January 2018; Accepted 23 January 2018 ⁎ Corresponding author. E-mail addresses: milos.matus@stuba.sk (M. Matúš), peter.krizan@stuba.sk (P. Križan), lubomir.soos@stuba.sk (Ľ. Šooš), juraj.beniak@stuba.sk (J. Beniak). Fuel 219 (2018) 196–204 0016-2361/ © 2018 Elsevier Ltd. All rights reserved. T