Combustion engineering
Approximately how large is limit layer for a spherical particle?
~ radius of the particle.
What will happen with the equilibrium if we increase the pressure?
According to Le Chatelier's principle, the equilibrium will counteract the change. Thus, an increase of pressure will result in that the equilibrium wants to decrease the pressure and thus shifting the equilibrium to the side which has the lowest amounts of moles.
What will happen with the equilibrium if the increase the temperature?
According to Le Chatelier's principle, the equilibrium will counteract the change. Thus, an increase of temperature will result in that the equilibrium wants to decrease the temperature and thus, shifting the equilibrium to the side which is endothermic. (heat is then consumed -> lowered temp)
How is a general conservation equation written?
Accumulation + convective transfer = diffusive transfer + source term
What is the meaning of Bi?
Bi - Biot number - Compares the external transport with the internal (Exist both mass and heat Biot numbers) High Bi - The external transport is dominating
Why is it not worth it to transport biomass long distances?
Biomass contains a lot of water which both increases its weight and lowers the heating value. However, as Margareta mentioned, it is worth it is the biomass is shipped by boat.
What kind of boundary conditions are usually used?
Dirichlet BC: A value at the boundary is given, X = Y Neumann BC: A normal gradient of the value is given, dX/dz = Y
What does dissociation mean?
Dissociation happen when the products of a reaction dissociate into other components. A dissociation reaction is an unimolecular reaction (only one reactant).
What is the evaporation depending on?
It can either be based on mass or heat, or a combination.
What is a proximate analysis for a solid fuel? What is an ultimate analysis?
It gives a rough estimation of the fuel by the content of moisture, volatile combustible matter, fixed carbon and ash in a fuel. The ultimate analysis provides the major elemental composition (C, H, O, S, N and Cl) of the fuel, reported on a dry and ash-free basis.
Describe the ideal thermodynamic cycle of a GT
Compressor -> Combustion chamber -> Turbine Assumptions: * The compression and expansion processes are isentropic (adiabatic and reversible) * No pressure losses in the components, which means that the pressure ratio over the compressor equals the compressor ratio over the turbine. * The ideal gas properties are assumed for the whole cycle * Constant mass flow throughout the cycle, which means that the fuel addition in the combustor is neglected.
How can you use the Dahmköhler number to distinguish between two cases of devolatilization?
Da is used to determine if the devol. is controlled by the heating rate of the particle or the rate of the kinetics. For Da <<1 the process is controlled by kinetics. For Da>>1 the process is controlled by the heating rate.
Define the smoke point for a liquid fuel.
The smoke point measures the tendency of a liquid fuel to form soot. The higher the smoke point, the lower is the tendency of the fuel to from soot.
When a solid fuel is combusted, what happens?
The solid fuel is injected in the form of a powder, heat is supplied by externally recirculated hot gases and by radiation from the flame. The heat will release the volatiles and ignite them. This will increase the temperature and trigger the char combustion.
How is a swirl burner designed?
(See Julias document for picture) Swirl burners are stabilized by rotation of the air by a set of vanes, which serve as a swirl generator. The upper picture shows a flame stabilized by a flameholder without swirl. Both methods create an inner recirculation vortex bringing hot gas to the injected fuel for ignition. The swirl widens the flame and creates a circulation flow in front of the burner, internal gas combustion. The external gas recirculation to the flame consists of partly burned gases from the surrounding combustion chamber.
How is the GT combustor built?
(See Julias document for picture) The length of the combustor is set by: * Residence time to evaporate the fuel * Ensure appropriate mixing * Ensure complete combustion The primary zone anchors the flame and provide sufficient time, temperature and turbulence for combustion. The primary zone operates at stoichiometric conditions and produces large amount of NOx due to the high temperature. In the secondary zone, the temperature is reduced by addition of air, which will burn out the soot, CO and other HC. In the third zone, the remaining air is added, which gives a temperature of the flue gases acceptable to be inserted in the turbine. One wants to increase the area at the inlet to reduce the velocity and thereby the pressure losses. It is also desirable to create circulation in the primary zone to reduce the velocity in order to create a flame. Secondly, it is also important to get an appropriate air to fuel ratio, which is achieved by adding only a part of the air in the primary zone and the rest in the secondary and dilution zone. Adding the rest of the air in the secondary zone and dilution zone will reduce the temperature to the turbine which is good due to material restrictions.
How is the jet burner designed?
(see figure in Julias document) The simplest arrangement for burning powders is by direct injection of a fuel-air mixture. The hot surrounding gases, and radiation from the flames, will ignite the mixture when the jet velocity has reached a sufficiently low level. In this type of jet burner, the jet velocity could be chosen so that ignition takes place at some distance from the nozzle. Another way: Fuel and air are injected separately through nozzles. In one type of design, the powder is injected together with some air to transport the powder in "ring" surrounding the primary air, and gas from the surrounding combustion chamber is mixed through the fuel layer.
Name some primary measures for NOx reduction.
* Air staging * Fuel staging (reburning) * Flue gas recirculation; exhaust gases are recirculated to decrease the combustion temperature and to utilize reburning. * Low excess air ratio: Limit the available oxygen that can react with nitrogen to NOx
How can the CO2 be transported?
* CO2 is compressed to super-critical conditions * Transported in pipelines or in ships
What three combustor types exists for a GT?
* Can: - Mostly used in industrial gas turbines since it is easy to maintain - Heavy - Used where maintenance is hard, e.g. in remote places * Turbo annular - A common air casing - More compact than a can type, lower weight - Common in industrial gas turbines - Robust - Easy to maintain * Annular - Difficult to obtain an even outlet temperature distribution - Compact - Even flow distribution
What characterizes a turbulent flow?
* Chaos * Unstructered flow * Fluctuating * Y-transport: advective * Has a laminar sublayer close to the plate, due to the non-slip condition(i.e. no velocity near the wall -> laminar region)
What are the two ways of spontaneous ignition?
* Chemical chain ignition * Thermal ignition
What are the arrangement for grate firing?
* Co-current * Counter-current * Cross-current
Describe fuel-N mechanism
* Dominating formation route in coal flames * Evolves form nitrogen contained in char and volatiles, which are formed during combustion
Describe thermal NO mechanism
* Free nitrogen (air borne) that reacts mainly with oxygen during high temperatures * Main reaction path in natural gas and liquid fuel combustion
What does the quenching distance depend on?
* Geometry of the walls ( mainly due to heat transfer and not diffusion of species) * Fuel type * Stoichiometry * Pressure * Reactant temperature * Turbulence
Prompt NO mechanism
* HC first react with the nitrogen under fuel-rich conditions * The nitrogen containing HC can then reactor to NOx * In general, of little importance
The combustion chamber of a boiler serves two purposes, which?
* It accommodates the combustion process with the aim of achieving complete combustion under the condition of minimizing harmful emissions. * It receives some of the heat released from combustion by heat transfer to the heat carrier and cools the combustion gases to a temperature which is sufficiently low for the ashes to have solidified before leaving the combustion chamber
What are some factors that decides if thermal ignition is possible or not?
* Lean limit - low concentration of oxygen * Rich limit - The adiabatic flame temperature decreases below the critical temperature * Temperature - At low T, no mix of fuel/oxidizer is enough to ignite
From where does the losses arise from in a real GT?
* Losses in compressor, combuster and turbine -> P is not constant, this i due to irreversibility in compressor and turbine. * Losses will give a larger work input in the compressor and a lower work output in the turbine * By material improvements, it is possible to have higher efficiency
What characterizes a laminar limit layer?
* Order * Streamlined flow * Non-fluctuating * Y-transport: diffusive
What are some measures to reduce CO2?
* Replacement of fossil fuels with other energy sources(renewables) *Reduce primary energy demand *Capture and storage of the CO2 formed from the combustion of fossil fuels. * Increase efficiency of conversion and use.
Name some secondary measures for NOx reduction
* SCR - selective catalytic reduction *SNCR selective non-catalytic reduction
Name som factors that influence the combustion of solid fuels.
* Size distribution of the fuel. The smaller the fuel, the thinner limit layer, which will increase the mass and heat transfer, which in turn will decrease the time for combustion. Different sized of fuel will give different times for combustion, smaller will have shorter time and larger a longer time. This can lead to that the depth of the furnace is not deep enough for the char to combust -> create layers (thus reducing transport). On the other hand, the depth might be too large, so the smaller particles are converted at half the depth, which can create cold zones (unbalances). * Properties of the fuel, especially the volatiles content. The ignition of volatiles promotes the ignition of char by creating a high temperature, so high volatile fuels are easier to ignite than low volatile fuels. * The burner design and the interaction with the combustion chamber (stabilization of flames)
What are the storage alternatives for CCS?
* Storage in aquifers * Gas fields * EOR * Mineral carbonation (reaction of CO2 with magnesium silicates to form carbonates)
Describe some measures that can be used to reduce the SOx emissions.
* The fuel can be pretreated to reduce the S-content. * Post combustion flue gas cleaning by: - Wet system: *Water-based sorbent slurries in scrubbers * Lime/limestone scrubbing most common process with gypsum as a bi-product - Dry system * Sorbent is directly mixed with SO2 -> no commercial bi-product.
What are the products of incomplete combustion and what are the problems related to them?
* The gross emissions of char and gaseous unburned combustibles, such as HC and CO, are important not only for pollution, but also for the heat balance in the boiler. In modern combustors, those emissions are small and the unburned char is taken out with the ashes. * Emissions of minor quantities of HC (VOC, PAH) are often regulated by maximum emission limits that vary from country to country. The most severe limits are for waste incineration plants. * Emissions from halogenated HC (F,Cl,Br and I). The halogens can form compounds that are harmful. * Fine particles. Too small particles will stay in the atmosphere for a long time before they settle. This can be a risk to human health.
What are the main factors controlling the emissions for a GT?
* The primary zone combustion and the equivalence ratio * The degree of mixing in the primary zone * Residence time * The combustor liner quenching characteristics
What influences the critical temperature?
* The vessel in which the mixture is contained. * Pressure * Fuel/oxidizer mixture
What are the four major types of engines in GT?
* Turbojet * Turbofan * Open rotor * Turboprop
Emissions can be reduced using three types of actions that has to do with the combustion process and its system, which?
1) Selection of fuel, cleaning or preparation of the fuel 2) Modification (usually improvements) of combustion conditions in the combustion chamber 3) Post combustion measures such as flue gas cleaning
What is a diffusion flame?
A diffusion flame arises from the combustion of separate gaseous fuel and oxidizer streams which combust as they mix. Diffusion flames are dominated by the mixing of the reactants, which can be either laminar or turbulent, and reaction takes place at the interface between the fuel and the oxidizer. A candle burns as a diffusion flame and is often yellow. In a candle, wax is melted and flows up the wick and is vaporized. Air flows upward due to natural convection. The reaction zone is between the air and fuel zones. Air diffuses inward and fuel outward. Soot particles are procuced giving rise to the yellow flame.
What is the characteristics and the design of a fluidized bed?
A fluidized bed consists of inert material (ashes, sand and sorbents). A fluid is fed from the bottom, ozygen mixtures if the purpose is combustion and oxygen lean mixtures if the purpose is to gasify the fuel. Depending on the velocity of the fluid, different characteristics is achieved; at a low velocity, a bubbling bed is created and at a high velocity, a circulating bed is created. For the latter case, a cyclon is needed in order to bring back the bed material that has left with the gases on top. In a BFB, a heat exchanger is submerged into the bed to maintain the desired bed temperature. In a CFD, which is narrower and taller than the SFB, the walls are covered with heat transfer surfaces. The large thermal capacity of the inert part of the bed evens out the temperature variations and forms a stable environment for combustion. The bed transfer heat produced by combustion to the heat transfer surfaces which are in contact with the bed.
What defines a fuel?
A fuel is a substance which, when heated, undergo a chemical reaction with an oxidizer (typically oxygen in air) to release heat
What is a premixed flame? How can you prevent flash back? Which applications use premixed flames?
A premixed flame arise from the combustion of gaseous reactants which are mixed prior to combustion. A premixed flame is a rapid, essentially constant-pressure, exothermic reaction of gaseous fuel and oxidizer which radiates light and heat and propagates as a thin zone with a speed less than a few meters per second. Laminar premixed flames have a unique burning velocity for a given fuel-oxidizer mixture. Turbulence increases the burning velocity. A premixed flame is bluer than the diffusion flame which is more yellow. The reaction is relatively fast and can be controlled by adding more air which will cool. No soot, no unburnt since there is an even heat distribution. If the velocity is too low, the flame will propagate backwards (flashback). This can be prevented by having a plate with small holes, which are smaller than the quenching distance. The small holes will increase the pressure drop over the plate and also the velocity. The turbulence will increase which increases heat transfer. If the velocity is too high the flame will blow out. Hydrogen and C2H2 (Acetylene) has higher laminar flame velocity which gives a smaller quenching distance. The laminar flame velocity is the lowest possible flame velocity of a premixed flame with a given mixture between oxidizer and fuel. The pressure difference across a flame front is small, which means that the pressure is assumed constant. This indicates that the conservation of momentum can be neglected. Premixed combustion takes place mainly in: gas-fired furnaces and stoves, gas fired turbines and automobile engines. See the different zones in figure 7.6 and read about those.
What is chemical equilibrium?
A reaction can occur both in the forward direction and the backward direction. When the rates of the forward reaction and the rate of the backward reaction are the same, the reaction is said to be in chemical equilibrium.
Describe pool combustion
A self-sustaining process. When the evaporated gases meet the air -> ignition. Heat transfer controlled.
How is the size of the limit layer affecting the transport?
A smaller limit layer results in a larger gradient, which gives more effective transport than for larger limit layers(i.e. for large particles)
What is an explosion?
An accelerating reaction, where more radicals are formed than consumed(i.e. chain branching)
What is an elemental reaction?
An elementary reaction is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step.
At which diameter does quenching occur?
At a diameter which gives Pe<8.
What does it mean if the flame is blown off?
At blowoff, the flame is no longer held or anchored by the heat transfer and fluid flow in the boundary layer, which forms its base, and the base of the flame rises above the burner.
Describe the p-T explosion diagram for hydrogen and oxygen.
At low pressures, the hydrogen/oxygen system does not ignite. Radicals, which are formed in the gas phase by chemical reactions, diffuse to the vessel wall where they recombine to stable species. Due to the low pressure, diffusion is fast. Thus, no ignition takes place. When the pressure is increased above a certain value (first ignition limit), spontaneous ignition is observed since the diffusion rate of radicals to the wall is reduced and becomes lower than the production of radicals. The first explosion limit depends on the concurrent process of chain branching in the gas phase and chain termination at the walls. The second explosion limit is governed by the competition of chain branching and chain termination in gas phase. At low pressure, hygrodgen atoms react with molecular oxygen in an important chain branching step: H+O2->OH+O. The products the chain-branching reaction, OH and O, rapidly react with fuel to produce H which can react in the reaction above to porduce more OH and O- The three-body reaction, H+O2+H+M->HO2+_M, is competing with the reaction above. The product HO2 is not as reactive as O, which means that it cannot break H2 or O2 molecules, which means that it mainly contributes to termination. When the pressure is increased, the three-body reaction will go fater than the one above, which will give rise to the second explosion limit. The third explosion limit, also called the thermal explosion limit, is governed by the competition between the heat produced from the reactions and the losses. The heat production per volume increases with pressure which will eventually cause an explosion.
What are the fundamentals of Sermenov's equation?
At the critical point, the heat generation rate is equal to the heat loss rate. From this, we can get the critical temperature, i.e. the ignition temperature.
What is the main purpose of burners?
Burners are designed to inject gaseous fuel, liquid fuel in the form of droplets and solid fuel int he form of powder but also a major quantity of combustion air.
What can be done to reduce the SOx emissions inside the furnace (in situ)? Is this possible for all types of furnaces?
By injection of finely ground limestone into the part of the furnace where the temperature is suitable for sulphation, the SOx emissions can be reduced. For grates and suspension fired furnace, this is not applicable since the efficiency is low. This means that flue gas clearning is the most suitable measure for those types. It is well suited for FB. The limestone is then added to the bed.
How can the CO2 be captured?
CO2 can be captures pre-combustion, post-combustion or by oxyfuel combustion.
Describe briefly the CSTR reactor. What assumptions are made and how do this simplify the conservation equations?
CSTR-Continuously stirred tank reactor: It is an ideal reactor in which perfect mixing is achieved inside the control volume so the concentration everywhere is the same. Common assumptions for a CSTR is that the temperature is constant through the reactor and that the flow is isothermal. This assumption makes the energy equation trivial because nothing changes. Also, the reaction is assumed to be equimolar. This makes the solution of the momentum and conservation equation trivial. The only equation that needs to be considered is the conservation equation for species. At the time you feed a stream with new composition to the CSTR, it appears in the outlet immediately, however it has a slow change, i.e. you will see a change immediately but it will take time for the outflow to stabilize the concentration according to the new inlet.
How can CO emissions be reduced in a GT?
Can be reduced by improved fuel atomisation by well-designed air blast spray nozzles. Air bleed through the liner in the intermediate combustion zone completes the combusiton of CO and CH, while cooling the wall. However, since non-uniform mixing occurs, this will generate cold spots where HC and CO are quenched and hot spot where NOx are produced.
What is the rank of coal?
Expresses the progressive metamorphism of coal from lignite (low rank) to anthracite (high rank). Rank is based on heating value, which is calculated ona dry ash-free basis for low-rank coals, and on percentage of fixed carbon, calculated on dry ash-free basis for higher-rank coals. Metamorphism is the change of minerals or geologic texture in pre-existing rocks under cristalline conditions, i.e. no melting of the soild has occurred. The change is mainly due to high pressure and temperatures etc.
How can the combustion process be controlled?
Flame limitation by cooling is a way of controlling the combustion process. This is in most cases achieved by passing the gas mixture through small passages, which prohibits the flame to propagate upstream, since the flame is quenched as the gas mixture is cooled during its passage. If the flame is to propagate, the energy released due to chemical reactions must keep the temperature of the reaction zone high enough to sustain a rapid reaction. If the heat transfer to the surrounding surface is high enough, the temperature will drop and the reaction will slow down. As the reaction slow down the energy release rate is lowered, the temperature drops below the ignition temperature and the flame will be quenched.
What is flame quenching?
Flame quenching is when favourable combustion conditions are taken away and occurs when: * Removal of oxidizer * Removal of fuel * Cooling * Lack of mixing * Consumption of radicals.
How is a flame stabilization achieved?
Flame stabilization is provided by swirl imparted to the primary inlet air. The swirl generates a recirculation of hot combustion products (a back mixing) For the flame to be stable, the flame velocity must be equal and opposite of the reactant velocity at each point along the flame. Flame holding is achieved by creating high inlet swirl, which causes back-mixing of hot products.
Describe the stages of conversion of a small fuel particle in a hot surrounding.
First the particle undergoes heating up to the temperature of the boiling point of the moisture. This is a rather fast process. When the temperature is at the boiling point of the moisture the temperature is constant during the time for drying. This is a fast process as well. After this the temperature rises again up to temperatures around 400-600°C depending on fuel type. At this point devolatilization takes place which is when volatiles in the fuel are released, e.g. CO, CO2 and H2. These are ignited and combusted. If the surrounding is oxygen free the fuel undergoes pyrolysis instead. The last step is at temperatures perhaps between 700-900°C and this process is slow. It is called char combustion or char gasification depending on in oxygen is present or not. Char gasification is slower than char combustion. Char combustion is exothermic, char gasification endothermic.
Combustion of liquid fuels can be seen as a two-step process, explain.
Firstly, the fuel needs to be evaporated and then gas combustion occurs.
Which three main types of combustion systems are used for firing solid fuels?
Fixed-bed, suspension and fluidized bed.
There is a burner technique that is special for gaseous fuels, which?
Flameless oxidation. Flameless oxidation is achieved by extreme external or internal recirculation of hot product gases. By dilution with recirculated gas, the combustion zone becomes extended in space and steep gradients of temperature are avoided. Consequently, the formation of thermal NOx is very low. In order to sustain combustion in the diluted zone a fairly high temperature is needed.
What does Bi>>1 and Bi<<1 imply for solid fuel conversion?
For Bi>>1: External heat transfer resistance is much smaller than the internal heat transfer resistance. Temperature will accumulate around the particle -> T_surf = T_surr. There will be a temperature gradient inside the particle. The temperature will increase gradually. Drying and devolatilization willl overlap each other due to dofferent temperatures in the particle. For Bi<<1: Internal resistance is much smaller than the external resistance. This will lead to a temperature gradient outside the particle since the particle is good at absorbing heat. The temperature inside the particle will thus increase homogenously throughout the particle. Drying and devolatilization will occur sequential since the temperature rise is the same. Characterized by small particles, like in fuel powders and PC boilers.
Describe the three char combustion regimes that exists and how they can be characterized. Hint: Dependent on Thiele modulus
For Th small, <1 (I guess): We have the shrinking density regime. This is also called the constant diameter case. The oxygen concentration in the fuel particle is almost equal to that od the surroundings. This case is associated with low temperatures and high porous particles. For Th high, >>1 (I guess): The shrinking core/sphere case. All oxygen is consumed in a narrow region close to the particle surface. This case is associated with high temperatures and large particles. It means that the density of the unreacted char is constant and instead the volume of the unreacted char decreases. The shrinking core case can be further split up into two regions, one restricted by mass transfer, when k_rC >> hm. Most common for large particles in a hot surrounding. The other case, when hm >> k_rC, the oxidation still takes place close to the surface but is restricted by the reaction rate on the surface.
How does the diffusion flame change with the jet velocity?
For free jet flames, as the velocity of the fuel jet is increased, the character of the flame changes. The laminar flame height increases linearly with the jet velocity up to a certain point where the flame becomes brush like (turbulent). Then, the flame height decreases due to more rapid turbulent mixing. In the fullly developed turbulent flame, the height is independent on velocity. To stabilize a free jet flame, one can increase the turbulence and the area of the flame by using a bluffbody or a swirl.
What criteria sets the height in small and large boilers?
For large boilers, it can be seen that the height of the boiler must be larger than that is necessary for the burnout of coal particles, in order to cool the gas properly; the design os heat transfer limiting. For small boilers, the heat transfer to cool the walls is not the limiting factor. Instead, the burnup of particles is a decisive criterion for the determination of the combustion chamber height.
How is the minimum ignition energy changing with the distance between the electrodes?
For large gaps between the electrodes, the heat transfer to the surrounding is increased and the minimum ignition energy increases. Also, for small distances, the minimum ignition energy is large since the heat transfer to the plaes does not allow ignition to take place. There is an optimum, where there is a gap giving the smallest ignition energy. As the gas is decreased, the energy is increased.
What is forced ignition?
Forced ignition occurs as a result of local energy addition in form of an external source. A flame is initiated locally near the ignition source and propagates into the rest of the mixture.
Which fuels are the most important ones for energy conversion?
Fossil and biomass fuels stand for almost 90% of the world energy production. The most important gaseous fuels are natural gas and liqeufied petroleum gas. Gaseous fuels are also produced from coal and wood, and from petroleum and natural gas. Liquid fuels are mainly derived from crude oil. In the future, these fuels may be derived from oil shale, tar sands, coal and biomass. Solid fuels are mainly biomass or fossil solid fuels, for example coal, lignite, bituminous and anthracite.
What is the definition of Fr?
Fr - Froude number - Tells how important the gravity is compared to the convection. If the gravity is small, the Fr number is large.
What is fragmentation?
Fragmentation is when fuel breaks a part into other particles with different sizes. This means that heat and mass transfer is improved. This means that time calculated for e.g. drying and devolatilization is for one particle and therefore the worst case scenario.
What is heat of reaction and how is it calculated?
Heat of reaction is the heat released or needed to be added to a reaction for it to occur. Is is the difference between the heat of formation for the products and the reactants. dHrxn = dHform_prod - dHform_react.
What is reactivity related to age?
I have no idea
What is a global reaction?
If a chemical reaction is happening during many steps, the reaction is said to be global. I.e. The reaction consists of many elementary reactions.
How can the performance of a GT be increased?
If the inlet temperature to the turbine is increased, the optimal pressure ratio is also increased. However, the gain in efficiency becomes marginal when the turbine inlet temperature is increased due to the high losses in the exhaust since the temperature out is high. By increasing the temperature of the inlet to the turbine, more work can be extracted and by increasing the pressure ratio, the efficiency can be increased as well. To increase the performance of a GT, the inlet temperature to the turbine should be increased and the pressure ratio as well.
What determines if the evaporation is heat or mass driven?
If the temperature of the surrounding is much higher than the boiling temperature of the fuel, the evaporation is heat transfer driven. If the temperature of the surrounding is much lower than the boiling temperature of the fuel, the evaporation is mass transfer driven.
What is the cetane-number?
In compression-ignition (Diesel) engines the time between start of injection and onset of combustion is known as the ignition delay. Cetane number ranks fuels accoding to their ignition delay when undergoing a standard test.
What are the different processes taking place in a grate fired boiler?
In grate boilers, only part of the combustion takes place on the grate. The process in the combustion chamber of a grate-fired boiler normally consists of several steps: 1. Fuel bed -> 2. Stirred volume for gas conversion -> 3. Plug flow volume for burn-up and cooling -> 4. Heat transfer pass for final cooling of the burned flue gas. The fuel is fed onto the sloping grate, where a fuel bed is formed. In the bed, the fuel is dried, devolatilized, the remaining char is burned and the ashes finally leave the grate at the lower end. The gases leaving the bed are stirred by air or gas jets in order to mix the fuel and air in the stirred reactor volume above the bed. The walls surrounding this space are to some extent refractory surfaces from which heat will be radiated back into the first part of the bed for drying, devolatilization and to the end of the bed for fuel burn-up. The fractions of wall covered by refractory and cooling surface depend on type of fuel and aims at producing a suitable temperature in the over-bed region. Air for final combustion is added above the stirred region and produces initial turbulence and mixing for the burn-out in the plug flow region, whose walls consists of heat receiving surfaces. In this region, the gases should burn out and attain a predetermined temperature before they reach the convection cooling surfaces.
Describe the two-stroke engine.
In the two-stroke engine the gas exchange takes place during the end of the expansion (exhaust I guess) and the beginning of the compression stroke (Intake).
How is turbulence included in the conservation equation?
In turbulence models, the Reynolds decomposition is often used. It means that the velocity and the pressure are divided into an average part and a fluctuating part. By inserting this into the conservation equations, and making some assumptions (an usual assumption is the Boussinesq approximation, where the turbulence is approximated to be a diffusive process that is isotropic. A turbulent viscosity is also assumed.) By assuming that the turbulent viscosity is much larger than the laminar, it is only one additional equation that is needed to solve the equations.
Describe with words, how a heat balance over a hot water boiler can be expressed.
In: - Cold water in - Air for combustion - Fuel input Out: - Fly ash including unburned fuel - Flue gas including unburned gas - Heated water out - Bottom ash including unburned fuel
Describe spray combustion
Increases the surface area by making smaller droplets. More effective heat and mass transfer which will make the fuel evaporate faster. Cannot go too low in droplet size due to pressure drop and expensive equipment. Mass transfer controlled.
Describe air-staging
Introducing air at different stages in the furnace. Firstly, in the primary zone, fuel and air is added in understoichiometric conditions. -> Reduce the amount of available oxygen in areas that are critical for NOx formation and reduce the peak temperature. In the secondary zone, more air is added in order to ensure complete fuel burnout. This zone is oxygen rich.
What is the flash point for a liquid fuel?
It is an indication of the maximum temperature at which a liquid fuel can be stored and handled without serious fire hazard. The minimum temperature at which the fuel will rapidly catch fire when exposed to an open flame loacted above the liquid.
What are the two main groups of pulverized fuel burners?
Jet burners and swirl burners
How is the efficiency changing in the ideal combustion engine?
Just like the ideal gas turbine it is a function of kappa = Cp/Cv and the compression ratio r. Äta = 1-1/r^(kappa-1) So by increasing the pressure ratio the efficiency increases.
Express Keq in the forward and backward reaction rates, kf and kb
Keq = kf/kb
What does it mean if a process is kinetically controlled and diffusion controlled respectively?
Kinetically controlled - The reaction rate is the limiting time step Diffusion controlled - The mixing is the limiting time step, -> "mixed is reacted"
How is the equilibrium constant defined for this reaction: aA+bB <-> cC + dD ?
Kp = p_C^c*p_D^d/(p_A^a*p_B^b) Kc = C_C^c*C_D^d/(C_A^a*C_B^b) p_i = partial pressure of component i C_i = Concentration of component i
What is the definition of Le?
Le - Lewis number - Compared the Sc and the Pr number, i.e. the thermal diffusivity and the mass diffusivity High Le - Thermal diffusivity dominates
How is the velocity in a GT combustor affected load changes?
Load increases -> larger mass flow of air(due to higher fuel flow) Load increases -> Pressure also increases -> the velocity in the combustor remains relatively constant with load changes.
How does the length of he fuel molecule influence the combustion process?
Long molecules: * Need more heat to evaporate and to create a combustible gas * Breaks easily down and ignites thermally Short molecules: * Easier to evaporate *Tightly bound together, so harder to thermally ignite.
What is a primary measurement to reduce NOx?
Lowering the temperature. This can be done by the design of the burners. Flue gas recirculation or dilution in the combustion zone is another way.
Which are the two ways that moisture can exist in soild fuel?
Moisture can exist as free water within the pores of the fuel which includes surface moisture. Also there is bound water, which is absorbed in the interior of the pore structure.
For large particles, does both the kinetic and the mass transport be taken into account?
No, for large particles, with large limit layers, the transport is very slow (due to a small gradient). Kinetics is often fast -> only need to consider the transport.
How is the liquid fuel burner designed?
Nozzles for atomization of the liquid fuel is important since the droplet size distribution is essential for the performance of the burner. The spray nozzle is inserted into a burner, which provides the air for combustion of the gas evaporation from the drops. The effect of a flame holder or a swirl are important to stabilize the flame.
What is the definition of Nu?
Nu - Nusselt number - Compares the convective heat transfer with the conductive heat transfer. High Nu - Convective heat transfer dominates
Describe briefly the PFR reactor. What assumptions are made and how do this simplify the conservation equations?
PFR-Plug flow reactor: It is a one-dimensional model with the following assumptions: 1. Steady state 2. No mixing in the axial direction. Implies that molecular and/or turbulent mass diffusion is negligible in the flow direction. 3. Uniform properties in the radial direction. So any cross-section in the flow is characterized by a single velocity, temperature, composition etc. 4. Ideal frictionless flow. 5. Ideal-gas behaviour. In the simplest form the reactor is assumed isothermal and equimolar which makes it possible to derive expressions for the PFR only from the species conservation equation. The delay time in the PFR is the residence time. So when you feed something new in the inlet with changed composition you will see all the changes at the same time, however it is delayed according to the residence time.
What two different kinds of liquid combustion exists?
Pool combustion and spray combustion
What is the definition of Pr?
Pr - Prandtl number - Compares the viscous diffusion rate with the thermal diffusion rate. High Pr - Viscous diffusion rate dominates.
Which factors influence the selection of fuels?
Price is a major criterion for selecting fuels, although convenience, emission control regulations and system availability are also important.
Describe fuel staging (reburning)
Primary zone: Fuel and oxidizer is added, lambda>1. Secondary zone: Additionally fuel is added, lambda<1. Burnout zone: additionally oxidizer is added, lambda>1. Reburning can be done at high temperatures and in large combustors where sufficient residence time is available. Reburning is achieved by injection some additional fuel just above the main combustion zone. This creates a fuel rich combustion zone where radicals can react with the NO from the primary zone to form Vol-N and since the amount of O2 is low in that zone, the Vol-N can react with the other NO and form N2. The remaining HCN and NH3 will be partially oxidized in the final combustion zone and NO will again increase. However, the net result will be a large reduction of NOx.
What is the definition of Re?
Re = Reynold number - Indicates how significant the diffusive (viscous) term is in relation ot the convective term. At high Re, the viscous term can be neglected compared to the convective term.
Ordinary BC in the different parts of a furnace? I.e. at the refractory walls, evaporating tubes, superheating tubes.
Refractory walls are thermally insulated -> use Neumann BC with no heat flux in these regions. Evaporator tubes: Use the Dirichlet thermal condition, i.e. T=T_sat. Assuming that the temperature inside and outside the water tube are the same.(false in reality) Superheater tubes: The wall temperature is not known, if local heat flux or temperature measurements are available, these can be used as BC. An alternative is to include models of the temperature on the steam side as BC. No-slip conditions are usually applied at the walls, -> The relative velocity between the fluid and the wall is zero. No penetration of mass is often true at the walls, i.e. no mass flux through the walls -> use Neumann BC.
Describe the emissions from the SI and the CI engine and how to reduce them.
SI: Both CO, HC and NO is a problem. Reduction of fuel-air ratio to leaner conditions will reduce CO and HC emissions as long as flame speed and ignitability do not become a problem. Lean operation also reduces NO formation because of lower flame temperatures. CI: The most troublesome emissions from diesel engines are soot (particulates) and NOx. Typically HCs and CO is are not a serious problem except at light loads. The optimum path of fuel/air ratio during combustion and rate of mixing of products with air is unknown. However, both retarded injection and exhaust gas recirculation produce more soot due to more diffusion burning, less oxygen and lower oxidation temperatures. Work is progressing on catalyst systems which can destroy NO in lean-burn exhaust mixtures, but such systems are not yet in use.
What happens to SOx and NOx when adding limestone in the furnace?
SOx will reduce, N2O will be reduced but NO is increased.
What is the definition of Sc?
Sc - Schmidt number - Compares the viscous diffusion rate (momentum diffusivity) with the molecular diffusion rate. High Sc numbers - Viscous diffusion dominates
What is the definition of Sh?
Sh - Sherwood number - Compares the convective mass transfer rate with the diffusion rate High Sh - Convective mass transfer dominates
What is appropriate BC for the inlet?
Since the inlet conditions are usually known, they are set as Dirichlet BC.
How can soot be reduced in s GT?
Soot is formed in fuel-rich zones near the fuel droplets. Most of the soot formed in the primary zone is oxidizer in the secondary zone. Injection more air into the primary zone will reduce the soot emissions at the expense of increased CO and HC. Soot emissions can also be reduced by improved mixing, water injection and increased residence time.
What is spontaneous ignition?
Spontaneous ignition occurs as a result of raising the temperature of a combustible mixture by containing a sufficiently large volume within hot boundaries or by subjecting it to an adiabatic compression. Once the generation rate of heat exceeds the heat loss rate, ignition occurs in the whole volume.
If one wants to reduce the sulphur content by preparing the fuel/selecting the fuel before the combustion, what can be done?
Switching to fuel with less S-content, switch to low S-content coal and oil from high S-content oil and coal. One can also remove the S before the fuel is combusted.
What is the thermal condition for ignition?
T_ad>T_cr for an explosion(ignition) to happen
What are the two major factors affecting the energy intensity?
Technical development and energy price.
Define the Thiele modulus
Th = r_0*sqrt(R_C,O2/D_AB,eff) The Thiele modulus is a dimensionless Dahmköhler number that relates the mass transport of the reactant into the particle with the reaction rate and gives a measure of the distribution of the reactant within the particle.
What can be assumed if the surrounding temperature is higher than the critical?
That the heat generated is much larger than the heat losses
How can you combine the CSTR and the PFR to describe a general combustion system?
The CSTR can be used where there is no temperature change or change in molar flow and in emission calculations since those reactions do not contribute significantly in heat or mass. CSTR reactors are used as a model in regions with extensive mixing, for example in areas with inflow of new air. Plug flow reactors are used when there are very little mixing along the axis. Example: For a grate fired furnace where you inject air through the grate the system can be described by a CSTR. Also, in the flow slightly downstream secondary air is injected, giving good mixing so can be described again with a CSTR. When the flow travels further downstream the mixing in the axial direction decreases, hence a PFR can describe the flow.
Describe the Diesel cycle and the corresponding four-stroke cycle
The Diesel cycle is an idealized cycle often called the constant-pressure cycle and corresponds to combustion at constant pressure. It is also called a compression ignition (CI) engine since the fuel is ignited through auto-ignition caused by compression. The four-stroke cycle is for the Diesel engine as follows: 1. Intake stroke that draws ONLY air past the open inlet valves into the cylinder. 2. A compression stroke with the valves closed which raises the temperature. The diesel is injected near the end of the compression stroke, evaporates, mixes with air and ignites. Sometimes the combustion take place before the piston is in its upper extreme position, top dead center (TDC). 3. An expansion or power stroke where combustion is completed. 4. An exhaust stroke that pushes the burned gases through the open exhaust valves.
Describe the Otto cycle and the corresponding four-stroke cycle
The Otto cycle is an idealized cycle often called the constant-volume and corresponds to instantaneuous combustion. It is also called a spark ignition (SI) engine since it is ignited using a spark. The four-stroke cycle is as follows: 1. Intake stroke that draws combustible mixture of fuel (e.g gasoline) and air past the open inlet valves into the cylinder. 2. A compression stroke with the valves closed which raises temperature of the mixture. The mixture is ignited at the end of the this stroke. 3. An expansion or power stroke where combustion is completed. 4. An exhaust stroke that pushes the burned gases through the open exhaust valves.
Which of the CSTR and PFR has highest conversion for the same inlet conditions? Explain why.
The PFR has the highest conversion. In the CSTR reactor the concentration of the inlet species goes instantly down to the concentration inside the reactor. Due to this the reaction rate lowers very fast which slows down the reaction. For the PFR the concentration varies along the axis which keeps the rate higher throughout the reactor so the reaction will go longer.
What is the Zeldovich mechanism?
The Zeldovich mechanism requires high temperature in order to break the bonds between the N2. N2 + O -> NO + N N + O2 -> NO + O N + OH -> NO + H This mechanism becomes significant in flame combustion. The lower combustion temperature, the less significant is this mechanism, like for FB where the temperature is lower than for coal fired furnaces.
What is the adiabatic flame temperature?
The adiabatic flame temperature is the temperature that would be if a reaction is operated adiabatically and that the reaction goes to completion.
Define the auto-ignition temperature for a liquid fuel.
The auto-ignition temperature the lowest temperature required to inititate a self-sustained combustion in a standard container in atmospheric air in the absence of a spark or a flame.
How can the NOx emissions be reduced in a GT?
The basic idea for reducing NOx is to reduce the peak temperatures during combustion and the time spent in high temperatures. This can be done by: * Water injection * Premixed lean burn combustion * Staged conbustion Note that emissions of HC is not a problem since there is a lot of air.
What is minimum ignition energy for forced ignition?
The basic idea in forced ignition is to rise a small amount of reactant to a temperature high enough to cause continued flame propagation after the ignition energy is removed. The volume of the gaseous reactant heated during ignition must be large enough, so when the ignition source is removed, the heat loss to the surroundings will not exceed the chemical energy release rate. That is, the ignition energy must be large enough for this to happen.
How is a pulverized coal-fired boiler designed?
The coal is pulverized before injected through the burners. The furnace walls consist of steam/water tubes welded together to form a membrane surrounding the combustion space. The heat transfer from the flames in the combustion chamber to the membrane tube walls is almost entirely by radiation. At the exit of the combustion chamber, superheater tube bundles are seen. Convective heat exchangers are located in the flue gas path: the economizer and the air preheater
What are the requirements of the combustor in a GT?
The combustor should: * Operate at high pressure and controlled temperature. * Provide rapid and reliable ignition * Operate over a wide range of mixtures without danger or blowoff. * Have minimum loss in pressure * Uniform exit temperature * Small in size and durable * Low emissions
In which conservation equation is Re*Pr included when writing it on dimensionless form?
The conservation of energy
In which conservation equation is Re*Sc included when writing it on dimensionless form?
The conservation of gas species.
What does a p-T explosion diagram show for spontaneous ignition?
The explosion(ignition) limits
Describe how the conversion in the two different types of reactors changes with residence time and load.
The expression for the outlet concentration from the CSTR is given as: C_out = C_in/(1+tau*k) for steady state and first order reaction. Therefore longer residence time inside the reactor and fast kinetics will lower the outlet concentration. The expression for the outlet concentration of the PFR is given as: C_out = C_i(0)exp(-k*tau) So also in this case, for a larger tau and fast kinetics, the outlet concentration will decrease.
How is a pulverized oil-fired boiler designed?
The features of the combustion chamber and the flue gas duct are similar to the case above. The outside, aslo like other boilers, covered by corrugated metal sheets, is insulated from the hot boiler tubes by a layer of insulating material. The burnup of the fuel requires less space in oil or gas fired boilers, and the combustion chamber is smaller than for solid fuels of similar outputs.
What are the different combustion regimes for liquid fuels and what number is used to differentiate between the regimes?
The group combustion number G is used. *G<10^-2: Isolated drop combustion. Here there is a separate flame envelope for each droplet. A small number of droplets are widely separated. *10-2<G<1 Internal group combustion. Small number of droplets are vaporized in a core of cloud, which is totally surrounded by a flame. *1<G<100 External group combustion. This happens when a single flame envelopes all droplets. For industrial burners *G>100 External sheath combustion. High number of droplets located close to each other. Only droplets at the edge of the cloud is evaporated.
Define the higher and the lower heating value (HHV, LHV)
The heating value is the heat realease per unit mass when the fuel, initially at 25°C, reacts completely with oxygen and the products are returned to 25°C. Higher heating value is referring to the heat released from the fuel when the water is condensed and lower heating value is when the water is not condensed.
What should the height of the combustion chamber ensure?
The height should be sufficient to accommodate a heat transfer surface to cool the gases in order to avoid melted ash particles depositing on downstream heat transfer bundles, and to allow the fuel to burn out before the intensive cooling in the tube bundles.
What is the minimum quenching distance?
The minimum distance where the flame can pass the hole, is when the maximum heat release is equal to the heat transfer to the walls.
What is the Octane number for a liquid fuel?
The octane number is a measure of the tendency of the fuel to knock as the compression ratio in a spark-ignition (Otto) engine is increased. Knocking is referred to when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.
What happens to the rate of evaporating if we have a burning droplet?
The time decreases significantly when we have a burning droplet. The flame provides heat which creates a larger gradient in heat. The flame also removes the evaporated fuel(by combusting it) which will increase the mass transfer gradient.
What is prompt NOx?
There is a possibility that HC-radicals react with air N2 to Vol - N(i.e. HCN). This can then further react to NO if there is enough oxygen. This might be important at high temperatures and in fuel rich zones, but is not common in combustion operation.
Briefly describe what affects drying, devol. and char conversion.
These phenomena depend on fuel type, fuel moisture content, size and heat and mass transfer to the particle. For small particles all these occurrs in sequence. For large particles they occur simultaneously but in different locations in the particle.
Give the background to the derivation of the expression for the laminar flame velocity of a premixed flame
This expression is derived by first assuming that heat conduction from the flame to the reactants was the rate limiting step. Assumption is made that the flame consists of a preheated zone and a reacting zone, and neglecting any molecular diffusion and chemical reactions in the preheated zone. The boundary condition between the two zones is assumed to be set b the ignition temperature, see Fig. 7.6. The reaction zone extends over a distance d. Moreover the thermal conductivity and the heat capacity is assumed constant. Assumption that chemical reactions starts at temperature T_ig.
What is chain initiation?
This occurs when molecules dissociate and creates radicals. Such reactions have high activation energy which means that they are sensitive for changes in temperature. Also, they need high temperature to start.
What is appropriate BC for the outlet?
To get the BC at the outlet, an insulated duct is often used. Thus, one can assume both thermal and fluid-dynamic equilibrium in the duct direction. -> Assume that there is no gradients in normal direction and at its end, the Neumann BC can be set =0.
Why does an oil lamp(or candle) have a wick?
To increase the surface area.
What influences the minimum ignition energy?
Turbulence and mixture composition influence the minimum ignition energy. The minimum energy is increased with increased flow velocity and turbulence intensity. When the turbulence intensity is increased, the burning velocity is increased in this decreases the energy required but the heat transfer also increase which increases the required energy.
What is turbulence?
Turbulence is characterized by rapid and chaotic variations of flow properties in space and time. Will never reach steady state -> accumulation term needed. Turbulent flow is 3D and dissipative, which means that kinetic energy is transferred from larger to smaller scales where it is finally dissipated. High Re-numbers.
What is the ash fusion temperature for a solid fuel?
When ash is heated up to a softened state it has a tendency to foul boiler tubes and surfaces. The ash fusion temperature is an indicator of the behaviour of the ashes. It depends on the composition of the ash. It is typically below the flame temperature, and above the steam and wall temperatures.
For small droplets, does both the kinetics and the mass transport be taken into account?
When having small particles, the limit layer will be small -> good mass transport. Kinetic will then be the limiting factor -> only need to consider the kinetics.
What is chain branching?
When one radical reacts with a molecule to form two radicals.
What is chain termination?
When the number of radicals is decreasing. No temperature dependency -> Ea=0
What is chain propagation?
When the number of radicals remain constant.
How is the presence of fuel particles included in the conservation equations?
When we have inclusion of fuel particles, the mass in is not equal to the mass out, since the combustion of liquids and solids generates mass. This will result in a generation term in the continuity equation and in the equation of conservation of species. The particle will also absorb heat from the surrounding as well as radiate heat, which also needs to be taken into account. Resulting in an additional Bdrag term in the momentum equation and a q_part in the conservation of energy equation. Adsorption on the other hand, will remove particles which will lead to a negative generation term.
Is the heat of reaction and the heating value connected?
Yes, if the reactants are fuel and oxidizer, the heat of reaction corresponds to the heating value.