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The need for finding renewable sources of energy together with the necessity of searching for new technologies to reduce the negative impact of waste accumulation has led to the possibility of using biomass as an alternate fuel, especially in electric utilities. Most of the industrial sources of pollution come from coal-fired power plants, which necessitate the need to find ways to decrease the Greenhouse gas emissions from these. Also, the ratification of the Kyoto Protocol and the tightening environmental regulations means that by the year 2008, countries would need to implement measures to meet these standards. One of the options that need to be considered is the application of cofiring technologies to coal fired power plants. In this paper, we seek to review the various cofiring technologies available and also at the methods by which these standards could be met.
ABSTRACT:
The need for finding renewable sources of energy together with the necessity of searching for new technologies to reduce the negative impact of waste accumulation has led to the possibility of using biomass as an alternate fuel, especially in electric utilities. Most of the industrial sources of pollution come from coal-fired power plants, which necessitate the need to find ways to decrease the Greenhouse gas emissions from these. Also, the ratification of the Kyoto Protocol and the tightening environmental regulations means that by the year 2008, countries would need to implement measures to meet these standards. One of the options that need to be considered is the application of cofiring technologies to coal fired power plants. In this paper, we seek to review the various cofiring technologies available and also at the methods by which these standards could be met.
KEYWORDS: Cofiring Technologies, Greenhouse
INTRODUCTION:
COFIRING, the Simultaneous combustion of a supplementary fuel with a base fuel, has been a traditional method for introducing new or different fossil fuels and opportunity fuels such as petroleum coke added to a coal based fuel in a boiler. Cofiring Biomass with coal is proving to be the lowest cost method for generating GREEN POWER in utility plant demonstrations. It also reduces the emission of fossil based CO2. Non woody bio fuels such as agribusiness wastes can also be cofired, given the proper fuel preparation and feed systems. The potential benefits of cofiring do not end at CO2 emission reduction, however it can reduce emissions of regulated pollutants specifically SO2 and NOx .
Technologies:
Cofiring involves the co-combustion of coal with a renewable source of energy. Cofiring technologies include combustion /gasification of
a) Coal and Bio mass
b) Coal and Bio gas
c) Coal and Biomass co gasified with plastic wastes
d) Coal and sewage sludge
e) Coal and Biomass-oil slurry fuels
EMISSIONS:
There is approximately a one to one relationship between SO2 reductions and the percentage of total heat input from biomass. For instance: By using biomass for 5% of a coal-fired power plants heat input would reduce SO2 emissions by almost 5% By carefully adjusting the combustion process, NOx reductions at twice the rate of biomass heat input have been documented. E.g.
Received on 09.08.2017 Accepted on 11.10.2017
If biomass is cofired at a 5% heat input rate, then the power plant could achieve NOx reduction by 10%Methane, which is the main component of natural gas is normally discharged in to the air but can be captured and used as a fuel to generate electricity and heat
BIOMASS COFIRING:
This involves the cofiring of coal with biomass. This could be done in the following three ways
a) Biomass blended with coal on fuel pile
b) Coal and biomass separately prepared and injected
c) Biogasification
The first technique is the low cost approach for cofiring. This can be approached at low percentages. In the second technique, the biomass is prepared separately and then injected in the boiler. Hence, the biomass bypasses the pulverizer. This approach requires careful attention to particle size. The third approach, namely gasification has significant potential since it permits the use of biomass in Natural gas fired systems. Here biomass is fed into a gasifyer to generate a producer gas. While this approach is the most capital-intensive approach for cofiring, it is also the most flexible, in terms of the base fuel used and the electricity generating system appropriate to its applications
In 1995, various tests were conducted at TVA sites across the U.S, following which commercialization of technology was discussed. Technology developments that resulted included "Design and development of a cofiring system for wall fired PCB's, Development of designer opportunity fuels for cyclone boilers, development of complete biomass fuel profiles for selected biofuels etc.". On studying the various fuels it was found that different biofuels present different opportunities for cofiring. It was seen that lumber mill sawdust was the most efficient fuel with furniture waste sawdust proving only slightly less so. The EPRI had also conducted tests with the objective of discussing biomass as a renewable fuel to supplement coal in utility boilers. In an analysis of the supply curve, it was found that cofiring is more effective for wall fired than Tangential Boilers. Analysis of NOx emission showed that the percentage of cofiring had an almost proportional increase on NOx reduction for wood / Eastern Coal but a negative NOx reduction for Wood chips / Western coal.
Thus it was concluded that from an emission point of view, the substitution of coal by a typical biomass is advantageous. The effects of cofiring of Biomass / Pulverized coal in PCF boilers focus on Ash deposition, NOx production, Corrosion and Carbon burnout. There is negligible sulphur content in most biomass fuels. SOx emissions uniformly decrease on cofiring in proportion to Biomass Thermal Load. Ash deposition efficiencies of herbaceous material exceed that of coal, while that of wood is lower. The main concern about corrosion is that the high temperature corrosion of superheater tubes induced by the presence of chlorine on the surface. The amount of chlorine in deposits decreases with an increase in Sulfur. In a test conducted on the co combustion of coal and waste wood, in an industrial steam boiler, it was found that the lowest CO and NO emissions were for a 60/20/20 (by mass) fuel blend of MDF - lignite and power poles.
The minimum SO2 emission was for an 80/20 (w/w) blend of natural wood and lignite. Tests indicated that NO emissions depended only on operating conditions and not affected by nitrogen content of fuel. Low SO2 emissions are observed with a decrease in lignite content of raw fuel.
A study conducted by the TVA shows that:
a) Using cofiring reduces the efficiency of boiler by only 1.5%, compared to coal only firing.
b) Coal consumption decreases as wood contributes to 27.4% of mass of coal
c) NOx emissions could decrease by as much as 10%
COFIRING OF COAL AND PLASTIC WASTES:
Plastic wastes have a high-energy content and are suitable for co-processing with coal. It has been found by Petra. E. Campbell et al, 2000 that the addition of 5% plastic waste to a coal bred PF power plant reduces the efficiency slightly from 44% to 43.7% due to the high moisture content of the plastics. It has been shown that plastic particles up to 2 mm can be cofired in a PF boiler without adversely affecting the degree of combustion (Christill et al, 1996).
COFIRING OF BIOMASS-OIL SLURRY FUELS:
This approach was investigated by Benter et al, where slurry fuel was produced from a mixture of oil and finely ground Biomass. Here, the wood was prepared by the convertech process, which was developed to continuously wash, auto hydrolyze and dry wood chips. Finely ground wood particles settle in fuels such as Diesel or Kerosene due to their greater density and thus need to be stabilized if long term storage is required. It was found that auto hydrolyzed wood particles can be prevented from settling by emulsifying the oil-particle mixture with a polar liquid. It was discovered that certain mixtures of kerosene, ethanol, water and wood form a stable emulsion without any additives for more than 30 days. A castor oil based thickener was found to successfully stabilize the slurry. If long-term storage without settling is required, the apparent viscosity remains low at higher shear rates. To achieve this, one or more additives which either directly or indirectly induce interactions between particles and thereby prevent them from settling Another way is to add a polar liquid such as water and/or a short chain alcohol to fuel oil to form an emulsion. Here, the particles get caught between the droplets in the continuous phase, which prevents them from settling. Thus, non-ionic surfactants (to stabilize the mixture as an emulsion) and thickeners (to stabilize the pure mixture) were identified. Also addition of ethanol and methanol gave promising results.
ENVIRONMENTAL BENEFITS:
Several potential benefits are the driving interest for cofiring. Leading the way is the possible environmental advantages of cofiring, particularly the ability to reduce emissions of acid rain precursors such as sulphur dioxide (SO2) and nitrogen oxides (NOx), as well as carbon dioxide (CO2) from fossil sources.
As wood residues have essentially no sulphur content, cofiring of wood reduces SO2 emission by the percentage of the heat content provided by the wood. Cofiring biomass with coal would allow power producers to earn SO2 emission allowances under the Clean Air Act Amendments. Allowances are earned for each ton of SO2 emissions avoided and for every gigawatt-hour produced by biomass in a cofired boiler. These allowances, currently priced at about $200 per ton, may be sold or traded on the open market.
Most wood fuel sources have low nitrogen levels, running one-third to one-tenth those of coal. In addition, moisture in wood can cool the coal combustion process, reducing the formation of thermal NOx. Further NOx reductions can be achieved by cofiring wood as a reburn fuel - downstream from the primary combustion zone - to destroy some of the NO produced upstream, according to laboratory combustion tests conducted under DOE sponsorship. Findings indicate that NOx reductions of 50 percent might be achieved in a full-size cyclone boiler and 30 to 40 percent in a PC boiler. The actual dollar value of these and other NOx reduction efforts will be site-specific, depending on the cost of alternative control techniques.
Another environmental benefit of cofiring involves reduction of CO2 emissions for greenhouse gas mitigation. While fossil-fueled-based systems add to the total atmospheric loading of CO2, biomass absorbs about the same amount of CO2, during its growing cycle as it is emitted from a boiler when it is burned. Net emissions are not actually zero, due to transportation of biomass fuels to power plants, but they are a small percentage of thee equivalent fossil fuel emissions. Experts estimate that the displacement of one megawatt of coal-fired generating capacity by biomass feedstock offsets about 6,000 tons of CO2 per year. A recent study found that cofiring biomass in the nation's coal plants at levels of 2 to 15 percent could reduce fossil fuel emissions of CO2 by 26.7 million metric tons per year.
COFIRING ECONOMICS:
While the environmental advantages of cofiring are promising, the economy of cofiring for any plant is sitespecific and depends on several factors. Some of these factors are related to the plant design - e.g., costs can increase significantly if plant retrofit includes addition of facilities for feedstock size reduction, wood drying or separate feed systems for wood fuels.
Generally, the capital costs for plant retrofitting to accommodate cofiring are relatively low - ranging from $50 to $400 per kilowatt. The lowest-cost opportunities, by far, are with cyclone boilers, where capital costs may be as low as $50 per kilowatt. For PC boilers, retrofit costs average $150/kW to $300/kW.
These costs are presented in terms of biomass capacity. For example, cofiring 10 percent biomass at a 300 MW power plant gives a biomass capacity of 30 MW.
However, determining whether a cofiring plant can be run economically enough to cover these capital costs is largely a matter of the availability and price of biomass feedstock's within 50 to 100 miles of the plant. The viability of cofiring at any coal-fired plant, in fact, can turn on the wood supply costs for that plant. Studies conducted for EPRI by Foster Wheeler showed that a 50mile change in plant location has significant impacts on the fuel supply costs. EPRI's Hughes estimates that to be economical, biomass fuel must be delivered at a price $0.25 to $0.40/MMBtu below the price of coal.
INDIAN SCENARIO:
Out of a possible 19050 MW of possible power generation, only a fraction of power is generated using biomass every year. This suggests the need to explore the use of biomass as an alternate source of energy. India, as one of the leaders of the G77 has always stressed on an increased North South cooperation. Bearing this in mind, India should, in principle accept the Clean Development Mechanism (CDM) and be ready to participate. It should be noted that a significant portion of carbon emission reduction could be undertaken as CDM projects, which will increase foreign exchange, flow. Again, it is possible to take advantage of regional cost difficulties in greenhouse gas abatement technologies which make it cheaper in developing countries Estimates show that by 2015, India will have between 1400 and 1700 MW of bio-power capacity. India is a good target for cofiring, because it has many older coal fired power plants where biomass cofiring could be used to economically improve environmental performance. Biomass cofiring experience have been tried out and implemented in paper mills and sugar mills in Tamilnadu. Efforts are also being taken to cofire coal and oil refinery wastes in Tamilnadu.
CONCLUSION AND FUTURE SCOPE:
The tests conducted so far have confirmed the technical feasibility of biomass cofiring and has documented performance and the potential for some benefits, such as lower NOx emissions (versus 100% coal) and lower costs of power generation (versus those of most renewable energy options and many CO2 capture/sequestration options). Ongoing and planned research will target continued progress on these and other technical and economical issues.
Key issues include:
* Influence of cofiring on boiler efficiency and load capacity. This would identify operational changes that might mitigate potential losses in boiler efficiency and loss of load capacity due to wet and lower-Btu fuel.
* Influence on flyash properties and potential effects on ash sales. Tests to date point minimal impact of cofiring on ash quality and on other technical issues that affect marketability of ash. However, an important, and current, ASTM standard has long required that ash used in cement manufacturing be made from coal only.
* Impact on boiler slagging and fouling patterns, and the extent of any associated maintenance requirements. These are not expected to be in issues for clean wood wastes, but could be issues in cofiring straw, grass, chicken litter or other high-alkali or high-chlorine biomass.
* Development of storage and fuel-preparation guidelines for specific boiler applications.
* Development of concepts that can reduce capital and operating costs.
* Maximizing the potential for NOx reductions.
* Level of biomass cofiring (i.e., the fraction of biomass fired with the coal) that is conducive to optimum and cost-effective operation, for example, pushing for 15% by heat.
On all of these issues, the most important advances will be:
1) Data coming from long-term, ongoing operations
2) Improved technology arising from improvements discovered by the operating and managing staff as a result of their hands-on experience.
REFERENCES:
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Copyright A&V Publications Oct-Dec 2017