## 4 Conclusions  
The basic parameter for assessing the environmental impact of domestic boilers is the amount of dust particles emitted. The amount of dust particles is related to the chemical nature of the particles (the presence of inorganic and organic components) and the form of the element presence and binding into the fuel matrix. A comparison between the combustion of fossil fuel (coal briquettes), biomass containing the minimum amount of waste wood (WM) which was prepared according to the requirements of the standard ISO 17225-7 [22], and briquettes from waste cardboard and paper shows that PB releases the highest amounts of major elements (up to 57 g/kg of fuel) into the atmosphere, while the corresponding values are 22-28 g/kg for WM and 22-23 g/kg of fuel for CB. Similar results were obtained for trace elements (As, Cu, Cr, Mn, Ni, Pb, and Zn) and chlorine.  
Burning 1 kg of WM released 0.38-0.41 g/kg into the atmosphere. In the case of PB, it is 0.63-0.82 g/kg, and, in the case of CB, 0.20-0.21 g/kg. Major elements and quantities of unburned carbon decisively influence the overall production of emitted particles. A comparison of BEC2 and BEC3 boilers shows that the modification of the combustion conditions (the amount of air added) is involved in the change in the phase composition of the particles in the deposits from CB. In the case of optimal oxidation conditions, the combustion of CB produces hematite in addition to maghemite. When burning WM and PB in BEC2 and BEC3, differences in the amounts of elements released into the air were generally not significant. At the present time, the quality of combustion fuel has a significant impact in terms of reducing the amounts of elements released into the air. Any addition of waste materials (including wood) contributes to increasing the amounts of elements released into the atmosphere. It is very important to follow the rules of the circular economy, which emphasize further recovery of waste, but their further thermal recovery must respect information on the behaviour of elements during combustion processes.  
Further research should include the verification of the possibility of producing \"clean fuel\" from cardboard by removing fillers and additives and an assessment of the economic and energy requirements of this production.  
Conceptualisation, H.R.; methodology, J.R. and D.J.; software, B.S., P.K. and M.K.; validation, K.S., H.R. and D.J.; investigation, P.K., B.S., J.R., M.K. and S.F.; writing--original draft preparation, H.R., P.K., D.J. and J.R.; writing--review and editing, K.S.; visualisation, B.S., S.F. and M.K.; supervision, H.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Ministry of Education, Youth and Sport of the Czech Republic, CZ.02.101/0.0/0.0/18_069/0010049 \"Research on the identification of combustion of unsuitable fuels and systems of self-diagnostics of boilers combusting solid fuels for domestic heating\".  
Not applicable.  
Data available in a publicly accessible repository.  
This work was supported by the project of the Ministry of Education, Youth and Sport of the Czech Republic--CZ.02.101/0.0/0.0/18_069/0010049 \"Research on the identification of combustion of unsuitable fuels and systems of self-diagnostics of boilers combusting solid fuels for domestic heating\".  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
A Ash BEC2 Boilers of emission class EC2 BEC3 Boilers of emission class EC3 CB Coal briquettes D Deposits EC Elemental carbon EC2,3 Emission class 2,3 EEA European Environment Agency EF Emission factors EMEP European Monitoring and Evaluation Programme F Fuel FC Fuel consumption MDF Medium-density fibrewood OC Organic carbon