Emissions from residential combustion of certified and uncertified pellets

Highlights

• Evaluation of pellet stove operating conditions and fuel certification on emissions.

• Thresholds set by the Ecodesign directive for gaseous compounds were surpassed.

• The combustion of two brands of ENplus A1 pellets resulted in distinct emissions.

• Coniferyl-type lignin pyrolysis products dominated indicating softwood combustion.

Abstract

The aim of the present study was to evaluate the relationship between fuel certification and gaseous and PM₁₀ emissions and their composition. Two pellets labelled as ENplus A1 (certified pellets R and P) and a non-certified brand of pellets were tested in a pellet stove. The impact of operating conditions on emissions was also evaluated. The highest carbon monoxide (CO, 2.7–6.1-fold) and total organic carbon (TOC, 1.9–11-fold) emissions were observed for certified pellets R. Nitrogen oxide (NO) emissions were higher for non-certified pellets (2.5–3.2-fold). The compliance of gaseous emissions with the Ecodesign thresholds was not ensured by certification. Certified pellets P generated significantly lower PM₁₀ emissions than the other two types of pellets at medium and nominal loads. Water-soluble ions represented from 36 to 68 %wt. of the PM₁₀ mass. The combustion of certified pellets P and non-certified pellets generated total carbon PM₁₀ mass fractions ranging from 23 to 50 %wt., whereas pellets R resulted in lower mass fractions (7–14 %wt. PM₁₀). The chromatographically resolved organic compounds were dominated by anhydrosugars and alkanols. Pyrene and retene were the most abundant among polyaromatic hydrocarbons.

Introduction

Increasing environmental concerns associated with fossil fuel consumption and EU bioenergy targets led to an increase in renewable energy production and utilisation [1]. Woody biomass meets an important share of Europe’s renewable-energy demand [2]. Among wood-based fuels, pellets offer numerous advantages and are particularly attractive [3], as evidenced by the strong growth of the pellet market over the years. In 2017, EU28 countries consumed around 24.1 million tonnes of wood pellets with nine million tonnes used for residential heating [4].

Key drivers behind the household’s heating system choice are the fuel price [[5], [6], [7]] and also the environmental benefits [5]. Despite the fact that pellet combustion heating devices allow achieving lower emissions compared with traditional firewood combustion in batch mode operated appliances [8], this technology is strongly dependent on stove/boiler design (e.g. fuel feeding system: bottom, horizontal or top fed, burner pot configuration) [[9], [10], [11]] and fuel physical (e.g. pellet length, particle density, durability, bulk density) [12,13] and chemical properties (e.g. fuel ash content, S, Cl and K contents) [[14], [15], [16]] for optimal performance. Furthermore, the user can still affect the emissions by adjusting the stove or boiler to operate at different heat output levels (nominal load or partial load). Generally, researchers found higher gaseous and particulate emissions when the pellet appliance is operated under partial loads [9,[17], [18], [19]].

The fuel quality is directly interconnected to consumer’s satisfaction with respect to pellet based residential heating [7]. In fact, besides its impact on emissions, the use of lower quality raw materials for pellet production, with high content of ash and critical inorganic elements (e.g. alkali metals, Cl, S and Si), can result in ash related problems (e.g. corrosion, slagging and fouling) [[20], [21], [22], [23], [24]]. Taking into account the importance of ensuring the quality of pellet fuels, many European countries, such as Germany, Austria, Sweden and Switzerland, developed their own standards [25]. In order to harmonise the European pellet market, the EN 14961–2 introduced a common standard for residential pellet fuels, which displaced all other national standards across the EU (e.g. DIN-52731, DIN plus, Ö-Norm M-7135, SS-187120). The European standard for wood pellets was followed a few years later by the international standard (ISO 17225–2). The ENplus certification scheme ensures consumers that the standard requirements are met [26]. Worldwide, the number of countries with ENplus certified producers increased from 7 to 44 between 2011 and 2017. The share of ENplus certified pellets on the European market was 70% in 2017 [4]. Over the years, several studies focused on assessing how the fuel labelling ensures compliance with the European standard requirements for a specific quality class [7,27,28]. Duca et al. [27] analysed chemical and physical parameters of commercial bagged pellets during a three-year period. The research results revealed that certification is useful to guarantee a high pellet quality. More recently, Pollex et al. [28] evaluated 249 high quality pellet samples to assess the potassium content. The authors reported huge variation in this ash-forming element among the tested samples and highlighted that the current limitation of fuel ash content is not enough to ensure low ash forming elements, such as potassium, in the pellets. Several studies have reported linear relationships between PM emissions and fuel K content [15,21]. Fine particles are mainly formed by vaporisation of ash forming elements (e.g. K, Na, S, Cl and easily volatile heavy metals, such as Zn and Pb) and subsequent nucleation and condensation due to flue gas cooling [14,29,30].

Several studies have focused on emissions from certified pellets. The main goal of most of them was to compare emissions from pellets manufactured from alternative raw materials with those obtained from the combustion of a high quality fuel [16,31,32]. Other studies reporting the performance of certified pellets aimed to evaluate the effect of combustion appliance operation on emissions [9,11,19]. More recently, Venturini et al. [17] investigated the emissions from the combustion of three wood pellets of distinct quality classes. The combustion of pellets with similar quality in the same combustion appliance and under similar combustion conditions was performed by Arranz et al. [33] and Fachinger et al. [34]. While the study of Arranz et al. [33] focused mainly on the gaseous emissions, the latter entails a more detailed characterisation, including PM emissions and chemical speciation. Fachinger et al. [34] reported slight differences in the gaseous emissions and PM chemical composition from the combustion of two types of pellets both complying with the threshold values defined for class A1 pellets (EN plus A1).

Despite the valuable data provided by the mentioned studies, the emissions from combustion of pellets sold in the Portuguese market are still poorly typified. The aim of this work was to evaluate the emissions (gaseous compounds and particulate matter) and the PM chemical composition from the combustion of two different brands of pellets with certification ENplus A1 and one brand of pellets without certification, all of them manufactured in Portugal. The effect of stove operation on emissions and PM chemical composition from the combustion of each type of pellets was also evaluated.