Article pubs.acs.org/EF
Enhanced Control of Fine Particle Emissions from Waste Biomass Combustion Using a Hybrid Filter Gaizka Aragon,† David Sanz,‡ Inaki Mugica,† Enrique Rojas,‡ Miren Larrion,† Jesus Rodriguez Maroto,‡ Raquel Ramos,§ Ricardo Escalada,§ Elena Borjabad,§ and Cristina Gutierrez-Canas*,† †
Department of Chemistry and Environmental Engineering, University of the Basque Country UPV/EHU, Alda. de Urquijo s/n, 48013 Bilbao, Spain ‡ CIEMAT, Avda. Complutense 40, 28040 Madrid, Spain § CEDER-CIEMAT, Autovia de Navarra A15, Salida 56, 42290 Lubia-Soria, Spain S Supporting Information *
ABSTRACT: This work demonstrates the feasibility of a robust and effective control of the emissions of particulate matter (PM) and heavy metals (HMs) from olive tree pruning combustion, as well as from its blends with two customary local components (a composted fraction of sewage sludge and a fraction of municipal solid waste with a high plastic content), in an approximated proportion of 50/50 (% w.t) to mimic the seasonal supply variability. The study was carried out during a 5 weeks experimental campaign in a 1MWth fluidized bed combustor. The combustor was equipped with a hybrid filtration system, consisting of an electrostatic precipitator and a bag filter module serially connected, for an enhanced control of the PM emissions when dealing with a realistic suite of fuel blends. Raw aerosol showed high concentrations in both PM and HMs, substantially dependent on the blend. However, the hybrid filter (HF) showed a good performance irrespective of these variable loads, leading to emission concentrations below 7 mg/Nm3 (max. 7% O2, total basis), ensuring compliance with the probable emission limit values. Both mass and number filtration efficiencies ranged within 95−99%. The fractional efficiency has been measured in quasireal time, thus enabling an assessment of the suitable operating conditions, as well as an evaluation of the reliability of HF operation. Removal efficiency for HMs was obtained and discriminated by size; results show differences among the elements considered, reflecting the differences in the chemical distribution over the size of the raw aerosol.
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INTRODUCTION A renewed interest in waste biomass combustion (forest and agricultural residues as well as waste biomass) is growing in Southern Europe1 as a result of a combination of different factors: not only CO2 neutrality and security of supplies, but of most importance is to advance toward sustainable land-use practices and fire prevention (deliberate, in-field combustion of agricultural wastes). In addition, the European Union bioenergy development policy has set a goal of 20% renewable energy by 2020. There is an opportunity to take advantage of this energy resource by means of a decentralized network of wellcontrolled, medium-scale combustion units adapted to the characteristics of the wastes from local crops or feasible blends throughout the year. Olive tree pruning residue is an important resource in the Mediterranean Basin, where basically the 8.6 million hectares of olive trees grown worldwide with are located.2 The estimated yield of pruning residue ranges from 0.3 t/ha, based on an average of 120 trees/ha and 25 kg dry pruning per tree, to 1 dry ton/ha.3,4 The use of olive tree pruning residue as an energy resource is growing and depends on the local development of systems for collection, processing, and delivery as well as on the feasibility of blends to ensure the operation on a yearly basis. Emission limits from biomass boilers with a capacity Si in the case of A, and Si > K in component B. The concentration levels of both K and Si are higher in blend components A and B than those in base fuel C. However, the individual ratio K/Si, indicating the potential to enhance nucleation,38 is higher in fuel C (2.01) than in components A (1.77) and B (0.30). Using the above-mentioned mixing ratios, the K/Si for blend AC (1.80) was close to that for fuel C (2.01), showing the lowest value for blend BC (0.46). Table 1 collates the trace metal concentrations of the blend components and their blends. In general, component B presents higher values for trace metals of concern, i.e., precursors of atmospheric pollution either directly, forming fine particles, or indirectly, as catalysts (Cu and Zn) for persistent organic pollutants. The concentration of Fe in component B reaches 6641 ppm, whereas for fuel C it is 229 ppm. The values for Zn and Cu in component B are respectively 660 and 1900 ppm. In component A, these concentrations are 84 ppm Zn and 12 ppm Cu. Fuel C shows contents of S (0.08% d.b.) and Cl (0.03% d.b.) substantially lower than those of its blends. Due to these differences in composition, blending is expected to have a substantial effect on the fly ash characteristics and, consequently, on the HF performance. The Experimental Setup. The experimental strategy encompasses real-time aerosol analysis, for monitoring purposes, with timeintegrated samples for further physicochemical characterization. Two sampling stations were set up (Figure 1), directly upstream and downstream of the HF, to enable simultaneous measurements and near-real-time analysis of both the raw and emitted aerosol. The raw gas was diluted using clean and dry air using a two-stage ejector system, the first of them being heated to avoid condensation and particle losses. The emitted aerosol sampling line had a similar configuration but with a single, nonheated diluter. Total concentration and chemical composition of the raw aerosol are obtained from isokinetically withdrawn samples on conventional 47 mm filters. Time-averaged aerosol mass and composition
Table 1. Composition and Heating Values for the Blend Components and the Fuelsa compost (A) moisture (% w.b.) ash content (% d.b.) volatile matter (% d.b.) fixed carbonb (% d.b.)
10.2
MSW (B)
olive prune (C)
Proximate Analysis 8.8 7.5
8.1
5.1
15.0
12.3
64.8
75.5
81.3
73.2
78.7
9.9
3.4
13.7
11.8
9.0
43.23 5.72 3.20 0.33 0.45 32.09
49.35 6.54 0.96 0.17 0.36 30.33
Al Ba Ca Fe K Mg Na P Si Sr Ti As Cd Co Cr Cu Mn Mo Ni Pb Sb Sn Tl V Zn
Elemental Analysis 897 15634 28 189 73221 34684 602 6641 17114 6131 7169 2163 8163 6574 35333 2670 9691 20712 202 102 8 2150
