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Sample Essay on Natural Gas Fluidized Combustion

Natural Gas Fluidized Combustion

Introduction

Fluidized bed combustion was first commercially applied in the early 80’s and has seen rapid growth to become a well established heat generation technology today (Johnsson, 2007). This technology originated from attempts to come up with combustion processes that checked emission of pollutants without the necessity of having external emission controls like scrubbers (U.S Department of Energy, 2013). Coal and natural gas are the two main fossil fuels that are employed in Fluidized-bed combustion systems for heat and electricity generation. The design, operating features and environmental performance of coal and natural gas fluidized bed combustors differs substantially. While coal operated fluidized bed combustors are the most widely applied technologies, there are increasing efforts focusing on the development of natural gas operated fluidized bed combustors due to their environmental friendliness and cost effectiveness when compared to coal based systems. This paper focuses on contrasting the design, operating features and environmental performance of natural gas fluidized bed combustor conventional pulverized coal unit.

Design

Conventional pulverized coal fluidized-bed combustor is designed to use coal as the main fuel for generation of heat and electricity, whereas natural gas combustors employ various forms of gas producing  biomass such as wood, crop remnants, wood pulp and chips, and municipal solid waste (Crawford, 2012). Regarding the Fluidized combustion technique, the bubbling fluidized bed (in which air is injected at a slightly higher velocity than the fluidization velocity) method is preferred in coal combustion units, natural gas on the other hand is processed using the circulating fluidized bed technique (here a very high air velocity in relation to the fluidization velocity leads to elucidation of particles from the bed and circulated back to the reactor via a cyclone). A more contrasting feature in the design of coal and natural gas fluidized bed combustors is in the turbine system; whereas coal operated systems are designed to use water-based turbines to produce steam, Fluidized-bed combustors that use natural gas relies on gas turbines for production of steam that is used in the generation of electricity.

Operational features

The natural gas fluidized bed combustors are mostly designed to use relatively low amounts of heat in the Image 2disintegration of biomass particles as compared coal-based combustors that require large amounts of heat to operate. Limestone is the main catalyst that is employed in the operation of fluidized bed combustors using coal for speeding up the process, on the other hand, catalytic reactions in natural gas based systems are made possible by use of a range of alkali metals. Natural gas systems essentially do not require large Sulphur Oxides (SOx) and Nitrogen Oxides (NOx) chambers due to the low levels of production of these toxic gases. Coal operated systems on the other hand require substantial efforts in their design and operation features to curb release of these toxic gases.

Absorption of Sulphur IV Oxide in a conventional pulverized coal combustion chamber is made possible by the presence of Limestone on its bed, on the contrary fluidized bed natural gas combustion units are surfaced with calcium and sand that are responsible for SO2 absorption. In terms of the amount of het required, pulverized coal combustion require large amounts of heat to disintegrate coal due to its nature. On the contrary, natural gas combustion systems are laced with relatively low heating features since only relatively low degrees in terms of temperature are necessary for natural gas combustion.

Environmental performance

In comparison to coal-based combustors, natural gas-based combustors are more efficient and environmental friendly, this is majorly due to a number of reasons;

  • Production and release of Nitrogen Oxides (NOx) gas to the environment can be put under check at a relatively low cost. Further more, NOx emission can be further reduced using add-on control technologies such as selective catalytic reduction (Pembina Institute, 2001).
  • The emission of particulate matter in biomass operated systems is relatively low, this implies that there is only a small amount of waste to be dumped and thus requires a small dumping space (Pembina Institute, 2001). Coal produces a large amount of solid waste in form of ash in contrast to natural gas whose level of ash-waste is usually low. Coal combustion is therefore costly due to the costs involved in the storage and proper disposal of the solid waste.
  • Biomass systems are more capital efficient due to the elimination of the steam cycle in their combustion process.
  • Natural gas driven fluidized bed combustors are more environmental friendly in term of green house gas (CO2) emission into the atmosphere. Research has proven that presence of large amounts of Carbon IV oxide in the atmosphere lead to the depletion of the ozone layer and an increase in global temperatures as a consequence. Natural gas contains a smaller amount of Carbon IV Oxide gas as compared to coal. Pulverized coal fluidized bed combustion therefore requires stringent measures to curb emission of the green house gases in order to reduce the green house effect.
  • Coal occurs naturally in a solid form, unless the combustion is done at the site of occurrence, transportation of coal to the processing plant is necessary. Due to its nature, coal is very heavy and requires road transport from site to factory. On the other hand, the gaseous nature of nature of natural gas allows it to be transported via underground and overhead systems making this a fast, cost effective and convenient form of transportation compared to road transportation of coal. Additionally, large storage space is required to store the raw materials in the case of coal compared to natural gas which is in most cases a valve-turn away and does not require storage space.

Since its introduction, fluidized bed combustion technology (a technology for reacting suspended fossil fuel particles with high velocity air) for heat and electricity generation have gained widespread acceptance. Coal, petroleum and natural gas constitute the main fossils utilized in electricity and heat generation.. Initially coal was the most widely used fossil fuel in fluidized bed combustion technologies, natural gas is rapidly gaining appreciation over mainly due to its non-bulky nature, cost effectiveness and environmental friendliness. Coal based fluidized combustors are designed to allow feeding of solid coal of reduced size while natural gas systems are designed to only allow injection of raw materials in form of gas. Both coal and natural gas systems emit SOx and NOx gases as waste produce, the mode of absorption of these gases in the chambers however differs significantly. Although coal is the most commonly used fossil, natural gas producing substances such as biomass have been proved to be more effective, environmental friendly and cost effective.

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References

Crawford, M. (2012, September). Fluidized-Bed Combustors for Biomass Boilers. Retrieved January 28, 2014, from ASME.org: https://www.asme.org/engineering-topics/articles/boilers/fluidized-bed-combustors-for-biomass-boilers?cm_sp=Boilers-_-Feataured%20Articles-_-Fluidized-Bed%20Combustors%20for%20Biomass%20Boilers

Johnsson, F. (2007). Fluidized Bed Combustion for Clean Energy. the 12 International Conference on Fluidization- New Horizons in fludization Engineering (p. 16). Goteborg: Engineering Conferences International.

Pembina Institute. (2001). Appendix 4: A Comparison of Combustion Technologies for Electricity Generation. Power for the Future , 12.

U.S Department of Energy. (2013). Fluidized Bed Technology – Overview. Retrieved January 28, 2014, from Energy.Gov: http://energy.gov/fe/fluidized-bed-technology-overview

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