It is verified that the work presented in the thesis entitled 'CIRCULATION OF FLUIDIZED BED COMBUSTION TOWARDS THE THIRD GENERATION OF OXY-FUEL COMBUSTION' submitted by Mr. By combining CFB combustion with oxygen and biomass utilization, CO2 can be extracted from the atmosphere.
Nomenclature
1 Introduction
Overview
Methods for reducing greenhouse gases emissions
Based on several previous techno-economic assessment studies, oxy-fuel CFB combustion appears to be the most energy- and cost-effective of the carbon capture technologies. Improve the overall efficiency of the power plant operating under the oxy-fuel CFB combustion conditions by increasing the oxygen concentration.
2 Literature Review
Fluidization regimes and classification
Most of the operational and environmental characteristics of CFBs are a direct result of hydrodynamic behavior. Fluidization is a function of several parameters such as particle shape, size and density, gas velocity, bed geometry, etc. the liquid just seeps through the empty spaces between the stationary particles.
Bed hydrodynamics and heat transfer
Suspension density decreased with riser height, in the same trend of pressure drop. Friction near the wall reduces the velocity relative to the riser core and separates the section into the annular zone (downward movement of particles); and core one (upward movement of particles).
Fluidization velocities
To calculate the minimum fluidization velocity (Umf) for minimum void (εmin), the Ergun equation is rearranged as follows, in Eq. While many researchers developed correlations to calculate the Reynolds number at minimum fluidization (Umf), Remf is given at the onset of fluidization by Eq.
Coal characteristics and classification
This section of literature review is dedicated to the discussion of coal and biomass fuel properties and combustion, oxy-fuel properties, evaluation and generations of oxy-fuel CFB combustion together with an overview of recent studies on oxy-fuel CFB combustion.
Biomass characteristic and classification
It can be concluded that there are similarities between coal and biomass in terms. Still the physical properties ie. density, porosity, internal surface area, and solubility vary, requiring different pretreatment techniques and feeding systems for fluidized beds. 2009) stated that neither technical nor economic nor environmental can be insurmountable barriers of sustainable biomass resources.
Combustion characteristics
Combustion Fundamental studies
They found that increasing O2 concentration led to decreasing soot cloud size; and led to increased devolatilization and combustion temperature; and led to decreased ignition delay due to increased local mixture reactivity. oxygen concentration in O2-N2 mixture and O2-CO2 mixture. The particle lifetime of 80 µm is almost the same for bituminous and lignite for 60% O2 concentration 60% and above (Maffei et al., 2013).
Ignition Characteristics
- Oxy-fuel CFB combustion
- Biomass combustion and co-combustion with coal
So far, oxy-fuel CFB combustion is considered the most promising CCS technology, and still the oxy-fuel combustion has challenges viz. As a result, improving the efficiency of oxyfuel CFB combustion is needed (Nsakala et al., 2004). The technical problem (control of the temperature) will be intensively studied in order to achieve the desired economic pure oxygen fuel CFB combustion.
This is to improve and clarify the oxy-fuel technology and its applications in CFB towards 3rd generation oxy-fuel CFB combustion. Definition and investigation of 3rd generation oxy-fuel CFB combustion process in CFB, for CCS. To summarize, the review of the previous work has clearly suggested that oxy-fuel CFB combustion is a promising technology for CCS.
3 Novel Method for Pure Oxy-fuel CFB Combustion
One CFB with multi-feeders along the riser
In the first arrangement, the main fuel feed is split into multiple fuel feeds along the riser using n-feeders (Figure 3.3). Rather than feeding the required fuel once at the lower part of the riser-1 and releasing the heat quickly under high oxygen concentration, we can feed Xn % fractions of the entire fuel feed rate where the fuel combustion starts and the temperature increases. Two or more feeders can be attached along the height of the rise; the reactants of the first reaction contain excess oxygen, which will react with fresh fuel at higher level of the riser under conditions of lower λ and oxygen concentration.
Series of mini-CFBs
The exhaust from the first mini-CFB is enriched with oxygen and forwarded to the second mini-CFB to react with the amount of fuel (under a stoichiometric ratio of 2) to result in flue gases enriched with oxygen at a percentage of 33%. Both of these techniques (multi-stage feeder and multi-mini-CFBs) lead to clean oxy-fuel CFB combustion with eliminated recirculation setup. The success of clean oxy-fuel CFB combustion will minimize the cost of oxy-fuel combustion.
Furthermore, the complexity of the oxy-fuel CFB power plant will decrease due to the elimination of the recirculation configuration. To the best of our knowledge, there are no CFBs operating under pure oxyfuel combustion conditions, even in R&D. Finally, thermodynamic analysis is performed to find the effects of clean combustion of oxygen fuel on
4 Experimental Setup and Procedure
- Body of the cold circulating fluidized bed unit
- Body of the hot circulating fluidized bed unit
- Measurement instruments A. Temperature measurement
- Materials
- Precautions for Running the Oxy-fuel experiments
- Pure oxy-fuel CFB combustion (100% O 2 )
- Oxy-fuel combustion Oxy-Coal CFB Combustion
A compressor (manufactured by Ingersoll Rand, Model No. S-01480) is used to supply primary and vent air to the CFB unit through the bottom of the riser and return leg. The sand inventory is fed into the riser through a removable transparent tube at the bottom of the riser. The upper outlet of the riser is connected to the aerocyclone (4) with a rectangular channel.
Both fuel and stock (sand of 160 µm particle size) are fed with a screw feed connected to the return leg. the stock of 750 grams is used. The same procedure from the previous section is also used in terms of heating, supply of fuel and stock. The rate of solid recirculation was in the range of 15-20 kg.m-2.sec-1. The exhaust gases are cooled by water tubular heat exchanger, filter, and samples are collected by Tedlar bags.
5 Results and Discussion
- Effects of the primary flow rate
- Effects of the operating pressure
- Effects of the aeration flow rate
- Effects of the particle size
- Effects of the bed inventory
- Effects of the O 2 -CO 2 concentrations in oxy-CFB condition
- Experimental Uncertainties
- Pure oxy-fuel CFB combustion
- Effect of stoichiometric ratio over concentration of CO 2 and O 2
The comparison of suspension density sand is shown in Figure 5.3 under the following operating conditions of working pressure of 200 kPa; three primary speeds of 2.72, 3.4 and 4.08 m/s;. The same general pattern is revealed, for example a higher clearance at the bottom of the riser. Furthermore, the suspension density increases with increasing particle size (Figure 5.10), these findings help to increase heat transfer in the bed (Basu, 2015).
In oxy-biomass CFB combustion, the temperature change through the riser height (with varying fuel mass flow rate) is related to distributed combustion along the riser. From the temperature profile, it is concluded that combustion takes place mainly at the bottom of the riser. In contrast to pure oxy-coal CFB combustion, the temperature profile along the riser (Figure 5.22) for pure oxy-biomass CFB combustion shows a peak at the level of the vent, which is due to the instantaneous ignition of the biomass particles, which have a lower ignition temperature. .
6 Fuel and Reaction Exergy Analysis for Oxy-fuel CFBC
- Calculation
- Exergy of different types of fuel
- Exergy and energy chemical reaction of air-fuel combustion Effects of exhaust (products) temperature over exergy and energy
- Chemical reaction exergy and energy of oxy-fuel combustion
This figure (Figure 6.1) may include other solid hydrocarbon fuels not categorized as coal or biofuel. Interestingly, Figure 6.3 shows the increasing agreement between LHV based on final and final-proximate analysis with reduced ash content. As shown in Figure 6.7, the exergy of a system is affected by the pressure and temperature of the reactants and products, and the heat transferred to the system.
On the other hand, in thermodynamic analysis, Figure 6.9 shows the energy loss with flue gases (exhaust gases); the same general trend of exergy loss is found. The preheating technique is used at the operating power plant level to improve the overall efficiency and improve the oxidant parameters, and at the chemical reaction exergy level, it has been found that preheating reactants from 298.15 K to 598 .15 K improves the exergy by 20.5% (Figure 6.10). And exergy increases as preheat levels increase, at different rates (Figure 6.14).
7 Conclusions and Future Scopes
- CFB hydrodynamic investigation (cold CFB unit study)
- Oxy-fuel CFB combustion investigation (hot CFB unit study)
- Thermodynamics analysis (exergy and energy analysis)
- Cold CFB unit Future Scopes
- Oxy-fuel CFB combustion Future Scopes
- Thermodynamics analysis (exergy and energy) Future Scopes
On the other hand, the hydrodynamic behavior under oxy-fuel CFB combustion conditions has been studied. The second proposed clean oxyfuel CFB combustion arrangement calls for a larger CFB with multiple feed ports. Mini-CFB is used for conducting CFB combustion experiments with pure oxyfuel, two adjustments can be applied for future work viz.
Consequently, long-term tests are needed to commercialize CFB combustion with oxygen and the proposed technique. A comprehensive economic analysis of CFB combustion with pure oxygen should be performed to promote this technology. In the first proposed arrangement of CFB combustion with pure oxygen (an array of connected mini CFBs), the number of mini CFBs is optimized, there can be only two or three connected mini CFBs.
Axial cavity profiles and identification of flow regimes in the rise of a circulating fluidized bed. Pilot-scale evaluation of oxycoal firing in circulating-fluidized bed and pulped coal-fired test facilities. Experiences of oxyfuel combustion of bituminous coal in a 150 kW th circulating fluidized bed pilot facility.
Emissions of SO2 and NOx during Oxy-Fuel CFB combustion tests in a mini-circulating fluidized bed combustion reactor. Experimental results for the combustion of oxygen and fuel with a high oxygen concentration in a pilot-scale circulating fluidized bed of 1 MW. Combustion progress in an oxygen-fuel circulating fluidized bed furnace: measurements and modeling in a 4 MWth boiler.
Appendices
Uncertainty in calculating voidage
A screw feeder is used to feed the sand stock initially, and solid fuel (biomass and coal) during the running experiments. The DC regulated power supply is used to control the supplied current to the electric motor, consequently the R.P.M is controlled, and subsequently the feed rate is controlled. For each fuel species, a correlation between the voltage and feed rate is derived as shown in Figure F.1.
After filling the feeder with required fuel, the DC motor current is increased from 0 up to V= 1 volt, then the weight of the accumulated fuel w[gram] is measured along with the time t [sec]. The entire process is repeated for increased voltage, and FD is calculated each time.
List of Publications
