Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a circulating fluidized bed combustor

doi:10.1016/j.fuel.2007.01.033

Fuel

Volume 86, Issue 15, October 2007, Pages 2308-2315

https://doi.org/10.1016/j.fuel.2007.01.033 Get rights and content

Abstract

Emissions of NO_x and N₂O were measured during mono-combustion of dried sewage sludge and co-combustion with coal in a bench-scale circulating fluidized bed combustor (CFBC). The results were compared with previous results obtained using a bubbling fluidized bed combustor (BFBC). The increase in NO_x with sludge ash accumulation in the combustor was less for the CFBC than the BFBC, partly because of the higher attrition rate of sludge ash in CFBC resulting from the higher gas velocity. The influence of sludge ash on the formation of NO_x in CFBC was less than that in BFBC during sludge combustion. The effects of fuel type on NO_x and N₂O emissions were also evaluated.

Introduction

Fluidized bed combustors (FBCs), both circulating (CFBCs) and bubbling (BFBCs), have been developed as combustion technologies that are useful for widely different fuels such as coal, wastes, and biomass. Municipal sewage sludge is a product of wastewater treatment. Increasing amounts of sewage sludge pose a problem for waste management in many developed countries [1]. Dried sewage sludge has moderate heating value. Therefore, the dried sludge can be burned in FBCs as a partial substitute for fossil fuels.

A salient problem of sewage sludge combustion is its high nitrogen content, which might engender high emissions of NO_x (NO + NO₂) and N₂O, the latter of which is known to be a greenhouse gas. High NO_x emissions from sludge combustion are also anticipated not only because of their high nitrogen content but also their high contents of metal oxides such as iron oxides and magnesium oxide in the ash; solids containing such metal oxides are known to boost NO_x emissions during coal combustion through oxidation of HCN and NH₃ to NO_x [2], [3], [4], [5], [6]. Indeed, the authors conducted dried sludge combustion in a BFBC and found that the sludge ash accumulation in the reactor significantly increased NO_x emissions [7]. Although emissions of NO_x and N₂O from co-combustion of sludge and coal or plastics have been reported in the literature for CFBCs [5], [8], [9], [10], the effect of ash accumulation on the emissions of NO_x and N₂O has not yet been fully elucidated: it remains uncertain whether ash accumulation influences NO_x emissions from CFBCs. The ash attrition rate is known to increase with increasing gas velocity [11]. Therefore, the ash attrition rate in CFBCs is expected to be much higher than that in BFBCs.

In the present work, mono-combustion of dried sewage sludge and co-combustion with coal were carried out and emissions of NO_x and N₂O were measured. A bench-scale circulating fluidized bed combustor was used. The effects of ash accumulation and fuel type on emissions of NO_x and N₂O were observed. The present results are compared with those in the previous work using the same fuels, but employing a different reactor type (BFBC).

Section snippets

Circulating fluidized bed combustor

A bench-scale (1.9 m high, 2.2 cm i.d.) circulating fluidized bed combustor (CFBC) was used. The top of the riser was connected to a cyclone, in which particles were separated from the gas stream. The captured particles were recycled to the bottom of the riser through a bubbling fluidized bed (loopseal). The riser and the loopseal were heated by electric heaters; the temperature in the riser was maintained at 1123 K. Quartz sand of 80–150 mesh (average size 0.15 mm), which is known to be inert for

Results and discussion

Fig. 1 shows results of mono-combustion of medium-volatile bituminous coal and dried sludge. First, coal combustion was conduced to attain the steady state. Then the fuel was switched to the dried sludge at t = t_f. Finally, the fuel was switched to the coal again at t = t_s. During the initial coal combustion, NO_x and N₂O emissions were almost constant. The ash from the present coal is considered to be inert or to be so fragile that the accumulation of coal ash did not affect the emissions of NO_x

Conclusions

Mono-combustion of dried sludge and co-combustion of dried sludge with coal were carried out in a bench-scale circulating fluidized bed combustor. The accumulation of sludge ash did not increase the NO_x emissions from mono-combustion of sludge and co-combustion of sludge with coal, but ash accumulation increased NO_x emissions from mono-combustion of coal. Conversion of fuel-N to NO_x during co-combustion showed no clear dependence on fixed carbon contents of fuel, but it had clear dependency on

Acknowledgement

This work was conducted in cooperation with Kobelco Eco-Solutions Co., Ltd.

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Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a circulating fluidized bed combustor

Abstract

Introduction

Section snippets

Circulating fluidized bed combustor

Results and discussion

Conclusions

Acknowledgement

Progr Energy Combust Sci

Fuel

Fuel

Fuel

Proc Combust Inst

Effects of NH3 on NOx reduction in homogeneous and heterogeneous reaction systems

Nenryo-Kyokai-Shi (J Fuel Soc Jpn)

Emissions of NO_x and N₂O during co-combustion of dried sewage sludge with coal in a circulating fluidized bed combustor

Effects of NH₃ on NO_x reduction in homogeneous and heterogeneous reaction systems