Nox Emissions and their Impact

Emissions of oxides of nitrogen, commonly referred collectively as NOx, are regulated because of their adverse effects on health and the environment. They play an important role in acid rain, the formation of harmful ozone and photochemical smog in the lower atmosphere and the depletion of the beneficial ozone in the upper atmosphere.

The most environmentally important oxides of nitrogen are:

NO, NO2, and, more recently, N2O.

N2O is a recent concern because it is a "greenhouse" gas which contributes to global warming and because it can aid in destroying the upper atmosphere ozone layer which protects us from ultraviolet radiation. Fortunately, very little N2O is emitted from the flame of a typical burner.

Over 90% of the NOx from a typical flame is in the form of NO and the remainder is NO2. However, since NO is eventually converted to NO2 in the atmosphere, most regulations treat all of the NOx as NO2.

NOx emissions from combustion sources are due to the oxidation of atmospheric N2 and the oxidation of nitrogen chemically bound in fuel molecules. Mechanisms for forming NO include the fuel NOx, prompt NOx and thermal NOx mechanisms. Some of these mechanisms are well understood, while others are still under investigation.

Although NO and NO2 molecules last only a matter of days in the atmosphere, N2O is a very stable species that can last 100 to 200 years in the lower atmosphere. Because of its long life span some N2O eventually reaches the upper atmosphere where it decomposes under ultra violet light and it's reaction products efficiently remove ozone from the upper atmosphere.

Heat transfer fluid contained in the collector loop

 Heat transfer fluid contained in the collector loop are :

 

WATER. As a heat transfer fluid, good quality water offers many advantages. It

is safe, non-toxic, chemically stable, inexpensive, and a good heat transfer medium.

Two drawbacks include a relatively high freezing point and a low boiling point.

Excessive scaling may occur if poor quality water is used.

 

GLYCOLS. Propylene or ethylene glycol is often mixed with water to form an

antifreeze solution. Propylene glycol has the distinct advantage of being nontoxic,

whereas ethylene glycol is toxic and extreme caution must be used to ensure that it is isolated from any potable water. For this reason, uninhibited USP/food-grade propylene glycol and water solution will be specified for any solar preheat system that requires an antifreeze solution.

Agnibaan SorTed Technology Brief

Linear Fresnel Solar Collector

 

India likely to add 30-34 GW of Renewable Energy

COLLECTOR EFFICIENCY AND PERFORMANCE

 

Collector efficiency is defined as the fraction of solar energy

incident upon the face of the collector that is removed by the fluid circulating through the collector. Several parameters are defined as follows:

Ti = heat transfer fluid inlet temperature

Ta = ambient air temperature

I = solar irradiance on the collector

Ac = solar collector surface area

FR = collector heat removal factor, a dimensionless parameter describing the ratio

of actual energy gained by the collector to that which would be gained, in

the limit, as the absorber plate temperature approaches the fluid inlet

temperature. This value is similar to a conventional heat exchanger's

effectiveness.

UL = overall heat loss coefficient. This factor describes the cumulative heat

transfer between the collector and the ambient surroundings.

t = transmittance of the glazing.

a = absorption coefficient for the absorber plate. Note that this value varies with

wavelength. A selective surface is one that absorbs short wavelength solar

radiation very well while emitting longer wavelength thermal radiation poorly.

Calculation of turbine efficiency and Rankine cycle efficiency for the given plant.

 

Steam Turbine efficiency

 

Efficiency= Energy output/ Energy input

 

Heat input = Steam inlet flow in TPH * Enthalpy of inlet steam (Kca/Kg)

                = 168*811

                = 136248 Kcal

 

Heat out put  = Power in MW* Heat equivalent of Power

                =30*860

                =25,800 Kcal

 

 

Efficiency of Steam Turbine =   Heat output/ Heat input

                                             = 25,800/136248

                                              = 18.93%

 

 

 

Rankine cycle efficiency for given plant:

 

W1=30 MW

 

W2= 1MW (assumed)

 

Q1= Heat input to a Boiler

     =m*(h1-h4)

     =336*(811-150)   (where 336 is total steam generation in Boilers)                                                     .    =222096

     =246773 Kcal(assuming Boiler efficiency of 90%) .

 

In a power plant that produces both Steam and power, we must give credit to Steam export also

 

Useful output as steam export= Steam export flow*Heat value (Kcal/Kg) 

                                            =150* 760

                                            =114000 Kcal                                                      

 

Efficiency of cycle is = ((W1-W2)+( Heat value of export steam)) /Q1

                                 =((60-2)*860)+114000)/ (246773) 

                                  =66.40%    

Transpired collectors

Transpired collectors use solar energy to preheat ventilation (outdoor) air as it is drawn into a building. The technology is ideally suited for buildings with at least moderate ventilation requirements in sunny locations with long heating seasons. Transpired collector technology is remarkably simple. A dark, perforated metal wall is  installed on the south-facing side of a building, creating approximately a 6-inch (15-cm) gap between it and the building’s structural wall. The dark colored wall acts as a large solar collector that converts solar radiation to heat. Fans associated with the building’s ventilation system mounted at the top of the wall draw outside air through the transpired collector’s perforations, and the thermal energy collected by the wall is transferred to the

air passing through the holes. The fans then distribute the heated air into the building through ducts mounted from the ceiling. By preheating outdoor air with solar energy, the technology removes a substantial load from

a building’s conventional heating system, saving energy and money.

A transpired collector is installed on all or part of a building’s south-facing wall, where it will receive the  maximum exposure to direct sunlight during the fall, winter, and spring. The size of the wall varies depending on heating and airflow requirements and climate, but in many applications, the transpired collector will cover the maximum south-facing area available. The amount of energy and money saved by a transpired collector depends on the type of conventional fuel being displaced, occupant use patterns, building design, length of heating season, and the availability of sunlight during the heating season. In general, each square foot of transpired collector will raise the temperature of 4 cubic feet per minute (cfm) by as much as 40°F, delivering as

much as 240,000 Btu annually per square foot of installed collector.

In addition to the metal sheeting that captures solar energy, the transpired collector heating system includes air-handling and control components that supply the solar heated air. The ventilation system, which operates independently of a building’s existing heating system, includes a constant-speed fan to draw air through the

transpired collector and into the distribution duct. Engineers typically use a 3-horsepower, 32-inch blade fan with about 10,000-cfm capacity.

Source: CED Engineering

 Where to Apply

The following are the most common transpired collector applications:

• Manufacturing plants

• Vehicle maintenance facilities

• Hazardous waste storage buildings

• Gymnasiums

• Airplane hangars

• Schools

• Warehouses requiring ventilation

 

What to Avoid

The following is a list of general applications and conditions that preclude the cost effective use of transpired collector technology:

• Outdoor air not required

• Shaded or insufficient south-facing wall area

• Buildings with existing heat recovery systems

• Locations with short heating seasons

• Multiple-story buildings (because of possible problems with fire codes).

SOLAR COLLECTORS and TYPES

 SOLAR COLLECTORS

A solar collector is a device that absorbs direct (and in some

cases, diffuse) radiant energy from the sun and delivers that energy to a heat transfer

fluid. While there are many different types of collectors, all have certain functional

components in common. The absorber surface is designed to convert radiant energy

from the sun to thermal energy. The fluid pathways allow the thermal energy from the

absorber surface to be transferred efficiently to the heat transfer fluid. Some form of

insulation is typically used to decrease thermal energy loss and allow as much of the

energy to reach the working fluid as possible. Finally, the entire collector package must

be designed to withstand ambient conditions ranging from sub-zero temperatures and

high winds to stagnation temperatures as high as 350 degrees F (177 degrees C).

 

COLLECTOR TYPES. The three major categories that have been used most

often are flat-plate glazed collectors, unglazed collectors, and evacuated tube

collectors. A general description of each collector type and its application is given

below.

FLAT-PLATE. Flat-plate solar collectors are the most common type used and

are best suited for low temperature heating applications, such as service water and

space heating. These collectors usually consist of four basic components: casing, back

insulation, absorber plate assembly, and a transparent cover. The absorber panel is a

flat surface that is coated with a material that readily absorbs solar radiation in the

thermal spectrum. Some coatings, known as "selective surfaces", have the further

advantage of radiating very little of the absorbed energy back to the environment.

Channels located along the surface or within the absorber plate allow the working fluid

to circulate. Energy absorbed by the panel is carried to the load or to storage by the

fluid. The absorber panel is encased in a box frame equipped with insulation on the

back and sides and one or two transparent covers (glazing) on the front side. The

glazing allows solar radiation into the collector while reducing convective energy losses

from the hot absorber plate to the environment. Similarly, back insulation is used to

reduce conductive energy loss from the absorber plate through the back of the collector.

UNGLAZED. Unglazed collectors are the least complex collector type and

consist of an absorber plate through which water circulates. This plate has no glazing or

back insulation. These collectors are often made of extruded plastic because they are

designed to operate at relatively low temperatures. Since they are not thermally

protected, these collectors should be operated only in warm environments where lower

thermal losses will occur. Swimming pool heating is the most common use of unglazed collectors.

EVACUATED TUBE. Evacuated tube collectors are best suited for higher

temperature applications, such as those required by space cooling equipment or for

higher temperature industrial process water heating. Convective losses to the

environment are decreased in this type of collector by encapsulating the absorber and fluid path within a glass tube that is kept at a vacuum. Tracking mechanisms and/or parabolic solar concentrating devices (simple or compound) are often used, resulting in somewhat higher equipment costs.

COLLECTOR ENERGY BALANCE.

 The collector parameters described

above allow an energy balance to be expressed as:

Energy Collected = Solar Energy Absorbed - Thermal Energy Losses to the

Environment

The energy balance can be written in a simple equation form using the efficiency

parameters described above:

Energy Collected = (FRta)(I)(Ac) - (FRUL)(Ac)(Ti - Ta) (Eq. 1)

Equation 1 shows that heat losses to the environment are subtracted from the net solar

radiation transmitted into, and absorbed by, the collector. Assuming that the efficiency

parameters are fixed for a given collector model, the main factors that affect the amount

of energy collected are I, Ti, and Ta. The geographical location and the season dictate

the weather variables I and Ta. The type of process load and system configuration

determines the relative circulation fluid temperature, Ti.

Typical collector efficiency curve

Typical solar collector efficiency plots

Why Industries set up their own power plants

 All the major basic process industries like Refinery, Petrochemical plants, Cement Industries,  Fertilizer industry , Steel manufacturing industry, Textile Industry, Paper Industry and Sugar industry etc need  huge quantity of Electric Power and Steam for their functioning.

 Power is required for various following purposes.

• Mainly for running equipment like Pumps, Compressors, Blowers etc. (90%)
• Other associated uses like Lighting, heating, air-conditioning etc.(10%)

 Steam is required for following purposes:

• Heating the process fluids (80%)
• Running turbines
• Flushing, sealing, ejectors etc

To meet this power and steam requirement they generally set-up their own Power Plants which produce steam along with power.

Most of the state electricity boards already have shortage in power supply.

Even when power is available, it is not reliable. These industries need continuous power because any stoppage results in huge economic loss and also damage to the equipment.

 

Typical Fuel quality requirements for a Gas Turbine:

A separate starting fuel is required for applications with single oil conventional burner if the fuel viscosity is above 4.1 mm²/s at 40°C.

The viscosity of the fuel at the injectors must not exceed 6.0 mm²/s (2nd gen. DLE burner) or 7.0 mm²/s (conventional burner) for good atomisation. A fuel with a higher viscosity than 6.0 respectively 7.0 mm2 /s, at the lowest temperature the fuel system may experience, requires preheating and trace heating of fuel piping in order to achieve correct viscosity at the fuel injectors.

Fuels with viscosity less than 2.0 mm2/s at highest fuel operating temperature, such as naphtha or kerosene, require dosage of anti-wear additives in order to prevent pump failures. 

The supply temperature of minimum 5°C prevents eventual free water from freezing in pipes and valves. The supply temperature shall also always be higher than the cloud point.

Fuels that are handled at temperatures close to or above the flash point require explosion proof equipment and a separate starting fuel. Special safety regulations will apply which vary from country to country.

The fuel must be kept at least 10°C above the cloud point.

In the specification polyaromatic compounds are defined as tricyclic compounds and higher, i.e. no di-aromatics included.

For fuels with higher water and/or sediment content separators should be installed.

Generally less than 0.5 ppm(w) heavy metals is required in order to conform to the condition for the ash sticking point. Somewhat higher contents can often be handled by employing corrosion inhibition additives. The quantity of the additive shall be enough to keep the ash sticking temperature above 950°C. However, a high content of additives requires additional cleaning of the turbine. Siemens will advise on equipment, additive type and dosage.

Generally less than 0.5 ppm(w) sodium + potassium is required in order to conform to the condition for the ash sticking point. Somewhat higher concentrations of sodium, potassium and calcium can be reduced to acceptable limits by fuel washing and/or separation. However, the washability of the fuel has to be checked. Siemens will advise on equipment.

The use of a fuel having an ash sticking point temperature below 950 °C introduces a great risk of high temperature corrosion for the gas turbine and must not be permitted even for a short period of time. The sticking point temperature can be raised by additive treatment of the fuel. Any such additive must be approved by Siemens 

NOx CONTROL

The major contributors to NOx emissions are thermal NOx and, if fuel bound nitrogen is present, fuel NOx. Most refinery process heaters in the US are fueled by refinery fuel gas and, thus, thermal NOx is the primary concern. As noted previously, thermal NOx is strongly influenced by peak flame temperatures and the key to controlling thermal NOx is to moderate peak flame temperatures.

Historically, thermal NOx control techniques have included excess air control, air or fuel combustion staging and flue gas recirculation. Low excess air operation provides only limited benefit. However, it is compatible with, and can be used together with, most of the other NOx control techniques. Combustion staging and flue gas recirculation have proven to be more beneficial. The combination of fuel staging and flue gas recirculation has proven to be the most beneficial combination until recent developments. Low NOx industrial burners have been developed utilizing these NOx control techniques

OPERATING PARAMETERS AFFECTING NOx

 Thermal NOx is the major source of NOx from the combustion of gaseous fuels. The parameters that influence the oxygen concentration in the flame zone or the temperatures achieved in the flame zone will affect thermal NOx emissions. The most important parameters are:

• Excess Air

• Fuel Composition

• Air Preheat Temperature

• Furnace Temperature

Excess Air

Excess air provides for additional oxygen beyond the stoichiometric air requirement and is generally required to minimize the emissions of CO and unburned hydrocarbons. It accomplishes this, however, by increasing the concentration of oxygen in the flame zone, which tends to increase NOx. Excess air also decreases the overall flame temperature and contributes to a loss in thermal efficiency. Figure 5 shows the effect of excess air, expressed as percent excess oxygen, on NOx emissions.

As the excess air is steadily increased the reduction in NOx due to the reduction in flame temperature finally overcomes the increase in NOx due to oxygen concentration and the NOx emissions peak. Further increases in excess air then reduce NOx emissions.

Fuel Composition

Fuel composition influences thermal NOx because of its direct effect on flame temperature. Different fuels are capable of achieving different flame temperatures and the maximum potential flame temperature for a fuel is best defined by the adiabatic flame temperature. The adiabatic flame temperature is the theoretical temperature attained when a fuel/air mixture is burned to completion and all of the sensible and chemical energy of the reactants is transferred to the products of combustion.

The variation in flame temperature with composition is apparent from the table, ranging from 3334oF for methane to 3652oF for hydrogen. Although practical flames transfer heat away from the flame zone, the adiabatic flame temperature provides a good method for evaluating the potential effect of fuel gas composition on flame temperatures and, therefore, the potential effect on thermal NOx emissions.

Air Preheat Temperature

Air preheat affects thermal NOx by its direct effect on flame temperature. Preheating the combustion air adds sensible heat to the flame reactants which increases the heat in the products of combustion and, thus, increases the flame temperature. Note that the NOx essentially follows an exponential increase with increasing air preheat temperature. A reasonably good rule of thumb from industrial experience is that the thermal NOx emissions will double as the combustion air temperature is increased from ambient to about 500 to 600 oF.

Furnace Temperature

Furnace temperature affects thermal NOx emissions by its effect on the rate of heat transfer from the flame and, as a result, it influences the actual temperatures attained within the flame zone. The lower the furnace temperature, the higher the heat transfer rate from the flame and the  lower the actual peak flame temperatures within the flame zone. Lower peak flame temperatures mean lower thermal NOx emissions.

Solar cooling systems

Solar cooling systems are attractive because cooling is most needed when solar energy is most available. If solar cooling can be combined with solar heating, the solar system can be more fully utilized and the economic benefits should increase. Solar cooling systems by themselves, however, are usually not economical at present fuel costs. Combining solar heating and cooling systems is not easy because of the different system requirements.

ABSORPTION COOLING. 

Absorption cooling is the most commonly used method of solar cooling. An absorption refrigeration machine is basically a vapor-compression machine that accomplishes cooling by expansion of a liquid refrigerant under reduced pressure and temperature, similar in principle to an ordinary electrically operated vapor-compression air conditioner. Two refrigerant combinations have been used: lithium bromide and water, and ammonia and water. There have been a number of proposed solid material absorption systems also.

In the absorption cooler, heat is supplied to the generator in which a refrigerant is driven from a strong solution. The refrigerant is cooled in the condenser and allowed to expand through the throttling valve. The cooled, expanded refrigerant receives heat in the evaporator to provide the desired cooling, after which the refrigerant is reabsorbed into the cool, weak solution in the absorber. The pressure of the resulting strong solution is increased by pumping and the solution is available to repeat the process.

The performance of the system is governed largely by the temperature difference between the generator and the condenser and absorber units. Since the generator temperatures in solar driven systems are only moderate, it is important to keep the condenser and absorber temperatures as low as possible. The LiBr system is preferred over ammonia systems for solar energy applications because of the lower generator temperatures required. Permissible generator temperatures for a water-cooled LiBr system range from 170 deg. F to 210 deg. F (76 deg. C-99 deg. C) compared to the 205 deg. F to 248 deg. F (95 deg. C-120 deg. C) temperatures required for a water-cooled ammonia absorption system. Most, if not all, of the commercially available absorption units use LiBr and water as the absorbent-refrigerant fluid pair. Because the LiBr will crystallize at the higher absorber temperatures associated with air cooling, these units must be water cooled. A prototype ammonia-water unit, amenable to direct air cooling, has been built by Lawrence Berkeley Laboratories.

Parabolic trough systems concentrate solar radiation, specifically direct normal insola(DNI), onto a receiver tube located along the focal line of a single-axis tracking parabolically curved, trough-like reflector. Heat transfer fluid flowing through the receiver tube absorbs the thermal energy. The heat is collected and used to generate steam which is produces electricity by a Rankine cycle turbine-generator. Troughcan be hybridized (natural gas can be burned to produce steam when the sun isn’t shining) or can use thermal storage to dispatch power to meet utility peak load requirements.

The operating temperature of trough plants is limited by the thermal property of the heat transfer fluid (HTF) that is suitable for pumping through miles of piping in the solar fIn typical applications, oil flowing through the receiver tube is heated to about 390°C and used to boil water to produce steam. The resulting steam is used in a Rankine power cycle and expanded through a turbine connected to an electric generator. As with any steam cycle, the exhaust steam is cooled and condensed back to liquid water to be recirculated in the cycle. The condensers can be either water-cooled or air-cooled, or a hybrid combination.

The IEA’s 7 key recommendations to scale up hydrogen

 1. Establish a role for hydrogen in long-term energy strategies. National, regional and city governments can guide future expectations. Companies should also have clear long-term goals. Key sectors include refining, chemicals, iron and steel, freight and long-distance transport, buildings, and power generation and storage.

2. Stimulate commercial demand for clean hydrogen. Clean hydrogen technologies are available but costs remain challenging. Policies that create sustainable markets for clean hydrogen, especially to reduce emissions from fossil fuel-based hydrogen, are needed to underpin investments by suppliers, distributors and users. By scaling up supply chains, these investments can drive cost reductions, whether from low-carbon electricity or fossil fuels with carbon capture, utilization and storage.

3. Address investment risks of first-movers. New applications for hydrogen, as well as clean hydrogen supply and infrastructure projects, stand at the riskiest point of the deployment curve. Targeted and time-limited loans, guarantees and other tools can help the private sector to invest, learn and share risks and rewards.

4. Support R&D to bring down costs. Alongside cost reductions from economies of scale, R&D is crucial to lower costs and improve performance, including for fuel cells, hydrogen based fuels and electrolysers (the technology that produces hydrogen from water). Government actions, including use of public funds, are critical in setting the research agenda, taking risks and attracting private capital for innovation.

5. Eliminate unnecessary regulatory barriers and harmonize standards. Project developers face hurdles where regulations and permit requirements are unclear, unfit for new purposes, or inconsistent across sectors and countries. Sharing knowledge and harmonizing standards is key, including for equipment, safety and certifying emissions from different sources. Hydrogen’s complex supply chains mean governments, companies, communities and civil society need to consult regularly.

6. Engage internationally and track progress. Enhanced international co-operation is needed across the board but especially on standards, sharing of good practices and cross border infrastructure. Hydrogen production and use need to be monitored and reported on a regular basis to keep track of progress towards long-term goals.

7. Focus on four key opportunities to further increase momentum over the next decade. By building on current policies, infrastructure and skills, these mutually supportive opportunities can help to scale up infrastructure development, enhance investor confidence and lower costs:

• Make the most of existing industrial ports to turn them into hubs for lower-cost, lower-carbon hydrogen.

• Use existing gas infrastructure to spur new clean hydrogen supplies. 

• Support transport fleets, freight and corridors to make fuel-cell vehicles more competitive.

• Establish the first shipping routes to kick-start the international hydrogen trade.

Solar collector- Parabolic Trough

 

Parabolic trough system schematic

 

Schematic Diagram of Hybrid Heat Rejection System

 

Schematic Diagram of Dry Heat Rejection System