Quantitative Measurement of Atomic Potassium in Plumes over Burning Solid Fuels Using Infrared-Diode Laser Spectroscopy

Abstract

Solid fuels, such as coal and biomass, comprise a large portion of the current annual world energy supply, roughly equal to the annual oil consumption. During solid fuel combustion, certain species, though they are fairly benign outside of the combustion system, can cause damage in and around the combustion chamber. Alkali release from coal and biomass is known to cause severe problems in solid fuel fired boilers, such as fouling and corrosion of the heat transfer surfaces. In this work, the amount of atomic potassium in the plume of burning single coal, wood, and straw pellets (∼170 mg/piece) was measured using tunable diode laser absorption spectroscopy (TDLAS) of the potassium D1 line at 769.9 nm. The sample pellets were burned in an environment with stable temperature and gas composition provided by a laminar flame burner. More atomic potassium existed in the plume of burning biomass pellets compared with coal pellets, and the two temporal concentration profiles were dissimilar. This was attributed to the difference in the respective combustible components, ash compositions, potassium concentrations, and potassium compound state found in the two fuels. A high proportion of fixed carbon in coal induces potassium release mostly during the char-burnout period. In contrast, biomass has its strongest release during the devolatilization stage since its potassium presents mostly in a water-soluble form. Measurements of atomic potassium concentration during wood pellet gasification were performed in an oxygen deprived hot flue gas environment. The concentration of atomic potassium was found to be halved relative to combustion. The distribution of atomic potassium in the plume at different height above the pellets was also measured and is discussed in brief.

TDLAS-based photofragmentation spectroscopy for detection of K and KOH in flames under optically thick conditions

“…The distribution of K(g) and KOH(g) in the plume resembled a Gaussian shape, from which the path length was estimated to 20 ± 1 mm. This is in accordance with [21], where pellets of similar size were used as point sources of K(g).…”

Ultraviolet Absorption Cross Sections of KOH and KCl for Nonintrusive Species-Specific Quantitative Detection in Hot Flue Gases

“…The concentration of K atoms was measured using the TDLAS system, based on the Beer–Lambert law: 7,16,33,34 where ν is the frequency of the light, I 0 (ν) and I (ν) are the intensity of the light before and after passing through the absorbers, respectively, and I n and I e are the frequency-independent backgrounds from the detector dark current and broadband emission, respectively. It was found that the broadband emission I e from the investigated methane flames was negligible, especially as our detector was placed ∼1.5 m away from the flame and an aperture was used in the beam path.…”

“…I n was assumed to be constant and obtained under the condition without laser beam in the flame. The frequency-dependent absorbance, α(ν), is proportional to the optical path length L and the number density N of K atoms: 7,35 where σ(ν) is the absorption cross section at frequency ν, which can be expressed aswhere h is the Planck’s constant, ν 0 the central frequency, B 12 the Einstein absorption coefficient, c the speed of light, χ(ν, N , T ) the area-normalized line shape function, and T the temperature. The line shape function can be described by a Voigt profile in the atmospheric environment.…”

Experimental study of potassium release during biomass-pellet combustion and its interaction with inhibitive additives

“…Weng et al [15] measured the release of atomic potassium from burning coal, wood, and straw pellets using tunable diode laser absorption spectroscopy (TDLAS). Qu et al [16] used the same technique to simultaneously measure the flame temperature, water vapor and atomic potassium distribution during entrained-flow biomass combustion and they observed that potassium species rapidly undergo primary ash transformation reactions even if the fuel particles reside in an oxygen-deficient environment.…”