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Ash aerosol and deposition formation mechanisms during air/oxy-combustion of rice husks in a 100 kW combustor Yueming Wang, Xiaolong Li, and Jost O.L. Wendt Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03127 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 4, 2018
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Energy & Fuels
Ash aerosol and deposition formation mechanisms during air/oxy-combustion of rice husks in a 100 kW combustor Yueming Wang a, Xiaolong Li a and Jost O.L. Wendt a* a
Department of Chemical Engineering and Institute for Clean and Secure Energy, University of Utah,
50 South Central Campus Drive, Salt Lake City, UT, 84112, USA; * Email:
[email protected] KEYWORDS: rice husks; oxy-combustion; ash aerosol; ash deposition; particle size distribution
ABSTRACT Pulverized rice husks are co-fired with natural gas in a 100 kW (rated) down-fired oxy-fuel combustor (OFC) under two conditions: 1) air combustion (denoted as Air); 2) oxy-combustion with 70% O2 and 30% CO2 in the inlet oxidant gas (denoted as OXY70). Studied in this paper are: 1) mechanisms governing the partitioning of inorganic matter within the fly ash aerosol, and 2) how these affect mechanisms of deposition on heat transfer surfaces. In each case, the ash aerosol particle size distributions (PSDs) were determined using electric mobility/light scattering instruments (SMPS/APS) and a Berner Low Pressure Impactors (BLPI), where the latter collected size segregated aerosol for subsequent analysis. The ash deposition rate was measured experimentally using a specially designed probe, and its relationship with aerosols was explored. The properties of the deposits were also investigated using a laser diffraction particle size analyzer for PSDs, and SEM/EDS and XRD for compositions. The data show that, compared to Air, OXY70 produces greater amounts of submicron aerosols, due to increased mineral vaporization at the higher flame temperature. The Cl and P are combined with K to form KCl and KPO3 in the submicron, but not in the super-micron, aerosols. For both conditions, the particle sizes within the more loosely bound outside deposits are much larger than
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those within the tightly bound inside deposits. Except for S and Cl, the deposit compositions do not differ much between Air and OXY70. Inside deposition rates show a positive correlation with concentration of submicron particles, which is consistent with previous findings on coal combustion. 1.
INTRODUCTION
Oxy-fuel combustion has been widely studied in recent years due to concerns of global warming. However, many past and current oxy-fuel combustion research projects focus on first generation oxy-coal combustion conditions suitable for retrofit. These require the peak flame temperature to be modulated by flue gas recycle and tend not to be economically efficient. Second generation oxy-combustion processes involve minimum recycled flue gas and higher levels of inlet oxygen concentrations at either atmospheric pressure, or at high (15-20 bar) pressure1-3 and are being proposed in order to improve the economics. Oxy-biomass combustion can be a process with negative carbon emissions when combined with carbon capture and sequestration. More importantly, this process is one of the very few that might allow energy production that consumes atmospheric CO2, using near conventional equipment4. Rice husks are an important biomass resource for renewable energy, since they are abundantly available in all rice producing countries. Rice husks have a fairly high ash content and low heating value, which could limit their application in utility boilers5. The work of Chao et al6 indicates that substantial energy released during the combustion of rice husks is from the reaction of volatile matter, while that energy for coal is mainly from char oxidation. The difference between rice husks and coal in combustion mechanisms might generate significant effects on gas temperature, combustion efficiency, formation of ash aerosols and ash deposits. The TGA test of Du et al7 suggested rice husks can improve the combustion performance of bituminous coal. Several drop tube furnace combustion tests have been completed on rice husks, Wang ACS Paragon Plus Environment
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Energy & Fuels
et al8 suggested the use of rice husk might induce slagging through the formation of low melting temperature ash; Branco et al9 discovered that burnout was improved for smaller fuel particle sizes, and particle fragmentation was not observed in that work; Zeng et al10 , however, observed the particle fragmentation during rice husks combustion and the PM1 formation in high sodium Zhundong coal was reduced when co-fired with rice husk. Many other studies are conducted on fluidized beds, entrained flow reactors and pulverized fuel combustors6, 11-15. For example, the fouling tendency of rice husk was reported to be fairly low due to the lack of alkali content13 and the ash particle size increased with larger rice husks particle size6. In contrast to these studies referenced above, the current work simultaneously addresses the formation of fly ash and of fouling ash deposits, with a view to understanding the relationships between the size segregated ash aerosol composition and the rates of deposition on heat transfer surfaces.
2.
MATERIALS AND METHODS
This work is performed on a 100 kW (rated) oxy-fuel combustor (OFC), a schematic of which is shown in Figure 1. The OFC is a down-fired laboratory combustor, and is designed to be large enough to be self-sustained (without external heating), yet still small enough to be systematically controlled. It operates at realistic stoichiometric ratios, with turbulent diffusion flames in the upper chamber causing realistic temperature/time profiles, although the flow becomes laminar downstream. It has been used in several systemic researches concerning oxy-coal combustion16-18. The OFC is slightly modified in this work. Previously installed ceramic plate heaters have been removed from the top section (Port 1 to Port 3) which has been insulated further with ULTRA-GREEN SR refractory (maximum operating temperature is 2143 K). This allows high flame temperatures to be expected under oxy-combustion conditions. The current internal diameter is 0.51 m rather than 0.61 m as in previous work16-18. In this
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study, pulverized rice husks with a mean particle size of 70 µm (a typical size of solid fuel in pulverized fuel furnace) are tested, and the compositions are described in Table 1 and Table 2. Composition analyses were performed according to approved standard methods for solid fuels, such as coal. Due to the low heating value and high ash content of rice husks, natural gas is co-fired during the tests in order to reach adiabatic flame temperatures and total ash concentrations comparable to coal combustion conditions. This work focuses on ash aerosol partitioning and ash deposit formation from combustion of rice husks at flame temperatures likely to be encountered in practice. The work does not address rice husk ignition issues directly. Therefore, natural gas was used as a supplemental fuel (90% thermal input) since it is a clean fuel without ash. Previous work has shown that flame temperature is a key variable in determining ash vaporization17. Furthermore, coagulation calculations for coal combustion19 have shown that there, little coagulation occurs in the flue gas far from the coal particle surface, suggesting that effects of diluting the flue gas on the shape of the measured ash PSDs would be minor, even though the concentrations are lower. The feeding rate of rice husks is set as 0.86 kg/hr, the flow rate of natural gas is 2.06 kg/hr, and a swirl burner is used in this work in order to increase the mixing between fuel and oxidant. Other detailed parameters are listed in Table 3. Note the actual thermal input during these tests is 33 kW. The ash aerosol is sampled by a water-cooled isokinetic probe at Port 9 with dilution ratio of ~80:1, and passes to a Scanning Mobility Particle Sizer (SMPS), an Aerodynamic Particle Sizer (APS) and a Berner Low Pressure Impactor (BLPI) to measure the particle size distributions and to collect size segregated particles. Condensation or adsorption of SO2 and/or HCl on particles is possible within the probe, although SO2 and HCl levels in the exhaust gas are very low (
