The Preliminary Studies of the Phase and Chemical Composition of Ashes Formed in the Co-combustion of Hard Coal and Wood Biomass in an Individual Domestic Furnace (Poland)

Danuta Smolka-Danielowska

University of Silesia,  Faculty of Natural Sciences,  Intitute of Earth Science, Poland

CitationCitation COPIED

Smolka-Danielowska D. The Preliminary Studies of the Phase and Chemical Composition of Ashes Formed in the Cocombustion of Hard Coal and Wood Biomass in an Individual Domestic Furnace (Poland). Pollut Prev Control. 2019 Oct;1(1): 102.

© 2019 Smolka-Danielowska D. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 international License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


This work presents the analysis results of ashes formed in the combustion of hard coal and wood biomass (birch) in an individual domestic furnace (upper combustion boiler) in Poland. The ash was sampled from under the grate (grain size from 100 µm to a 2 cm) and from the upper chamber at the chimney (max. grain size 20 µm). A mineral composition was determined in ash samples by an X-ray diffraction method and a powder method and the phase composition was determined by means of the scanning electron microscope. The samples of hard coal and wood biomass ashes were analysed in terms of the content of potentially toxic elements (PTE): Pb, Cd, Cu, Ba, As, Zn, Ni, Mn by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) . 


ard coal, Wood biomass, Individual domestic furnace, Minerals phase, Trace elements


A dominant impact on the size and structure of the air pollution emission in Poland is exerted by commercial energy and hard coal combustion in individual domestic furnaces. The emission and ashes from individual domestic furnaces constitutes 35.8% (share in total emissions) at the national level [1]. This results from the fact that individual domestic furnaces do not have any devices for fumes de-dusting. Solid combustion products in the form of coarse and fine dusts, which are discharged at municipal landfills, also constitute a serious problem. Individual heating consumes more than 10 mln tonnes of hard coal annually (approximately 14% of the national consumption). It is estimated that 1 tonne of hard coal produces about 0.4 tonne waste (ashes) [2]. During the heating season, ashes emitted by individual domestic furnaces and related PTE contribute to air pollution, causing adverse effect on the environment and human health. 

The chemical and phase composition of ashes formed in the co-combustion of hard coal and biomass in individual domestic furnaces differs basically from ashes formed of commercial energy plants (power plants, heat and power stations) [3-6]. A dominant component in ashes formed during the combustion of hard coal in commercial energy plants is SiO2 [7-9]. According to Vassilev et al. [10] fluidised ashes from the co-combustion with biomass may include more elements, such as: Ag, Au, Cd, Cr, Cu, Mn, Ni and Zn compared to the ashes from coal combustion. However, this is not a rule because the trace elements content is mostly influenced by such parameters as: the physical and chemical properties of hard coal, the chemical composition of biomass and combustion technology. 

It is estimated that approximately 480 mln tonnes of ash from the combustion and cocombustion of biomass may be generated worldwide annually, assuming that the biomass combusted amounts to 7 billion tonnes/year [11-13]. The ash content is usually lower in biomass than in hard coal; yet, this is not a rule.

In individual domestic furnaces, the process of hard coal combustion takes place in lower temperatures due to the insufficient quantity of oxygen. This process leads to the formation of ashes with a different chemical and mineral composition and variable PTE content [14]. Wood combustion is an important source of organic and in-organic solid particles emitted to the environment [15-17]. 

The aim of the present work was to study the concentrations of potentially toxic elements of the ash generated from the combustion of hard coal and wood biomass. To establish what phase mineral are present in ash from individual domestic furnaces.


Sample collection and preparation

Coal ash, hard coal and biomass samples come from individual domestic furnaces using Upper Silesia Basin coals (Poland) as a fuel for central heating purposes, one located in the Rybnik town (sample codes A1–A9, B1-B2 and C - unburned coal in Table 1). A total of 12 samples were taken.

The city of Rybnik is the centre of a metropolitan area, the Rybnik Coal Region (Figure 1). Geographical co-ordinates defining the Rybnik town location are 500 5’ northern latitude and 180 33’ of eastern longitude.

Ash was sampled in November, December and January of 2018/2019. The biomass burned in the furnace was softwood (birch). The percentage of wood biomass in the combustion process is 35%. Wood usually is not available in a dried state and it is characteristic for varied humidity which ranges from 15% to 60% (the Author’s own research) depending on the seasoning duration. The hard coal net calorific value is 23 GJ/t. This is energy coal of a walnut size (commercial assortment). The net calorific value of birch wood is about 17 GJ/t [18].

Hard coal was a combusted in the boilers of the Zębiec SWK-21. The SWK boiler with the upper combustion of solid fuels ensures the efficient heating of any type of building: from a garage, a house and utility buildings to commercial spaces. Indicative thermal power in depedence from stove 21 kW and admissible pressure was 0.15 MPa. Temperature of combustion gas in boilers was from 3000 C to 4000 C. The hard coal combusted in the individual domestic furnace comes from the coal mines belonging to the Upper Silesian Coal Basin (USCB). Its calorific value is low (19 MJ/kg) (data from the Rydułtowy-Anna coal mine).

 The grain - size analysis was performed with the application of a laser particle size meter, type Analysette 22 Micro Tec plus (University of Silesia, Faculty of Natural Sciences).

X-ray powder diffraction 

The mineral composition of the samples was analyzed with the X-ray diffractometry (XRD) method ussing an - X’Pert Pro MPD PW3040/60 diffractometer, utilizing Co-K α radiation with a graphite monochromator. The lamp electric voltage was 40 kV with current intensity of 40 mA. Impulse time counting in the method was 100 s and the rate of a tape movement was 0.01/min. The estimated concentrations (%) of the analyzed amorphous phase are given using the X’PERT computer program. 

Scanning electron microscopy

Qualitative phase analysis of ashes and biomass was carried out with the method of scanning electron microscopy equipped with EDAX EDS using PHILIPS XL 30 under the following analytical conditions acceleration voltage 15-20 keV, beam current intensity 20 nA

Chemical analysis

The content of the potentially toxic elements was analysed at Activation Laboratories Ltd.’s Global Headquarters (Ancaster, Ontario, Kanada) by means of inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectroscopy (ICP-MS) methods using the Perkin Elmer SCIEX ELAN 6000 and Varian 735 ES (ICP-OES). ICP-OES and ICP-MS analyses by 4-acid (hydrochloric, nitric, perchloric and hydrofluoric) digestion are “near total” digestions.One blank is analyzed per work order. International certified reference materials USGS GXR-1, GXR-2, GXR-4 and GXR-6 are analyzed at the beginning and end of each batch of samples. 

Figure1: Location of sampling for testing (https://www.google.com/search=Poland)


In ashes from hard coal and wood biomass combustion, the share of large fraction (> 100 μm) amounts to 52.21–78.42%, and that of fine fraction (< 6 μm) is at the level of 17.25–25.24% (samples A1- A5). The average values in ash samples are given in Table 2. The percentage fractions of the grain fraction were calculated for the upper grain size limit.

Mineral composition of wood biomass ash

The wood biomass ashes (B1-B2) show different mineral composition and variable content of respective components. Calcite (25.6%-69%), quartz (2.1%-17%) and fairchildite (2.3%-8.6%) are the main mineral components in ash from wooden biomass. Monetite, arcanite, apatite, periclase, sylvine, portlandite and lime constitute from 1.8% to 5.5%. A high share of an amorphic substance was found in the tested wood biomass ashes (55.7% - 62.4%). Samples of ashes difractogram s show hight background and a hump, characteristic for amorphous substance, with maximum at 20o -27o 2Θ and 40o -50o 2Θ. The chemical and phase differences among the specified natural and anthropogenic biomass groups and sub-groups are significant and they are related to diverse biomass source and origin [19]. 

Mineral composition of hard coal and wood biomass ash 

The fine ash samples (A6-A9) constitute from 25.6% to 30.8% of the total ash mass obtained after the combustion of hard coal with wood biomass. The main mineral components identified in the analyzed ashes (A1-A9) are anhydrite, hematite, magnetite, mullite, pyrite, galena, gypsum, quartz, calcite, feldspars, kaolinite. Fairchildite and monetite occure in to the non-combusted coal substance. The amorphic substance (aluminosilicates and carbonates) in the examined ash samples constitutes from 56.7% to 63.7%.