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Lecture No: 8
Environmental Considerations in Incineration Plant
Environmental Considerations
The Urban Waste-to-Energy plants have to meet stringent standards of pollution
control regulations. The typical limiting values of pollutants discharged by a Waste-to-Energy
plant are:
Table 8.1 Units of Pollutions
Pollutant Limit as per EC Standard
mg/Nm3
Particulates
SO2
HCL
MF
Ph + Cr + Cu + Mn
Ni + As
Cd + Ag
30 mg/Nm3
300 mg/Nm3
50 mg/Nm3
2 mg/Nm3
5 mg/Nm3
1 mg/Nm3
0.2 mg/Nm3
The equipment provided in a typical power plant for controlling pollutants are:
Electro-Static Precipitators (ESP) For controlling particulates
Bag house Filter For controlling particulates
NOx Scrubber For removal of NOx
Chemical treatment For removal of chemicals such as HCL, SOx
Environmental Aspects.
The environmental aspects of pyrolysis/gasification plants will be considered under the
following six headings:
Air
Water
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Land
Noise
Visual Amenity
Worker Protection.
7.3.1Air Emissions
All pyrolysis and gasification processes produce flue gases, since the product gases
are normally used as fuel. The processes currently in use burn the gases directly, although
some processes, now abandoned, did first purify them, usually by wet scrubbing.
The potential air pollutants from pyrolysis/gasification processes are:
Particulates, including some metals
HCl, HF, H2S, NH3, HCN or on combustion
Particulates, including metals
SO2,NOx and HCL.
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Lecture No: 9
Wood and waste for Co-generation- Harvesting Super trees and Energy Forests
9.1. Wood and Wood Wastes as Primary Energy Sources
Presently the largest use of wood and wood-waste is by paper, pulp and lumber
industry. These industries burn wood to obtain process heat. In some industrial units, wood
and wood waste is burnt for producing heat and electricity to cogeneration plants.
Residential fuel in rural and tribal areas is mostly fire-wood.
During the coming decades wood and wood waste is expected to be increasingly used
as the fuel for wood-fired thermal electric power plants.
An EPRI sponsored project+ aims at making the wood economically competitive as a
fuel for wood-fired thermal electric power plants
The main strategy in such cost reduction is
1. To produce dedicated crops of fast growing tree species having higher energy density
(MWh/kg)
2. To harvest, transport, store, dry-out, and eventually burn the wood as whole trees.
3. (Earlier, the trees were cut into sections, chips, and then burnt resulting in increase in
fuel cost by 25%)
The process is as follows:
1. A self-propelled harvesting machine circular sew and manipulator arm in the front
cuts the trees and loads them in the trucks.
2. The trees are stacked in air supported balloon like Drying Structure with a rotating
crane at the centre.
The trees are dried in this structure over a period of about one month for reducing
their moisture content by about 40 to 50 %.
3. The crane then loads the dried trees onto conveyor system in feeds them whole into
the furnace of the boiler.
In the above process first come, first burnt rule is followed in the inventory system in the
drying-structure as the rotating-cranes loads and removes the trees round a broken circle.
The essential steps include:
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Process Heat
Feeling of trees Drying Burning Electrical Power
Generation
The heat obtained from combustion is used as a process heat in the industry and for
producing electrical energy in thermal electric plant
9.2. Harvesting Super trees and Energy Forests. The renewable energy source trees needs
planned harvesting to ensure steady, regular supply for several decades and to maintain
environmental balance. The fast growing tree species are selected for harvesting (Eucalyptus,
Willow, Popal, Gulmohor, etc). Recently, fast growing species of trees are being developed
by genetic manipulation. Some varieties have been selected for woody crops. These treescan grow by about 10 meters in 3 years and can give average kg of wood per square
kilometer per year.
A 100 MW Wood-Fired Thermal Electric Power Plant can be perpetually operated by
a forest-wood harvested in an area within a circle of 10 km radius.
Use of short-cycle woody crops have environmental advantages. Every year, forest is
harvested and replanted keeping the original number of trees without reduction. The profits
are re-invested in expanding the forests.
The forests are used as an important source of renewable energy. They clean the
environment and are necessary for good rain, soil conservation, etc.
Burning of wood produces carbon-dioxide. The carbon-dioxide is recycled by the
green trees. This gives oxygen to the environment. Hence Wood-Fired Power Plants will
improve environmental balance. Moreover, the wood fire boilers do not emit SOx, NOx etc.
into atmosphere and therefore the pollution does not increase.
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Lecture No: 10
Co-generation Plant - Wood/Wood-Waste Incineration Co-generation
10.1. Wood/Wood-Waste Incineration Co-generation Plant
The biomass technology of converting wood/wood waste or forest produce is
emerging as an important renewable energy technology in USA Europe and countries having
excess forests and wood industries.
The energy routes of converting wood/wood waste into energy are:
Incineration of wood/wood waste to produce heat, steam and electrical energy.
Gasification of wood/wood waste to produce methane and carbon dioxide gas, and
charcoal.
One kg of dry plant material yields about MJ energy when burnt directly. (The same kg of
plant material will yield about 0.2 kg of methane by anaerobic digestion.)*
The incineration Plants are designed to burn wood/wood waste to produce hot water,
steam and electrical energy. Several types of wood-waste mixtures, wood from energy crops
etc. are used as fuels.
---- wood logs
---- hogged bark chips, hogged trim ends
---- planner sharings ----- saw dust.
The saw mills built is Forest Based Wood Fired Thermal Electric Power Plants which use the
wood incineration plant to produce heat steam and electrical energy. Table 10.1 gives a date
regarding species of trees grown in India and used as firewood.
10.2. Processing of Wood-waste for Feeding to the Incineration Plant.
The Incineration Plant is usually located in the forest and near saw mill. This reduces
the expenditure of transportation of wood and makes it competitive as a fuel for producingelectricity. The steps in the process are:
------ Felling of Trees in the Forest
------ Segregating logs, tree-barks, leaves etc.
------ Transporting the logs and other residue to central store.
------ Storing the logs in a circular store with a circular crane at the centre.
------ Drying of wood in the circular store.
------ Collecting dried wood by means of central crane in the Circular Store
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and transporting the wood to power plant for incineration.
------ Shredding (Making smaller pieces).
------ Feeding to furnace.
------ Process shown in Fig. 6.1
Similar process is used for incineration of wood waste. Wood waste is generally burnt in
Fluidized Bed Combustion Boilers (FBCB) .
Table 10.1.Species of Trees used in India for Firewood
%Growth rate Abundance
1. Eucalyptus (Nilgiri) Fast Increasing
2. Leucaena
3. Casurina
4. Acacia (Babul)
5. Auericus (Oak)
6. Ficus bengalensis (Vad) slow less
7. Syzygium cumini (Jambhul) slow more
8. Mangifera indica (Amba) slow more
9. Phyllostachya (Bamboo) fast more
10. Anogeissus latifolia (Dhavada) slow more
11. Bombax ceiba (Sawar) fast more
12. Dalbergia sissoo(Shisam) slow medium
13. Azadirachta India (Neem) fast more
14. Terminalia paniculata (Kinjal) fast more
15. Lagerstroemia parviflora (Nana) slow more16. Ficus racemosa (Umbar) slow more
17. Terminalia Tomentosa (Ain) fast more
18. Sesbania aegyptica (Shivari) fast more
19. Cocos nucifera (Coconut) slow more
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Lecture No: 11
Fluidized Bed Combustion Boilers (FBCB)* for Burning Solid Biomass and Fossil
Fuels
Biomass burning process has been simplified by FBCB. Fluidized Bed Technologyhas been developed during 1970s and has become very successful all over the world for
burning solid fuels. A variety of fuels can be burnt in a fluidized bed boiler (Fig. 11.1).
Fig 11.1. Fluidized Bed Combustion Boilers (FBCB)
Table 11.1 Variety of Solid Fuels used in Fluidised Bed Boilers
Rice husk Peat
Wheat husk, nut-shells Saw dust
Sugar-cane bagasse Low grade coals
Municipal solid wastes Wood piecesStraw of rice, wheat, bamboo Dried cow dung pieces
Fluidized bed is a layer of solid particles of fuel and ash in turbulent motion of air-
swirl forced into the bed from bottom. Solid pieces of fuel are added in the bed and gets
burnt.
Heat is produced by swirling churning solid particles (ash) (which are only about 99%
of bed). Fuel particles constitutes only 1% of bed volume, gets heated and burnt. Heat us
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transferred to water and steam flowing through the tubes which are in intimate contact
with the solid particles. Some tubes are in the path of hot gases.
Advantages of FBCB are:
1. Coal burnt in the presence of limestone at relatively low temperature does not giveobjectionable SOx, NOx etc.
2. Lower temperatures (app.850C) gives lesser SOx, NOx and longer life of materials,
reduces maintenance cost.
3. A variety of fuels can be accepted.
4. Quick cold start with auxiliary fuel burners and slightly slower start without auxiliary
burners.
5. No need for costly pollution control equipment for SOx, NOx removal.
6. Lower installation cost maintenance cost.
7. Low objectionable emission product. Hence can be located in the large cities.
8. Calcium oxide in limestone absorbs sulphur oxides (SOx). fly ash is collected by
ordinary fly ash collecting equipment such as fabric filters.
9. Superheated steam even at low ends.
10. No pulverization of coal is needed. Small pieces upto a fes cm. dia. of coal can be
used.
11. Can be used with combined cycle power plants for giving heat to HRSG and
producing steam.
Ref. fig. 11.1. The Furnace (F) integral with boiler (B) is a tall fabricated tank made of
boiler-plates forming water-walls.
Water tube banks are located horizontally in (1) Fluidized Bed (combustion zone) and
(2) upper part of the combustion chamber. Steam drum has water in lower half and steam
in upper half. Super heater (not shown) tubes are also located in the combustion zone of
furnace.
Crushed coal or any other dry biomass fuel mixed with limestone and particulates (fly
ash) form the fluidized bed. Hot air is let in through strategically located nozzles and to
obtain agitation of the bed-particles in the combustion zone and the fuel starts burning.
For rapid start, or low loads auxiliary oil firing burners are also located (not shown).
Summary
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The Urban Waste is disposed off suitable by Waste-to-Energy conversion systems
including:
----Landfill Gas Energy Plants.
----Waste Incineration Cogeneration Plants.
----Biochemical Conversion Plants.
Waste Incineration Energy Co-generation Plants produce hot water, steam and
electricity. The waste is processed and combustible portion (fuel) is burnt a furnace/fluidized
bed boiler. Steam is used by steam turbine generation plant to produce electricity.
Heat Recovery Steam Generator (HRSG) collects heat from exhaust gases and
supplies hot water/steam to process industries. There are strict rules regarding pollutants in
exhaust gases. Electrostatic Precipitators, Filters, Scrubbers, Chemical Treatment Plants etc.are installed. *Such plants are located in large cities and have rating of 50 MW to 150 MW.
The combustion of urban waste is in the range of 1000 t/day to 8000 t/day. Such
plants supply hot water, process steam and electrical energy to consumers located in the city.
Presently, there are about 120 Waste-to-Energy Incineration Power Plants in the world.
Several other mega-cities are planning to have such plants for solving their waste disposal,
environmental and energy problems.
Wood-waste and wood from energy forest is converted with energy by incinerationplants. Super trees are especially grown in energy forests. Waste-wood (chips, saw dust etc.)
from saw mills are furniture industry is incinerated to produce heat, steam and electrical
energy.
Fluidized bed combustion boilers (FBCB) are useful for burning solid dry waste fuels
at relatively low temperature. The fluidized contains a layer of solid ash particles. Fuel pieces
are injected into the bed along with hot dry air. The particles of ash and fuel are subjected to
swirling action which results into heat. The fuel gets burnt and the heat is delivered to the
water circulating in the tubes embedded in the fluidized bed. The steam produced in FBCB is
delivered to the process plant or thermal power plant. Typical ratings of FBCB plants are 10
MW, 15 MW 50 MW. Recently Pressurized Fluidized Bed Combustion Boilers (PFBCB)
have been commercialized (1998).