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High-Phosphorus Fuel Combustion: Effect of Oxyfuel Conditions on PM10 Emission from Homo- and Heterogeneous Phases Sui Boon Liaw and Hongwei Wu* Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia ABSTRACT: Volatiles and char were prepared from the pyrolysis of a biosolid (with a phosphorus content of ∼2.3 wt %) at 1000 °C and then combusted separately in air and oxyfuel (30% O2 in CO2) in a drop-tube furnace at 1300 °C. The aim is to understand the effect of oxyfuel conditions on the emission of particulate matter with aerodynamic diameters of ≤10 μm (PM10) from separated combustion in homo- and heterogeneous phases, respectively. For volatiles combustion in homogeneous phase that leads to only PM1 (dominantly PM0.1) emission, a change from air to oxyfuel results in an increase in PM1 emission as a result of a higher yield of Na, K, S, and P, likely resulting from enhanced sulfation of alkali species under oxyfuel conditions (with a higher O2 content), but leads to a negligible effect on the release of trace elements (As, Cd, Pb, V, and Zn). On the contrary, for char combustion in heterogeneous phase that contributes to both PM1 and PM1−10 emissions, a change from air to oxyfuel conditions leads to a reduction in PM1 emission but little change in the PM1−10 yield. Such a reduction in PM1 is contributed by reductions in the yields of Na, K, and P, most likely as a result of part of volatilized P to react with CaO to form non-volatile Ca3PO4. For trace elements during char combustion in heterogeneous phase, oxyfuel conditions lead to reductions in As and Cr released as PM1 (most likely as a result of the enhanced formation of Al/Fe/Ca arsenate and iron chromate) but have little effect on the release of Co, Cu, Mn, Ti, and V. The results show that P plays an important role in PM10 emission. For volatiles combustion in homogeneous phase, P is present in PM0.1 (contributes to most of the PM1 formed) in the form of both (Na,K)PO3 and P4O10 that is slightly favored under the oxyfuel conditions as a result of a higher O2 content. However, for char combustion in heterogeneous phase, P is present in PM0.1 dominantly as (Na,K)PO3, with little P4O10 under both air and oxyfuel conditions. Phosphorus in PM1−10, which is only produced during char combustion, is in the form of Mg3(PO4)2 and Ca3(PO4)2 under both air and oxyfuel conditions.
key feature of the biosolid sample is that it contains very high P but very low Cl. 2.2. Separated Combustion of Volatiles in Homogenous Phase and Char in Heterogeneous Phase. The experimental program considers both air (21% O2 in N2) and oxyfuel (30% O2 in CO2) conditions. Oxyfuel with 30% O2 in CO2 is chosen for the experimental program because such an O2 concentration provides particle temperature, particle ignition, and devolatilization properties similar to those from combustion in air.1,6 Investigating the effect of oxyfuel conditions on separated homo- and heterogeneous phases requires the supplies of separated volatiles and char for individual combustion. In this study, a drop-tube/fixed-bed quartz reactor7 was used as a volatiles generator to produce char and volatiles from biosolid fast pyrolysis at 1000 °C. Briefly, the reactor was preheated at the pyrolysis temperature, and then the biosolid sample was injected into the reactor at ∼0.05 g/min using 1 L/min ultrahigh-purity (UHP) argon. Biosolid particles experience fast pyrolysis, with the volatiles produced in situ passing through the quartz frit, while the char remains on the frit. An additional stream of 0.5 L/min UHP argon is supplied to the reactor to prevent the volatiles from back flowing in the quartz reactor. It is noted that the pyrolysis temperature of 1000 °C is restricted by the limit of the quartz working temperature. Figure 1 presents the distribution of major and trace elements in char and volatiles after biosolid pyrolysis. It can be seen that some Na, K, P, and As plus most Cl, S, Cd, Pb, and Zn in biosolid are released in the
1. INTRODUCTION Oxyfuel combustion is considered a key step-change technology to retrofit existing solid-fuel-fired power plants for CO2 capture.1 A large quantity of sewage sludge/biosolid can be produced from wastewater treatment plants, offering a good opportunity for utilization as a fuel source. A key feature of sewage sludge/biosolid is its high phosphorus content,2 which may have some important effect on the emission of particulate matter with an aerodynamic diameter of
