2.0 LITERATURE SURVEY
Types of Technology
A. Pyrolysis
B. Plasma Gasification
C. Bioremediation
D. Thermophilic Digestion
A. PYROLYSIS
Pyrolysis is the heating of an organic material, such as biomass, in the absence of oxygen. Because no oxygen is present the material does not combust but the chemical compounds (i.e. cellulose, hemicellulose and lignin) that make up that material thermally decompose into combustible gases and charcoal. Most of these combustible gases can be condensed into combustible liquid, called pyrolysis oil (bio-oil), though there are some permanent gases (CO2, CO, H2, light hydrocarbons). Thus pyrolysis of biomass produces three products: one liquid (bio-oil), one solid (bio-char) and one gaseous (syngas).
Processes and Reactions Involved
The pyrolysis process consists of both simultaneous and successive reactions when organic material is heated in a non-reactive atmosphere. Thermal decomposition of organic components in biomass starts at 350 °C–550 °C and goes up to 700 °C–800 °C in the absence of air/oxygen. The long chains of carbon, hydrogen and oxygen compounds in biomass break down into smaller molecules in the form of gases, condensable vapours (tars and oils) and solid charcoal under pyrolysis conditions. Rate and extent of decomposition of each of these components depends on the process parameters of the reactor temperature, biomass heating rate, pressure, reactor configuration, feedstock, etc.
Depending on the thermal environment and the final temperature, pyrolysis will yield mainly bio-char at low temperatures, less than 450 °C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 °C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil.
Products:
1) Bio-oil: Bio-oil is a dark brown liquid and has a similar composition to biomass. The oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel.
2) Bio-char: Bio- char is used on the farm as an excellent soil amender that can sequester carbon
3) Syngas: Syngas can be used directly as an energy source.
Advantages and Disadvantages
Advantages:
1) Used for all types of solid products and can be more easily adapted to changes in feedstock composition
2) Conducted as a batch, low pressure process, with minimal requirements for feedstock pre-processing
3) Designed to produce minimal amounts of unusable by-products
4) Produce potentially valuable chemicals and chemical feedstock
Disadvantages
1) Technology is still evolving
2) Markets are yet to be developed for char product and pyrolysis liquids
3) Pyrolysis plants require a certain amount of materials to work effectively
B. PLASMA GASIFICATION
Plasma gasification is an emerging technology which can process landfill waste to extract commodity recyclables and convert carbon-based materials into fuels. Plasma arc processing has been used for years to treat hazardous waste, where it ionizes gas and catalyse organic matter into synthetic gas and solid waste (slag).
Processes and Reactions Involved
The Plasma Gasifier produces a gaseous product and an inert solid by-product; the individual amounts of which will depend on the type of waste being feed into the gasifier.
Synthesis gas (syngas), the main output of the plasma gasifier, can be used as a fuel source in power plants, or treated further to generate hydrogen. It can also be used in the rural and industrial sector in the production of a wide range of polymers, chemicals, biofuels (including ethanol), fertilizers, pressure agents and more.
Slag is an obsidian-like silicate material which can be used in concrete or asphalt, even shaped into bricks or pavement stones for use in the construction industry or even be used in the abrasives industry. Metals can also be extracted and on sold to various industries.
Rock wool is a highly efficient insulator with higher energy efficiencies than fibre glass, it is highly absorbent and lighter than water, with many potential methods of use in both the industrial and environmental segments
Advantages and Disadvantages
Advantages:
1) Handle any combination of solid, liquid and gaseous wastes, hazardous and non-hazardous wastes.
2) Production of value-added products (metals) from slag
3) Ecologically clean. The lack of oxygen prevents the formation of many toxic materials, and the high temperatures in a reactor also prevent the main components of the gas from forming toxic compounds
Disdvantages:
1) Too much inorganic material such as metal and construction waste increases slag production, which in turn decreases 2) syngas production.
3) Large initial investment costs relative to landfill
4) The plasma flame reduces the diameter of the sampler orifice over time, necessitating occasional maintenance
$240M PLASMA GASIFICATION WASTE TO ENERGY DEAL SIGNED IN BARBADOS
Guernsey-based Cahill Energy has signed an agreement with the government of Barbados to build and operate a plasma gasification waste to energy facility on the Caribbean island.
Cahill claimed that once operational the plant will be capable of generating up to 25% of the island’s energy, providing the government of Barbados several hundred million dollars in estimated savings over the 30 year lifetime of the contract.
“Cahill Energy offers us a real solution to becoming energy independent, while at the same time reducing our massive oil import bill,” explained Barbados’s minister of environment, the honourable Dr Denis S. Lowe.
“Indeed, this project will help Barbados significantly to reach this target ten years earlier than planned,” he continued. Cahill added that it received legal advice on the deal from international law firm Taylor Wessing.
C. BIOREMEDIATION
Bioremediation is the use of living organisms, primarily microorganisms, to degrade the environmental contaminants into less toxic forms. It utilizes naturally occurring bacteria and fungi or plants to degrade or detoxify substances hazardous to human health and/or the environment.
Reactions Involved
Microorganisms are capable of catalyzing a variety of reactions including dehalogenation.
Cl2C = CHCl + H+ ClHC = CHCl + Cl–
Successful Practitioners
Department of Energy (DOE) currently is responsible for remediating 1.7 trillion gallons of contaminated groundwater. The researchers comprises of Stefan Green, Om Prakash Sharma, Puja Jasrotia and Niki Norton.
Advantages and Disadvantages
Advantages:
1) Many compounds that are legally considered to be hazardous can be transformed to harmless products.
2) Less expensive than other technologies that are used for clean-up of hazardous waste.
Disadvantages:
1) There are some concerns that the products of biodegradation may be more persistent or toxic than the parent compound.
2) Limited to those compounds that are biodegradable.
Recommendation
The full potential of bioremediation to treat a wide range of compounds cannot be realized as long as its use is clouded by controversy over what it does and how well it works. By providing guidance on how to evaluate bioremediation, the committee hopes this report will eliminate the mystery that shrouds this highly multidisciplinary technology and pave the way for further technological advances.
D. THERMOPHILIC DESIGN
This process utilize the thermal decomposition process by combining present microorganism to increase the decomposition process of biological waste which consists of complex biochemical structures into simpler chemical constituents. These product would include biogas, carbon dioxide , methane and hydrogen. This process can be done in both aerobic and anaerobic method.
Process
Before the process is started, pre-aeration is needed as it will increase the temperature to a certain degree as to improvise the thermal condition for the reaction. Charles, Walker and Cord-Ruwisch (2009) stated that pre-aeration in 48 hours will increase the temperature of the organic fraction of municipal solid waste (OFMSW) to 60 , thus pre-heating process for thermophilic anaerobic reaction can be neglected to save energy consumption. Another approach would be pre-heating without any pre-aeration process, this would be time-saving despite the fact of higher energy consumption. Monson, Esteves , Guwy and Dinsdale (2007) further illustrates the pre-treatment process by collecting of kitchen waste and passed to the collection pit where the wastes will be shredded . Magnetic ferro-separator is implemented to remove any metals from kitchen waste. Manure and organic waste stream is differentiated and stored in different storage tank for improving stability during digestion process via process control.
In case of kitchen waste anaerobic thermophilic digestion process conducted by Charles , Walker and Cord-Ruwisch(2009), continuous process is conducted in digester under temperature parameter of 55 , volume of 2400 , range of pH between 8.0-8.3. Another unit operation component which further includes boiler which utilizes the biogas and oil as heat supply unit, heat exchanger to maximize heat transfer between inlet and outlet waste stream as well as minimize the required heat supply to reduce biogas consumption, and process control unit. For BioMax’s Rapid Thermophilic Digestion Technology(RTDT), the aerobic thermophilic process is done in batch reactor under temperature in 70-80, addition of enzymes as suggestion to accelerate digestion process and working under 24 hours to produce organic fertilizers.
Product , Yield and Implementation
Themophilic Digestion produces biogas , carbon dioxide , hydrogen and methane. Despite that , solid products also formed and can be used as fertilizers for agricultural purposes. The biogas produced from thermophilic digestion is reused as the heating component for the feed stream and additional biogases can be diverted into local power plant for electrical energy production (Charles,Walker & Cord-Ruwisch,2009).
Advantages and Disadvantages
Advantages:
1) Lower retention time
2) Faster rate of reaction
3) Provides better pathogen control due to high temperature condition
Disadvantages:
1) Expensive to operate as require high energy for heating
2) High sensitivity to operational and environmental conditions.
REFERENCES
Goswami, D. (1986). Alternative energy in agriculture. Boca Raton, Fla.: CRC Press.
Phoenix Energy, Plasma Gasification. Retrieved 30th July 2015 from http://www.phoenixenergy.com.au/plasma-gasification/
Review of Technologies for Gasification of Biomass and Wastes (June, 2009). E4tech.
Tendler, Michael; Philip Rutberg; Guido van Oost (2005-05-01). "Plasma Based Waste Treatment and Energy Production." Plasma Physics and Controlled Fusion 47 (5A): A219.
T. Cairney. Contaminated Land, p. 4, Blackie, London (1993).
R. B. King, G. M. Long, J. K. Sheldon. Practical Environmental Bioremediation: The Field Guide, 2nd ed., Lewis, Boca Raton, FL (1997)
