**1. Introduction**

In recent years, the monitoring of air pollution has been heavily focused on particulate matter (PM) [1,2]. The negative impact of fine and ultra-fine particles has been studied by many authors. Ultra-fine particles are dangerous to human health [3] and have much greater active surface than larger particles with comparable weight and, therefore, have a greater ability to bind to other harmful substances [4]. Particles smaller than 1 micrometre (PM1) are less than the minimum size for the lung's self-cleaning ability. The smallest particles can even diffuse through the wall of alveolar sacs and into the bloodstream [3]. The ultra-fine particles are produced during combustion processes due to a rise in temperature. Internal combustion engines and small furnaces are the main producers of ultra-fine particles in urban areas [5] and most of the particles produced by these combustion processes are PM1 [6], which becomes increasingly problematic in urban areas with a high density of residents using cars and small furnaces. The production of ultra-fine particles from these sources is significant and the final emission concentrations are dependent on the conditions of dispersion. Unfortunately, city buildings are a grea<sup>t</sup> obstacle for air flow at the canopy layer of air and the low air velocity creates conditions for a long residence time of particulates in air [7]. Ultra-fine particles do not sediment and can remain in the environment for days and weeks. They are separated from the air by touching a solid surface like walls, roads, vegetation, or a liquid surface like water bodies or raindrops. The ultra-fine particles can grow to fine particles by (i) coagulation mechanisms, that aggregates colloidal and macromolecular organic particles into larger clusters, (ii) by agglomeration, where particles bonding is

based on the adhesion of the surfaces, by (iii) oxidation reactions or by (iv) condensation of condensable vapours on the particle surface [8].

This paper focuses on the emissions of ultra-fine particles released by the combustion process occurring in a small biomass furnace. There is a vast variety of those furnaces and in each of them are different combusting conditions. Some parts of the furnaces have intense air flow that causes the flying of ash. Sides of a combustion chamber are intensively heated by exothermic reactions as there is a high concentration of volatiles in the combustion chamber. These volatiles undergo incomplete combustion due to a lack of intake air and then cool down and condense – a mechanism resulting in the nucleation of ultra-fine particles.

The measurement of the ultra-fine particles dispersed in the flue gas is important for identifying the effects of furnace operating parameters (heat power output, a surplus of combustion air, temperature) on the resulting emissions of the particles. Such measurements are also used for comparison of different fuels in one furnace [9], but these experimental measurements do not provide detailed insight at the sub-stages of the process of nucleation and subsequent agglomeration of particles. Numerous studies published by different authors deal with this topic, but the process of particle nucleation is still not sufficiently explained and clarified [10–12]. It is necessary to continue with the experimental investigation of numerous samples and analyse the result for a better understanding of the nucleation and the growth process. These processes are studied in laboratory conditions with ideal combustion of fuel samples. This experimental research is the main target of this paper and uses thermogravimetric analysis (TGA) for the identification of particles emitted from a small sample of wood. The measuring instrument provides the particle size distribution of particulate matter. It is very important to understand the nucleation and creation of these particles in the combustion process to create viable strategies for their elimination.

An earlier study by the authors [13] dealt with a laboratory investigation of fine particulate matter production from the controlled heating of beechwood samples in the atmosphere with 21% oxygen. This paper extends this previous experimental investigation by using lower oxygen content in the atmosphere during laboratory combustion. The limiting test case uses an inert atmosphere with 100% nitrogen.

#### **2. Particulate Formation**

When heat flux is applied to the solid phase of woods it is divided into three distinctive layers: (i) the char layer, (ii) the pyrolysis layer and (iii) raw wood. Between the char and pyrolysis layer is identified the char front in which occurs transition from raw wood into char by thermal decomposition (pyrolysis). This transition is usually considered to take place at the 300 ◦C isotherm, called the char-line [14].

When external heat flux affects the wood surface, part of the heat is reflected from the surface. Convective heat transfer between surrounding gases and the wood surface also occurs. In the pyrolysis layer, water evaporation occurs first and is later accompanied by pyrolysis reactions and the production of gas volatiles. Vapour and gas volatiles penetrate through pores and leave the wood.

Released volatile components have different values of partial pressure. When they reach their respective saturation point, the formation of a new phase begins – this is the nucleation process. The molecules are clustered into ultra-fine particles up to 0.1 μm in size. These particles can further grow by mechanisms mentioned in the introduction section. Particles that are formed in the combustion chamber are called primary particles. Particles formed in the flue gas duct and in the atmosphere are called secondary particles. Organic particles have different levels of volatility that divides them into volatile organic compounds (VOC) and semi-volatile organic compounds (SVOC) [15].
