Kent Davidsson

Optical and mass spectrometric study of the pyrolysis gas of wood particles

A detailed experimental investigation has been made of the pyrolysis—the first step in biomass combustion—of single birchwood particles. In addition to mass spectrometric and gravimetric analysis, the pyrolysis volatiles were characterized by different optical techniques. Absorption measurements showed a nearly featureless absorption in the ultraviolet spectral region with a continuously stronger absorption for shorter wavelengths. Using different excitation wavelengths, laser-induced fluorescence measurements revealed generally broad spectra in the spectral region from 300–500 nm, which are characteristic spectral signatures for larger hydrocarbons. The optical data were monitored at different times in the pyrolysis process of the particles and compared with the results from the mass spectrometric and gravimetric analysis. The sensitivity of the optical techniques for differentiation between specific molecules was rather low, although formaldehyde could be observed both in absorption and fluorescence spectra. Laser-induced fluorescence measurements were also made for two-dimensional visualization of the pyrolysis volatiles emitted from heated birchwood particles, indicating much higher flows along the fiber direction than across.

Sulfur recirculation for increased electricity production in Waste-to-Energy plants

Sulfur recirculation is a new technology for reducing boiler corrosion and dioxin formation. It was demonstrated in full-scale tests at a Waste to Energy plant in Göteborg (Sweden) during nearly two months of operation. Sulfur was recirculated as sulfuric acid from the flue gas cleaning back to the boiler, thus creating a sulfur loop. The new technology was evaluated by extensive measurement campaigns during operation under normal conditions (reference case) and operation with sulfur recirculation. The chlorine content of both fly ash and boiler ash decreased and the sulfur content increased during the sulfur recirculation tests. The deposit growth and the particle concentration decreased with sulfur recirculation and the dioxin concentration (I-TEQ) of the flue gas was reduced by approximately 25%. Sulfuric acid dew point measurements showed that the sulfuric acid dosage did not lead to elevated SO3 concentrations, which may otherwise induce low temperature corrosion. In the sulfur recirculation corrosion probe exposures, the corrosion rate decreased for all tested materials (16Mo3, Sanicro 28 and Inconel 625) and material temperatures (450 °C and 525 °C) compared to the reference exposure. The corrosion rates were reduced by 60-90%. Sulfur recirculation prevented the formation of transition metal chlorides at the metal/oxide interface, formation of chromate and reduced the presence of zinc in the corrosion products. Furthermore, measured corrosion rates at 525 °C with sulfur recirculation in operation were similar or lower compared to those measured at 450 °C material temperature in reference conditions, which corresponds to normal operation at normal steam temperatures. This implies that sulfur recirculation allows for higher steam data and electricity production without increasing corrosion.

Fate of Phosphorus During Co-Combustion of Rapeseed Cake with Wood

Recent studies show that deposit formation and agglomeration in fluidized bed boilers may be aggravated by a high phosphorus content besides alkali metals, chlorine and sulphur in a fuel. This paper presents the fate of phosphorus during co-combustion of wood chips and wood pellets with rapeseed cake pellets, a high phosphorus fuel in a 12MW CFB boiler. 12 hour tests with 12% and 18% (energy basis) of rapeseed cake with wood were performed with and without limestone addition. All fuels were characterised by means of standard fuel analyses combined with chemical fractionation. Retrieved ash samples were analysed using wet chemical analysis complemented with SEM/EDXA. Gaseous alkali metal chlorides as well as HCl and SO2 were measured upstream of the convective pass at a flue gas temperature of 800oC where also the deposit samples were collected with a deposit probe. The composition of deposits was studied with SEM/EDXA. Analyses of bed material particle cross-sections showed phosphorus compounds present within a K-silicates matrix between the agglomerated sand particles, indicating direct attack of gaseous potassium compounds on the bed surface followed by adhesion of rich in phosphorus ash particles. Build-up of the deposits took place mainly on the windward side of the probe; where up to 9 wt-% of phosphorus was present. SEM/EDXA shows that rapeseed cake addition caused an increase of K, Na besides P indicating presence of low melting phosphate salts in the deposits. During limestone addition in the deposit samples the increase of Cl could be noticed however no significant change in P content was observed. This paper shows that agglomeration and fouling when co-firing rapeseed cake may be linked to its high content of organically bonded phosphorus - phytic acid salts; together with high content of water soluble fraction of alkali metals chlorides and sulphates in the fuel mixture.