Impact of operating wood-burning fireplace ovens on indoor air quality

Highlights

• Wood-burning fireplaces are potential sources of indoor air contaminants.

• Especially the concentration of ultrafine particles increases during operation.

• The effects of NO_x, CO and CO₂ were comparatively small.

• In addition to the combustion process itself firelighters are a potential source for benzene.

• In some case increased concentrations of benzo[a]pyrene were measured.

Abstract

The use of combustion heat sources like wood-burning fireplaces has regained popularity in the past years due to increasing energy costs. While the outdoor emissions from wood ovens are strictly regulated in Germany, the indoor release of combustion products is rarely considered. Seven wood burning fireplaces were tested in private homes between November 2012 and March 2013. The indoor air quality was monitored before, during and after operation. The following parameters were measured: ultra-fine particles (5.6–560 nm), fine particles (0.3–20 μm), PM_2.5, NO_x, CO, CO₂, formaldehyde, acetaldehyde, volatile organic compounds (VOCs) and benzo[a]pyrene (BaP). Most ovens were significant sources of particulate matter. In some cases, an increase of benzene and BaP concentrations was observed in the indoor air.

The results illustrate that wood-burning fireplaces are potential sources of indoor air contaminants, especially ultra-fine particles. Under the aspect of lowering indoor air exchange rates and increasing the use of fuels with a net zero-carbon footprint, indoor combustion sources are an important topic for the future. With regards to consumer safety, product development and inspection should consider indoor air quality in addition to the present fire protection requirements.

Introduction

Wood-burning fireplaces have regained popularity in homes over the past years. These ovens create a cozy, warm ambience while offering an attractive lower cost, ecological alternative to other forms of heating. In times of increasing energy prices, wood-burning ovens can be competitive on running costs and conserve resources when compared to fossil fuels. Combustion of wood as a renewable resource is also close to climate-neutral in terms of carbon dioxide (CO₂), since in the ideal case only the CO₂ which was drawn in during the tree’s growth and stored in the wood is released into the atmosphere again.

It is, however, natural that combustion takes place more or less incompletely and causes undesirable by-products to form. The more complex the combustion material is, the more difficult the reaction process is. Natural gas can relatively easily be burned to produce carbon dioxide and water, but burning oil and coal in an environmentally friendly manner requires much more effort (Lackner et al., 2013). Wood and other kinds of biomass are among the oldest combustion materials but are also some of the most problematic. From a chemical standpoint, wood is a polymer of celluloses, polyoses and lignin with proportions of other minerals and extracts depending on the wood species (Fengel and Wegener, 1989). The heat energy from firewood (beech, oak, birch) is around 17–19 MJ kg⁻¹. Depending on how the oven is operated, set up and maintained, and the firewood itself, a wide range of other combustion products are released besides carbon dioxide and carbon monoxide (McDonald et al., 2000, Schauer et al., 2001, Hedberg et al., 2002, Shen and Gu, 2009). These particularly include the usual combustion products of the cellulose (Shen and Gu, 2009), aldehyde (Cerqueira et al., 2013) and particulate matter (Hedberg et al., 2002, Tissari et al., 2008). At high burn-off temperatures (<1200 °C), which however rarely occur in wood combustion, NO_x can form from atmospheric nitrogen. Low temperatures (<500 °C), in contrast, lead to de novo synthesis of aromatic compounds (Choudry and Hutzinger, 1983) and the formation of poly chlorinated dioxins/furans when chlorine is present (Salthammer et al., 1995).

It has generally always been the case that fire sources in rooms cause an elevated level of indoor air pollution. Moreover, a heat source might influence the air movement in a room (Ardkapan et al., 2014). Reports in the past have looked closely at the effects of sources such as gas burners (Wallace et al., 2008, Wallace and Ott, 2011), candles (Glytsos et al., 2010), incense sticks (Wang et al., 2007), kerosene ovens (Carteret et al., 2012), pizza ovens (Buonanno et al., 2010) and especially open fireplaces in homes (Lahiri and Ray, 2010). Publications were made 30 years ago concerning the influence of wood combustion on indoor air quality (Sexton et al., 1984). In more recent times, Noonan and colleagues have paid particular attention to the formation of PM_2.5 in indoor air when operating wood-burning fireplace ovens (Ward and Noonan, 2008, Noonan et al., 2012, McNamara et al., 2013). Carvalho et al. (2013) have studied the formation of ultrafine particles from wood-burning stoves.

With fine and ultra-fine particulate matter being increasingly in the focus of public discussion recently, the German Federal Environment Agency published a guidance value for evaluating fine particulate matter (PM_2.5 fraction) in indoor air of 25 μg m⁻³ (Salthammer, 2011). This directly concerns wood-burning fireplace ovens as potential sources of particles (Jalava et al., 2010). Consequently, WKI carried out a study with a total of seven test series in living rooms with wood-burning fireplace ovens. The results are presented in this paper and discussed with regard to current indoor air hygiene criteria. Another paper reports the effects of ethanol fireplaces on indoor air quality (Schripp et al., 2013).