Fate of fuel-bound nitrogen and sulfur in biomass-fired industrial boilers

G5 Doctoral dissertation (article)


Internal Authors/Editors


Publication Details

List of Authors: Emil Vainio
Publisher: Åbo Akademi University
Place: Åbo
Publication year: 2014
ISBN: 978-952-12-3009-7
eISBN: 978-952-12-3010-3


Abstract

The use of biomass as an energy source for the production of heat and power is one way to decrease dependency on fossil fuels and increase energy self-sufficiency. The utilization of fossil fuels in energy production is also the major source of CO2 emissions, and CO2 is the main anthropogenic greenhouse gas. Biomass, on the other hand, is regarded as a CO2-neutral energy source. However, the nitrogen and sulfur in the biomass forms pollutants such as NOX and SO2. These must meet the stringent emission limits set by emission directives. Furthermore, the sulfur in the fuel can both cause and prevent corrosion of an incinerator, depending on the fuel and combustion.

The main objective of this work was to gain knowledge about the fate of fuel-bound nitrogen and sulfur in industrial-scale biomass combustors during combustion of various biofuels. This was achieved by full-scale measurement campaigns: in a bubbling fluidized bed (BFB) boiler combusting various fuel mixtures and in a Kraft recovery boiler. A four-meter-long quenching probe connected to a Fourier transform infrared (FTIR) gas analyzer was used to determine the gas composition in the furnace measurements. In-furnace measurements of NO, NH3, HCN and HNCO were carried out, to understand how the fuel-bound nitrogen is released and to learn how and where the reduction of the released nitrogen species occur. This kind of information is essential for the validation of models, e.g., those based on computational fluid dynamics (CFD), to enable the construction of cleaner and more efficient boilers.

The measurements in the BFB boiler showed that NH3 was the main reactive nitrogen species at reducing conditions in the furnace, and the highest concentrations of NH3 were measured above the fuel inlet. No significant reduction of the nitrogen species to N2 took place in the lower furnace, however, a drastic reduction was observed over the secondary air jet level. The final reduction of fuel bound nitrogen to N2 was over 90%, although only air-staging was applied to minimize the NOX emissions. Furthermore, the reduction of reactive nitrogen species in the air jets in the bubbling fluidized bed boiler was studied with kinetic modeling using a detailed reaction mechanism. The modeling work showed the importance of mixing the combustion gases with the air jets, when modeling the NOX formation and final emissions of a boiler.
Another objective was to gain knowledge about the formation of SO2 and sulfation of ash components during combustion of biofuels with different ash properties. This was done by in-furnace measurements of the gaseous sulfur species and by detailed sampling and analysis of the fuel mixtures, ashes, and fine particles. All the fuels combusted in the BFB boiler had a high sulfur capturing potential, i.e., a high Ca/S ratio, and the emissions of SO2 were low. Furthermore, the sulfur in the fuel played an important role in sulfating alkali chlorides, which are known for enhancing deposit formation and may also accelerate superheater corrosion. Deposit formation on heat transfer surfaces reduces the boiler efficiency and may result in unplanned shutdowns of the boiler. Sulfation of alkali chlorides was observed in the co-firing case with bark, sludge, and solid recovered fuel (SRF). The SRF had a fairly high chlorine content, while sludge had the highest sulfur content. Most of the chlorine was found as gaseous HCl in the measurements, which implies that sulfation of alkali chlorides occurred. This was seen in the furnace measurements as a rise in the HCl concentration and a decrease in SO2 when moving up in the freeboard. The work showed the benefits of co-combustion of fuels with different properties. Despite the high sulfur content and low heating value of sludge, the ash components in the sludge play an important role in combustion. The sulfur has the positive effect of sulfating alkali chlorides when chlorine is present in biomass combustion.

The measurement campaign at the Kraft recovery boiler resulted in valuable data regarding the nitrogen and sulfur species in the furnace. The main nitrogen species at reducing conditions was NH3. Considerable amounts of HCN were measured at the black liquor spraying level. This HCN is believed to be formed via re-burning of NO, since HCN has not been found as a pyrolysis species in earlier laboratory studies. The main sulfur intermediates close to the fuel inlet were H2S and methyl mercaptan. In the flue gases, the only nitrogen species measured was NO and virtually all the sulfur was captured in the ash.

The presence of gaseous sulfuric acid (H2SO4) in the cold end of a Kraft and sulfite recovery boiler was also studied, due to low temperature corrosion seen in these boilers. A small part of the SO2 formed in combustion forms SO3, which reacts with water vapor to form H2SO4 as the flue gas temperature drops. Sulfuric acid in the flue gas may lead to severe corrosion of components, such as economizers, air preheaters, and the flue gas duct, if their material temperature is below the sulfuric acid dew point temperature. If the H2SO4 concentration in the flue gas is known, the dew point temperature can be calculated. The objective was first to evaluate various SO3/H2SO4 measurement techniques in the Chalmers 100 kWth oxy-fuel test unit during air-fired and oxy-fuel conditions. The SO3 in these experiments was generated by combusting propane and injecting SO2 in the feed gas. A salt method showed promising results in these measurements. In this method a salt is used to capture H2SO4 in the form of sulfate, and the amount of sulfate formed in the salt is determined after the measurement. The salt method was further studied and developed in laboratory conditions. Various salts´ - NaCl, KCl, K2CO3, and CaCl2 – ability to capture H2SO4, without the interference from SO2, was studied. In this case, a synthetic flue gas was used and H2SO4 was generated by evaporating a weak solution of sulfuric acid. Both NaCl and KCl proved to be suitable for the measurement of low quantities of H2SO4 in a flue gas environment. Furthermore, an in-situ implementation of the salt method was used to study the presence of H2SO4 in the cold end of a Kraft and sulfite recovery boiler. The measurements revealed that during normal operation of these boilers, there existed no risk of low temperature corrosion due to condensation of H2SO4


Keywords

Bubbling fluidized bed combustion, co-combustion, fuel-nitrogen conversion, Full-scale measurements, kraft recovery boilers, NH3, nitrogen oxide, SO2

Last updated on 2019-23-07 at 06:49