Catalytic oxidation of ammonia to nitrogen

In this chapter the problems related to the emission of NH3, the sources of emission and the possibilities for their removal are discussed. The emphasis is on the removal of ammonia from gas phase. The selective catalytic oxidation (SCO) of ammonia is believed to be a most efficient and promising method for abating ammonia emissions. The state of the art with respect to catalytic solutions to the problem is discussed in detail with emphasis on the catalyst components and performance. Finally the mechanisms of ammonia oxidation on metals and metal oxides are reviewed. The scope of the investigation is also described briefly. General Introduction 2 1. The NH3 pollution problem The emissions of nitrogen oxides (NOx) and sulphur oxides (SOx) give rise to acidification of the environment. NOx and SOx are converted in the atmosphere to give nitric and sulphuric acid. However emission of ammonia causes acidification of the environment in an indirect way. Reaction of ammonia with acidic aerosols in the atmosphere, such as aerosols of sulphuric acid or nitric acid, gives aerosols containing ammonium sulphates or ammonium nitrates [1]. Oxidation of ammonia and ammonium-containing aerosols by microorganisms in the soil then gives acidic HNO3 [2]. Emission of nitrogen oxides and sulphur oxides are the major sources of acidic deposition in most countries. However emission of ammonia from intensive agricultural activities, e.g. cattle breeding, makes a significant contribution to the acidification of the environment in The Netherlands. About 94% of the ammonia emitted originates from agricultural sources in The Netherlands [3]. The damage caused by acidification in The Netherlands is serious: about half of the forests and much of the heather are affected; most of the fens have turned acidic; the nitrate concentration in the groundwater has increased and is still rising [4]; and the nitrogen balance in the ecosystem is seriously disturbed [5]. Ammonia can be health damaging and has an irritating smell. It is known to be a primary pollutant causing severe irritation and is suspected to have long-term effects such as bronchitis. Furthermore, ammonia is an undesired impurity for many industrial processes because it can cause corrosion and plugging of instrumentation. The major source of ammonia emission has been attributed to the intensive farming areas and notably to livestock manure. This has led to a recent study of the role of ammonia in the formation of a rural version of urban smog [6]. In contrast, NH3 is used beneficially in industry to reduce NOx emissions by the so-called selective catalytic reduction process (SCR). It is added to the effluent gas as reductant in order to perform the following reaction: NH3 + NO + 1/4 O2 → N2 + 3/2 H2O. However, the reaction is complete only with an excess of NH3, giving rise to NH3 slip. The unreacted ammonia is present in the off-gas and must therefore be removed in a secondary step [7,8]. Ammonia emissions are also caused by various other sources like: soda production; nitric acid production; metallurgical industry; coal or biomass gasification. Ammonia is also a potential source of nitrogen oxides (NOx) when involved in conventional combustion [9]. A typical example is the production of energy by combustion of the gasified biomass. Several projects addressing combined heat and power generation using turbines and heat-recycling system are ongoing [10,11]. The General Introduction 3 use of biomass as a primary fuel is particularly interesting since it offers the possibility of no net emission of CO2. It is also flexible for use in small units for remote locations where no other fuel is available. However biomass contains significant quantities of fuel-bound nitrogen. Gasification results in the formation of NH3 (600-4000 ppm) in addition to CO (9.8-17.2%), H2 (9.8-13.2%), CH4, CO2, H2O and N2. Subsequent combustion of this NH3 can lead to the formation of an equivalent amount of NOx, jeopardising all the advantages of the process. The emission limits for the emission of ammonia to the air are described in the Dutch Emission Directives (Nederlandse emissie Richtlijn, NeR). For the industrial sector, ammonia is classified in the class gaseous inorganic 4 (gA.4). This means that, according to the NeR, the concentration in the outgoing gas flow may not be larger than 200 mg/Nm. This limit applies to polluted gas flows of at least 5,0 kg/hr. However there might be plans to group ammonia in the class gA.3. This would mean that the concentration must not be larger than 30 mg/Nm, valid for polluted gas flows of at least 0,300 kg/hr. The NeR-limits for the agricultural sector are much stricter. The ammonia concentration in the outgoing gas flow may not be larger than 5 mg/Nm. This applies to all polluted gas flows. The ammonia emission for the Netherlands was 188.000 tons in 1996, 188.000 tons in 1997 and 177.000 tons in 1998 [12]. 2. Removal of ammonia Removal of ammonia can be divided in two groups (i) removal of ammonia from liquid phase and (ii) removal of ammonia from gas phase. 2.1 Removal of ammonia from water Conventional wastewater treatment plants have only limited capacity for nitrogen removal. Plant effluents thus contain large concentrations of nitrogen, with ammonia being the main form. Ammonia concentrations in biologically treated wastewater are generally in the range 12-35 mg/L [13]. Several technically and economically feasible methods are available to reduce this concentration. They are characterized in Table 1. General Introduction 4 Table 1 Comparison of ammonia removal process [14,15] Process Advantage Disadvantage Efficiency (%) Relative cost Biological nitrificationdenitrification N2 end product All forms of nitrogen removed Long detention time Sensitive to inhibitors, changing flow and temperature 70-95 1.00 Breakpoint chlorination Complete removal can be obtained Temperature variation and inhibition proof Chemicals added Dechlorination is often necessary Continous control equipment required 90-100 -Ammonia stripping Simplicity of operation Ease of control No byproducts Large Scale Inefficient at low temperature 50-95 1.32 Selective ion exchange on clinoptilolite Wide temperature range and no inhibition By-product formation(brine) 90-97 1.54 The relative costs of the treatment processes listed in Table 1 are controversial, and some authors claim that ammonia desorption is by far the least expensive ammonia removal process [16]. It should be noticed that the ammonia desorption method is actually change the liquid phase problem into a gas phase problem. Other methods such as catalytic wet oxidation and electrochemical oxidation are still under development [17,18,76]. 2.2 Removal of ammonia from gas Two main sources of waste gas containing ammonia can be identified: gas from industrial processes and gas from agriculture. Examples of industrial processes that produce ammonia are gasification, carbonisation, HDN process and thermal treatment of coal, liquid petroleum, shale oil, biomass and tar sands. The name of the contaminated gas flow is Cokes Oven Gas (referred to as COG-gas). The main source of agricultural pollution in Noord-Brabant is ventilation from sties, stables and manure processing factories. The name of the contaminated gas flow is Sty-gas. General Introduction 5 The gas generated from COG-gas usually contains high concentration of ammonia. A conventional method of removing ammonia from COG is conducted by washing the COG with dilute sulfuric acid to recover the ammonia as ammonium sulfate. However the demand of ammonium sulfate for fertilizer has decreased and the market price greatly lowered. As a result, the profit is remarkably inferior and this process is now almost worthless from the industry viewpoint. At present, the ammonium sulfate production process is reduced and changed to orther processes such as the Phosam process or Chevron WWT process to produce highly pure liquid ammonia, the Koppers process to separate ammonia followed by direct combustion or the Carl Still process to burn ammonia in the presence of a catalyst. However in the case of the direct combustion process, it is difficult to inhibit the production of NOx [19-21]. An alternative method to remove NH3 from COG is the decomposition of NH3 at high temperature in the presence of a catalyst [22-24]. A few common techniques are schematically presented in Fig.1. Fig.1 Basis steps in ammonia removal Biological and acid scrubbing are two current techniques that are available for the removal of NH3 from Sty-gas, which usually contains low concentrations of ammonia (<1000 ppm). In Noord-Brabant only one manure processing plant is operational (CNC) and another pilot plant is in experimental state (Ferm-O-Feed). Acid scrubbing and biofilters are techniques that are used in manure processing. The scale of the gas flow varies strongly per plant (10,000  300,000 m/hr). Biofilters are mainly used for General Introduction 6 odour removal. The scrubbers are able to remove ammonia to acceptable concentrations. From an economical point of view the acid scrubbing is the most favourable one, because it is much faster. Furthermore the system is more accurately controllable. The biological system produces a larger liquid waste flow. This flow is mixed with manure waste, and the farmer has to pay per m to sell the waste. Apart from COG and Sty-gas, many industrial processes, such as textile treatment, soda production, SCR process and nitric acid production, produce emission streams containing ammonia. These streams usually contain excess O2, H2O and very low concentrations of NH3 (<1000 ppm). The conventional method for NH3 removal is again by acid scrubbing. It should be pointed out that the acid scrubbing actually change a gas phase problem into a liquid phase problem. At present, the scrubbing liquid containing either NH4NO3 or (NH4)2SO4 is sold as a fertilizer. As discussed above, the market for such