Research articleWood washing: Influence on gaseous and particulate emissions during wood combustion in a domestic pellet stove
Introduction
Wood pellets have become an important fuel in domestic heat generation, since the costs of fossil fuels are rising and the emissions are nearly CO2 neutral. All households are incited to turn to the use of biomass as energy source for domestic heating. In contrast to other wood based fuels, the utilization of pellets is easy and automatic feeding to stoves and boiler is possible. Furthermore, the pelletization densifies the wood and produces a fuel with high energetic density [1]. On the one hand, wood pellets are a convenient choice for domestic fuel because of its simplicity of implementation and its low cost [2]. But on the other hand, wood pellets combustion, and more generally wood combustion, is a source of fine particles (PM2.5) and gaseous compounds as carbon monoxide (CO) or other incomplete combustion gas as nitrogen oxides (NOx) or Non-Methane Volatile Organic Compounds (NMVOC) [[3], [4], [5], [6], [7], [8]]. But most emissions from pellets combustion are lower than wood log combustion [3,9] because in the case of wood logs, parameters as wood species, humidity could disfavor the combustion process and generate big amounts of incomplete combustion products. As example, pollutant emissions from wood log combustion in domestic devices (open and closed fireplaces, traditional and advanced stoves) range from 20 and 120 g·kg−1 for CO, are close to 1 g·kg−1 for NOx, range from 2 to 20 g·kg−1 for NMVOC and range from 0.2 to 2 g·kg−1 for PM emissions [[3], [4], [5], [6]]. The high magnitude of emissions from small domestic devices well depends on several parameters as fuel quality and operating conditions (wall material of the combustion chamber, natural draft, primary and/or secondary air, operational practices, etc.). Whereas values from pellets combustion range from 1 to 5 g·kg−1 for CO, are close to 1 g·kg−1 for NOx, range from 60 to 100 mg·kg−1 for NMVOC and range from 0.2 to 0.5 g·kg−1 for PM emissions [3,7,8]. More generally, combustion of wood pellets, due to a better complete combustion, is clearly less emissive in comparison with wood logs combustion. The automatic feed of the fuel and the possible presence of a lambda probe are advanced technologies that contribute to complete combustion [7,8].
In the last decade, numerous studies were devoted to the evaluation of emission factors for gaseous compounds or particulate matter from residential wood combustion appliances [3,8,10]. The main gaseous pollutants usually followed from wood combustion are carbon monoxide, Volatile Organic Compounds (VOCs) [8]. Trace pollutants as Polycyclic Aromatic Hydrocarbons, benzene, toluene, ethylbenzene, xylenes, trimethylbenzene (BTEXT), phenols, aldehydes and ketones are often analyzed too [3,10]. Phenol and methoxy-phenols as syringol and guaiacol are also used as tracers of wood combustions [11]. Regarding particulate emissions, Total Suspended Particles (TSP) and PM2.5 are constantly measured because it is well known that wood burning is one of the most emitting sources of fine particles [3,9,12].
Nowadays, several techniques are developed in order to reduce pollutants from wood combustion. Primary and secondary technologies must be distinguished: on the one hand, primary solutions focus on fuel quality and the conception of stoves and boilers, on the other hand, secondary solutions focus on the post-treatment of fumes [[13], [14], [15]]. Secondary technologies largely dominate using technologies as electrostatic filter, catalytic filter, or cyclones [16,17]. Secondary technologies aim principally at reducing particles emissions, but some techniques as catalytic filter allow to reduce gaseous emissions as carbon monoxide and VOCs [17]. Concerning primary techniques, important efforts from the 2000s were done in order to complain with different National and European standards [[18], [19], [20], [21]] in terms of conception of stoves and boilers [22], choice of wood species and optimization of the air/fuel ratio [23]. One of the primary solutions to reduce pollutant emissions from biomass combustion could be a washing pre-treatment of wood. Indeed, by washing wood, some compounds as minerals or extractives from wood could be removed. Deng et al. [24] have studied demineralization of some biomasses by a washing process with deionized water at temperatures ranging from 30 °C to 90 °C and results show that minerals as potassium or calcium could be extracted. For example from rice straw, extraction efficiency reaches respectively 87 and 19% at 30 °C for both elements. Jiang et al. [25] have also studied rice straw demineralization by several solvents as deionized water, chloride acid or phosphoric acid for example. Results show that, depending on the solvent, demineralization efficiency varies: calcium can be removed by 17% using deionized water versus 98% using chloride acid. Some extractives as phenols or carboxylic acids are also removed by wood washing [[25], [26], [27]].
Most of the studies have attempted to compare pollutant emissions as a function of the nature of the wood or the technology of the domestic heating appliances [22,23]. Main literature data focused on the influence of demineralization process on devolatilization phases of lignocellulosic polymers during thermogravimetric analysis [24,25,28]. Investigations on the influence of the wood preparation, particularly the washing process on gaseous and particulate pollutants emitted in the exhaust are scarce at real domestic scale [28]. This study aims at studying the leaching at room temperature of woods and their combustion (raw and washed biomasses) in a pellet stove to observe variations of gaseous and particulate emissions thanks to demineralization of wood. This work is a preliminary study of a French project supported by the National Agency of Energy and Environment (ADEME). The ultimate goal of this project is to study the influence of the natural leaching by rain of wood logs stored outside in forest for several months to a few years on gaseous and particulate pollutants during combustion in domestic devices as inserts and stoves. In order to get free of the variability due to the wood log fuel, it was decided to study the influence of leaching at laboratory scale using a pellet stove. Wood chips were then washed using demineralized water before preparing wood pellets. The stabilized conditions of a pellet burning system allow the detection of even small differences between different fuel types. This preliminary study on pellets washing aims to really understand and clearly explain the role of leaching on wood combustion with respect to gaseous and particulate emissions. Combustion of natural leached wood logs in a domestic stove will follow in a near future in order to compare these results.
Section snippets
Washing and pelletization protocols
Washing and pelletization protocols of biomasses are schematized in Fig. 1. Pre-treatment of beech, oak and fir chips was carried out at laboratory in a pilot according to the standard leaching protocol EN 12457/2 of May 2002 [29]. A quantity of 20 kg of dried wood chips was introduced into the pilot filled with 200 l of deionized water and mechanically shaken for 6 h. This operation was repeated three times but without drying wood samples between each cycle. For each cycle, the washing water
Effect of washing pre-treatment on standardized pollutants
Fig. 3 presents the influence of the washing pre-treatment on CO concentrations and its comparison with French and European standards. Concentrations of CO are relatively high for raw biomasses except for DIN+ Certco pellets which are standard solid fuel used for standardized combustion tests on small domestic pellet stoves. The tests carried out with pellets of raw fir, beech and oak wood lead to concentration of CO Two to three times higher than DIN+ pellets results. Comparison of the real
Conclusions
This work aimed to study the impact of wood washing on combustion emissions of a recent wood pellet stove. Three wood varieties, as fir, oak and beech were burned in their raw and washed form. Natural wood pellets according to the DIN CERTCO standard were also combusted as a reference during combustion tests. Particulate and gaseous emissions like CO, THC, PAH, biomass tracers as phenols or sugars, and some organic compounds as BTEXT and aldehydes were measured in the fumes.
Concerning CO,
Acknowledgments
The authors thank Mrs F. Proharam from the Agence De l'Environnement et de la Maitrise de l'Energie (ADEME) for the financial support of the PREPABOIS project (no 1501C0039), Sirs S. Labbé and F. Balay from LORFLAM industry and Mr N. Adam from AGRIVALOR industry.
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