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  • Amir Muhaisen

Bio-Gas : Domestic Approach

in this article, bio-gas process is analyzed with an aim to enhance efficiency thus to revolutionize domestic production



Presently, economic transition is based on circular economy in which hydrocarbon fuel price volatility is required to make alternatives feasible ; as european energy crisis have contributed to green hydrogen market feasibility via renewables, waste-to-chemicals via environmental legislations, and a contemporary trend for bio-gas especially as small-scale domestic appraoch


Normally, formation of methane or sometimes called bio-methane from substrate requires an anaerobic condition in presence of metabolic activity via syntrophic mico-organisms (bacteria), hence methane fromation is bio-chemical process that is illustrated in conventional reactions below, notice how methane content increases upon substrates rich in long-chain hydrocarbons


Bio-Gas is term used to refer to methane, carbon dioxide and impurities produced via anaerobic digestion of substrates, in which composition, calorific value, and other properties are illustrated


Critical to mention here that common substrates are feces and bio-residues (meat, crops, leftovers) in which feces and meat residues provide higher methane content in comparison to crop residues


Practically, 0.3 kilogram bio-gas per kilogram of substrate is common estimate as 1.0 kilogram of bio-gas is sufficient for 4-hours of domestic cooking due to its methane content. while 1.0 kilogram of liquified petroleum gas is sufficient for approx. 9-hours of domestic cooking


Process Design


In reference to bio-chemical reaction model ; methane formation via anaerobic syntrophic micro-organisms (bacteria) requires multiple reactions in which hydrolysis (1), acidogenesis (2), and acetogenesis (3) are sequential to reach methanogenesis (4)


Note that hydrolysis (1) takes place as undissolved substrate components -carbohydrates, proteins, and glycerides- are cracked into monomers by exo-enzymes of facultative and obligatorily anaerobic bacteria. Beneficially, facultative anaerobic bacteria consume dissolved oxygen in water hence necessary for obligatorily anaerobic bacteria. In addition, covalent bonds are split in chemical reaction with water as hydrolysis of carbohydrates takes place within hours, yet hydrolysis of proteins and lipids within days


Consequently, acidogenesis (2) takes place as monomers formed in hydrolysis reactions are taken up by different facultative and obligatorily anaerobic bacteria into short-chain carboxylic acids (propionic, butyric, valeric, i-valeric, capronic, lactic) , alcohols (ethanol, glycerine) , hydrogen , and carbon dioxide


Furthermore, acetogenesis (3) takes place as obligatory anaerobic bacteria turns carboxylic acids, alcohols, and carbon dioxide into acetic acid esters (acetates) and hydrogen thus pH increases. Finally, methanogenesis (4) takes place in which methanogen obligatory anaerobic bacteria consume acetic acid esters (acetates) and reduces carbon dioxide via hydrogen into methane as hydrogen is produced in previous acetogenesis reactions


In reference to chemical reaction engineering ; hydrolysis (1) and acetogensis (3) control overall reaction rate thus reaction rates must be equal. Otherwise if hydrolysis is faster then carbon dioxide yield increases and pH decreases hence hinders acetogenesis. On the other hand, if acetogenesis is faster then methane yield decreases upon deficient reactants


In an engineering sense, it is recommended to use multiple reactors ; hydrolyzer to carry out hydrolysis and acidogenesis, plus digester to procceed into acetogenesis and methanogenesis, hence ideally for lignocellulose substrates (crop-residues) as hydrolysis enhancement is top-priority. Note that conventional domestic bio-methane process is illustrated

Critical to mention here that sequencial reactions mentioned above take place within substrate-concentration profile, reactors operate @ ambient pressure, high residence times are proportiental to yield, and single reactor is sufficient for protien-rich substrates (meat residues) upon tolerable pH variations. Moreover, reactor operational parameters illustrated


Practically, product purification is required to remove impurities ; ammonia, hydrogen sulfide, and carbon dioxide to eliminate effects, thus bio-methane is preferable as clean fuel compared to pre-treatment bio-gas

In reference to industrial-scale production, absorption is utilized as bio-methane impurities are captured via sequence of scrubbers ; in which di-ethanolamine captures carbon dioxide and traces of hydrogen sulfide, while water captures traces of ammonia

However, carbon dioxide capture proved inefficient as for domestic-scale production in terms of energy consumption via adsorption or absorption. Therefore, domestic-scale production focus on traces removal as end-product to be moisture-free, ammonia-free, and de-sulfurized bio-gas via adsorption @ atmospheric conditions ; efficient desiccant and activated carbon


Furthermore, bio- methane holder operates @ ambient pressure since bio-methane compression is inefficient as for expensive equipment required. Note that typical cylinders are operational till approx. 12 bar. On the other hand, methane cylinders hold compressed methane to 137 bar or more, besides exceptional insulation required as methane liquifaction requires -162 °C thus cryogenic apparatus


According to Chemiprobe, flame-resistant poly-vinyl chloride inflatable holders have recently revolutionized bio-methane production in rural parts of china as for design and safe transport


Attention should not be restricted upon product purification though, substrate pre-treatment is critical as for efficiency because substrate size-reduction contributes to more than 30 % increase in yield, not to mention that in case standard pumps are used for liquid transport then substrate (dry matter) should be mixed with water within (2% – 12%) ratio to ensure functionality. Keep in mind that antibiotics, therapeutics, and anti-septics found in substrate impair operations. Another observation is that co-substrates or in other words "substrate additives" such as potato or apple peels contributes to impurities reduction within (2% - 7%) decrease in carbon dioxide content


Market Trend


In middle-east, conventional bio-methane business requires tolerable capital as for industrial-scale upon substrate availability, therefore only profitable if substrate is sourced from location within (9 km – 20 km) from production facility. On the other hand, domestic-scale business is expected to gain public attention upon fuel price volatility as competent prototypes are offered such as homebiogas inflatables and methane-x ; in an effort to promote eco-lifestyle. Critical to mention here that domestic septic tank approach recieved backlash years ago upon reported incidents as bio-gas posed an explosion hazard once accumulated in piping system


In europe, bio-methane business is promising upon efforts to produce genetically-modified anaerobic bacteria strains to enhance process yield as multiple bio-catalyst start-ups have had access to research funds



substrate : suitable biomass for bio-methane production >> feces, leftovers, and bio-residues


anaerobic : absence of free oxygen


syntrophic : various species of micro-organisms acting together to degrade substrate in which each type of micro-organisms cannot degrade substrate by itself


exo-enzyme : extra-cellular enzyme that is secreted by micro-organism cell to function outside that micro-organism such as hydrolase that accelerates hydrolysis reaction


facultative anaerobic bacteria : able to perform aerobic respiration if oxygen is present yet switch to fermentation if oxygen is absent thus resembles obligatory anaerobic bacteria


silica gel : an efficient desiccant with 40% capacity, captures 40 mass units of moisture per 100 mass units of packed-bed @ atmospheric conditions, regenerated via heat @ 150 C >> 0.2 kilogram packed-bed is sufficient for + 90 operation days for domestic-scale reactors


activated carbon : an efficient adsorber with 20% capacity, captures 20 mass units of hydrogen sulfide and ammonia per 100 mass units of packed-bed @ atmospheric conditions, exhibited preferential adsorption for ammonia , hydrogen sulfide , and even carbon dioxide compared to bio-methane, regenerated via super-heated steam >> 0.2 kilogram packed-bed is sufficient for + 200 operation days for domestic-scale reactors


liquified petroleum gas cylinder (Jordan) contains12 kilogram of liquified mix

>> (65% butane) (35% propane)


HomeBioGas is an israeli company headquartered in Beit Yanai ; specializes in domestic-scale bio-gas systems


methane-x is Jordan-based start-up on a mission to provide efficient domestic-scale bio-gas systems


References


1. Ghanavati, H. Tabatabaei, M. (2018) biogas: Fundamentals, process, and Operation. springer


2. Deublein, D. Steinhauser, A. (2008) biogas from waste and renewable resources. wiley-vch


3. Gerardi, M.H. (2003) the microbiology of anaerobic digesters. wiley-Interscience

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