12-12-2012, 03:17 PM
A SEMINAR REPORT ON FLUIDIZED BED TECHNOLOGY FOR BOILERS
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ABSTRACT
This seminar gives a brief overview of the status and prospects for fluidized bed combustion (FBC) for clean energy, with focus on power and heat generation. The paper summarizes recent development trends for the FB technology and makes an outlook into the future with respect to challenges and opportunities for the technology. The paper also identifies areas related to fluidization, which are critical for the technology and, thus, will require research.
The main advantage with the FBC technology is the fuel flexibility. A compilation of 715 FB boilers (bubbling and circulating) worldwide illustrates the two main applications for the FBC technology: 1. Small and medium scale heat only or combined heat and power boilers (typically of the order of or less than 100 MW thermal), burning biomass or waste derived fuels, including co-firing with coal and 2. larger (up to 1,000 MW th) power boilers using coal (black coal or lignite) as fuel.
Emerging development includes circulating fluidized beds with supercritical steam data (power boilers) with the first project coming on-line in the near future and research on oxy-fuel fired circulating fluidized beds for CO2 capture (O2/CO2 recycle schemes as well as chemical looping combustion).
INTRODUCTION
‘Getting rid of waste’ was the ultimate goal when the fluidized bed combustion (FBC) technology was introduced. This goal evolved over time
to ‘clean energy for the future’. Since its introduction in the 1970s the technology has gained acceptance in various industrial applications. It is known f‘Getting rid of waste’ was the ultimate goal when the fluidized bed or its ability to burn low-grade fuels with low calorific value, high ash content and high moisture content. Other advantages are fuel flexibility, emission performance, re-use of non-hazardous by-products (e.g. gypsum) and the possibility of the technology to be implemented in an existing plant (retrofit).
As the technology evolved, three variants of this technology were developed. The bubbling fluidized bed (BFB) was the first version of FBC technology Practical experience over the past three decades confirms that BFB technology can be well suited to the utilisation of ‘difficult’ fuels such as high moisture fuels (e.g. wastes and sludge’s), high-ash fuels (e.g. some types of municipal solid waste and refuse-derived fuel) and low volatile fuels (including anthracite, culm and petroleum coke). Although BFB technology has faced increasing competition from the circulating fluidized bed (CFB) variant in recent years, it has maintained an important position in the market.
TECHNOLOGY OVERVIEW
The technology is named after the state of the matter within the fluidized bed boiler and 22 J. Koornneef et al. / Progress in Energy and Combustion Science 33 (2007) 19–55 determines the way in which the combustion process managed. The fluidisation process begins when a bed of inert material (usually sand), which is a solid granular particle, is suspended by a flow of air or gas (air). This flow is injected into the boiler from the bottom and from the side. When the velocity of the gas stream increases, the flow suspends the individual particles in the bed. At this stage, the fuel with a (optional) sorbent (mainly limestone or dolomite) can be injected into the boiler. All bed particles are now in a ‘liquid state’
The base principle has been utilised in two major variants of the FBC technology (BFB and CFB). Although they share the same principle, design parameters of the installations vary considerably. The design of the FBC installation depends mainly on the fuel and required steam conditions, though also influenced by emission requirements, manufacturer and site conditions. The main differences in design parameters are summarised in . A more detailed overview of design, operating and economical variables of FBC installations is given in
the Appendix in. In this section the main components and design parameters of the major variants of FBC, being BFB and CFB, are discussed. Further reading is suggested for detailed insight into the design considerations of the FBC technology.
HISTORY OF FBC
In general, the rate of adoption of a new technology often follows a standard pattern. When the rate of adoption is plotted cumulatively against time, the resulting distribution is often S-shaped (which is also termed a logistic substitution or diffusion curve. According to Rogers this rate of adoption and curve are found for most new technologies. This curve can be divided into different stages. In the invention stage no applications have been introduced into the market. The time lag between invention and innovation can range between 10 and 60 years. The rate of adoption in the innovation phase is low. In 1922, the development of the FBC started with the Winkler patent for gasification of lignite. The technology has been used for different applications since then. Efforts in the 1960s ultimately resulted inthe design of three coal firing test units. The first BFB test facility was commissioned in 1965. This test unit was used to conduct experiments to
establish the potential for controlling emissions of sulphur dioxide. In that same year the Atmospheric FBC Program started in the USA. Subsequently, the USA founded the Environmental Protection Agency (EPA) in 1970, which gave the FBC technology with lower emissions the advantage over conventional coal combustion technologies. FBC could meet the new SO2 and NOx emission restrictions without the use of auxiliary equipment. new restrictions concerning environmental control in the USA were regulated by the Clean Air Act issued in 1971. The development of FBC was not limited to the USA. Other countries like the UK, Finland, Germany and China also started programmes to develop FBC, as they wanted to establish a technology, which was able to burn low-grade fuels with low emissions. Indications for these incentives may be the rising research and development (R&D) budgets for coal technologies in USA and Germany in the period between mid-1970s and early 1980s [25]. R&D of circulating FBC type began in Europe, funded mostly by private industry.
FLUIDIZATION
Fluidization is a technique through which fine solid particles behaves like a fluid through contact with liquid or gas or both. Under the fluidized state, fluidized state, gravitational pull force on solid particles is offset by the fluid drag force.
In fluidized condition particles remain in a semi suspended condition. term 'fluidization' is usually associated with two or three phase systems, in which solid particles are fluidized by a liquid or gas stream flowing in the direction opposite to that of gravity. In these classical fluidized bed systems, the solid particles have a higher density than the fluid. Fluidization where the liquid is a continuous phase is commonly conducted with an upward flow of the liquid in liquid-solid systems or with an upward co-current flow of the gas and the liquid in gas-liquid-solid systems. Under these fluidization conditions, a bed of particles with a density higher than that of the liquid is fluidized with an upward flow of the liquid counter to the net gravitational force of the particles.
it is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like state. This process occurs when a fluid (liquid or gas) is passed up through the granular material.