BiogasThe Anaerobic Digestion Process

Andrew Gabriel and Tidasate Success

What exactly is biogas (composition)? What is the biological process by which it is

created? What type of substance is used in this

process? What are the three stages of the biogas

production process?

Things you should learn from this presentation

Biogas is a gas composed of methane and carbon dioxide

It is desired for its highmethane content foruse as a fuel

What is biogas?

The commercial recovery of biogas hasn’t been a main concern in industry until the mid 1970’s the fuel’s potential was realized. Today, farm based anaerobic digestion facilities are the most common in biogas production industry. Most of these facilities use the biogas as an energy source for their own usage.

History

The process by which biogas is produced Uses carbonaceous organic matter In the absence of oxygen

C6H12O6 → 3CO2 + 3CH4. (Organic Material) (Carbon Dioxide) (Methane)

Anaerobic Digestion

sewage sludge solids refuse food waste biomass including plants Crops plant and crop wastes industrial liquids and solids anything else organic

Anaerobic Digestion:Carbonaceous Organic Matter

The three main stages are: Hydrolysis/fermentation Acid phase digestion Acetogenesis Methane phase digestion

Anaerobic Digestion

Legend 1. Feeding header

2. Gas header3. Gas collecting system

Methanisation stage

Hydrolyse

stage

Methanisation stage

Feeding piping

Evacuated into the

substrate storage

Anaerobic Digestion

Bioreactor type: fixed dome and floating drum◦ Other types include balloon, horizontal, earth-pit

ferrocement plant

Bioreactor Design Parameters

Parts of the plant:◦ Mixing pit ◦ Digester◦ Strirring system◦ Gas holder◦ Slurry storage◦ Heating system◦ Pumps◦ Rings

Bioreactor Design Parameters

Solids content◦ High or low◦ Wet or dry

Temperature:◦ Psychrophilic◦ Mesophilic◦ Thermophilic

Operating Parameters

C:N ratio 15:1-25:1 Excess Carbon

◦ organic acids: lower pH Excess Nitrogen

◦ Ammonia: higher pH

Operating Parameters

Acidity: Carbon Nitrogen ratio

◦ 30:1◦ Excess Carbon

organic acids: lower pH◦ Excess Nitrogen

Ammonia: higher pH

Operating Parameters

Daily◦ Gas pressure, substrate temperature, gas

production, strong sludge odor Weekly

◦ Clean gas appliances, lubricate movable parts, control gas appliances

Monthly◦ Maintenance of slurry agitator

Annually◦ Check for corrosion

Maintenance

Main producers of biogas come from: Farm based facilities Waste water treatment plants Sewage treatment plants

Biogas Producers

In 1977 Pacific Gas and Electric Co. and Southern California Gas Co. contracted Bio Gas of Colorado

Project used waste from 30,000 head of cattle

Two 21,300 gallon digesters 1.5 million standard cubic feet per day of

biogas (60% methane composition) average-year gas cost was $3.44/million Btu Average retail cost that year was $10.99

Imperial Valley Biogas Project

contribute to a number of health hazards to humans.

These emissions are regulated by The United States Clean Air Act and Title 40 Part 59 of the Code of Federal Regulations (CFR).

volatile organic compounds (VOCs)

For employers to ensure employee safety dealing with flammable or explosive materials:◦ Dangerous substances◦ Control measures to remove risks.◦ Controls to reduce the effects of any incidents.◦ Plans and procedures to deal with accidents,

incidents and emergencies.◦ Employees are trained to deal with the risks.◦ Classify areas where explosive atmospheres may

occur and avoid ignition sources in those areas.

DSEAR Guidelines

Number of Reactors Single Stage

◦ Entire process in one step the two main phases can conflict with each other

Multi-Stage◦ hydrolysis and acid phase steps are combined,

and the methane phase digestion proceeds separately. Better pH regulation

Other Considerations

The size of the digester◦ Vd = Sd x RT [cubic meter/days x number of days]

Daily gas production◦ Based on volatile solids content G = VS x Gy◦ Weight of the moist mass B: G = B x Gy◦ STD gas yield values per livestock unit

Digester loading◦ Daily total solids input

The size of the gas holder◦ depends on the relative rates of gas

generation/consumptions

Calculations

Biogas is made up of methane and carbon dioxide

Biogas is produced through the process of anaerobic digestion

The substance used in this process is carbonaceous organic matter

The three stages of biogas production are ◦ Hydrolysis/fermentation◦ Acid phase digestion◦ Methane phase digestion

Conclusion

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Questions