What Are The Mechanisms Of Ozone Depletion Environmental Sciences Essay

The mechanism of global warming can be described by the Greenhouse effect; by which, solar radiation enters through the Earth’s atmosphere and is retained, resulting in increased temperatures. Solar radiation entering the Earth’s atmosphere is partially absorbed by the Earth’s surface and partially re-radiated back into space. However, having lost energy to the Earth’s surface, the infrared radiation is of an increased wavelength and is consequently absorbed by certain gasses (greenhouse gasses) in the Earth’s atmosphere. This results in the radiation being recycled within the Earth’s atmosphere, leading to an increased average temperature of the Earth’s near surface air and oceans.

Ozone depletion is a result of the catalysed reactions between ozone and atomic chlorine or bromine. Chlorofluorocarbons (CFC) and bromofluorocarbons have the greatest ozone depleting potential as they form atomic chlorine and bromine upon photodissociation.

Carbon dioxide, methane and nitrous oxide can be considered to have the greatest global warming potential due to their abundance. From these gasses, nitrous oxide is 310 times more effective in absorbing radiation than carbon dioxide, and methane is 21 times more effective than carbon dioxide.

What is the difference between ‘winter’ and ‘summer’ smog? Explain the mechanism by which they are generated.

Winter and summer smog can be differentiated by their constituents and thus their environmental impacts. Winter smog is made up of sulphur dioxide, partially oxidised organics and particulate matter (PM), the concentrations of which are typically increased in winter months due to increased heating from sulphur rich fossil fuels such as coal and oil. It is also referred to as ‘reducing’ smog. These pollutants can affect the respiratory system and form secondary pollutants. Sulphur dioxide can form acid rain from oxidation catalysed by PM or free radicals of oxygen and nitrogen.

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Summer, or ‘photochemical’ smog, tends to occur as a result of increased nitrogen oxides or hydrocarbon concentrations in the atmosphere due to exhausts from internal combustion engines. Nitrogen oxides can be broken down by sunlight to form radicals causing low level ozone formation, nitric acid, peroxides, aldehydes and ketones.

Both types of smog are more likely to form in cities and as a result of lack of wind. The lack of air movement can result in a temperature inversion which causes a layer of still warm air to cover a layer of cool air, trapping any pollutants below the warm layer near ground level.

What is the difference between stratospheric and tropospheric ozone? Explain the role of NOx in the generation of tropospheric ozone.

The ozone layer typically occurs in the stratosphere and is naturally formed and decomposed from the reactions between oxygen and oxygen free radicals which are formed from the decomposition of oxygen by ultraviolet light. The stratosphere is typically located from around 20 to 50 km above the Earth’s surface. The occurrence of ozone at this level helps absorb harmful ultraviolet light.

Tropospheric ozone refers to ozone occurring in the troposphere (up to 12 km above the Earth’s surface). Ozone at this level can be poisonous and also acts as a greenhouse gas with a heat trapping effectiveness of 2000 times greater than CO2.

The breakdown of nitrogen dioxide by ultraviolet light can lead to the formation of oxygen free radicals.

NO2 = NO + O*

O* + O2 = O3

Describe the mechanisms for acid rain and eutrophication, respectively. What are the effects of these two environmental impacts?

Acid rain is primarily formed from SOx and NOx. Sulphur oxides are oxidised in the presence of ultraviolet light to form sulphur trioxide gas, through reacting with water, this can then form acid rain in the form of sulphurous acid and/or sulphuric acid. Nitrogen oxides are oxidised to nitrogen dioxide which may then lead to the reaction between water and nitrogen dioxide allowing for the formation of nitric and nitrous acid. Acid rain can effect vegetation, lakes and rivers, buildings and human health.

Eutrophication is the process that occurs due to excessive growth of habitats to an extent where the growth becomes destructive to the environment. Use of artificial fertilisers from farming leads to increased levels of nitrates and phosphates accumulating in lakes and rivers. The nitrates and phosphates are nutrients that increase the growth of plants and algae. When these plants and algae die they can decompose aerobically to form carbon dioxide and water. With excessive decomposition, oxygen will be depleted and anaerobic decomposition will occur, leading to formation of hydrides such as ammonia and hydrogen sulphide. More species will die due to poisoning caused by the hydrides and may eventually lead to the death of the entire habitat.

Which of the above environmental impacts do nitrogen oxides contribute to? Explain how.

Nitrogen oxides contribute to the formation of acid rain either in the form of nitric or nitrous acid.

Upon absorbing ultraviolet light nitrogen dioxide breaks down to form nitrogen oxide and oxygen radicals. These radicals can combine with water to form hydroxyl radicals which may then react with nitrogen oxide to form nitric acid.

Air pollution prevention and control

What options exist for the prevention and clean-up of acid gas emissions?

Emissions of nitrogen oxides can potentially lead to the formation of acid rain, with several options available for its prevention and clean-up. These primarily include selective catalytic reduction (SCR), non-selective catalytic reduction (NSCR), and selective non-catalytic reduction (SNCR).

The power industry uses SCR for post-combustion NOx clean up and/or low NOx burners and SNCR for prevention of NOx formation in the combustion stage. SCR involves reacting ammonia or urea with NOx over vanadium oxide catalysts in a temperature range of 300 to 400 °C and can remove up to 95 % of NOx. SNCR involves injecting ammonia/urea in the furnace at temperatures of 900 to 1100 °C, with a removal efficiency of only around 30 %.

Non-selective catalytic reduction removes NOx in a method analgous to the three-way catalytic converter used in the automotive industry. This is typically applied in the chemical industries.

Sulphur oxides can also lead to acid rain. Flue gas desulphurisation (FGD) plant involves scrubbing the gasses to remove sulphur oxides. For example, limestone scrubbing is one method of FGD, which converts sulphur oxides into calcium sulphate dihydrate (gypsum).

Which stages in the life cycle of an installation must be considered within the IPPC Directive?

All stages of the life cycle should be considered; i.e. from cradle to grave. In order to provide an integrated approach, no stage can be left out, and a full assessment of the environmental, social and economic impacts should be carried out for the raw materials, processing, storage and transportation stages involved.

Which industrial sectors are regulated by the IPPC Directive? Why do you think these sectors have been included under the IPPC Directive?

The industries covered by the IPPC directive include:

Energy Production

Refineries

Production & Processing of Metals

Production of Cement & Lime

Activities involving Asbestos

Glass, Glass Fibre and other Mineral Fibre Manufacture

Ceramic Production

Organic & Inorganic Chemical Production

Fertiliser & Biocide Manufacture

Pharmaceutical Manufacture

Explosives Manufacture

Storage of Bulk Chemicals

Incineration

Landfill

Paper Manufacture

Tar & Bitumen Processes

Coating, Printing and Textile Activities

Dye, Ink and Coating Material Manufacture

Timber Activities

Rubber Activities

Processing of Food; and

Intensive Farming.

These sectors have been included under the Directive due to the requirement of controlling and limiting the environmental impact these industries can have during the manufacturing process of their relevant products.

Case study: Identifying BAT for the prevention and control of NOx emissions

Nitric acid manufacture

What influences the yield of nitric acid? Why is it important to maximise its yield?

The yield of nitric acid is effected by:

The efficiency of the catalytic oxidation of ammonia to nitrogen monoxide

The efficiency of the oxidation of nitrogen monoxide to nitrogen dioxide

The absorption of nitrogen dioxide in water to produce nitric acid

Maximising its yield allows for an efficient production; thus generating more nitric acid at the same operating costs to achieve larger profits. Maximising yield means minimising unreacted nitrogen oxide which is consequently released to atmosphere.

Why are the reaction (6) and the reverse of reaction (4) undesirable in this process?

The reverse of reaction 4 results in a lower NO2 yield as the reaction will tend towards the NO and O2 through a shift in equilibrium to the reactants. As the absorption of NO2 is limited by NO2 concentration, it is desirable to ensure the forward reaction in reaction 4 occurs in order to maximise HNO3 yield. In addition to this, the occurrence of a reverse reaction (4) in which NO is formed allows for the possibility of nitrous acid formation (reaction 6). Again this is undesirable as NO2 is consumed in producing an unwanted product, consequently leading to a lower NO2 concentration and lower HNO3 yields.

In addition to NOx and N2O emissions, what other releases to air, water and land can be expected from nitric acid manufacture? What environmental impacts can these releases cause?

Carbon dioxide emissions from burning fossil fuels for energy requirements and transportation should be accounted for, as well as ammonia, nitric acid, nitrous acid leakages.

Pollution prevention and control of NOx emissions

Pollution prevention: Process design and operation

The efficiency of NO2 absorption to produce HNO3 can be increased further by increasing pressure. Discuss the advantages and disadvantages of pressurised systems in terms of their technical complexity, environmental impacts (including noise) and economic costs.

As stated, the main advantage of a pressurised system is the improved absorption of NO2 to produce HNO3, this results in less NO2 being released to the atmosphere. However, pressurised systems require a more complex design as a result of the serious safety considerations associated with them. Failure of pressurised systems can lead to death or injury of workers on-site as well as the release of NO2 to atmosphere.

Pressurised systems would require increased compression and pumping duties, contributing to increased noise pollution, operating costs and maintenance costs. The absorption column may not have been designed for higher pressures, i.e. materials of construction, column thickness, and column closures may not be suitable, and thus changes would have to be made, resulting in increased capital costs. These costs must be compared with the improved nitric acid yield in order to fully asses this option.

Discuss the advantages and disadvantages of supplying pure oxygen instead of air for oxidation of NO in the HNO3 absorption tower. Address the following issues:

What are the advantages of using pure oxygen instead of air in terms of oxidation efficiency, gas flowrates, column volume etc?

Using pure oxygen rather than air (21% oxygen, 79% nitrogen), allows for the volumetric gas flowrate of this stream to be reduced by 79% due to the elimination of nitrogen. This results in an improved oxidation efficiency, allowing more NO2 to be formed by reducing the amount of NO. The reduced gas flowrate results in a reduced column volume, assuming a constant gas hourly space velocity.

Unlike air, oxygen does not come for free – it has to be produced by separating nitrogen from air (you may remember this from the last year’s coursework on Waste Water Treatment within the module Introduction to Sustainable Development). This is usually done in a large scale cryogenic process (separation by cooling). What are the implications of this in the context of IPPC (i.e. taking into account all life cycle stages associated with this process option)?

The cryogenic separation of oxygen from air is a highly energy intensive process. The main stages of the process include compression, cooling, and distillation. From a life-cycle perspective, the raw material (air) is free; however, it is the processing stage which incurs the majority of environmental impact. Energy is required in the compression and distillation stages of the process, thus, assuming energy is obtained from fossil fuel sources, greenhouse gas emissions become a concern. Further to this, the requirement of refrigeration may have environmental impacts depending on the refrigerant used. Transportation of the oxygen to the nitric acid plant may have potential impacts; however, in comparison to the processing stage, any environmental impacts are likely to be relatively small. Therefore the production of oxygen does have an environmental impact to an extent; however, the IPPC does not apply to this industry, so due to the lack of IPPC regulation in the sector inefficiencies or environmental concerns may indirectly effect the nitric acid manufacturing process. The key concern of using pure oxygen is the additional cost.

Pollution prevention: Extended absorption

Explain the idea behind extended absorption. What is the link between the number of transfer units (NTU) in the HNO3 absorption tower and the NOx emissions?

Extended absorption (EA) allows for any unreacted nitrogen dioxide to be absorbed in a second tower, thus increasing nitric acid production and reducing NOx emissions. Oxygen can also be injected to oxidise any nitrogen monoxide so that it can be absorbed in the new absorption column. An increase in NTU in the HNO3 absorption tower results in lower NOx emissions.

How would you calculate the required height of the absorption column to increase the HNO3 production yield and reduce the emissions of NOx from the manufacture of nitric acid? What information and data would you need to do that? (See the Appendix.)

In order to calculate the required height of the absorption column, the NTU must be calculated. This requires the gas mole fraction at the top of the column (yT) and at the bottom (yB). The gradient of the operating line, R is also required, this is obtained from the equilibrium of the operating line, the molar gas flowrate, and the molar liquid flowrate per unit tower area.

This value is then multiplied by the height of a transfer unit which is given by:

Where G is the molar gas flowrate per unit tower area, ky is the overall mass transfer coefficient, and a is the interfacial area per unit packed volume.

Height = HTU x NTU

Pollution control: Absorption in sodium hydroxide

IPPC requires consideration of wider impacts of an activity, which means consideration of a number of life cycle stages of a process. In the case of NOx scrubbing by sodium hydroxide, what parts of the life cycle must be included for BAT assessment? Analysing this system, explain why absorption of NOx in NaOH is unlikely to represent BAT.

All stages of the life cycle must be considered for a through and exhaustive analysis of BAT assessment. The final stage of the life-cycle for the sodium hydroxide scrubbing option is a key concern. The disposal of the sodium nitrite-nitrate effluent is a key concern. If no suitable disposal or recycle method can be identified for the effluent, then along with the increased costs incurred from NaOH feedstock and increased pumping, this option does not represent BAT.

Pollution control: Non-selective catalytic reduction (NSCR)

Non-selective catalytic reduction (NSCR) is quite efficient in reducing the emissions of NOx from the manufacture of nitric acid. However, it also generates additional environmental impacts. Identify these impacts for hydrogen and natural gas as reducing agents, respectively, and explain the origin of these impacts.

The requirement of a reducing agent such as hydrogen and natural gas has significant environmental implications. Hydrogen is likely to have been produced from steam reforming of fossil fuels, and natural gas is a fossil fuel, therefore, consideration should be given to the process of extracting the fossil fuel, transporting it and processing it. In the case of hydrogen, the processing stage is likely to be a key concern as it is an energy intensive process which results in further emissions and additional environmental impacts. For example, steam reforming of methane to produce hydrogen and carbon monoxide is an endothermic reaction requiring heat generated often from burning fossil fuels. However, using methane as a reducing agent leads to the formation of CO2 in the process.

Calculate the amount of carbon dioxide in t/yr generated in the NSCR process which removes 1.5 t NO2/day from the nitric acid tail gas using natural gas. Calculate the potential for global warming from these CO2 emissions. Compare that with the global warming avoided by the removal of the equivalent amount of N2O per day from the same tail gas. What do you conclude?

2CH4 + O2 + 2NO2 +2N2O = 2CO2 + 4H2O + 3N2

Mass of NO2 = 1500 kg/day

Molecular Weight of NO2 = 46.01 kg/mol

Moles of NO2 = 32.60 kmol/day

Moles of CO2 = (2/2) * Moles of NO2 = 32.60 kmol/day

Molecular Weight of CO2 = 44.01 kg/kmol

Mass of CO2 = 1.4 t CO2/day

Assuming that the plant is operational for 365 days per year, the annual amount of carbon dioxide generated by the NSCR process can be estimated as 524 tonnes per year.

N2O has a heat trapping effectiveness of 150 times that of CO2; thus the saving made by converting N2O to CO2 is substantial with regards to global warming potential. The N2O produced would be equivalent to 78,551 tonnes of CO2 per year.

Use the example in the previous question to calculate the equivalent acidification impact that would be avoided by the removal of NOx with the NSCR process. Compare this avoided impact with global warming generated through the use of natural gas to remove the NOx. What do you conclude? How should we approach situations like these, where reducing one environmental impact causes another?

The classification factor for NOx is estimated as 0.7 kg/kg. Therefore acidification savings can be estimated as 1050 kg/day which is equivalent to 383 tonnes per year. In comparison to the 524 tonnes of CO2 produced per year, it can be noted that the reduction of the acidification impact does not outweigh the CO2 produced in the process.

Pollution control: Selective catalytic reduction (SCR)

What are the main environmental and safety concerns associated with the selective catalytic reduction (SCR) process?

SCR typically uses ammonia or urea to catalytically convert nitrogen oxides to molecular nitrogen and water; however, the production of these reactants have key environmental considerations. Ammonia production typically involves steam reforming of a hydrocarbon feedstock such as natural gas or naphtha. Thus, fossil fuel depletion and the issues surrounding fossil fuel extraction, transportation and processing are all key sustainability concerns. However, ammonia produced from naphtha would be more of a concern than ammonia produced from natural gas due to the additional refining process from which it is produced. Ammonia production also involves the emission of carbon dioxide, a greenhouse gas.

Gas preheating also has an impact on energy requirements and thus greenhouse gas emissions.

On-site ammonia or urea storage would be required; any leakages of ammonia could prove fatal as it is toxic upon inhalation.

Catalyst disposal may also have various environmental impacts depending on the toxicity of the used catalyst.

Why is it important to minimise ammonia “slip” from the SCR process for NOx control?

The occurrence of ammonia slip results in inefficient use of feedstock. Costs can be notably reduced by ensuring minimum ammonia slip throughout operation. Ammonia may also react with other compounds to form unwanted products. For example, ammonium bisulphate formation in power industry often results in damage to air heaters. Therefore, this should be further investigated for the application of nitric acid production to ensure any unwanted compounds are not formed. Further to this, as ammonia is toxic, any ammonia slip would result in release to atmosphere; therefore potential health concerns exist.

Balancing environmental and economic costs

What would be your answer to the above question on the comparison between SCR and NSCR?

From table 2, it is evident that SCR can provide an improved NOx removal (exit level of 100 ppm) in comparison to NSCR (205 PPM); however, SCR’s failure to remove N2O may be a key concern when compared to NSCR which does remove the compound. However, unlike NSCR, SCR does not result in significant CO2 emissions. The economics of both options demonstrate that when combined with EA, NSCR is significantly more expensive than SCR and only provides an improvement of 0.3 %. Therefore SCR would be chosen over NSCR.

Analyse the data shown in Tables 3 and 4 and make your own choice of BAT for NOx prevention and control. Explain and justify your choice.

The significant costs of NSCR in comparison to SCR would result in increased nitric acid prices, to the extent where it would not be profitable to manufacture. The small gains in removal efficiency made using NSCR over SCR with EA do not justify the large difference in price; therefore, the choice of BAT is between EA and EA with SCR. With EA adding a cost £ 230 per tonne of acid, and EA+SCR adding a cost of £ 880 per tonne of acid, from an economic perspective it would be most suitable to chose EA due to its high NOx removal of 94.8 %. However, the improved removal efficiency of SCR (98.5%) does allow for an argument to be made for its choice as BAT. With regards to cost, EA+SCR does provide increased costs and lower profits; however, its ability to sufficiently meet IPPC targets cannot be overlooked, thus for this reason, it should be chosen as BAT.

The social implications of the pollution prevention and control techniques have not been considered above. Can you identify them for each option? Do the social considerations change your choice of BAT?

Both EA and EA+SCR share common social implications such as the construction of new plant absorption column for EA, and a reactor for SCR. This involves construction vehicles travelling through the area, resulting in increased noise pollution. Other social impacts of EA are minimal as an increased compression requirement may result in slightly increased noise pollution. If energy is generated on-site for the compressors, then higher greenhouse gas emissions may be a concern. With regards to SCR, ammonia slip is a key concern, as the gas is toxic when inhaled. Further to this, the energy required for gas pre-heating also contributes to increased greenhouse gas emissions. Overall the EA option proves more suitable in terms of reducing social impacts and so this would be the option for BAT.

Choosing BAT

Why do you think the company has chosen to consider these two options and not any other described above?

Choosing to modify the absorption column in order to operate at higher pressures would be a lot cheaper than employing the extend absorption option which would require the construction of an additional column, thus resulting in increased capital and operational costs. In addition to this, this option would be chosen over the NaOH absorption process, again due to the increased capital costs resulting from an additional absorption tower and the increased operational costs incurred from a NaOH feed. Increasing operating pressure does not have any significant on-site environmental releases/impacts other than an increased compression duty. SCR may have been chosen due to its ease to be retrofitted and its high removal efficiency. In comparison to other options, SCR provides the best performance for NOx removal.

Choosing BAT: Environmental considerations

Consider the LCA results shown in Figure 5 and answer the following questions:

Why do you think the SCR option has higher fossil fuel and ozone depletion than the base case?

The requirement of natural gas for the production of ammonia is likely to be the primary cause of increased fossil fuel depletion for SCR. NOx formed during ammonia production may also be a cause of the increased ozone depletion associated with SCR. In addition to this, ammonia slip may result in ammonia being released to atmosphere which then reacts with ammonia to produce ammonium nitrate.

The fact that SCR is better for some impacts but worse for the others when compared with the base case makes it more difficult to chose BAT. If you as a plant operator had to choose between these two options, which one would you choose? Justify your choice by discussing the ‘significance’ of global impacts (such as ozone depletion and fossil fuel depletion) and regional and local impacts (such as acidification and photochemical smog).

In comparison to the base case, SCR only has a slightly higher fossil fuel and ozone depletion; whereas other impacts such as acidification and human toxicity are significantly reduced through using SCR. As efficiencies are made in the ammonia production process, SCR could potentially have a reduced impact on fossil fuel depletion. In addition to this, the production of ammonia using energy from renewable sources is also an option to reduce fossil fuel depletion, as well as the ability to generate hydrogen from the electrolysis of water using renewable energy.

Based purely on environmental considerations, which process out of the three options (base case, HP and SCR) would you choose as BAT? Explain why.

Based only on the environmental considerations, HP demonstrates BAT. In comparison to the base case and SCR, HP has the least environmental impact for fossil fuel depletion, global warming, ozone depletion, acidification, photochemical smog, and human toxicity.

Compare now the SCR and HP options in terms of the level of NOx emissions that they can achieve. Which process would you as an operator choose? Why?

The HP modification has a removal efficiency of 84%, removing 1202 ppm of NOx; however, SCR has a greater removal efficiency of 93%, removing 1332 ppm of NOx. As the base case plant produces 1432 ppm of NOx per hour, the outlet concentrations for the HP and SCR modifications are 230 ppm and 100 ppm, respectively. SCR’s effectiveness in removing NOx is notably greater than that of the HP option; therefore, from an operational point of view, SCR would be chosen.

Combine both the environmental impacts and the levels of NOx emission that each option can achieve and make an overall choice of BAT. Justify your choice by taking into account the IPPC principles.

Both options satisfy the IPPC principles to a certain extent. However, HP has greater compliance than SCR with the IPPC principles. HP and SCR both provide protection for the environment as a whole by reducing NOx emissions from the nitric acid plant. Although, SCR has improved removal efficiency over HP, the impact of ammonia production fails to demonstrate SCR as the BAT in comparison to HP in this case. HP provides a better case for pollution prevention, by reducing NOx formation throughout the process; whereas SCR demonstrates a case for pollution control. As prevention is preferred to control, HP again provides the better option. In providing a balance between the environment, economics and social impacts, HP provides the better option. Significant reductions in global warming potential, acidification and human toxicity are made through employing HP over SCR, with notable gains in reducing fossil fuel depletion, ozone depletion and photochemical smog. Although SCR demonstrates an improved NOx removal efficiency, overall, the environmental impact as a whole can be reduced by employing HP modifications. Further to this, the income generated by HP modifications, can contribute to further plant modifications leading to improved process efficiencies. Therefore, considering its wider compliance with IPPC principles HP would be chosen as BAT.

Choosing BAT: Socio-economic considerations

Choose your preferred NOx prevention or control option considering the internal (MAC) and external costs (MDC) shown Tables 6 and 7. Justify your choice.

With regards to the marginal abatement costs (MAC) for NOx emissions, the HP option clearly proves to be the more attractive option due to the savings made ( MAC = £ -92 per ppm). The surplus energy generated and increase in acid produced leads to improved revenue which consequently reduces the operating cost of the plant. However, SCR increases operating cost by £ 225 per ppm of NOx removed. In order to asses if SCR’s improved removal efficiency can justify such a cost, the marginal damage cost of both options was compared. The results indicate that MDC due to NOx emissions was lower for SCR (£ 35,770 per year) than for HP (£ 82,320 per year). However, the MDC fails to consider environmental impacts other than NOx emissions. If costs were considered for the impacts of ammonia production, it is likely that the MDC for SCR would be much higher. Considering this, HP was chosen as the best option.

Choosing BAT: The whole picture

List all criteria relevant for choosing BAT in the above case study. On the basis of the results obtained, rank the three options in order of their desirability. You may wish to create a ranking table, assigning a number to each technology to indicate the order of preference on a scale from 1-3 (e.g. number 1 indicates the best option and 3 the worst); this ranking should be done for each criterion you have listed.

NOx removal efficiency/Marginal Damage Costs

SCR

HP

Base Case

Marginal Abatement Costs

HP

Base Case

SCR

Life-Cycle Environmental Impacts

HP

SCR

Base Case

Compliance with IPPC Principles

HP

SCR

Base Case

Identify BAT for the conditions of this case study, assuming that you are Head of Engineering and Technology Division in a company. Justify your choice.

Based on the criteria listed in question 1, HP demonstrates the most suitable option for IPPC compliance, reducing overall environmental impacts as well as providing savings in operating costs. Although SCR has a greater NOx removal efficiency, HP is consistently and in some cases significantly better than SCR in reducing environmental impacts such as fossil fuel depletion, global warming, ozone depletion, acidification, photochemical smog, and human toxicity. In addition to this, choosing HP not only reduces the plant’s environmental impact, but allows for improved energy generation onsite. Therefore, gains in efficiency can be made, reducing reliance on volatile energy prices. The HP option does not require additional feedstock, unlike SCR’s ammonia requirement, and is therefore not as directly exposed to price fluctuations. Thus, the option of HP modification is econo