Ilkka Hippinen

The emissions and control of NOx and N2O in pressurized fluidized bed combustion

Using the Otaniemi pressurized fluidized bed combustion test rig a study has been made of the emissions and control of NOx (NO + NO2) and N2O in different operating conditions for three types of solid fuel (peat, bituminous coal and brown coal). The influence of operating parameters, including bed temperature, excess air level, pressure, etc., were investigated. The results showed that NOx emissions were low and the excess air level was a dominant variable, and N2O emission was mostly dependent on the burning temperature. Ammonia injection and air staging are effective as NOx reduction techniques. Both techniques can reduce the NOx up to 60% dependent on the fuel type and the amount of ammonia or degree of air staging. The oxygen level and temperature of ammonia injecting position were significant factors in NOx reduction in ammonia injection operations. The N2O emissions, however, were always increased as a consequence of NOx reductions.

Drying of Activated Sludge Under Partial Vacuum Conditions—An Experimental Study

Abstract A typical wastewater treatment system in a pulp and paper mill in Finland treats wastewater both mechanically and biologically. Sludges resulting from these processes have to be disposed of. One possible way of doing this is to incinerate them with solid fuel in the power plant of the mill. To minimize the amount of sludge and to make the use of the sludge energy efficient, it has to be dried before incineration. Mechanical drying of the sludge from biological wastewater treatment is difficult to carry out. Using secondary energies may provide a competitive way of arranging drying: a method of doing this by using partial vacuum evaporation to utilize the low temperature secondary heat in sludge drying is under development. A laboratory study to examine the behavior of activated sludge under partial vacuum evaporation conditions was carried out using a laboratory rotating evaporator to analyze the drying of activated sludge from three mills at 40–80°C boiling temperatures. This article presents the results from the tests. These are promising; it was possible to reach high dry solids content. Also, no boiling point temperature rise was detected, fouling of the evaporator seemed low and easily avoidable, condensate from the evaporator weas relatively clean and returnable to the processes of the mill.

Control of NOx and N2O in pressurized fluidized-bed combustion

Experiments with a test rig on the formation of NOx and N2O and their reduction in pressurized fluidized-bed combustion of bituminous coal are reported. The emission of NOx is controlled by two NO reduction methods—ammonia injection and staging of combustion air—and the emission of N2O by increasing the combustion temperature. Both ammonia injection and staging of combustion air are effective in NO reduction. The emission of NOx decreases with increasing ammonia flow, and ∼75–85% NOx reduction can be achieved at an NH3NOx molar ratio of 4–5. However, ammonia injection increases N2O emission and ammonia slip occurs. In the staging of combustion air a maximum NO reduction of 75% can be achieved with primary air ratio ≈ 1. A further decrease in primary air ratio to sub-stoichiometric conditions leads to an increase in NOx concentration, and has other negative effects on sulfur capture and combustion performance. No great change in N2O emission is observed when air staging is changed. Increasing combustion temperature reduces N2O emission significantly. Extremely low N2O concentrations are obtained at high bed temperature and pressure. Freeboard temperature profile also plays an important role in N2O emission.

Experiences of a Fibrous Ceramic Candle Filter for Hot Gas Cleaning in Pressurized Fluidized Bed Combustion

A fibrous, layered Al2O3/SiO2-based ceramic candle filter has been tested for pressurized fluidized bed combustion (PFBC) hot gas cleaning at the Otaniemi PFBC test rig. The fuels used have been bituminous coal and peat. The exhaust gas from a pressurized fluidized bed reactor was passed through two cyclones before entering the ceramic filter unit, which was operated at a temperature range from 853 to 1013 K (580 to 740 °C) and at a pressure range from 0.6 to 1.0 MPa. Total exposure time of the two candle filter elements was about 340 hours.