دانلود رایگان ترجمه مقاله انتشارات گازی از احتراق همزمان لجن فاضلاب و زغال سنگ/چوب در بستر سیال (نشریه الزویر سال 2004)

انتشارات گازی از احتراق همزمان لجن فاضلاب و زغال سنگ / چوب در یک بستر سیال

Gaseous emissions from co-combustion of sewage sludge and coal/wood in a fluidized bed

Abstract

Co-combustion of sewage sludge together with coal or wood has been investigated in two circulating fluidized bed (CFB) plants, a laboratory scale plant and a pilot scale 12MWth CFB boiler, in both of which the gas residence times are comparable to those in commercial plant. The investigation focuses on emissions of harmful gases from co-combustion compared to mono-combustion in CFB and the influence of air supply. The result shows that co-combustion can be carried out in CFB plant designed for the base fuel without exceeding EU or German emission limits for sludge energy fractions of less than 25%, except for the chlorine emission that may have to be reduced by flue gas treatment. Although sewage sludge contains large quantities of nitrogen and sulfur, the beneficial properties of CFB lead to considerable reduction of nitrogen oxides, and only a few percent of the nitrogen was effectively converted to NO or N2O. Sulfur can be captured by conventional limestone addition, but for wood as a base fuel this method is not as efficient as for coal.

1 Introduction

Sludge combustion has been practiced for decades in many types of combustors [1]. However, co-combustion of sludge with another fuel (the base fuel for which the plant was designed) has been suggested only recently. Moreover, the environmental regulations have become increasingly severe, and the addition of sludge to a combustion plant designed for a base fuel may require expensive modifications to meet the legislative conditions. The emissions of both heavy metals (including mercury) and harmful gases are of concern. The present work aims at investigating the feasibility of co-combustion of sewage sludge with base fuels, such as coal or biofuels (wood), with respect to the gaseous emissions, using fluidized bed combustion, a suitable combustion device for handling various types of fuel, with a good capability for NOx reduction.

6. Summary of results and conclusions

Dried sewage sludge can be handled together with the base fuels without any technical problems, but if the moisture content exceeds 10% the sludge is difficult to store for longer periods because of odors and biological activity that increases the temperature and reduces the calorific value. Co-combustion with dried sludge worked well, using the conventional fuel feed system, and only small differences from the over-all performance of the base fuels were observed, despite fractions of added fuel of up to 50% energy. A necessary condition is that the ash handling system of the plant can receive the increased ash flow resulting from the sludge. Moreover, fly ash from sludge combustion is stickier than ash from coal or wood.

With coal and wood as base fuels German and EU emission limits for CO, NOx, and SO2 were not exceeded for energy fractions of additional fuel of less than 25%. To fulfill the extremely low EU regulation for SO2 emission from co-combustion with wood, high limestone addition is needed.

The emission behavior of the base fuel plays a dominant role, particularly for energy fractions of additional fuel of less than 25%.

Sewage sludge contains large quantities of nitrogen, and high emissions of NO may occur, especially during mono-combustion. However, the reduction in a CFB combustor is also high, particularly in the presence of char, and emissions are only moderate for waste energy fractions of less than 25%; the conversion of fuel nitrogen to NO was only a few percent. Also the conversion to N2O was small.