Thermochemical product yield. The production of tar products

Thermochemical
processing involves the thermal conversion of organic components in biomass to
yield fuel products. This is achieved through various processes like thermo
chemical liquefaction, gasification and pyrolysis.

 

1.5.1.1 Thermochemical
liquefaction

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Thermochemical
liquefaction is a low-temperature (300-350°C) high pressure (5-20 MPa) process
aided by a catalyst to convert wet algal biomass into liquid fuel, in the
presence of hydrogen (Patil et al. 2008, Goyal et al. 2008). The process
utilizes the high water activity in sub-critical conditions to decompose into
small molecular material with a higher energy density (Clark and Deswarte,
2008). The main disadvantage of this method is that it requires complex and
expensive reactors. Several investigations were done on algal biomass as a
feedstock for Thermochemical liquefaction. Dote et al. (1994) successfully used
thermochemical liquefaction on B. braunii at 300°C to achieve maximum yield of
64% dry wt. basis of oil with HHV of 45.9 MJ kg-1. The same
experiment was conducted on Dunaliella
tertiolect, giving 42% dry wt. yield of oil with 34.9 MJ kg-1 of
HHV (Minowa et al. 1995). The positive energy balance for both the processes
(output/input ratio) is 6.67:1and 2.94:1,
respectively. These results showed that thermochemical liquefaction is a viable
method for algal biomass conversion to liquid fuel.

 

1.5.1.2
Gasification

Gasification is a partial oxidation process involves the production of
combustible gas mixture at high temperature (800-1000°C) from biomass (Clark et
al. 2008). In gasification process biomass reacts with steam of water and
oxygen to generate syngas (a mixture of CO, H2, CO2 and
CH4), which is a low calorific gas (typical 4-6MJ m-3)
(Demirbas, 2004). Syngas can be burnt directly or used as a fuel for gas
turbines or gas engines. The main advantage of gasification process is the
production of syngas from a wide variety of potential feedstocks (Clark et al.
2008).  Hirano et al. 1998 partially oxidized spirulina at high temperature ranging
from 850-1000°C and found that algae biomass gasification at 1000°C
produced highest theroretical yield of 0.64g methanol from 1g of biomass.  Sometimes during the gasification of biomass,
some unwanted products (water, tar and ash) are formed which cause various
problems with the main product yield. The production of tar products ranging from
0.1% to 20% depending on the gasifier agent and type of reactor used. 

 

 

 

1.5.1.3
Pyrolysis

Pyrolysis
is the conversion of algal biomass in the absence of air to syngas, charcoal
and bio-oil at medium to high temperature (350-700 °C) (Goyal et al.
2008). Fast pyrolysis is the heating of biomass at high heating rate (1000 °C
min-1) and short residence times (10-20s) compared to slow heating
rates (5-80 °C min-1) and longer residence times (5-30
min). Flash pyrolysis (moderate temperature of 500°C) is a viable
method for future replacement of fossil-fuels with biomass derived liquid fuels
because of higher conversion rate of 95.5% (biomass to liquid conversion rate)
(Demirbas et al. 2006). However there are some technical challenges in
pyrolysis i.e the oil derived from it, is highly viscous, unstable, acidic in
nature and contain solids and chemically dissolved water (Chiaramonti et al.
2007). Therefore, oil derived from pyrolysis requires upgrading hydrogenation
and catalytic cracking to remove alkalis and reduced oxygen.

 

1.5.2
Biochemical processing

Biochemical
process of energy conversion of biomass into other fuels includes
transestrification, fermentation, anaerobic digestion and hydroprocessing.

 

1.5.2.1 Transestrification

Transecterification
is a physiochemical process which involves reaction of an alcohol with
triglycerides to form fatty acid alkyl esters in the presence of a catalyst
(acid catalyst, basic catalyst or enzymatic conversion).  In acid-catalyzed reactions, strong acids are
used in transestrification like H3PO4, H2SO4
or HCl, while in basic catalysis reactions, NaOH or KOH are commonly used. The
basic catalyzed reactions have several advantages over acid catalyzed reactions
like it has high conversion rate and conducted at low temperature and pressure.
Despite of these advantages, the process is energy intensive, and have problem
in removal of alkaline catalyst from the final product. These problems can be
overcome by the use of biocatalysts, such as lipases (Ravishankar et al. 2012).  Lipase based transestrification has been used
because of its mild reaction conditions, moderate temperature and pressure
requirements, and have great free fatty acids (FFAs) concentration tolerance.
After the reaction, biodiesel is easily separated from glycerol, no need of
additional purification steps. This is an efficient method for biodiesel
production in high yield per unit production cost due to reuse ability of
enzymes (Jegannathan et al. 2008). Despite of various advantages, enzyme-based
systems also face several challenges such as concentration of substrates and
enzymes, pH of the reaction, destabilization of enzyme and interaction
distances between enzyme and substrates (Suali and Sarbatly, 2012). These
conditions are carefully monitored and optimized to produce maximum yield (Fig.
2). 

1.5.2.2 Fermentation

Microalgae
are suitable source for bioethanol production. Some microalgae contain
carbohydrates (generally not cellulose) beside of their lipid content; can be
used as a carbon source or substrate for fermentation. Under stress conditions,
certain algae start accumulating starch which can be exploited for ethanol
production by fermentation. The bioethanol production processes from microalgae
are much similar with that of first generation technology which use sugarcane
and corn derived feed stocks (Schenk et al. 2008).  Ethanol can be produced either from algal
cake or from algal biomass. Despite of this, the research on microalgae
fermentation (algae biomass to bioethanol) is limited in the literature
(Bezergianni et al. 2010).

 

1.5.2.3 Anaerobic digestion

Anaerobic
digestion is an engineered biochemical process which converts organic matter to
biogas (a mixture of methane and carbon dioxide). The microalgae with low lipid
content are more suitable for biogas production using anaerobic fermentation.
Anaerobic digestion process is best suitable for high moisture content organic
wastes (80%-90% moisture). The Anaerobic digestion process occurs in three
sequential stages i.e hydrolysis, fermentation and methanogenesis. The complex
compounds are firstly broken down into soluble sugars (Hydrolysis process)
which are utilized by bacteria to convert into alcohols, acetic acid, volatile
fatty acids (VFAs) and gas (H2 and CO2) (fermentation).
In methanogenesis stage, methanogen metabolized H2 and CO2 into
primarily CH4 (60-70%) and CO2 (30-40%) (Brennan and
Owende, 2009). The anaerobic digestion can achieved 250 m3 t?1
yields of methane from algae. Sometime in case of marine species, the high
level of saline conditions may inhibit growth or productivity of the anaerobic
microorganism in the fermenter. This problem can be overcome by mixing algal
biomass with other type of biomasses, to dilute the saline concentration.
Another problem faced by algae especially green microalgae is the formation of
H2S (due to the high sulphate concentration in the species), which
can be reduced by applying iron based chemicals (Streefland et al. 2010). There
are three problems to digest microalgae are: the biochemical composition of
cell wall, the nature of cell wall and higher cellular protein content (release
ammonia causing toxicity).  Seaweeds
produced methane in the range of 0.14 m3 kg?1 to 0.40 m3 kg?1 volatile solids
(Roesijadi et al. 2010).

 

1.5.2.4 Hydroprocessing

Hydroprocessing
technology is used to convert lipid feedstocks into distillate fuels such as
petroleum diesel and propane in the gas phase stream (Bezergianni et al. 2010).
Various hydroprocessing technologies has been utilized to processed algal oil
into kerosene like fuel which is very similar to petroleum derived jet fuels
and to remove impurities. This fuel is consider as a jet fuel for various
reasons; the heating content is greater, the propane byproduct is preferable
over glycerol byproduct, fuel have superior cold weather properties, and the
cetane number is greater (Guzman et al. 2010). The advantage of hydroprocessing
method is that it requires infrastructure which is widely available in all
refinery units (Chernova et al. 2010).