Course No. & Title: ACHEM 2121 – Agroindustrial Chemistry
Course Teacher:
Professor Dr. Md. Mokhlesur Rahman
Department of Agricultural Chemistry
Bangladesh Agricultural University Mymensingh
Bioconversion of biomass to biogas Biogas
Biogas is an environmental friendly renewable energy source produced by the breakdown of organic wastes or biodegradable materials by certain bacteria under anaerobic digestion.
Biogas is a mixture of gases primarily consisting of methane and carbon dioxide generated from various raw materials such as agricultural waste, manure, municipal solid waste, energy crops, green waste and food waste.
Biomethanation in Bangladesh
In Bangladesh, Prof. Dr. A. Karim (Dept. of Agricultural Chemistry, BAU, Mymensingh) was the pioneer to produce biogas as alternate fuel from cow dung and organic wastes.
Afterwards, Bangladesh Council of Scientific and Industrial Research (BCSIR) was installed biogas digesters.
Different NGO’s - BASA, IDCOL and Grameen Shakti are working to establish biogas plants in rural areas of Bangladesh.
Raw materials for biogas production
The following raw materials are used for bioconversion of biomass or biowaste to biogas:
1)Cow dung
2)Poultry dropping
3)Water hyacinth
4)Rice or wheat straw
5)Green grass
6)Municipal solid waste
Biogas production
The following four steps are involved for converting biomass or biowaste to biogas:
Step I: Hydrolysis
Step II: Acidogenesis
Step III: Acetogenesis
Step IV: Methanogenesis
Stage - I: Hydrolysis
During hydrolysis, the waste material of plant or animal origins having the insoluble polymerized organic compounds like carbohydrates proteins and fats are decomposed into soluble monomers and dimers such as monosaccharides, amino acids and fatty acids This stage passes through extracellular enzymes like amylases, proteases and lipases produced by appropriate strains of hydrolytic bacteria. This stage is also known as polymer breakdown stage. As for example, the cellulose consisting of polymerized glucose is broken down to dimeric and then to monomeric sugar molecule (glucose) by cellulolytic bacteria.
Stage - II: Acidogenesis
The monomer such as glucose is fermented under anaerobic condition into various short-chain organic acids such as acetic acid and propionic acid with the help of enzymes produced by the acid forming bacteria.
Stage - III: Acetogenesis
In this process, the acetate bacteria convert acid phase products into acetate and hydrogen, which may be used by methanogenic bacteria.
Stage - IV: Methanogenesis
This phase consists in methane production by methanogenic bacteria. Methane is produced from substrates, which are the products of acetic acid. A vast majority of CH4 arising in methane digestion process results from acetic acid conversion. The overall reaction of converting organic matter when cellulose is the starting point becomes: (C6H10O5)n + nH2O →3nCO2 + 3nCH4 The key methanogenic reaction is as follows: CH3COOH → CH4 + CO2
Methanogenic bacteria
Four bacteria have been identified as fermenting the key intermediate - acetate to methane and carbon dioxide. 1)Methanobacterium soehngeii
2)Methanosarcina methanica
3)Methanosarcina barkeri
4)Methanococcus mazei
Types of biogas plant
The following two types of basic designs of biogas plants are found in Bangladesh:
1) Floating dome biogas plant
2) Fixed dome biogas plant
Factors affecting biogas production
The following factors are responsible for biogas generation:
1)C/N Ratio
2)pH
3)Temperature
4)Loading Rate
5)Retention time
6)Toxicity
1.C/N Ratio
C/N ratio ranging from 20:1 to 30:1 is considered optimum for anaerobic digestion. An optimum C/N ratio is required for satisfactory decomposition of organic substrates by microorganisms, those require carbon for energy source and nitrogen for nutrients.
2. pH
The optimum biogas production is achieved when pH of input mixture in the digester is from 6 to 7. Methanogenic bacteria are very sensitive to pH and do not thrive below pH 6.5 value. When methane production level is stabilized and pH range remains between 7.2 and 8.2.
3.Temperature
Satisfactory gas production takes place at a temperature of 25 to 30oC. Methanogens are inactive in extreme high and low temperatures. When the temperature goes down to 10oC, gas production virtually stops.
4.Loading rate
The loading rate is another important factor which is the amount of raw materials fed per unit volume of digester capacity per day. If the plant is overfed, acids will accumulate and methane production will be inhibited. Similarly, if the plant is underfed, the gas production will be low.
5.Retention time
Retention time is the average period that a given quantity of input remains in the digester to be acted upon by the methanogens for generation of biogas. Considering climatic condition, the retention time of 50 to 60 days is desirable. It is also dependent on the temperature. The higher the temperature, the lower is the retention time.
6.Toxicity
Heavy metal ions are toxic materials that inhibit the normal growth of microorganisms in the digester. Small quantity of mineral ions stimulates microbial growth, while high concentrations of metal ions will have toxic effect.
Application of biogas
Biogas are used for the following purposes:
1)For lighting: Biogas provides a clean fuel for lighting purpose
2)For cooking: Biogas is used for cooking purpose.
3)For electricity generation: Biogas is utilized for generating electricity.
4)For engine fuel: Biogas provides reasonably efficient fuel for engines.
Utilization of bioslurry
The benefits of bioslurry/biogas slurry or effluent are as follows:
1)Bioslurry is a cheap source of plant nutrients and can offer extra benefits to soil fertility.
2)It can ultimately reduce the production cost of agricultural products when applied as organic fertilizer.
3)It being richer in nutrients can reduce the use of chemical fertilizers for crop cultivation.
4)It reduces environmental pollution caused by the application of chemical fertilizers.
Effect of biogas as bioenergy on environment
Sustainable bioenergy development in Bangladesh brings the following significant environmental benefits: 1)Reduction of higher level of deforestation
. 2)Production of pollution free biofuel.
3)Reduction of net greenhouse gas emissions.
4)Improvement of air quality and reduction of acid deposition.
5)Improvement of soil quality.
Recommended Books:
1)Halford, N.G. 2015. An Introduction to Bioenergy. Imperial College Press, London, UK.
2)Li, Y. and Khanal, S.K. 2017. Bioenergy: Principles and Applications. John Wiley & Sons, Inc., New Jersey, USA.
3)Meisam, T. and Hossein, G. (Eds.) 2019. Biogas: Fundamentals, Process and Operation. Springer International Publishing, New York, USA.
4)Lee, S. and Shah, Y.T. 2013. Biofuels and Bioenergy: Processes and Technologies. CRC Press, Boca Raton, Florida, USA.
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