Impacts of co-firing biomass on emissions of particulate matter to the atmosphere

Abstract

Gas and particle emissions from co-firing coal and two types of biomass versus coal was evaluated in a circulating fluidized bed boiler operating with a constant energy input. Compared to coal, co-firing 50% oat hulls (by weight) significantly reduced the emission of particulate matter (PM) by 90%, polycyclic aromatic hydrocarbons (PAH) by 40%, metals by 65%, and fossil carbon dioxide by 40%. In contrast, co-firing 3.8% wood chips (by weight) had a negligible impact on the emissions of PM and PAH, but caused a 6% reduction in metals. Fuel-based emission factors for PM, metals, and organic species including biomass burning markers retene and levoglucosan, were determined. Enrichment factors (EF) were computed to examine the distribution of metals across PM, fly ash, and bottom ash and demonstrated enrichment in volatile metals (e.g. Fe, Al, and Cr) in PM and fly ash. Co-firing 50% oat hulls led to a significant depletion of K in PM and its enrichment in bottom ash. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX) analysis revealed a wide heterogeneity in particle sizes and compositions across particles for all fuel types. Overall, this study demonstrates that co-firing a 50% oat hulls with coal provides several benefits to air quality and outlines important changes to PM composition when biomass is co-fired with coal.

Introduction

Biomass is a renewable fuel that holds significant potential for electrical energy production. Biomass co-firing is the practice of adding biomass as a partial substitute for coal in high efficiency boilers. Co-firing in existing boilers is a practical approach for increasing the use of biomass as fuel, because it draws upon widely-available, existing infrastructure and presents immediate opportunity for the production of low cost renewable energy [1], [2], [3], [4], [5]. Prior studies have demonstrated the feasibility of co-firing coal with a range of biomasses for electricity generation including wood chips [6], straw [7], switch-grass [8], rice husk [9], sugar cane bagasse [10], and saw dust [11]. Effective use of biomass fuels requires optimizing the type and amount of biomass, fuel introduction method, equipment maintenance, and air pollution control devices. In particular, understanding how co-firing alters emissions of gases and particulate matter (PM) to the atmosphere is needed to assess the potential environmental and health impacts of co-firing.

Among the greatest benefits to co-firing biomass is the direct reduction in the release of fossilized carbon to the atmosphere in the form of carbon dioxide (CO₂). Biomasses sequester atmospheric CO₂ during photosynthesis from the atmosphere that is re-released when burned. When regrown, biomass again serves as a CO₂ sink from the atmosphere, thus having a significantly smaller impact on atmospheric CO₂ concentrations than fossil fuels [9], [12], [13]. When sustainable growing practices are used, biomasses are locally-sourced, and biomass-to-coal blending ratios are high, the reductions in fossil CO₂ become more substantial.

Co-firing also has the benefit of reducing air emissions of criteria pollutants compared to traditionally coal-fired power plants [14]. Biomasses contain less sulfur, and thus emit less sulfur dioxide (SO₂), a greenhouse gas and the primary precursor for acid rain [15], [16]. Combustion produces nitrogen oxides (NO_x) that are toxic and reactive gases, the levels of which depend on fuel type and combustion conditions. When co-firing coal with biomass, NO_x has been shown to decrease [15], [16], but can increase when biomass is combusted alone [17]. NO_x emissions mainly depend on combustion temperature, which is influenced by fuel properties such as its moisture and volatile matter content [6].

Similar to NO_x, emissions of PM highly depend on the biomass to coal blending ratio. For example, co-firing 10% cedar chips increased PM emission, whereas co-firing 50% cedar chips reduced PM emission [18]. In addition, co-firing has been shown to increase the average size of PM compared to coal-only, which generally improves the particle capturing efficiency by bag houses and other particulate control systems [16], [19]. Furthermore, co-firing has been shown to alter the chemical composition of the PM that is formed [18], [20].

Polycyclic aromatic hydrocarbons (PAH) are semi-volatile organic compounds that have mutagenic and carcinogenic properties. PAH are emitted by burning carbonaceous fuels; the magnitude of their emissions and the distribution of species depend on the fuel type, combustion temperature, and air mixing ratio [21]. Generally, biomass contains higher volatile matter than coal which has been shown to combust more quickly and reduce PAH emissions [9], [22].

Co-firing also alters emissions of trace elements, by reducing coal inputs in favor of biomass [18]. Coals naturally contain heavy metals, such as Hg, As, and Se [23]. When co-firing biomass that contains low amounts of these metals, their overall emissions are reduced. Meanwhile, the high Cl content in biomass promotes formation of volatile metal chloride species (e.g. Zn and Pb), that condense upon fine particles and exit the stack as particulate matter [24]. Further, alkali metals in biomass, such as K in the presence of Cl, S, and Si undergo undesirable reactions and can deposit on the boiler surface and increase the rate of corrosion [25], [26]. Hence, emissions from co-firing cannot be represented as the sum of emissions from coal and biomass, because they do not account for the selective chemical reactions occur when biomass and coal are co-fired.

Enrichment factors (EF) provide a means of comparing the metal content of the PM, fly ash, and bottom ash generated by co-firing to the input fuel. EF correct for increases in metal content due to carbon loss by normalizing sample and fuel to a reference species in each matrix [27], [28]. EF are defined as the ratio of the concentration of an element (X) to the concentration of a reference element (R) in phase i (either PM, fly ash, or bottom ash) relative to the input fuel (j) as shown in Eq. (1):

$$\text{EF}=\frac{{(X/R)}_i}{{(X/R)}_j}$$

EF for fly ash and PM are interpreted as follows: EF > 1 indicates that volatile metals have condensed or absorbed, while EF < 1 indicates metals are leaving particles through a selective process to the gas or ash phases. For bottom ash, EF < 1 indicates that the metal is lost from the matrix due to volatilization, whereas EF > 1 reflects that the reference species may not be conserved or indicates an additional source of that element [27]. The comparison of EF across PM, fly ash, and bottom ash reveals the distribution of an element across small and large particles. Through the comparison of EF from co-firing to coal only, the effects of co-firing biomass with coal on trace element composition may be identified.

The aim of this study is to assess how the chemical composition of PM and ash generated by firing coal is affected by the co-firing of oat hulls and wood chips in a fluidized bed boiler. This study provides emissions data on co-firing oat hulls for the first time. With an advanced combination of analytical tools, we examine the magnitude of PM emissions, its metal and organic composition, and particle heterogeneity, and their relationships to the chemical composition of fuel input. EF are used to examine the distribution of metals across PM, fly ash, and bottom ash, and how these distributions are affected by co-firing. Fuel-based emission factors for PM, PAH, metals, and other organic species are determined, and include biomass burning markers for the first time. Through a deeper understanding of co-firing on the emission of PM to the atmosphere and its composition, a better understanding of its environmental and health impacts is gained.