Catalysis for NOx abatement

Research in the field of NO_x abatement has grown significantly in the past two decades. The general trend has been to develop new catalysts with complex materials in order to meet the stringent environmental regulations. This review discusses briefly about the different sources of NO_x and its adverse effect on the ecosystem. The main portion of the review discusses the progress and development of various catalysts for NO_x removal from exhaust by NO decomposition, NO reduction by CO or H₂ or NH₃ or hydrocarbons. The importance of understanding the mechanism of NO decomposition and reduction in presence of metal ion substituted catalysts is emphasized. Some conclusions are made on the various catalytic approaches to NO_x abatement.     

The influence of hydrogen-enriched gas on the performance of lean NOx trap catalyst for a light-duty diesel engine

Modern diesel engines have improved engine fuel economy and significantly reduced nitrogen oxides (NO_x) and particulate matter (PM) emissions achieved by advances in both combustion and exhaust aftertreatment technologies. Recently, it has been shown that the vehicle emissions can be further improved by several catalytic systems including fuel reformers and aftertreatment systems, such as the Lean NO_x Trap (LNT). This NO_x removal system, called LNT, absorbs NO_x under lean exhaust gas conditions and releases NO_x under rich conditions. This technology can provide high NO_x conversion efficiency, but the right amount of reducing agent should be supplied into the catalytic converter under appropriate conditions.     

Exergy-based assessment for waste gas emissions from Chinese transportation

As an effective measure for environmental impact associated with the waste emissions, exergy is used to unify the assessment of the waste gases of CO, NO_x, and SO₂ emitted from fossil fuel consumption by the transportation system in China. An index of emission exergy intensity defined as the ratio of the total chemical exergy of the emissions and the total converted turnover of the transportation is proposed to quantify the environmental impact per unit of traffic service. Time series analyses are presented for the emission exergy and emission exergy intensity of the whole Chinese transportation as well as for its four sectors of highways, railways, waterways and civil aviation from 1978 to 2004. For the increasing emission exergy with CO taking the largest share, the highways sector was the major contributor, while the railways sector initially standing as the second main contributor developed into the least after 1995. The temporal and structural variations of the emissions are illustrated against the transition of the transportation system in a socio-economic perspective, with emphasis on policy-making implications.     

Analysis of exhaust waste heat recovery from a dual fuel low temperature combustion engine using an Organic Rankine Cycle

This paper examines the exhaust waste heat recovery potential of a high-efficiency, low-emissions dual fuel low temperature combustion engine using an Organic Rankine Cycle (ORC). Potential improvements in fuel conversion efficiency (FCE) and specific emissions (NO_x and CO₂) with hot exhaust gas recirculation (EGR) and ORC turbocompounding were quantified over a range of injection timings and engine loads. With hot EGR and ORC turbocompounding, FCE improved by an average of 7 percentage points for all injection timings and loads while NO_x and CO₂ emissions recorded an 18 percent (average) decrease. From pinch-point analysis of the ORC evaporator, ORC heat exchanger effectiveness (?), percent EGR, and exhaust manifold pressure were identified as important design parameters. Higher pinch point temperature differences (PPTD) uniformly yielded greater exergy destruction in the ORC evaporator, irrespective of engine operating conditions. Increasing percent EGR yielded higher FCEs and stable engine operation but also increased exergy destruction in the ORC evaporator. It was observed that hot EGR can prevent water condensation in the ORC evaporator, thereby reducing corrosion potential in the exhaust piping. Higher ? values yielded lower PPTD and higher exergy efficiencies while lower ? values decreased post-evaporator exhaust temperatures below water condensation temperatures and reduced exergy efficiencies.     

Optimizing thermal performances and NOx emission in a premixed ammonia-hydrogen blended meso-scale combustor for thermophotovoltaic applications

As a carbon-free energy carrier, ammonia has attracted significant interest in the combustion field as a potential substitute for fossil fuels. However, the focus has been given to the application at meso-scale conditions, particularly with regard to thermal performance and NO_x emissions. Therefore, the present study numerically investigates a 3-dimensional time-domain premixed ammonia/oxygen meso-scale combustor to optimize its' thermal performance and NO_x emission for power generation applications. The numerical model is firstly validated by using experimental data available in the literature. Then, the effects of 1) the inlet pressure (P_in), 2) the equivalence ratio, and 3) the hydrogen blended ratio on the temperature uniformity, the combustor outer wall mean temperature (OWMT), NO emission, and exergy efficiency are examined. The results indicate that increasing P_in intensifies the mixing process of the mixture gases, thus reducing the residence time for the high-temperature flame in the combustion chamber. The optimized OWMT and NO emissions are up to 26% and 40.3% respectively, with only 9% compensation of the standard deviation achieved, when the inlet velocity is set to 0.5 m/s and P_in is 3.0 bar. Furthermore, varying the equivalence ratio in the range of 0.95–1.1 has a minor influence on improving thermal performances, but a significant impact on mitigating the NO_x emission performance. Additionally, blending less than 15% hydrogen has a significant reduction in the maximum NO_x emission (up to 53%); however, the influence on the OWMT can be neglected. Further exergy analysis reveals that elevating P_in results in a decrease in the exergy efficiency due to the increased inlet exergy. In general, this work provides a preliminary method for improving the thermal performance and NO_x emission of an ammonia/hydrogen-oxygen-fueled meso-scale combustor for power generation purpose.     

Internalisation of external cost in the power generation sector: Analysis with Global Multi-regional MARKAL model

The Global MARKAL-Model (GMM), a multi-regional “bottom-up” partial equilibrium model of the global energy system with endogenous technological learning, is used to address impacts of internalisation of external costs from power production. This modelling approach imposes additional charges on electricity generation, which reflect the costs of environmental and health damages from local pollutants (SO₂, NO_x) and climate change, wastes, occupational health, risk of accidents, noise and other burdens. Technologies allowing abatement of pollutants emitted from power plants are rapidly introduced into the energy system, for example, desulphurisation, NO_x removal, and CO₂ scrubbers. The modelling results indicate substantial changes in the electricity production system in favour of natural gas combined cycle, nuclear power and renewables induced by internalisation of external costs and also efficiency loss due to the use of scrubbers. Structural changes and fuel switching in the electricity sector result in significant reduction of emissions of both local pollution and CO₂ over the modelled time period. Strong decarbonisation impact of internalising local externalities suggests that ancillary benefits can be expected from policies directly addressing other issues then CO₂ mitigation. Finally, the detailed analysis of the total generation cost of different technologies points out that inclusion of external cost in the price of electricity increases competitiveness of non-fossil generation sources and fossil power plants with emission control.     

Supply- and demand-side effects of power sector planning with demand-side management options and SO2 emission constraints

This paper examines the implications of SO₂ emission mitigation constraints in the power sector planning in Indonesia—a developing country—during 2003–2017 from a long term integrated resource planning perspective. A decomposition model is developed to assess the contributions of supply- and demand-side effects to the total changes in CO₂, SO₂ and NO_x emissions from the power sector due to constraints on SO₂ emissions. The results of the study show that both the supply- and demand-side effects would act towards the reduction of CO₂, SO₂ and NO_x emissions. However, the supply-side effect would play the dominant role in emission mitigations from the power sector in Indonesia. The average incremental SO₂ abatement cost would increase from US$ 970 to US$ 1271 per ton of SO₂, while electricity price would increase by 2–18% if the annual SO₂ emission reduction target is increased from 10% to 25%.     

Post combustion methods for control of NOx emissions

This paper is a review of stack gas treatment methods for the control of NO_x emissions. Particular emphasis is placed on status of development and factors affecting the performance of the processes. Catalytic, noncatalytic, and scrubbing processes are compared on a uniform engineering basis. Most of the active process development work is taking place in Japan. The three leading stack gas treatment techniques for NO_x control are catalytic reduction with ammonia, noncatalytic reduction with ammonia, and direct scrubbing of NO with simultaneous absorption of SO₂. The wet processes are much less developed than the dry processes.     

Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation)

The effects of biodiesel (rapeseed methyl ester, RME) and different diesel/RME blends on the diesel engine NO_x emissions, smoke, fuel consumption, engine efficiency, cylinder pressure and net heat release rate are analysed and presented. The combustion of RME as pure fuel or blended with diesel in an unmodified engine results in advanced combustion, reduced ignition delay and increased heat release rate in the initial uncontrolled premixed combustion phase. The increased in-cylinder pressure and temperature lead to increased NO_x emissions while the more advanced combustion assists in the reduction of smoke compared to pure diesel combustion. The lower calorific value of RME results in increased fuel consumption but the engine thermal efficiency is not affected significantly. When similar percentages (% by volume) of exhaust gas recirculation (EGR) are used in the cases of diesel and RME, NO_x emissions are reduced to similar values, but the smoke emissions are significantly lower in the case of RME. The retardation of the injection timing in the case of pure RME and 50/50 (by volume) blend with diesel results in further reduction of NO_x at a cost of small increases of smoke and fuel consumption.     

Catalytic reduction of NOx in gasoline engine exhaust over copper- and nickel-exchanged X–zeolite catalysts

Catalytic removal of NO_x in engine exhaust gases can be accomplished by non-selective reduction, selective reduction and decomposition. Noble metals are extensively used for non-selective reduction of NO_x and up to 90% of engine NO_x emissions can be reduced in a stoichiometric exhaust. This requirement of having the stoichiometric fuel–air ratio acts against efficiency improvement of engines. Selective NO_x reduction in the presence of different reductants such as, NH₃, urea or hydrocarbons, requires close control of the amount of reductant being injected which otherwise may be emitted as a pollutant. Catalytic decomposition is the best option for NO_x removal. Nevertheless, catalysts which are durable, economic and active for NO_x reduction at normal engine exhaust temperature ranges are still being investigated.Three catalysts based on X–zeolite have been developed by exchanging the Na⁺ ion with copper, nickel and copper–nickel metal ions and applied to the exhaust of a stationary gasoline engine to explore their potential for catalytic reduction of NO_x under a wide range of engine and exhaust conditions. Some encouraging results have been obtained. The catalyst Cu–X exhibits much better NO_x reduction performance at any temperature in comparison to Cu–Ni–X and Ni–X; while Cu–Ni–X catalyst exhibits slightly better performance than Ni–X catalyst. Maximum NO_x conversion efficiency achieved with Cu–X catalyst is 59.2% at a space velocity (sv) of 31 000 h⁻¹; while for Cu–Ni–X and Ni–X catalysts the equivalent numbers are 60.4% and 56% respectively at a sv of 22 000 h⁻¹. Unlike noble metals, the doped X–zeolite catalysts exhibit significant NO_x reduction capability for a wide range of air/fuel ratio and with a slower rate of decline as well with increase in air/fuel ratio.     

NOx formation and selective non-catalytic reduction (SNCR) in a fluidized bed combustor of biomass

Caledonian Paper (CaPa) is a major paper mill, located in Ayr, Scotland. For its steam supply, it previously relied on the use of a Circulating Fluidized Bed Combustor (CFBC) of 58 MW_th, burning coal, wood bark and wastewater treatment sludge.It currently uses a bubbling fluidized bed combustor (BFBC) of 102 MW_th to generate steam at 99 bar, superheated to 465 °C. The boiler is followed by steam turbines and a 15 kg/s steam circuit into the mill. Whereas previously coal, wood bark and wastewater treatment sludge were used as fuel, currently only plantation wood (mainly spruce), demolition wood, wood bark and sludge are used.Since these biosolids contain nitrogen, fuel NO_x is formed at the combustion temperature of 850–900 °C. NO_x emissions (NO + NO₂) vary on average between 300 and 600 mg/Nm³ (dry gas). The current emission standard is 350 mg/Nm³ but will be reduced in the future to a maximum of 233 mg/Nm³ for stand-alone biomass combustors of capacity between 50 and 300 MW_th according to the EU LCP standards. NO_x abatement is therefore necessary.In the present paper we firstly review the NO_x formation mechanisms, proving that for applications of fluidized bed combustion, fuel NO_x is the main consideration, and the contribution of thermal NO_x to the emissions insignificant.We then assess the deNO_x techniques presented in the literature, with an updated review and special focus upon the techniques that are applicable at CaPa. From these techniques, Selective Non-catalytic Reduction (SNCR) using ammonia or urea emerges as the most appropriate NO_x abatement solution.Although SNCR deNO_x is a selective reduction, the reactions of NO_x reduction by NH₃ in the presence of oxygen, and the oxidation of NH₃ proceed competitively.Both reactions were therefore studied in a lab-scale reactor and the results were transformed into design equations starting from the respective reaction kinetics. An overall deNO_x yield can then be predicted for any operating temperature and NH₃/NO_x ratio.We then present data from large-scale SNCR-experiments at the CFBC of CaPa and compare results with the lab-scale model predictions, leading to recommendations for design and operation. Finally the economic impact is assessed of implementing SNCR-technology when applying an NH₃ SNCR or urea SNCR to the CFBC at CaPa.     

NOx reduction and NO2 emission characteristics in rich-lean combustion of hydrogen

Hydrogen is a clean alternative to conventional hydrocarbon fuels, but it is very important to reduce the nitrogen oxides (NO_x) emissions generated by hydrogen combustion. The rich-lean combustion or staged combustion is known to reduce NO_x emissions from continuous combustion burners such as gas turbines and boilers, and NO_x reduction effects have been demonstrated for hydrocarbon fuels. The authors applied rich-lean combustion to a hydrogen gas turbine and showed its NO_x reduction effect in previous research. The present study focused on experimental measurements of NO and NO₂ emissions from a coaxial rich-lean burner fueled with hydrogen. The results were compared with diffusion combustion and methane rich-lean combustion. Significant reductions in NO and NO₂ were achieved with rich-lean combustion. The NO and NO₂ reduction effects by rich-lean combustion relative to conventional diffusion combustion were higher with hydrogen than with methane.     

Reducing inefficiency and emissions of large steam generators in the United States

Combustion process modifications to reduce inefficiency and emissions in contemporary large boilers are constrained by a number of important design and operating considerations such as steam temperature, ash fusion and material limitations. Plant efficiency gains, obtainable by further increasing combustion heat release efficiency, are in many cases negligible. However, depending upon standard operation, a large potential exists for increasing overall efficiency by lowering excess air levels, while at the same time decreasing pollutant emissions. Dramatic increases in fuel prices have now made investment in such improvements more attractive in the United States than it was in the past. Reduction of excess air level can in addition effect reductions in emissions of NO_x and SO₃, and even of particulates, through equipment and operating improvements. High fuel prices have also made investment in air preheaters and economizers attractive for a larger number of boilers than ever before.Air pollutants can originate from components of the fuel, from incomplete combustion, or even through complete combustion processes. These various forms of pollution can be controlled in varying degrees during the combustion process by wet ash handling, SO₂ to SO₃ conversion minimization, or mixing and temperature control in the case of combustibles and nitric oxide. NO_x reduction techniques, applied to a large gas-fired utility boiler, resulted in NO_x emissions of 33 ppmV or less at power outputs up to 355 MW. For coal- and oil-fired boilers, NO_x scavenging by ammonia injection, recently developed, can give dramatic reductions in NO_x emissions without affecting the combustion process.     

Removal of NOx by microwave reactor with ammonium bicarbonate and Ga-A zeolites at low temperature

Microwave reactor with the mixture of ammonium bicarbonate (NH₄HCO₃) and Ga-A zeolites was set up to study the removal of nitrogen oxides (NO_x) from waste gas with excess oxygen concentration (14–19%) at low temperature (80–120 °C). The results showed that the microwave reactor filled with NH₄HCO₃ and Ga-A zeolites could reduce NO_x to nitrogen with the best purifying efficiency of 95.45% and the best denitrification amount of 89.28 mg h⁻¹. The optimal microwave power and residence time (RT) on denitrification was 259–280 W and 0.259 s, respectively. Microwave denitrification effect of the experiment using ammonium bicarbonate and Ga-A zeolites was much higher than that using ammonium bicarbonate or Ga-A zeolites only. The mechanism for microwave-induced NO_x reduction can be explained as the microwave-induced catalytic reaction between NO_x and ammonium bicarbonate with Ga-A zeolites being the catalyst and microwave absorbent.     

Control of nitrogen oxide emissions from coal combustion

The paper gives a discussion of the regulatory framework currently being used to limit nitrogen oxide (NO_x) emissions, with emphasis being placed on EC and UN measures. It then follows with a discussion of the procedures being adopted at present to control or limit these emissions. The topics discussed include combustion modification measures, new combustion technologies, flue gas treatment, selective catalytic reduction, selective noncatalytic reduction and combined SO_x/NO_x processes.     

Evaluation of the performance of a multi-fuel domestic boiler with respect to the existing European standard and quality labels: Part-1

Emissions from a multi-fuel domestic boiler (40 kW), fired with nine different agro-biomass pellets have been compared. The pellets include apple pomace (Malus domestica), reed canary grass (Phalaris arundinacea), pectin waste from citrus shells (Citrus reticulata), sunflower husk (Helianthus annuus), peat, two types of straw pellets and two types of wood pellets. The measurements of emissions comprised carbon monoxide (CO), nitrogen oxides (NO_x), unburned hydrocarbons (C_xH_y), sulphur oxides (SO_x) and flue dust mass concentration (by DINplus and isokinetic sampling methods). Comparison of experimental emission values with relevant quality labels (Blue Angel and Swan Mark) and standard (EN-303-5) showed that the boiler satisfied the emissions requirements of Blue Angel, Swan Mark and EN-303-5 when using wood pellets-1 (except CO emission), reed canary grass and citrus pectin waste pellets as fuel at nominal load. The wood pellets-1 yielded the highest boiler efficiency of 92.4%. Dusts emission varied as a function of fine content and elemental constituent of the pellets and was the highest with sunflower husk. CO and C_xH_y emissions were maximum with peat pellets. NO_x emissions were below the concerned permissible values with all experimental pellets. Emissions of NO_x and SO_x were found maximum with straw pellets.For agro-pellets, statistical differences in ash contents were significant. High ash contents and low ash melting temperature made straw pellets less suitable for domestic applications. Reed canary grass, citrus pectin and apple pellets were the most suitable agro-pellets for small scale boilers with reasonable less ash contents and less emissions as compared to others.     

Application of the Miller cycle to reduce NOx emissions from petrol engines

A conceptual analysis of the mechanism of the Miller cycle for reducing NO_x emissions is presented. Two versions of selected Miller cycle (1 and 2) were designed and realized on a Rover “K” series 16-valve twin-camshaft petrol engine. The test results showed that the application of the Miller cycle could reduce the NO_x emissions from the petrol engine. For Miller cycle 1, the least reduction rate of NO_x emission was 8% with an engine-power-loss of 1% at the engine’s full-load, compared with that of standard Otto cycle. For Miller cycle 2, the least reduction rate of NO_x emission was 46% with an engine-power-loss of 13% at the engine’s full-load, compared with that of standard Otto cycle.     

Supply- and demand-side effects of power sector planning with CO2 mitigation constraints in a developing country

In this paper, the implications of CO₂ emission mitigation constraints in the power sector planning in Indonesia are examined using a long term integrated resource planning model. An approach is developed to assess the contributions of supply- and demand-side effects to the changes in CO₂, SO₂ and NO_x emissions from the power sector due to constraints on CO₂ emissions. The results show that while both supply- and demand-side effects would act towards the reduction of CO₂, SO₂ and NO_x emissions, the supply-side options would play the dominant role in emission mitigations from the power sector in Indonesia. The CO₂ abatement cost would increase from US$7.8 to US$9.4 per ton of CO₂, while the electricity price would increase by 3.1 to 19.8% if the annual CO₂ emission reduction target is raised from 10 to 25%.     

Experimental study of the performances of a modified diesel engine operating in homogeneous charge compression ignition (HCCI) combustion mode versus the original diesel combustion mode

Homogeneous charge compression ignition (HCCI) combustion mode provides very low NO_x and soot emissions; however, it has some challenges associated with hydrocarbon (HC) emissions, fuel consumption, difficult control of start of ignition and bad behaviour to high loads. Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production in diesel and HCCI combustion mode. However EGR has different effects on combustion and emissions, which are difficult to distinguish. This work is intended to characterize an engine that has been modified from the base diesel engine (FL1 906 DEUTZ-DITER) to work in HCCI combustion mode. It shows the experimental results for the modified diesel engine in HCCI combustion mode fueled with commercial diesel fuel compared to the diesel engine mode. An experimental installation, in conjunction with systematic tests to determine the optimum crank angle of fuel injection, has been used to measure the evolution of the cylinder pressure and to get an estimate of the heat release rate from a single-zone numerical model. From these the angle of start of combustion has been obtained. The performances and emissions of HC, CO and the huge reduction of NO_x and smoke emissions of the engine are presented. These results have allowed a deeper analysis of the effects of external EGR on the HCCI operation mode, on some engine design parameters and also on NO_x emission reduction.