New approach to common removal of dioxins and NOx as a contribution to environmental protection

Abstract

Meeting environmental limits represents the most important issue in the field of waste processing. Our primary effort consists either in eliminating hazardous emissions or in prevention of their production. However, this is not feasible in most cases therefore the so called secondary methods have to be applied. Technologies based on adsorption of hazardous compounds using activated carbon, deNOx/deDiox technologies as well as technology of catalytic filtration using a special material REMEDIA^® proved itself to be very efficient. The latter technology consists in using a baghouse with bags manufactured from a special material (two layers – membrane from ePTFE and felt with bound in catalyst) called REMEDIA^® which has successfully been used for removal of PCDD/F during recent period. However, it has been found that this technology can partially remove NOx as well. Based on our experience from operation industrial incineration plants it has been proved that even after more than three years' operation the activity of filtration material was not decreased and efficiency of dioxins removal from flue gas ranges from 97 to 99% (Pařízek et al., 2008).

Based on industrial experience and new findings it has been decided to focus continuing research and development on deNOx experiments by applying SCR using the above mentioned efficient filtration material. This type of material is primarily designed for reduction of PMs and PCDD/F. Experiments with this filtration material should test possibilities for simultaneous reduction of PMs, PCDD/F and NOx. Tests are performed in MSW incineration plant utilizing new experimental unit and under standard conditions. Waste processing capacity of the incineration plant amounts to 15 t/h. Tests result in evaluation of overall reduction efficiency and negative factors that might influence the efficiency. Catalytic filtration is further compared to other types of deNOx methods.

Thus we have obtained qualitatively new knowledge about this method the value of which is emphasized by full scale industrial testing.

Introduction

New European Union regulations (Council Directive 2000/76/EC, 2000) as well as other ones worldwide have resulted in more stringent environmental limits for all pollutants originating during the thermal processing of various types of waste. Ways of how to achieve energy reduction and consequently emissions reduction are discussed by Dovì et al. (2009). Regulations require that hazardous components have to be efficiently removed before they enter the ambient (Council Directive 2000/76/EC, 2000). Sufficiently low levels of most of the unwanted components can be reached by common methods of absorption and adsorption gas cleaning. The most toxic components contained in flue gas created by combustion of MSW and industrial waste are PCDD/F.

However, NOx also represent a serious issue. If a general fuel is considered, combustion of N₂ instigates its reaction with oxidant, i.e. with auxiliary combustion air which contains approximately 78% of N₂. Combustion forms NOx mostly represented by nitric oxide (NO) and partially by nitrogen dioxide (NO₂), there may also be nitrous oxide (N₂O) present. NO is colorless toxic gas which quickly reacts with oxygen (O₂) in the atmosphere to form NO₂. Legislation then usually expresses NOx concentration as NO₂ concentration. NO and NO₂ in the atmosphere create undesired ozone in lower atmosphere (which is desired in the upper atmosphere) as well as smog and acid rain, where NO₂ reacts with water (H₂O) and forms acids such as nitrous acid (HNO₂) and nitric acid (HNO₃).

Amount of formed NOx is influenced by several factors such as temperature of combustion, flow velocity in combustion space, combustion space design, burner design, fuel composition, ratio of fuel and air dosage, heat removal rate from combustion space, etc. (Baukal, 2004).

Methods of NOx control may be categorized into primary methods which prevent formation of NOx; secondary NOx control methods which effectively reduce already formed NOx. Among the primary methods there are (Baukal, 2004):

• usage of fuel with low nitrogen content,

• usage of fuel additives,

• pretreatment of fuel,

• usage of different oxidant (with lower nitrogen content),

• reduction preheating of added air,

• reduction air excess,

• distribution fuel dosing into several steps (staging),

• flue gas recirculation,

• water/steam dosage,

• auxiliary dosage of different fuel for reduction of NOx formation,

• low NOx burners,

• and others.

Primary methods may be successfully used in the so called clean applications where only fuels are combusted, e.g. for preheating of other types of media, etc. However, their efficiency is limited when it comes to heterogeneous waste. This type of waste requires secondary methods for deNOx. Among the most common secondary deNOx methods there are (Heck et al., 2002, European IPPC Bureau, 2008a):

• selective non-catalytic reduction of NOx (SNCR),

• selective catalytic reduction of NOx (SCR) – depending on the catalyst used, there are SCR technologies utilizing fixed-bed catalytic reactor and technologies with catalyst applied on fabric or ceramic filter (the so called catalytic filtration), technologies with fluidized bed are not very common,

• flue gas cleaning using NOx absorption (Dalaouti and Seferlis, 2005).

All these methods may be combined, which increases efficiency of NOx reduction as well as NOx formation. Selection of individual method or combination of methods is always conditioned by economic balances and legislation.

Based on literature sources and previous experience any contribution bringing an added value, and/or a novel approach to deNOx/deDiox system is valuable.

We will now focus on following technologies.

Efficient technology for removal of dioxins is their catalytic decomposition, occurring together with SCR by means of ammonia (NH₃) (Fino et al., 2003) according to the following stoichiometric equations:

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

C₁₂H_nCl_8−nO₂ + (9 + 0.5n)O₂ → (n − 4)H₂O + 12CO₂ + (8 − n)HCl

The reactions leading to the concurrent destruction of both NOx and dioxins (deNOx/deDiox) proceed in a catalytic reactor at temperature interval from approximately 200 to 300 °C (Goemans et al., 2003). The efficiency of destruction of NOx is high in the catalytic reactor but the reactor has also a certain disadvantage in its sensitivity to catalytic poisons and in a necessity to include it into the technology line only at a point where the flue gas is free of PMs which practically means after mechanical and chemical cleaning.

This configuration requires reheating of flue gas up to the temperature necessary for reactions taking place in the deNOx/deDiox reactor (see Fig. 1).

Based on evaluation of applications it has been found that the method of dioxin removal by catalytic filtration REMEDIA^® (Pranghofer and Fritsky, 2001, Bonte et al., 2002) is still highly effective even after long time of operation, which considerably reduces the total annual cost. The method of catalytic filtration is based on a special GORE-TEX^® material which is used for the filtration bags of the fabric filter, where solid matters of fly ash are successfully separated and at the same time dioxins present in the filtered gas are broken down. The outer filter layer which is made out of a membrane from ePTFE, can separate up to 96.6% of fly ash particles also containing compounds of heavy metals in the filtered gas. The cleaned gas enters the inner layer of the filtration layer which has in its structure built in components acting as catalysts breaking down dioxins with 98.8% efficiency (at a level from 0.01 to 0.03 ng TEQ/N m³). The filtration material is cleaned by a pulse-jet cleaning method.

The implemented alternative of flue gas cleaning technology at the incineration plant in question is obvious from Fig. 2.

When testing efficiency of PCDD/F reduction in the above mentioned MSW incineration plant (maximum waste processing capacity of 15 t/h and flue gas production of 65,000 N m³/h) using SNCR method for NOx reduction, it was discovered that residual NOx reacts with residual ammonia in the above mentioned filtration material, which further lowers NOx concentration.

Catalytic filter in this test is positioned in between ESP and wet scrubber of flue gas. Residual ammonia does not interfere with structure of filtration material and even lowers amount of NOx in flue gas; more tests could be then performed to find out deNOx efficiency under various conditions.

Reduction of NOx by applying SCR method is usually done in temperatures ranging from 175 to 600 °C depending on type of catalyst (Heck et al., 2002). REMEDIA^® filtration material should work in temperature conditions ranging from 220 to 240 °C (Smejkal et al., 2009). Key feature of SCR technology is the injection of NH₂-X compounds (X being H, CN or CONH₂) into gas stream (flue gas stream) (European IPPC Bureau, 2008a). Consequently, NOx is reduced to nitrogen (N₂) and water (H₂O) at catalyst (principle of catalytic filtration is displayed at Fig. 3). Most common reaction agent is ammonia water (NH₄OH, water solution of ammonia) or pure ammonia. Among other types of reaction agents there are urea solutions, nitrolime and cyanamide (European IPPC Bureau, 2008a). Catalysts involve noble metals such as Pt/χ-Al₂O₃, compounds such as V₂O₅/TiO₂, V₂O₅–WO₃/TiO₂, V₂O₅–MoO₃/TiO₂, MnOx/CeO₂, Rh₂O₃/CeO₂ or zeolites (Fino et al., 2004). V₂O₅–WO₃/TiO₂ catalyst for REMEDIA^® filtration material is located on ePTFE layer (Weber et al., 2001). ePTFE shows high resistance to most types of loading such as chemical, thermal, UV attack as well as good abrasion resistance; it also does not absorb water. Filtration material is covered with membrane which captures PMs. This reduces the amount of PMs in flue gas and also protects catalytic layer from fouling. Catalytic filtration requires specific filter (baghouse) design which allows reaching 0.8–1.4 m³/m².min of filtration velocity (European IPPC Bureau, 2008a). Decomposition of NOx requires over-stoichiometric amount of NH₃, optimum ratio is NH₃:NO = up to 1.1 mol/mol (European IPPC Bureau, 2008a). Decomposition of NOx gives rise to desired reactions (see (3), (4), (5), (6), (7), (8), (9)) which reduce the amount of NOx; however it also gives rise to parallel undesired reactions (10), (11), (12), (13) which increase the amount of NOx or which may reduce amount of reaction agents necessary for the reaction.

Desired reactions of NOx with NH₃ (Schnelle and Brown, 2001):

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

6NO + 4NH₃ → 5N₂ + 6H₂O

2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O

6NO₂ + 8NH₃ → 7N₂ + 12H₂O

NO + NO₂ + 2NH₃ → 2N₂ + 3H₂O

Desired reactions of NOx with urea solution/(NH₂)₂CO/ (European IPPC Bureau, 2008b):

4NO + 2(NH₂)₂CO + 2H₂O + O₂ → 4N₂ + 6H₂O + 2CO₂

6NO₂ + 4(NH₂)₂CO + 4H₂O → 7N₂ + 12H₂O + 4CO₂

Undesired reactions (Busca et al., 1998):

4NH₃ + 3O₂ → 2N₂ + 6H₂O

2NH₃ + 2O₂ → N₂O + 3H₂O

4NH₃ + 5O₂ → 4NO + 6H₂O

4NH₃ + 4NO + 3O₂ → 4N₂O + 6H₂O

Following authors have recently dealt with the issue of NOx elimination from flue gas or waste gas stream by catalytic filtration method: Nacken et al. (2007) and Heidenreich et al. (2008) have discussed possibilities of using V₂O₅–WO₃/TiO₂ catalyst located in ceramic filtration element. Laboratory tests of the filtration element then showed up to 96% deNOx efficiency with optimum temperature at 300 °C. Kim et al. (2007) have also dealt with usage of modified V₂O₅–WO₃/TiO₂ catalyst.

Kang et al. (2009) have used MnOx catalyst in a filter which results in subsequent 92.6% deNOx efficiency and temperature of 150 °C. Park et al. (2009) have incorporated an adjusted CuMnOx catalyst in a ceramic filter. Laboratory tests then showed up to 94% deNOx efficiency with temperature of 200 °C.

Zürcher et al. (2008) have researched possibilities of kinetics of deNOx in catalytic filters and Döring et al. (2008) have discussed impact of filtration velocity on deNOx efficiency at catalytically impregnated filtration elements; he has also described dependency of this reduction on various operational conditions.