Laboratory Measurements of Alkali Metal Containing Vapors Released during Biomass Combustion

Abstract

Alkali metals, in particular potassium, have been implicated as key ingredients for enhancing fouling and stagging of heat transfer surfaces in power generating facilities that convert biomass to electricity. When biomass is used as a fuel in boilers, the deposits formed reduce efficiency, and in the worst case lead to unscheduled plant downtime. Blending biomass with other fuels is often used as a strategy to control fouling and stagging problems. Depending on the combustor, sorbents can be added to the fuel mixture to sequester alkali metals. Another possibility is to develop methods of hot gas cleanup that reduce the amount of alkali vapor to acceptable levels. These solutions to fouling and stagging, however, would greatly benefit from a detailed understanding of the mechanisms of alkali release during biomass combustion. Identifying these alkali vapor species and understanding how these vapors enhance deposit formation would also be beneficial.Our approach is to directly sample the hot gases liberated from the combustion of small biomass samples in a variable-temperature quartz-tube reactor employing a molecular beam mass spectrometer (MBMS) system. We have successfully used this experimental technique to identify alkali species released during the combustion of selected biomass feedstocks used in larger scale combustion facilities. Multiple combustion conditions have been investigated to target those conditions that minimize alkali metal release. The results of these laboratory studies indicate that initial feedstock composition has the most pronounced effect on alkali metal released during combustion. Four mechanisms of alkali metal release have been identified depending on the feedstock composition. Primary alkali metal release in the combustion of relatively low alkali metal containing woody feedstocks is through the vaporization or decomposition of alkali sulfates. Alkali metal chlorides are the primary alkali metal species released during combustion of herbaceous feedstocks, grasses, and straws with high alkali metal and chlorine contents. For feedstocks with high alkali metal and low chlorine content, alkali metal hydroxides are the most abundant alkali vapor released. If a high alkali content is coupled with high levels of fuel-bound nitrogen, the dominant form of alkali metal vapor is the alkali cyanate. In general, the chlorine content of biomass has been identified as an important parameter that facilitates alkali release.

A Plug Flow Model for Chemical Reactions and Aerosol Nucleation and Growth in an Alkali-Containing Flue Gas

“…The assumption is based on our intuitive recognition of the chemical reactivity of the listed species. It is supported, though, by experimental studies which show that KCl and KOH are the dominant high temperature compounds of potassium (Dayton and Milne 1996) in agreement with equilibrium calculations (cf. Figure 3).…”

“…We also assume that NO x does not interfere with the gas reactions or the particle formation. However, the possible catalytic effect of NO x on the rate of SO 2 -oxidation cannot be ruled out, nor can the possible formation of vapor phase KOCN, observed by Dayton and Milne (1996) at high NO x -concentrations. The effect of these reactions-most likely of secondary signi cance-cannot at present be quanti ed, and the reactions are neglected in our study.…”

Bamboo: an overlooked biomass resource?

“…In fact, the alkali index of these samples (defined as kg alkali oxide per GJ energy content) ranges from 0.1-0.3, generally below the empirical limit of 0.17-0.34 kg/GJ known to cause adverse fouling and slagging in combustion systems (Miles et al, 1996;Baxter et al, 1998). Chlorine in biomass has been shown to increase the volatility of alkali metals during combustion (Dayton and Milne, 1996), and the low chlorine content of the bamboo samples suggests that the potassium that is present is unlikely to be volatile, and therefore not problematic in terms of ash deposition. The low chlorine content of the bamboo samples means that burning them is unlikely to enhance high temperature corrosion in biomass combustion systems.…”

Investigations on biomass slagging in utility boiler: Criterion numbers and slagging growth mechanisms

“…In addition, some researchers argued that K 2 SO 4 is the dominant substance that nucleates first when gas temperature is lowered [6]; KCl then condenses on the nucleon of K 2 SO 4 at lower temperature [7]. The contradictions in the presented studies are primarily caused by the different biomass species, planting environments, combustion atmospheres, and temperatures [8][9][10][11][12]. Therefore, the extensive research on slagging remedies including co-firing [13,14], additives [9,15], and chemical pretreatment [16,17] are all targeted in certain condition.…”