Năng lượng từ rác thải: Một nguồn năng lượng tiềm năng mới

WASTE-TO-ENERGY (WTE) A NEW SOURCE OF RENEWABLE ENERGY

 Tạp chí Khoa học năng lượng – IES 

(Số 04-2014)

Bài báo đã xuất bản tại: THE THIRD INTERNATIONAL SCIENCE CONFERENCE ON SUSTAINABLE ENERGY DEVELOPMENT, Hanoi, 2013. ISBN: 978-604-913-137-0, Publishing House for Science and Technology, PP.29-35

 Prof. Le Xuan Que1, 2, PhD. Doan Van Binh2

1Institute for Tropical Technology, 2Institute of Energy Science

 Abstract

 Waste-to-energy (WTE) is an efficient way to treat municipal waste (MW). The technology has been considered as a perfect environmental solution, beside reducing landfill area, generating electricity and also minimizing the greenhouse gas emission.

 I. INTRODUCTION

 Dally household waste (domestic waste, garbage, or simple wastes) is a part of the modern life. The concentration of population at high density and the urbanization have been causing a large quantity of municipal wastes at a limited area and in short time, and continuously 7/7. The quantity and density (per person and per surface) of the wastes constantly increase. Quantitatively the growth of the wastes usually arises about 10% - 20 % per year. Especially for the developing countries the increasing rate widely exceeds that of the industrial pays. However the necessary facilities for the waste treatments of those countries are limited, on both inverts and technology.

 Table 1. Illustration: The number of urban types from 2005 - 2015 (*)

 

Year

Special type

Type 1 (City)

Type 2 (City)

Type 3 (City)

Type 4 (Town)

Type 5 (township)

Total

2005

2

4

14

22

52

621

715

2007

2

4

13

43

36

631

729

2010

2

9

13

43

43

624

734

2011

2

10

12

47

50

634

755

2015

2

9

23

65

79

687

870

Note: Figures from the Department of Urban Development, Ministry of Construction, 2011.

The problems of municipal wastes are considered in an international scale. However the national solution just has been the most important, decisive. Even for the poor countries, in general, the finance invert does not possess the key position. Resolution of the wastes problems exclusively depend on the ‘waste politics’ and technological application and development.

Many countries have controlled waste problem, especially they have transferred the municipal waste into row materials of industrial measure. Namely, the Scandinavia Countries, Singapore have been applied WTE relatively for 100% municipal waste. Some head industrial countries, such as Japan, America the U.S states, Germany…, have developed sophistic technology for using and recycling municipal waste.

Outstanding type of waste treatment technology must be mentioned transforming the waste to electricity. Electrical energy itself has been considered as the clean energy, with the highest values​​ and technological content in the all form of energy. However, nuclear power, electricity from fuel minerals contain in itself the problems can not be solved. The coal and oil depletion, climate change has changed the perception of the modern world. The coal and oil depletion, climate change has changed the perception of the modern world, do appear clean energy needs, and the concept of renewable energy, sustainable development was born. Renewable energy includes types original from sun emission (solar sells, wind electric system, hydro power, biomass), from earth (geothermal, tide - wave energy), from hydro, biofuel, and specially, from household waste. Waste-to-energy is just perfected strategy to the environment and green energy.

II. MUNICIPAL WASTES

Municipal wastes include organic wastes such as paper, cardboard, food, yard trimmings, and plastics, and inorganic wastes such as metal and glass. The average ratio of the waste composition depends on some factors, in general on cities, regions, seasons, years. In the U.S the total quantity of wastes 2010 is 250 million tons, the ratio of the composition of waste is presented in figure 1.

Breakdown of MSW generated in the United States in 2010

Source: EPA, http://www.epa.gov/epawaste/nonhaz/municipal/images/index_pie_chrt_900px.jpg

Figure 1. Breakdown of MSW generated in the United States in 2010

Table 2. Waste composition of  Hanoi

Source: Brief report of the municipal waste management Hanoi, 2005

TT

Composition

URENCO data,  %

JICA data, %

1

Organic (food,)

50.27

47.51

2

Paper

2.72

7.28

3

Plastic, rubber

0.71

7.47

4

Wood, textile

6.27

1.92

5

Bone, shell, snail

1.06

0.96

6

Brick, stone, concrete,…

7.43

4.41

7

Glass

0.31

0.77

8

Metal, cans

1.02

0.38

9

Others

30.21

29.32

Average pH: 6,5-7; Average Humidity: 60-67%; Density: 0.38-0.416 tons/m3

In Vietnam, urban waste problem has appeared in many places, from many years ago (an illustrative in tables 2 and 3). The government has identified disposal strategy for 2015 and beyond visibility (Decision of the Prime Minister No 2149 December 17, 2009), concentrated management of MSW with the aim of strengthening recycling technology.

The analysis results of the wastes composition of some cities of Vietnam are presented in table 2 and 3. It í clear that the waste compositions are principally the some for all 5 cites exceptionally HCM city.

Table 3. Solid Waste Composition (weight %) of some cities of Vietnam

TT

Composition

Ha Noi

Hai Phong

Ha Long

HCM city

Da Nang

1

Organic waste

56,1

52,58

50,1

51,25

51,5

2

Nylon, plastic, rubber

5,5

4,52

3,7

8,78

7,5

3

Paper, textile, carton

4,2

7,52

5,5

14,83

6,8

4

Metal, cans

2,5

0,22

0,5

1,55

1,4

5

Glass, ceramics

1,8

0,63

4,1

5,59

1,8

6

Others

29,9

34,53

36,1

18

31

Properties

7

RH , % wt

47,7

45-48

40-46

27,18

39,05

8

Ash, % wt

15,9

16,62

11,0

58,75

40,25

9

Density, T/m3

0,43

0,45

0,57-0,65

0,412

0,385

Source: Report on the environmental monitoring stations and national environmental analysis, 1998.

III. PRINCIPAL TECHNOLOGY FOR MOW TREATMENT

Technologies utilized as part of the criteria for recycling and energy generation include

+ Thermochemical: - Combustion, - Gasification, - Pyrolysis, - Plasma

+ Biochemical: - Anaerobic digestion, - Fermentation

+ Landfill

These technologies can be combined, and are used with emission control equipment and monitoring systems to substantially reduce emissions to meet the stringent air emission limits established through the permitting process with the specific Environmental Protection Agency (EPA) air district and local air management district in each state.

The primary challenge facing these technologies is the heterogeneous nature of MSW, which creates a widely varying chemical constituency of the energy products generated from these processes. This variance affects the ability to efficiently extract energy. It has been focused for the feedstock preparation, shredding, and/or mixing MSW to make the feedstock more homogeneous, especially for the thermo-chemical transformation. This homogeneity will be reflected in the energy product(s) and help improve its utility.

Many communities as well as local and state governments have implemented zero-waste strategies, where they utilize the reduce, reuse, recycle, and compost (or 3RC) strategy, WTE, and landfill as a path to minimize the potential for pollution of air and ground water.

However the landfill technology possesses up to 60%, Figure 2. Recycling efforts are also being implemented successfully by many organizations.

Hanoi MSW in 2011 reached to more than 6,500/day, the main component is organic matter: vegetables, kitchen garbage, and other substances. This waste was processed by landfill (mostly), by recycling in small companies (only 60 tons/day), and only 10% were recycled spontaneously in the craft villages (Hanoi URENCO, 2011, and the National Environmental Report 2011).

CPDVMT Thang Long Company has successfully researched technological process, systems and equipment handling solid waste disposal by burning technology with heat recovery. Compared with other incinerator technologies in Vietnam, the company's technology to achieve many new features such as using cutting equipment to recycle waste evenly, creating conditions for waste drying process with high efficiency; Thu heat recovery from waste flue gas for drying and hot air drying for incinerators. In addition, this process technology has shown through innovative waste classifications such as uniform size and composition of solid waste ; uniformity of litter size prior to drying to enhance the result of the drying process , eliminate waste unburned components of the waste prior to combustion.

 

Recovery and discards of materials in MSW

 Figure 2. Recovery and discards of materials in MSW, 1960 to 2010

Especially with the ability to recycle drying and air drying, the technology could help save a part from the burning fuel through the garbage and litter drying air entering the kiln with the temperature of the environment. Furthermore, the disposal of this technology is effective for handling environment allows the maximum amount of waste can cause pollution, emission reduction of 75 % of greenhouse gasses and reduce the range 85 % of land for waste disposal than landfilling.

The factory s waste disposal in Son Tay cityFigure 3. The factory's waste disposal in Son Tay city; Illustrative

(JSC Environmental Services Thang Long)

With the ability to widely apply in industrial scale, easier to fabricate devices using readily available local materials, easy to manage and operate, low cost investment and the equipment to available alternative technology, this research has been applied in waste treatment plant Son Tay with a capacity of 300 tons/day, official activities from 01/01/2012, disposal for Hanoi. The processing capacity has reached 83 % design capacity.

1. Combustion

Direct combustion is the most common method of producing heat, power, or CHP from MSW resources. In a direct combustion system the MSW is burned to generate heat. The heat is then used to boil water in a boiler, which can be used for heating/cooling applications, other technique applications, or for driving steam turbines to generate electricity.

 2. Gasification

 Gasification is an emerging WTE technology in which fuel is heated in a limited-oxygen environment, otherwise known as partial combustion.

 Gasification is a high-temperature process that is optimized to produce a fuel gas from dry biomass with a minimum of liquids and solids. Gasification consists of heating the feed material in a vessel with partial addition of oxygen or air. Water might or might not be added. Thermochemical reactions take place, and a mixture of hydrogen and carbon monoxide (CO) are the predominant gas products, along with water, methane, carbon dioxide (CO2), nitrogen (if air is used), and other hydrocarbons such as C2H2, C2H4, and C2H6. The resultant gas is called variously producer gas or syngas (synthetic natural gas).

3. Pyrolysis

Pyrolysis is a high-temperature process that is optimized to produce pyrolysis oils, bio-char, and synthesis gas from dry biomass. Pyrolysis consists of heating the feed material in a vessel without the addition of oxygen. Decomposition reactions take place, and a mixture of hydrogen and CO are the predominant gas products. Other products include pyrolysis oil, water, methane, and CO2. The resultant gas is called variously biogas, producer gas, or syngas (synthetic natural gas). The composition of the resultant fuels is determined by a combination of the initial mixture of feedstock constituents, temperature, and time within the reactor.

IV. POWER GENERATION

 The process of steam generated through direct combustion includes the generation of steam at a set pressure, then adding additional heat to produce a superheated steam. For the direct gasification - generation the process includes the gasification - cleaning gas - engine -generator. This process obtains higher energy efficiency. Basing on the calculated average heat value of MOW, about 7000 - 1100kJ/kg, with an average heat value of about 8000kJ/kg, heating or burning of gas will transform urban organic waste becomes important fuel source.

Table 4. Heat value of some waste component, Dry Weight

Component

kJ/kg

Btu/lb

Food Waste

4648

2000

Paper

16731

7200

Cardboard

16267

7000

Plastics

32533

14000

Textiles

17428

7500

Rubber

23238

10000

Leather

17428

7500

Garden trimmings

6507

2800

Wood

18590

8000

 In Vietnam total quantity of solid waste in urban, across the country, increased average 10 ÷16% per year. In most urban areas, the volume of SW accounts for about 60-80% of the total urban SW (some urban areas this percentage up to 90%). Table 5 shows concerned urban situation on the MSW. It is important that only in urban areas the total quantity of MOW arise 3-5 minion tons per year.

Table 5. Solid Waste in Urban of Vietnam, years 2007 - 2010

Parameters

2007

2008

2009

2010

Urban Population (million)

23,8

27,7

25,5

26,22

% Urban population over the country

28,20

28,99

29,74

30,2

Urban SW (kg / person / day)

~ 0,75

~ 0,85

0,95

1,0

The total urban SW (*1000 tons/ day)

17.680

20.850

24.230

26.240

MSW 70% of  ( 1000 tons/ year)

4455.36

5254.2

6105.96

6612.48

OW=75% MSW (1000 tons / year)

3341.52

3940.65

4579.47

4959.36

 (Hanoi URENCO, 2011, the National Environmental Report 2011).

 

 Taking average heat value of municipal organic waste 8000kJ/kg, theoretical energy from WTE can be calculated, table 6. A comparison of some principal gasification technology helps to review suitable type for the first power engine using gas fuel. In practise for small scale it seems better to choose fluidised bed gasifier, table 7. 

Table 6. Calculated*) Waste-to-Energy, urban of Vietnam, years 2007 – 2010

Average heat value taken is 8000kJ/kg

Parameters

2007

2008

2009

2010

Total MOW (1000 tons / year)

3341.52

3940.65

4579.47

4959.36

Total calculated Energy of the MOW, GJ/y

26732.16

31525.2

36635.76

39674.88

*)Average values were taken, even different ratio of organic waste,

 Table 7. Comparison of some gasification technology types, (* bad, ****very good)

Gasifier types

Materials

Gas product

Technology

Perspectives

Economy

EF

Entrained flow

** d<1mm, low ash, Humid. 15%,

*** Poor CH4, C2+ and tars, rich of H2 and CO

*** Demo, experienced industrial applications

**** Large plant, centralized

*** High efficiency, low - medium and small scale.

BFB

Bubbling fluidised bed

*** <50 -150mm, Humid. 10-55%, high ash

** C2+  and tars, High H2/CO

** Well applied 

*** Large plants

** High investment, efficiency

CFB

Circulating fluidised bed

***

<20mm, Humid. 5-60%, high ash

** C2+  and tars, High H2/CO

** heat and power,

*** Large plants

*** Investment

Dual

Dual fluidised bed

***

<75mm, Humid.  10-50%, ash

**** C2+ and tars, High H2 and CH4.

** Small scale, low development

** Small plant

*** Low investment, low cost

Plasma

**** No speciality

**** Low CH4, C2+ and tar, high H2 and CO

** New technique

* Small scale, module

* Extremely high investment low efficiency

V. CONCLUSION

 Previously waste has been considered as garbage, not worth, were disposed indiscriminately into the environment, including urban environments. Today a new industry – recycling - has been established and the waste become highly valuable materials. Energy crisis, depletion of mineral oil and coal has spurred research new energy, renewable energy. And the urgent need to limit greenhouse gas emissions is the new dynamic force promoting the development of clean energy. Technology transforming the waste to energy (WTE), especially to electricity, possesses dual special effects which are particularly difficult to obtain in other similar areas: Solving environmental problems caused by waste, reduce landfill area, reducing methane emissions, reduce secondary pollution due to landfills, and significantly contributing to meet the needs of green energy, the demand for mini energy system for geographic/economic regions, for small and medium businesses, the farms, and also households family…

Problem for the WTE has been not practically a finance question; mostly it depends on the applied technology.

REFERENCE 

1. The National Environmental Report 2011, Hanoi URENCO, 2011, Brief report of the municipal waste management Hanoi, 2005

2. Decision No 445/QD-TTg dated 07/4/2009 of the Prime Minister

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