Energy From Solid And Liquid Wastes - V

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 Particulates

Limit as per EC Standard mg/Nm3 30 mg/Nm3

SO2

300 mg/Nm3

HCL

50 mg/Nm3

MF

2 mg/Nm3

Ph + Cr + Cu + Mn

5 mg/Nm3

Ni + As

1 mg/Nm3

Cd + Ag

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

•

Land

•

Noise

•

Visual Amenity

•

Worker Protection.

7.3.1 Air 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.

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:

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 trees can 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.

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 producing electricity. 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

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

Fast

Increasing

1.

Eucalyptus (Nilgiri)

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

more

16.

Ficus racemosa (Umbar)

slow

more

17.

Terminalia Tomentosa (Ain)

fast

more

18.

Sesbania aegyptica (Shivari)

fast

more

19.

Cocos nucifera (Coconut)

slow

more

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 Technology has 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 Municipal solid wastes Straw of rice, wheat, bamboo

Low grade coals Wood pieces Dried cow dung pieces

Fluidized bed is a layer of solid particles of fuel and ash in turbulent motion of airswirl 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

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 give objectionable SOx, NOx etc. 2. Lower temperatures (app.850°C) 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

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 incineration plants. 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).