In most research papers, the ground biomass is first treated with acid, then with alkaii and subsequently saccharification by fungus or commercial enzymes. How can we categorize the treatments into: Pretreatment, hydrolysis and fermentation. Please comment.
Dr. Nath,
Treatment methods for biomass can be divided into physical, chemical, and biological categories. It is better to categorize the methods in this way since the three categories are based on different mechanism and have different functions.
Maohong Fan
Dear Nath,
The steps you mentioned i.e, Pretreatment, hydrolysis and fermentation, are the different stages that could be followed during the lignocellulosic biomass conversion to biofuels (bioethanol, biohydrogen or biobutanol). As Prof. Fan said the pretreatment methods have been categorized into physical, chemical and biological treatments based on the action mechanism. For example the acid treatment tends to hydrolyse hemicellulose to little extent of cellulose and acid soluble lignin. The alkali treatment dissolves lignin (non-carbohydrate) and hemicellulose (up to 60-70%). The If we employ any of these physic-chemical methods the desired cellulose will be converted into spinoffs such as HMF, levulenic acid and formic acids, which are also inhibitors to the fermentation process. Hence we use enzymes for the conversion of cellulose to glucose, which is called saccharification.
Cheers..
Chiranjeevi
Thank you Prof. Fan and Dear Chiranjeevi for your prompt reply. I had few more queries. Please comment on physiochemical and thermochemical method of pretreatment.
Regards,
Pranab
Apart from above methods as mentioned by Dr Chiranjeevi, which are sequence of steps, biomass needs to be collected, and stored, followed by size reduction prior to pre-treatment which require considerable time and money for any bioprocessing industry in the investment point of view.
Thermochemical methods (Pyrolysis, carbonization, Gasification, Liquefaction and combustion) give biooils, biochar,syngases from various biomass whereas physicochemical methods use the combination of thermal/mechanical/physical method with chemical method is used to solubilize hemicellulose to little extent of cellulose and acid soluble lignin.
Thank you Dear Selvaraju.
I have one more query, anybody can kindly reply.
1) Could you please differentiate
Torrefaction, Pyrolysis, gasification, syngas and liquefaction. Are they product of the same process (Heating at high temp)? are same process only with the difference in the applied temp and other conditions?
2) Also the distinction between "Thermochemical ethanol via direct gasification and that of via indirect gasification."
Dear Nath,
Acid pretreatment of biomass is a process of treating biomass with an acid (generally sulfuric) either diluted or concentrated with the aim of preparing it for cellulose enzymatic hydrolysis. As result of the acid pretreatment the reactivty of cellulose towards cellulolytic enzymes is significantly improved. At the same time, hemicelluloses are partially hydrolysed resulting in formation of different sugars.
Thermochemical conversion is the controlled heating and/or oxidation of biomass as part of several pathways to produce intermediate energy carriers or heat. Thermochemical conversion technologies are classified by their associated oxidation environment, particle size and heating rate, ranging from heating biomass in an oxygen-free environment (endothermic) to full exothermic oxidation of biomass.
(i) Combustion
The direct combustion of biomass is still the dominant bio-energy pathway, worldwide (Gaul 2012). Complete combustion involves the production of heat as a result of the oxidation of carbon- and hydrogen- rich biomass to CO2 and H2O. However, the imperfect combustion results in the release of intermediates including environmental air pollutants such as CH4, CO, and particulate matter. Additionally, fuel impurities, such as sulphur and nitrogen, are associated with emissions of SOX and NOX (Robbins et al 2012).
(ii) Carbonization
Carbonization involves the production of charcoal via the partial oxidation of biomass with long residence time (Bailis 2009). An alternate approach utilizes an aqueous environment at moderate temperatures (200-600C) and high pressures (5-40 MPa) to decompose biomass to solid, liquid, and gaseous intermediates (Peterson et al 2008; Brown 2011).
(iii) Gasification
Gasification is the exothermic partial oxidation of biomass with process conditions optimized for high yields of gaseous products rich in CO, H2, CH4, and CO2. The gas can be cleaned and used directly as an engine fuel or upgraded to liquid fuels or chemical feedstocks through biological fermentation (Datar et al 2004) or catalytic upgrading via the Fischer-Tropsch process (Boerrigter & Rauch 2005; Huber et al 2006;Wang et al 2008).
(iv) Liquefaction
Liquefaction or torrefaction is the low temperature (200-300C) pyrolysis of biomass in order to remove water and volatiles, increasing its energy density and susceptibility to mechanical pretreatment (Van der Stelt et al 2011).
(v) Pyrolysis
Pyrolysis is the thermal decomposition of biomass into highly heterogeneous gaseous, liquid, and solid intermediates in the absence of oxygen and the process is endothermic. The bio-oil is a heterogeneous mixture characterized by high oxygen content and alkalinity, which can be upgraded to fuels or chemicals. The bio-char can be used as a fuel or soil amendment (Field et al 2013).
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