Ash Aerosol and Deposition Formation Mechanisms during Air/Oxy-combustion of Rice Husks in a 100 kW Combustor

Abstract

Pulverized rice husks are co-fired with natural gas in a 100 kW (rated) down-fired oxy-fuel combustor under two conditions: (1) air combustion (denoted as air) and (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 (scanning mobility particle sizer/aerodynamic particle sizer) and a Berner low-pressure impactor, 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 scanning electron microscopy with energy-dispersive X-ray spectroscopy and X-ray diffraction for compositions. The data show that, in comparison to air, OXY70 produces greater amounts of sub-micrometer aerosols as a result of increased mineral vaporization at the higher flame temperature. Cl and P are combined with K to form KCl and KPO3 in the sub-micrometer but not in the super-micrometer aerosols. For both conditions, the particle sizes within the more loosely bound outside deposits are much larger than 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 the concentration of sub-micrometer particles, which is consistent with previous findings on coal combustion.

Comparison Study of Ash Partitioning and Deposition Behavior between Two Rice Husk Fuels under a 100 kW Combustor

“…The PSDs of these two rice husks are shown in Figure 2. The results from RH_US have been previously published, 15 and they are included in this paper for comparison purposes. The composition and ash analyses of both rice husks are shown in Tables 1 and 2, respectively.…”

“…26−28 However, the experimental results from the authors of this paper suggested that there exists some extent of fouling tendency for rice husks co-firing with natural gas in a downfired combustor. 15 These discrepancies in ash partitioning and fouling behavior for rice husks may be caused by the variations in furnace type, the particle temperature history, and the ash properties of rice husks from different locations. Therefore, it is important to further conduct a systematical study on rice husk combustion to investigate the formation mechanism of ash aerosol and ash deposits.…”

“…This work attempts to achieve this goal by burning two different rice husks under the same combustion conditions through a self-sustained laboratory combustor. Experimental results from one rice husk has been published, 15 whereas that work studied the particulate matter concentrations and ash deposition rates. However, the influence of fuel properties (diameter, ash composition, etc.)…”

“…In this work, rice husks were chosen as the biomass of interest for two reasons: first, they are prevalent in all rice-producing countries, and second, the ash content is mostly dominated by silicon, , leading to significant amounts of sub-micrometer ash particles rich in silicon , rather than in sodium, which is the case for coal under most conditions . Some studies have been conducted to determine the combustion performance and formation of aerosols and ash deposits under rice husk combustion.…”

Particulate Matter Formation and Alkali and Alkaline Earth Metal Partitioning in a Pressurized Oxy-fuel Fluidized-Bed Combustor

“…In recent years, there exists increasing interest in pressurized oxy-fuel combustion technology because it can reduce the overall plant efficiency and reduce the CO 2 capture cost at the same time. The advantage of this technology has been validated by many researchers through both simulation methods , and experimental studies. − A variety of research topics have been studied for atmospheric pressure oxy-combustion, such as (1) operating conditions, ignition, and flame stability, − (2) heat transfer, emissivity, and thermal capacity of exhaust gases, − (3) gaseous emissions and effects on pollution formation [NO x , SO 2 , and particulate matter (PM)], − and (4) ash deposition and slagging and fouling on the wall and on the heat transfer surface. − However, to the best knowledge of the authors, the PM formation from a pressurized oxy-fuel combustion process has not yet been reported. Studies concerning PM formation are necessary because it causes ash deposition on the heat transfer surface and may cause serious problems that threaten safety operation of the power plant. , Therefore, the target of this paper is to shed light on PM formation in pressurized oxy-fuel combustion.…”