Lecture No: 5

Urban Waste-to-Energy by Incineration Process And Energy fromIncineration of Wood

5.1. Introduction.

One of the most important biomass conversation technologies is incineration

(combustion). The following two similar technologies are described in this chapter.

Waste-to-energy by incineration process.

Wood fired steam thermal power plants and energy technology.

5.2. Application of Incineration Process are:

Table 5. 1. Application of Incineration process

Biomass

Resource

Conversion

Technology

Products Applications

Wood

Wood waste

Forest matter

Dry solid biomass

Incineration

(Burning)

Heat *

Steam *

Electricity

Co-generation plants, heat

steam, electricity for industry

and utility

Urban solid waste

(Municipal refuse)

Incineration Heat *

Steam *

Electricity

Disposal of urban waste

Energy to urban

consumers

*In cold countries heat is supplied by the utilities for heating the houses. Steam is

required for process industries.

The other processes of waste to energy and wood to energy are

Production of biogas from waste, by methane fermentation process

Production of wood gas from wood,

Production of biogas from landfills, etc.

These are described in a separate chapters.

5.3.Urban Solid Waste (Municipal Waste, Municipal Refuse)

The number of mega-cities is rapidly increasing in various arts of the world. The

domestic waste (Refuse) in these cities is usually sent to landfill sites located far away (50 to

200 km) from the centre of the city. Large cities like Delhi, Bombay have large amount of

waste and increasing waste disposal problems. The emerging solution is to produce useful

thermal and electrical energy by waste-to-energy plants (WTE) located in

Fig 5.1. Energy routes of urban waste to energy

the heart to the city. Such energy plants are rated in MW range (50 to 200 MWe) and serve

the following functions Safe and economical disposal of urban waste.

Supply of electrical and thermal energy to the consumers in the city.

Environmental protection from urban waste

WTE plants have been successfully built and or being operated in several mega-cities

in the world since 1985 and several such plants are being built during recent years.

Composition of Urban Waste. The composition of urban waste depends on the

standard of living and style of living of the people. Typical composition of urban waste in

Europe is given in Table.5.2.

Table 5.2. Typical Composition if Urban Waste in Europe

Waste Material % Waste Material %Paper 51 Plastic, rubber 4Food-rubbish 20 Wood 2

URBAN SOLID WASTE

LANDFILL GAS PLANT

WASTE INCINERATION CO-GENERATION PLANT

SCRAPBIO-CONVERSION PLANT

BIOGAS

BIOCHEMICALS

BIOGAS BIOGAS

BIOCHEMICALSHOT WATER

STEAM

ELECTRIC POWER

FOR RECYCLE

RESIDUE, LANDFILLS, MANURE

RESIDUE, LANDFILLS, MANURE

ASH TO LANDFILLS. MANURE

Metal-scrap 10 Textile 2Glass 9 Miscellaneous 2

The heat content of refuse varies very widely, sometimes by 125%. The average heat

value of the urban waste is only about 50% of that of coal on mass basis.

The heat of combustion of urban waste is 5 to 8 MJ/kg and is likely to increase to 10-

12 MJ/kg.

5.4. Waste-to-Energy Incineration Process

The energy route of the waste-to-electrical energy by incineration process is as

follows:

Biomass

energy

from

nature

Thermal

energy

from inci-

nerators

Mechanical

energy

from

steam

Electrical

energy

from

generator

Electrical

energy to

user or

grid

5.5. Choice of In-feed, Range and Location of Plants

The incineration Process accepts a wide variety of biomass inputs including:

Semi dried wood, trees, tree residues, wood-chips, saw-dust

Semi dried garbage (urban waste)

Semi dried farm waste (dried cow-dung, straw, sugar, bagasse, etc.)

Mixtures of fossil fuels and biomass for higher heat content of the infeed.

Steam is supplied to steam-turbine power plant (50 to 150MW)

Heat (hot water) is supplied for district heating in cold countries.

Steam is supplied to process industry.

Waste Incineration Power Plant is usually located near the source of waste thereby

minimizing the cost of fuel transport.

Table 5.3. Locations of Waste-to-Power Plants

In Feed Location of Plant Output

Forest Produce

Trees, Tree residue

Wood

Wood waste

Forest

Near furniture

industry

Electric power

Heat/steam for

furniture industries

Sugar bagasse Near sugar

Producing plants

Electric power

Heat, steam for

sugar plant

Urban waste In a large city Electric power

Heat and steam for

urban consumer

Lecture No: 6

Schematic of a Waste Incineration Energy plant - Furnace and boiler in waste

incineration plant

6.1. Waste Incineration Process

The waste incineration plant has following process-sequence. (Fig.6.1)

Fig 6.1. Waste to energy by Incineration Process

1. Receiving and storing of dry biomass and preparation of Refuse Derived fuel (RDF)

2. Shredding ( Making pieces of about 2.5 cm dia)

3. Air Classification: Passing air stream for segregation of Refuse Derived fuel ( lighter

pieces and non-combustible heavier matter (metal, glass pieces). These are reclaimed

and recycled.

4. Combustion of Refuse Derived Fuel (RDF) is the furnace to produce steam in boiler

from feed water.

5. Steam turbine drives rotor of synchronous generator to produce electrical energy.

6. Electrical power is supplied to the network.

7. Flue gases are passed through

Electrostatic Precipitators for removal of particulate.

Chemical treatment plant and scrubber for removal of CO, NOx, SOx.

Heat is recovered from flue gas by heat Recovery Steam Generator (HRSG). The

recovered heat is used for heating feed water. The ash derived from furnace is sent ot special

landfills for disposal.

6.2. Schematic of a Waste Incineration Energy plant

The schematic diagram of the entire process is shown in Fig. 6.2 (a)

Fig 6.2 a. Schematic of a Waste to Energy plant for urban waste incineration

1. Furnace Cum Biler

2. Heat Recovery Steam Generator

3. Stack (Chimney). Flue gases flow through (3’)

4. Steam turbine

Electro Static Separator

Bag Filter Scrubber, Chemical Treatment Plant

5. Electrical Generator

6. Steam condenser

7. Cooling Tower

8. Cooling water loop

9. Feed Water pump

10. Stack (Chimney)for exhaust gases

Urban Refuse may be treated in a waste treatment plant before dispatching it to the

power plant. Solid, Treated Dry biomass (Wood, Urban Waste, Agricultural or Aquatic

Waste) is received and stored (1) The Shredder (2) makes small pieces of the raw biomass.

The air classifier (3) has flowing air which separates dry light fuel pieces from heavy

metal/glass and other by strong air stream. (Horizontal or vertical). The lighter fuel pieces are

sent to the furnace (5). The heavy metal pieces, glass pieces etc. are sent to material recovery

chamber.

The furnace (5) burns the dry shredded biomass and the boiler (6) produces steam.

The superheated steam (B) is supplied to the steam generator (13). The synchronous

generator (14) produces electrical energy from mechanical shaft energy.

Flue glasses (C) from the furnace are passed through

Heat Recovery Steam Generator (7).

Electrostatic precipitator (8) and filters for removal of particulates (D).

Scrubber & Chemical treatment plant (9) for removal of SOx and NOx,

Induced Draft Fan (10) sends the cleared exhaust gases through the stack (11) to the

external atmosphere (E).

Steam from turbines (13) is condensed in condenser (12) and is re-cycled through the

boiler (6).

6.3. Furnace and boiler in waste incineration plant

Fig.6.2 (b) illustrates the furnace-cum-boiler. Municipal ( Domestic) waste is

collected and fed into the hopper (1).

Fig.6.2.b. Furnace cum boiler and accessories in an Urban waste to energy plant

1. Feed Hopper for dry, selected, shredded waste

2. Ran feeder (feeds the fuel to 3)

3. Incineration grate

4. Slag prevention system

5. Forced draft system for air

6. Secondary air system

7. Ignition and auxiliary firing system

8. Boiler plant

9. Ash Conveyor

It is burnt in grate (3).

Forced air draft (5) from bottom of grate (3) helps in combustion.

Firing is initiated and sustained of auxiliary oil fired burners (7).

Particulars of combustion products are collected from the gases by electrostatic

precipitators, fabric filters and dumped into ash-conveyors (9).

Fuel gases are cleared by SOx, NOx, CO removal system and let into atmospheric via a

stack. Waste incineration Plants provide solution to utilize domestic waste, municipal waste

and produce useful energy. The plants are usually located within large cities so that

transportation of waste is minimized. Another method is obtain biogas from Wasteland fills

and use the gas as fuel.

Lecture No: 7

Electrical Scheme of Urban Waste-to-Energy Plant

The electrical power generated by the synchronous generators in the waste-to-energy

electric power plant is supplied to the local distribution system or the local consumer (e.g. a

sugar factory). The electrical system comprises of.

---- Main electric circuit equipment through which the electric power flows from the

generator flows to the load. This circuit is at higher voltage (3.3 kV, 6.6 kV, 11 kV).

---- Auxiliary electric power circuits for feeding electric power in generator auxiliaries,

boilers auxiliaries, station auxiliaries etc. These circuits are at high, medium and low

voltages.

----- Auxiliary electric control and protection circuits. These are at low AC and DC

voltages.

Table 7.1Main Technical Data of an

Urban Waste-to-Energy Plant at Newstadf, Germany

Furnace,Number of furnace Incineration capacityGrate widthGrate areaGrate driveRange of lower heating value of Refusal FuelCombustion Air System,SystemSteam capacitySteam pressure+Steam temperature at outletBoiler feed inlet water temperatureBoiler design pressureBoiler cleaningFuel gas cleaning processCleaning capacity

TurbineInlet steamOutput (nominal)Steam throughoutExhaust steam pressureSpeed of rotorGenerator,Nominal power rating

18000kg/hour3,6m30m2

Hydraulic, stepless6.6 to 15 MJ/kg

Integrated two-bars corner tube boiler26 t/h42 bar420°C130°C55 barrappingCiba-Geigy/Von roll66000 Nm3/h

40 bar, 400°C5.3 MW25 t/h0.12 bar10400 rev/min

6300 KVA0.8

Power factorNominal voltageFrequencyShaft speedMain transformer,Nominal power Voltage ratioCooling (Oil natural, Air natural)Emergency diesel generator set,Nominal outputNominal voltageClosed Circuit Cooling Water Plant,Flow rateHeat exchanged

11 KV50 Hz1500 rpm

5000 KVA11 kv/30 kvONAM

350 kVA0.4 kV

150 m3/hour3GJ/hour

Table 7.2Comparison of various Renewable Resources

for Producing Electrical Energy. (Considering favorable location only)

Renewable EnergySource

Cost per kWh of ElectricalEnergy Present

(1991)Mid-range(1995-2000)

Long-Term(Beyond 2000)

Biomass *

Solar thermal

Solar PV

Wind

Geothermal

5¢**

10¢

35¢

8¢

6¢

5¢

8¢

15¢

5¢

6¢

4¢

7¢

6¢

3.5¢

5¢

Note * Requires subsidy for tipping fee.

The two main routes include 1. Burning of trees, 2. Burning of urban waste.** ¢ Cents (American currency, 1 $= ¢)

Fig 7.1. Electrical scheme of a Urban Waste Incineration Power Plant