Comparison of emissions and toxicological properties of fine particles from wood and oil boilers in small (20–25 kW) and medium (5–10 MW) scale

Highlights

• The use of fossil fuels versus wood fuels was compared.

• Gaseous and particle emissions and the cytotoxicity of PM₁ were examined.

• HFO combustion PM₁ was found to evoke the highest toxicity responses.

• Toxicity of emissions should be considered when authorities regulate emission limits.

Abstract

The aim of this study was to compare four alternatives for providing decentralized energy production in small communities in terms of their flue gas emissions and toxicological properties of the emissions. In this study, two different size classes of boilers were examined and the use of fossil fuel oils was compared against wood fuels. The lowest PM₁ emission, 0.1 mg MJ⁻¹, was observed from small-scale light fuel oil combustion. In medium-scale wood combustion, PM₁ emission values from a grate fired wood combustion boiler (10 MW) without particulate filtration were the highest (264 mg MJ⁻¹) but were substantially reduced down to 0.6 mg MJ⁻¹ due to the usage of an electrostatic precipitator (ESP). The wood combustion particles were mainly formed of potassium salts. In light fuel oil combustion, one of the main components in the particles was sulphate whereas in heavy fuel oil combustion also significant amounts of V and Ni were emitted. Pellet combustion produced the lowest PAH emissions. Overall, oil combustion produced higher amount of PAHs than wood combustion. This was indicated also as a higher cytotoxicity of the oil combustion samples when compared to those from wood combustion in the corresponding scale of boilers. However, when calculated on an equal mass basis, the particles collected after ESP were even more cytotoxic which can be explained by the altered chemical characteristics of the emissions in the ESP. Due to the variation in the emissions and in the toxicity of the emissions, we propose that in the long term, not only the emission levels but also the toxicity of the emissions should be taken into account in the regulations of the emission limits of the combustion plants.

Introduction

Fossil fuel combustion is considered to be problematic because it is a significant source of greenhouse gas emissions into the atmosphere and because of the limited resources of the fossil fuels. An increase in biomass use as an energy source holds the potential to reduce these emissions, since biomass combustion is currently not considered to affect the carbon dioxide (CO₂) balance in the atmosphere. In addition, biomass combustion can have political support in countries where there are no fossil fuel deposits but are endowed with substantial biomass resources (Obernberger and Thek, 2010). However, one of the major problems with the biomass combustion is its tendency to emit an abundance of fine particles. For example in Finland, it has been estimated that 25% of all fine particle (aerodynamic diameter <2.5 μm) emissions originate from domestic wood combustion (Karvosenoja et al., 2008). In addition, small and medium scale district heating units may generate substantial amounts of PM₁ (particles <1 μm in aerodynamic diameter) emissions if they are not equipped with efficient filtration systems (Sippula et al., 2009a). Different filtration techniques such as electrostatic precipitators (ESPs), flue gas scrubbers and fabric filters reduce the emissions from biomass combustion to a moderate or even to a low level.

In addition to particulate emissions, both biomass and fossil fuel combustion can release varying amounts of nitrogen oxides (NO_x) and sulphur dioxide (SO₂) emissions into the atmosphere. SO₂ emissions are generally related to the fossil fuel combustion but they can be significant also in the combustion of many field biomasses due to the high sulphur content of that kind of fuel (Tissari et al., 2008). NO_x and SO₂ emissions induce secondary particle formation and therefore they can have significant effects on air quality and climate. The climatic effects are caused both by direct sunlight scattering effects and by indirect effects due to the influence of secondary aerosols on the cloud formation and their properties. It has been estimated that 46–82% of SO₂ will form sulphate particles in the atmosphere (IPCC, 2001). In one of our previous studies, Sippula et al. (2009c) compared the particle emissions from small heavy fuel oil (HFO) and wood-fired boilers. One conclusion of that study was that the quantity of the particle emissions as well as their physical and chemical properties are clearly different between heavy fuel oil and wood combustion. Thus, the use of wood and fossil fuel oils would be predicted to have a very different impact on both human health and the environment. With efficient wood combustion systems, the particle emissions are mainly composed of ash species such as alkali metal chlorides and sulphates, but unfortunately when the combustion conditions are not optimal, more organic compounds such as polycyclic aromatic hydrocarbons (PAHs) are formed (Tissari et al., 2008; Sippula et al., 2009a; Hays et al., 2011; Aurell et al., 2012; Kinsey et al., 2012). The particles from the oil combustion usually contain soot, organic material and significant amounts of sulphur, depending on the oil sulphur content and combustion quality (Hays et al., 2008; Sippula et al., 2009c). The HFO particles typically contain also substantial amounts of heavy metals (Hays et al., 2009; Sippula et al., 2009c).

Previous epidemiological studies have shown that ambient particles which originated from fossil fuel combustion are especially harmful to health (Laden et al., 2000; Lanki et al., 2006; Penttinen et al., 2006). Toxicological studies have also shown that particles from fossil fuel combustion may cause adverse effects (e.g. Jalava et al., 2006; Happo et al., 2008). The toxicity of the emissions from oil combustion is often associated with the high metal concentrations, especially Ni and V from heavy fuel oil having been associated with toxic effects (e.g. Molinelli et al., 2006). However, also emissions from wood combustion have exerted toxic properties in several studies (Kocbach Bølling et al., 2009; Danielsen et al., 2011; Tapanainen et al., 2011; Jalava et al., 2012; Tapanainen et al., 2012). In epidemiological studies, it has been confirmed that small-scale wood combustion is associated with adverse health effects (Andersen et al., 2007; Naeher et al., 2007; Sarnat et al., 2008). Wood combustion emissions may also have an influence on the emergence of asthma and other respiratory symptoms (Boman et al., 2003; Allen et al., 2008; Ghio, 2008).

Even though there are many studies examining energy production related emission sources, there are no comparisons available helping to decide which energy sources should be favoured in order to reduce the adverse health effects in humans. If one wishes to estimate the optimal way to produce energy, one needs to perform a comprehensive examination of the emissions. In other words, the toxicity of emissions from wood and fossil fuel combustion need to be compared, not simply the overall emission levels.

This work is a continuation study to evaluate the toxicological effects of the emissions from fossil fuel oil and wood combustion and to compare the new results with those obtained in the previous study. The aim of this study was to compare four alternative ways of producing decentralized energy for use in small communities with respect to their flue gas emissions and the toxicological properties of emissions. Two different size classes of boilers were examined, the small scale (20–25 kWth) and the medium scale (4–15 MWth), and the use of fossil oil fuels was compared against wood fuels. A modern small-scale pellet boiler and a small-scale district heating plant with a rotating grate combustion unit represented the energy production of the wood combustion, and a small-scale light fuel oil (LFO) burner and heating boiler with a HFO burner represented fossil fuel combustion. Gaseous and particulate emissions were measured from the flue gas and the cytotoxicity of the particulate emissions was examined.