Biomasa

Final Examination

JEE 651 Biomass for Heat and Power

Semester 1/2006

1.      Give your explanation to support the statement “Biomass is renewable and zero or near zero CO₂ emission”. Also give an example.

Solution

Biomass comes from all plant and animal matter on the earth. Biomass includes trees, crops, algae and other plant, as well as agricultural and forest residues plus other waste (animal waste, sewage sludge, domestic and municipal sewage from mainly human waste). So, if we use biomass as energy resources, we can renew it’s by replanted the plant. For example if we use agriculture residues such as bagasse from sugar cane production as fuel to produce heat, we can renew it’s with replanted a cane or if we use waste from pigs to produce biogases, we can renew it’s because pigs always produce waste every day.

According from the carbon cycle, using biomass as a fuel means that carbon dioxide (CO₂) which was absorbed from the air while the plant was growing is released back into the air when the fuel is burned. The system is said to be carbon neutral. Providing the balance is maintained between the plant growth and biomass use, the system is sustainable. For example if we use wood as fuel in steam generator (boiler) we must burn wood and produce CO₂, by planted the tree, CO₂ from the air can absorb while respiration process. So, emission of CO2 is zero or near zero.

2.      Biomass Technology

a)      Anaerobic Digestion Technology for Treatment of Municipal Solid Waste

-        Concept of the technology

Anaerobic digestion occurs naturally wherever high concentrations of wet organic matter accumulate in the absence of dissolved oxygen. Only waste of organic origin can be processed in an anaerobic digester. As this makes up 30-60% of household waste there is a considerable benefit in diverting this waste from landfill. A wide variety of systems have been developed to an aerobically treat MSW. They can be split into a variety of categories such as: wet or dry, batch or continuous, single step or multi step, co-digestion with animal manure/ digestion of MSW alone.

-        Industry that currently applies the technology or technology application

Arge biogas (Austrian), OWS-Dranco (Belgian), Kompogas (Swiss), Steinmuller Valorga Sarl (French), Ecotec (Finish), BTA (German), C.G.Jensen (Danish), Paques Solid Waste Systems (Dutch), Unisyn Biowaste Technology (USA), WMC Resource Recovery (UK), Ionics Italba (Italian), SWECO/VBB (Swedish)

-        A kind of biomass are being used or can be use

Municipal solid waste is composed of:

1) Digestible Organic Fraction – Readily biodegradable organic matter, e.g. kitchen scraps, food residue, grass cuttings etc.

2) Combustible Fraction – Slowly digestible organic matter such as coarser wood, paper, cardboard. These are lignocellulosic materials which do not readily degrade under anaerobic conditions and are better suited to aerobic digestion, i.e. composting. (Opinions vary over the digestibility of paper, which depends on the lignin content, some forms of paper are much more digestible than others – generally only paper that is too contaminated with organic waste to be recycled, is considered for digestion). The combustible fraction also consists of indigestible

3) Inert Fraction – Stones, glass, sand, metal, etc. Some of these products are suitable for recycling, the remainder can be landfilled.

-        Advantages and disadvantages

Advantages of MSW anaerobic digestion:

1) Makes landfills easier to manage by removing problematic organic waste material which is responsible for gaseous and liquid emissions.

2) Enclosed system allows all of the biogas to be collected, unlike on landfills where recovery only yields 30-40% of gas generated. Methane is a greenhouse gas with twenty times the impact of carbon dioxide.

3) An end product that can be used as a soil conditioner is produced.

Disadvantages

1) Nature of organic waste may vary according to location and time of year. In wet season horticultural plant cutting levels may be higher than in the dry season. This may lead to a variation in the C/N ratio and affect the rate of gas production.

2) Inadequate mixing of refuse and sewage can affect efficiency of system.

3) Blockage of pipes can be caused if large pieces of waste enter the system; this causes problems, particularly in continuous systems.

-        Why or why not the technology is suitable use in Thailand

One small scale system that has been identified as having potential for application in Thailand is the Rottaler Model. This appears to be a flexible design that can be sized to suit the situation, without adversely affecting the process performance.

-        Suggestion to improve the system.

Mixing the refuse with animal dung improves the system efficiency and allows for a more simple process design, improving the economic viability of the system. This is due to the improved nitrogen content that is achieved by mixing with dung.

b)     Municipal Solid Waste Incineration

-        Concept of technology

Main process of MSW incineration

Drying	Degassing	Ignition/Combustion	Gasifying/Combustion
Heating of the waste by flue gas, and radiation heat, evaporation of water	Drive out of volatile components oxygen reduce atmosphere, gasses stream in combustion chamber	Oxidation of out driven gas by mixing with combustion air	The remaining carbon is gasified to CO and CO2, CO and other gasses are burned with further access of air nearly complete mineralizing of organic material

-        Industry that currently applies the technology or technology application

Shenzhen MSW incinerating plant in Guangdong province, The Pudong MSW incinerating plant in Shanghai, Ningbo MSW incinerating plant in Zhejiang province, Longgang MSW incinerating plant in Guangdong province, The Xiaoshan MSW incinerating plant is located in Hangzhou, Zhejiang province. Haerbin MSW incinerating plant is located at the suburban of Haerbin, Helongjiang province. Beijing Changping incineration plant, and Sichuan Yueshan incinerator.

-        A kind of biomass are being used or can be use

The physical compositions of MSW are complicated and include paper, fiber, plastic, rubber, etc. In China, the MSW is generally divided into organic matter, inorganic matter, paper, fiber, timber bamboo, plastic, rubber, glass and metal. The composition of MSW for some cities in China was shown in table below.

-        Advantages and disadvantages

Advantages:

1.      Technology is derived from power plant technology

2.      Best available technology

3.      reliable and experienced technology

4.      Modular system – adaptable to real local condition

5.      Control, maintenance, and repair are well developed

6.      High efficiency

7.      Several 1000 plants are operation in worldwide

Disadvantages

1.      Still high investment and operational cost (app 0.5 mil. $ per 1000 t annual capacity)

2.      Expensive gas purification

3.      Weak partial load behavior

4.      Minimum capacity for economic operation appropriate 30.000 – 50.000 t/a

-        Why or why not the technology is suitable use in Thailand

Municipal Solid Waste Incineration technology is the potential technology to apply in Thailand particularly in Bangkok city. MSW incineration can reduce the mass of waste to 25 – 35%, and decrease landfill that use to MSW.

-        Suggestion to improve the system

The major source for initiation of dioxin formation is due to residences present from incomplete combustion. It is generally considered that at temperature above 800^oC any dioxin present in the feed will be destroyed, but for effective and complete combustion of particulate carbonaceous material a temperature greater than 1000^oC should be achieved. In addition to a high combustion temperature, it is recommended that a sufficient residence time should be allowed to ensure complete destruction of combustion zone greater than 1 s is specified and at 850^oC a residence time of 2 s is recommended. A further stipulation to ensure the complete destruction and combustion of any fuel particulates in the hot flue gas is recommendation that a 3–6% excess oxygen level is utilized. Further efforts to ensure complete combustion can be made by creating turbulence in the combustion chamber. A Reynolds number greater than 10,000 is recommended and the most effective incineration furnaces have Reynolds number of 50,000 or greater.

From this, it is possible to retrofit existing Cement rotary kilns to MSW incineration to minimize dioxin production. The nature of the rotary kiln, which allows flame residence times of the order 2–5 s and reaches temperature as high as 2000^oC make cement kilns a competitive alternative to commercial incinerators for MSW. The rotary kiln rotates the waste in a cylindrical furnace in order to optimize mixing and provide a uniform burn. At the same time, solid wastes of various shapes, sizes and heat values can be fed into a rotary kiln either in batches or continuously. In incineration, an after-burner often follows the kiln to guarantee the complete combustion.