The varieties of available technologies
In the production of the biofuels (biogas, bio oil, methane) from the biomass, there are a lot of technologies that are useful to be used. In this literature review, we would like to introduce the usage of anaerobic reaction technology, hydrothermal liquefaction (HTL) and pyrolysis technology.
First and foremost, ANAEROBIC DIGESTION is a multistep biological and chemical process that is beneficial in not only waste management but also energy creation such as biogas. Biogas is a biofuel produced from the anaerobic fermentation of carbohydrates in plant material or waste (example: food peelings or manure) by bacteria. It is mainly composed of methane, with some carbon dioxide and other trace gases. However, the proportion of methane within the biogas can vary between 50% and 80%, depending on whether some oxygen is able to enter at the beginning or during the process. If some oxygen is present, the bacteria will respire aerobically and will produce a gas with a higher proportion of carbon dioxide and a lower proportion of methane. Biogas can be produced on a small scale in a biogas generator/digester, which can be made of simple materials (GSCE BiteSize, 2014).
The reaction of anaerobic reaction begins from the organic waste. The organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so the process could occur. The process of anaerobic digestion consists of three steps. The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar .The second step is the conversion of decomposed matter to organic acids.Finally, the acids are converted to methane gas. Process temperature affects the rate of digestion and should be maintained in the mesophillic range (95 to 105 degrees Fahrenheit) with an optimum of 100 degrees F. It is possible to operate in the thermophillic range (135 to 145 degrees F), but the digestion process is subject to upset if not closely monitored (Sethi, n.d.).
In the production of the biofuels (biogas, bio oil, methane) from the biomass, there are a lot of technologies that are useful to be used. In this literature review, we would like to introduce the usage of anaerobic reaction technology, hydrothermal liquefaction (HTL) and pyrolysis technology.
First and foremost, ANAEROBIC DIGESTION is a multistep biological and chemical process that is beneficial in not only waste management but also energy creation such as biogas. Biogas is a biofuel produced from the anaerobic fermentation of carbohydrates in plant material or waste (example: food peelings or manure) by bacteria. It is mainly composed of methane, with some carbon dioxide and other trace gases. However, the proportion of methane within the biogas can vary between 50% and 80%, depending on whether some oxygen is able to enter at the beginning or during the process. If some oxygen is present, the bacteria will respire aerobically and will produce a gas with a higher proportion of carbon dioxide and a lower proportion of methane. Biogas can be produced on a small scale in a biogas generator/digester, which can be made of simple materials (GSCE BiteSize, 2014).
The reaction of anaerobic reaction begins from the organic waste. The organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so the process could occur. The process of anaerobic digestion consists of three steps. The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar .The second step is the conversion of decomposed matter to organic acids.Finally, the acids are converted to methane gas. Process temperature affects the rate of digestion and should be maintained in the mesophillic range (95 to 105 degrees Fahrenheit) with an optimum of 100 degrees F. It is possible to operate in the thermophillic range (135 to 145 degrees F), but the digestion process is subject to upset if not closely monitored (Sethi, n.d.).
There are several studies regarding anaerobic reaction to produce biofuels. University of Cincinnati researchers who are turning food waste into biodiesel. The researchers of the university have since developed a breakthrough synergistic technology that uses anaerobic digestion to turn nutrient-rich organic materials into fuel (biogas), fertilizer, or soil conditioner, while using the carbon dioxide fraction of the biogas to grow algae. Simultaneously, lipid oils in the algae are also extracted and converted to biodiesel. This novel process, which essentially integrates algae production with anaerobic digestion, allows researchers to almost completely utilize the carbon found in food waste in a renewable manner. McAvoy explains, “The anaerobic digestion of food waste coupled with algae production seems to be an attractive alternative for not only reducing greenhouse gas emissions, but also for the production of renewable energy” (Researchers Turn Food Waste into Biodiesel, n.d.).
Throughout the research it has been found that the advantages of anaerobic reaction are the lower emissions of carbon dioxide from the process (because some complex compounds are not digested) and there will be no production of such toxic products as dioxins (Potts). The disadvantage of this anaerobic technology is that you need to control emissions from burning biomass materials to prevent local air pollution. Any system you install must comply with legislation such as the Clean Air (Northern Ireland) Order (Generate Your Own Renewable Energy, n.d.) . Secondly, it is slow, so it needs large, costly digesters (Potts,2013).
Throughout the research it has been found that the advantages of anaerobic reaction are the lower emissions of carbon dioxide from the process (because some complex compounds are not digested) and there will be no production of such toxic products as dioxins (Potts). The disadvantage of this anaerobic technology is that you need to control emissions from burning biomass materials to prevent local air pollution. Any system you install must comply with legislation such as the Clean Air (Northern Ireland) Order (Generate Your Own Renewable Energy, n.d.) . Secondly, it is slow, so it needs large, costly digesters (Potts,2013).
Moving on to HYDROTHERMAL LIQUEFACTION (HTL), which is also known as hydropyrolysis (Smith & Keener,n.d.), is one of the thermochemical conversion (TCC) processes in which it converts biomass into liquid fuels by processing the biomass in a high temperature and high pressurized water environment to break down the solid biopolymeric structure to liquid components for a sufficient time (Elliot et al., 2015). HTL is different from other TCC processes in the sense that it occurs at elevated pressures and it is performed on wet biomass, in which it uses water as an important reactant in the decomposition process (Smith & Keener). The conditions that are mostly used for hydrothermal liquefaction process are temperatures between 523K to 647K and operating pressures from 4 to 22 MPa. This process is mainly applied to treat wet materials without the need of drying and to access ionic reaction conditions by maintaining a liquid water processing medium (Elliot et al.).
Hydrothermal liquefaction is divided into three separate processes, relying on the operating conditions. It is defined as hydrothermal carbonization when the operating temperature is below 520K in which the main product formed is a hydrochar, which has the same properties as a low rank coal. When the operating temperature used is between 520K and 647K, the process is known as hydrothermal liquefaction which leads to the production of a liquid fuel known as biocrude. Whereas for temperature higher than 647K, the process is defined as hydrothermal gasification, resulting in the production of synthetic fuel gas (Elliot et al.).
Hydrothermal liquefaction is divided into three separate processes, relying on the operating conditions. It is defined as hydrothermal carbonization when the operating temperature is below 520K in which the main product formed is a hydrochar, which has the same properties as a low rank coal. When the operating temperature used is between 520K and 647K, the process is known as hydrothermal liquefaction which leads to the production of a liquid fuel known as biocrude. Whereas for temperature higher than 647K, the process is defined as hydrothermal gasification, resulting in the production of synthetic fuel gas (Elliot et al.).
There are also researches done by the researcher to convert biomass to biofuels using the HTL. This technology has been successfully applied by Aarhus University in Denmark this year. On 22 May 2015, Aarhus University has launched an HTL pilot plant at AU Foulum (Andreasen, 2015). This newly launched plant is proven to have the ability to convert organic materials such as organic waste into bio-crude oil, which can replace fossil oil for producing fuels and chemicals. “The new plant is based on a flow reactor, with a steady inflow of biomass and an outflow of bio-oil from the other end. The process takes place in a 120-meter-long pipe, where the watery biomass is heated to 450 degrees and subjected to a pressure of up to 350 bar,” says Associate Professor Ib Johannsen, Department of Engineering, who is responsible for designing the plant (Andreasen).
Based on the researches that have been carried out by far, the benefits of HTL are consuming approximately 10-15% of energy from the feedstock biomass and producing an energy efficiency of 85-90% and HTL oil recovers more than 70% of the feedstock carbon content (Aarhus University, 2013). The setback of this HTL technology is that the technology is still under development. Besides, HTL requires high pressure and high temperature for the processes; hence, it might lead to a high cost and possible safety concerns (Smith & Keener).
Based on the researches that have been carried out by far, the benefits of HTL are consuming approximately 10-15% of energy from the feedstock biomass and producing an energy efficiency of 85-90% and HTL oil recovers more than 70% of the feedstock carbon content (Aarhus University, 2013). The setback of this HTL technology is that the technology is still under development. Besides, HTL requires high pressure and high temperature for the processes; hence, it might lead to a high cost and possible safety concerns (Smith & Keener).
Lastly, PYROLYSIS is a thermochemical conversion technology that used to produce energy from biomass, household and commercial waste and residue. It is fundamental chemical reaction that involves the superheated decomposition of carbon-rich organic in the absence of reagent, especially oxygen in order to produce a mixture of bio char, pyrolysis liquid (bio oil) and a synthetic gas such carbon monoxide, hydrogen, carbon, dioxide and methane. Pyrolysis differs from other processes like combustion and hydrolysis in that it usually does not involve reactions with oxygen, water, or any other reagents.
In pyrolysis reaction, the decomposition of the product from biomass is determined by temperature and residence time of the process. Lower process temperature and longer vapor residence times are optimum for the production for charcoal; high temperatures and longer residence times favor the production of gas; moderate temperatures and short vapor residence time increase biomass conversion to liquid (bio-oil). However, pyrolysis process can be divided into two main categories, which are slow pyrolysis and fast pyrolysis. Slow pyrolysis is a reaction which being carried out in a temperature range of 400 to 600 C with a slow heating rate of 0.1 to 1 C/s and a residence time anywhere form hours to minutes (Dickerson & Soria, 2013). It takes longer period of time compare to fast pyrolysis, and yields biochar was the main product. Although generally pyrolysis does not release heat, however the slow pyrolysis reaction for converting biomass mainly to biochar can be slightly exothermic and it will consequently leads to increase amount of greenhouse effect emission. For fast pyrolysis, it is a relatively new, promising technology to produce bio oil. Currently, fast pyrolysis which maximizes the production of liquid has gaining interest rather than the production of biochar from the slow pyrolysis. This is because the reaction is endothermic and it involves rapid heating rate of 10 to >1000C/s, short residence times of less than 2s, and higher temperature of 400-650C compared to slow pyrolysis (Dickerson & Soria, 2013). Due to its short residence time, the main products from biomass fast pyrolysis are ethylene-rich gases that can be condensed to produce bio-oil and alcohols rather than char and tar. Fast pyrolysis can convert up to 75 percent of biomass input into bio-oil, yielding about 135 gallons of bio-oil per ton of biomass (Sadaka).
In pyrolysis reaction, the decomposition of the product from biomass is determined by temperature and residence time of the process. Lower process temperature and longer vapor residence times are optimum for the production for charcoal; high temperatures and longer residence times favor the production of gas; moderate temperatures and short vapor residence time increase biomass conversion to liquid (bio-oil). However, pyrolysis process can be divided into two main categories, which are slow pyrolysis and fast pyrolysis. Slow pyrolysis is a reaction which being carried out in a temperature range of 400 to 600 C with a slow heating rate of 0.1 to 1 C/s and a residence time anywhere form hours to minutes (Dickerson & Soria, 2013). It takes longer period of time compare to fast pyrolysis, and yields biochar was the main product. Although generally pyrolysis does not release heat, however the slow pyrolysis reaction for converting biomass mainly to biochar can be slightly exothermic and it will consequently leads to increase amount of greenhouse effect emission. For fast pyrolysis, it is a relatively new, promising technology to produce bio oil. Currently, fast pyrolysis which maximizes the production of liquid has gaining interest rather than the production of biochar from the slow pyrolysis. This is because the reaction is endothermic and it involves rapid heating rate of 10 to >1000C/s, short residence times of less than 2s, and higher temperature of 400-650C compared to slow pyrolysis (Dickerson & Soria, 2013). Due to its short residence time, the main products from biomass fast pyrolysis are ethylene-rich gases that can be condensed to produce bio-oil and alcohols rather than char and tar. Fast pyrolysis can convert up to 75 percent of biomass input into bio-oil, yielding about 135 gallons of bio-oil per ton of biomass (Sadaka).
The technology of pyrolysis is noteworthy and has been tested and experimented over decades to meet the requirement to produce renewable liquid fuels, chemicals and derived products. For example, in 15 September 2009, the researches in University of Georgia Research Foundation have successfully developed an innovative way to turn dead trees into a liquid fuel by using pyrolysis technology. This low-cost, on-site process which has turns waste biomass into sustainable and low-sulphur vehicle fuel had represented a lap forward for the biofuel industry and now it is being successfully implemented by the people in Califonia. Besides, researches of United States Department of Agriculture are now one step closer in developing mobile pyrolysis processing systems that may one day be used on farms to produce “green” biofuel (Perry, A. 2013). The standard pyrolysis process is being further modified where oxygen requirement and acidity is being reduced and no additional catalyst is needed. Experimental researches has been carried out and found out that bio-oils produced form oak and switchgrass by this new process system has considerably higher energy content than those produced by conventional fast pyrolysis. “It’s becoming one of the most promising methods for extracting the energy from tough plant materials to produce liquid fuels,” says Agricultural Research Service chemist Charles Mullen.
Based on experimental researches so far, there are many advantages associated with pyrolysis technology. The main attractive feature of this technology is that it produces only few air emissions due to the limited use of oxygen. The amount of greenhouse gas emissions is significantly low until it only leads to minimal risk of health consequences. Besides, solid biomass and waste which are very difficult and costly to manage can be decomposed and transformed into renewable fuel product by using this technology and this reduce the amount of waste transferred to landfills. The constraint of this technology is that it’s still evolving and under development. Besides, phase-separation and polymerization of the liquids and corrosion of containers make storage of these liquids difficult (Robert and Jennifer).
In conclusion, we would like to choose pyrolysis technology in our study because it is so far the best and commercial technology in converting the biomass to the biofuels as we not only taking the consideration about the biofuels production alone but about the safety and the environmental friendliness of the technology as well.
Based on experimental researches so far, there are many advantages associated with pyrolysis technology. The main attractive feature of this technology is that it produces only few air emissions due to the limited use of oxygen. The amount of greenhouse gas emissions is significantly low until it only leads to minimal risk of health consequences. Besides, solid biomass and waste which are very difficult and costly to manage can be decomposed and transformed into renewable fuel product by using this technology and this reduce the amount of waste transferred to landfills. The constraint of this technology is that it’s still evolving and under development. Besides, phase-separation and polymerization of the liquids and corrosion of containers make storage of these liquids difficult (Robert and Jennifer).
In conclusion, we would like to choose pyrolysis technology in our study because it is so far the best and commercial technology in converting the biomass to the biofuels as we not only taking the consideration about the biofuels production alone but about the safety and the environmental friendliness of the technology as well.
References
- Aarhus University (February 6, 2013). Hydrothermal liquefaction: the most promising path to a sustainable bio-oil production. Retrieved 25 July, 2015 from http://www.eurekalert.org/pub_releases/2013-02/au-hl020613.php
- Andreasen, C. (May 5, 2015). New plant converts biomass to bio-oil. Retrieved 25 July, 2015 from http://dca.au.dk/en/current-news/news/show/artikel/nyt-anlaeg-omdanner-biomasse-til-bioolie/
- Elliot, D., Biller, P., Ross, A., Schmidt, A., & Jones, S. (October 13, 2014). Hydrothermal Liquefaction of biomass: Developments from batch to continuous process. Retrieved 26 July, 2015 from http://www.sciencedirect.com/science/article/pii/S0960852414013911
- Generate Your Own Renewable Energy. (n.d.). Retrieved from NetRegs: http://www.netregs.org.uk/library_of_topics/carbon_reduction__efficiency/generate_renewable_energy/biomass_energy_and_ad.aspx
- GSCE BiteSize. (2014). Retrieved from BiteSize: http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/humans_and_environment/biofuels/revision/4/
- Perry, A. (2013). New and More Economical Pyrolysis Techniques for Bio-oil Production. Retreived 25 July, 2015 from http://agresearchmag.ars.usda.gov/2014/apr/pyrolysis
- Potts, L. G. (2010). Anaerobic Digestion , Gasification and Pyrolysis. Waste Management and Minimization, 4-5.
- Researchers Turn Food Waste into Biodiesel. (n.d.). Retrieved from Domestic Fuels.com: http://energy.agwired.com/2015/04/21/researchers-turn-food-waste-into-biodiesel/
- Robert, C.B. & Jennifer H. (n.d.) Fast Pyrolysis and Bio-Oil Upgrading. Retreived 25 July, 2015 from http://www.ars.usda.gov/sp2UserFiles/Program/307/biomasstoDiesel/RobertBrown&JenniferHolmgrenpresentationslides.pdf
- Sadaka, S. (n.d.). Pyrolysis and Bio-Oil. Retreived 25 July, 2015 from https://www.uaex.edu/publications/PDF/FSA-1052.pdf
- Sethi, P. (n.d.). Anaerobic Reaction. Retrieved from California Energy Commission: http://www.energy.ca.gov/biomass/anaerobic.html
- Smith, M. and Keener, H. (n.d.). Manure Processing Technologies. Hydrothermal Liquefaction. Retrieved 25 July, 2015 from http://www.oardc.ohio-state.edu/ocamm/images/MPT_3.7_hydrothermal_liquefaction.pdf