Biological Conversion Technologies

Anaerobic Digestion

Training onTechnologies for Converting Waste Agricultural Biomass into Energy

Organized by

United Nations Environment Programme (UNEP DTIE IETC)

23-25 September, 2013San Jose, Costa Rica

Surya Prakash ChandakSenior Programme Officer

International environmental Technology CentreDivision of Technology, Industry and Economics

Osaka, Japan

CONTENT

Introduction

Fermentation

Anaerobic Digestion

Basic Process

Factors Influencing Biogas Production

Types of Biogas Digesters

Biogas Yield

2

INTRODUCTION• Biological Conversion– Conversion of the biomass to fuel by exposing

biomass to certain microorganisms is called biological conversion.

– The secondary fuels are produced as a result of metabolic activity of the microorganisms.

– Fermentation and anaerobic digestions are the two most common biological conversion processes and products of these processes are ethanol and biogas.

– Today, ethanol is widely used as an alternative source of liquid fuels for the transport sector in countries and regions like USA, Brail, EU and China.

– Ethanol as a fuel offers many advantages such as high octane number (99) than petrol (80–100), low emission.

INTRODUCTION• Biogas – Biogas originates from the bio-degradation of organic

material under anaerobic conditions. – Today biogas has several applications such as

industrial and household cooking, lighting, radiant heaters and incubators for agricultural purposes and absorption refrigerators.

– Biogas system provides a whole range of benefits for their users, the society and the environment in general.

INTRODUCTION• Biogas – Benefits:

Production of energy (heat, light, electricity), Transformation of organic waste into high quality fertilizer, Improvement of hygienic conditions via reduction of

pathogens,worm eggs and flies, Increase of productivity, mainly for women, in firewood

collection and cooking, Environmental advantages through protection of soil, water,

air and woody vegetation, Micro-economical benefits through energy and fertilizer

substitution, Additional income sources and increasing yields of animal

husbandry and agriculture, Macro-economical benefits through decentralized energy

generation, import substitution and environmental protection.

FERMENTATION• Overview – Fermentation is a natural process initiated by

microorganisms, similar to common yeast cultures, under anaerobic conditions.

– Ethanol can derive from any material which contains sugar.

– In the Fermentation process, sugar elements such as glucose, fructose and sucrose are converted into ethanol and carbon dioxide as metabolic waste products.

– The net chemical equation for the production of ethanol from glucose is:

2526126 2CO OHHC2OHC

FERMENTATION• Bio-ethanol– The raw materials used in the production of ethanol

via fermentation are mainly classified into three types as sugars, starches, and cellulose materials.

– Sugars (extracted from from sugarcane, sugar beets, molasses, and fruits) can be converted into ethanol directly.

– But starches (from corn, cassava, potatoes, and root crops) and cellulose (from wood, agricultural residues, waste sulfite liquor from pulp, and paper mills) are needed to pre-treat prior the fermentation.

– The final product, ethanol (C2H5OH) mixed with small amounts of methanol or higher alcohols and diluted in water.

FERMENTATION• Bio-ethanol

Flow diagram of bioethanol production

Bio Ethanol

Enzymetic hydrolysis

Acid hydrolysis

Sugars (From sugar

cane, molasses

and fruits)

Starches(From corn,

cassava, potatoes, and

root crops

Cellulose(from wood, agricultural

residues)

Enzymetic hydrolysis

Fermantable Sugars

Fermentation Distillation

GasificationSynthesis

gas

ANAEROBIC DIGESTION• Basic Process– A simplified stoichiometry for anaerobic digestion of

biomass is;

– The whole process of biogas production from organic wastes occurred in main three steps namely hydrolysis, acidification, and methane formation.

– Three types of bacteria namely fermentative, acetogenic and methanogenic are involved in hydrolysis, acidification and methane formation, respectively.

2425106 3CO CH3OHOHC

• Basic Process– The three stage anerobic fermentaton of biomass

ANAEROBIC DIGESTION

ANAEROBIC DIGESTION• Hydrolysis– In hydrolysis aerobic micro-organisms convert

complex organic compounds (carbohydrates, proteins and lipids) into simple forms which are soluble and can be consumed by the micro-organisms.

– As an example, Polysaccharides are converted into monosaccharides, lipids to fatty acids, proteins to amino acids and peptides.

• Acidification– In the second step, acid-producing bacteria

(acetogenic bacteria), convert the intermediates of fermenting bacteria into mixture of acetic acid (CH3COOH), H2, CO2, alcohols, organic acids, amino acids and hydrogen sulphide.

ANAEROBIC DIGESTION• Acidification– The oxygen requirement for producing acetic acid is

fulfilled by the oxygen solved in the solution or bounded-oxygen.

– The acid-producing bacteria create an anaerobic condition which is essential for the methane producing microorganisms.

• Methane formation– In the third step, methane-producing bacteria utilize

hydrogen, carbon dioxide and acetic acid formed in acidification process to form methane and carbon dioxide.

ANAEROBIC DIGESTION• Factors influencing the biogas production– Substrate temperature

Optimal temperature range for biogas production: 20-28°C. This can be achieved in a satisfactory level only where mean

annual temperatures are around 20°C or above or where the average daily temperature is at least 18°C.

If the temperature is below 15°C, gas production will be so low that the biogas plant is no longer economically feasible.

– Changes in temperature The process is very sensitive to changes in temperature. Most of the biogas plants are builds in underground in order

to overcome this issue. The temperature fluctuations between day and night are no

great problem for plants built underground, since the temperature of the earth below a depth of one meter is practically constant.

ANAEROBIC DIGESTION• Factors influencing the biogas production– Available nutrient

In order to grow, bacteria need organic substances as a source of carbon and energy.

In addition to carbon, oxygen and hydrogen, the generation of bio-mass requires an adequate supply of nitrogen, sulfur, phosphorous, potassium, calcium, magnesium and a number of trace elements such as iron, manganese, molybdenum, zinc, cobalt, selenium, tungsten, nickel etc.

"Normal" substrates such as agricultural residues or municipal sewage usually contain adequate amounts of the mentioned elements.

Higher concentration of any individual substance usually has an inhibitory effect, so that analyses are recommended on a case-to-case basis to determine which amount of which nutrients, if any, still needs to be added.

ANAEROBIC DIGESTION• Factors influencing the biogas production– Retention time

The effective retention time may vary widely for the individual substrate constituents depending on the vessel geometry, the means of mixing, etc.

Selection of a suitable retention time depends on process temperature as well as on the type of substrate used.

For liquid manure undergoing fermentation, the following approximate values apply:

- liquid cow manure: 20-30 days

- liquid pig manure: 15-25 days

- liquid chicken manure: 20-40 days

- animal manure mixed with plant material: 50-80 days If the retention time is not maintained properly and it is too

short, the bacteria in the digester are "washed out" faster than they can reproduce, and fermentation practically comes to a standstill. This problem rarely occurs in WAB systems.

ANAEROBIC DIGESTION• Factors influencing the biogas production– pH Value

The best condition for the methane-producing bacteria is neutral to slightly alkaline conditions.

Once the process of fermentation has stabilized under anaerobic conditions, the pH will normally take on a value of between 7 and 8.5.

If the pH value drops below 6.2, the medium will have a toxic effect on the methanogenic bacteria.

– Nitrogen inhibition Nitrogen in the substrate inhibits the process of

fermentation. Noticeable inhibition occurs at a nitrogen concentration of

roughly 1700 mg ammonium-nitrogen (NH4-N) per liter substrate.

The main prerequisite is that the ammonia level does not exceed 200-300 mg NH3-N per liter substrate.

ANAEROBIC DIGESTION• Factors influencing the biogas production– C/N ratio

Microorganisms required both nitrogen and carbon for assimilation into their cell structures.

Various experiments have shown that the metabolic activity of methanogenic bacteria can be optimized at a C/N ratio of approximately 8-20, whereby the optimum point varies depending on the nature of the substrate.

– Substrate solid content Solid content of the substrate impaired the mobility of the

methanogens within the substrate. Therefore the biogas yield decreases with the increase of

solids content. No generally valid guidelines can be offered with regard to

specific biogas production for any particular solids percentage.

ANAEROBIC DIGESTION• Types of Biogas Plants– There are various types of biogas plants available in

the world and they are classified mainly based on feeding method and construction.

– Based on the feed method, three different forms can be distinguished: Batch plants, Continuous plants, Semi-batch plants.

– Based on the construction, two main types of simple biogas plants can be distinguished: Fixed-dome plants, Floating-drum plants.

ANAEROBIC DIGESTION• Types of Biogas Plants– Batch type digesters.

In batch type plants, materials fed into the digester at a time and sealed only allowing the gas to exit and then emptied completely after a fixed retention time.

Each design and each fermentation material is suitable for batch filling, but batch plants require high labor input.

The major disadvantage of this type is unsteady gas-output.

Figure 5.3: Batch type biogas digester

1 - Digester.

2 - Gasholder.

3 - Gas pipe.

1

3

2

ANAEROBIC DIGESTION• Types of Biogas Plants– Continuous type digesters.

Once the process started, regular quantity of waste are fed and regular quantity of material discharged, continuously.

They empty automatically through the overflow whenever new material is filled in.

Therefore, the substrate must be fluid and homogeneous. This technology is suitable for both medium and large scale waste treatment and large scale biogas production.

Advantages of this type are constant and higher gas production

ANAEROBIC DIGESTION• Types of Biogas Plants– Semi-batch type digesters.

If the two materials which have completely different digestion rates (such as straw and dung) are to be digested together, a biogas plant can be operated on a semibatch basis.

The slowly digested straw-type material is fed in about twice a year as a batch load. The dung is added and removed regularly.

ANAEROBIC DIGESTION• Types of Biogas Plants– Fixed Dome type digesters.

A fixed-dome plant comprises of a closed, dome-shaped digester with an immovable, rigid gas-holder and a displacement pit.

ANAEROBIC DIGESTION• Types of Biogas Plants– Fixed Dome type digesters.

The gas is stored in the upper part of the digester. When gas production commences, the slurry is displaced

into the displacement tank. Gas pressure increases with the volume of gas stored, i.e.

with the height difference between the two slurry levels. If there is little gas in the gasholder, the gas pressure is low. The digesters of fixed-dome plants are usually masonry

structures, structures of cement and ferro-cement exist. Main parameters for the choice of material are technical

suitability (stability, gas- and liquid tightness), cost-effectiveness, availability in the region and transport costs and availability of local skills for working with the particular building material.

ANAEROBIC DIGESTION• Types of Biogas Plants– Fixed Dome type digesters.

Currently, various types of fixed dome plants are available such as Chinese fixed dome plant, Janata model, Deenbandhu and CAMARTEC model.

Currently, various types of fixed dome plants are available such as Chinese fixed dome plant,

Janata model, Deenbandhu and CAMARTEC model. Chinese fixed-dome plant, shown in

Figure 5.5, is the archetype of all fixed dome plants and extensively use in China. The

digester consists of a cylinder with round bottom and top.

Chinese fixed dome type biogas digester

Janata

Fixed dome biogas digester: CAMARTEC model

ANAEROBIC DIGESTION• Types of Biogas Plants– Fixed Dome type digesters.

Fixed dome biogas digester - Nicarao design

Automatic overflow

Waste Biogas Gas collector,

fixed dome

Slurry

10

4

5 6

7

8

9

3

1

2

1. Mixing tank with inlet pipe and sand trap.

2. Digester. 3. Compensation and removal

tank. 4. Gasholder. 5. Gas pipe. 6. Entry hatch, with gastight seal. 7. Accumulation of thick sludge. 8. Outlet pipe. 9. Reference level. 10. Supernatant scum

ANAEROBIC DIGESTION• Types of Biogas Plants– Floating drum type digesters.

Major difference between fixed dome and floating drum type plant is, a floating-drum plant consists of a floating gas-holder, or drum.

ANAEROBIC DIGESTION• Types of Biogas Plants– Floating drum type digesters.

This floats either directly in the fermenting slurry or in a separate water jacket.

The drum in which the biogas collects has an internal and/or external guide frame that provides stability and keeps the drum upright. If biogas is produced, the drum moves up, if gas is consumed, the gasholder sinks back.

Floating-drum plants are used mainly in continuous feed mode of operation.

They are used most frequently by small- to middle-sized farms (digester size: 5-15m3) or in institutions and larger agro-industrial estates (digester size: 20-100m3)

ANAEROBIC DIGESTION• Types of Biogas Plants– Advantages of fixed dome type.

Produce just as much gas as floating-drum plants, if they are gas-tight.

Low cost operation Simple design Long life of the plant (20 years or more)

– Disadvantages of fixed dome type. Utilization of the gas is less effective as the gas pressure

fluctuates substantially. Labor-intensive design Not easy to build. Difficult to achieve gas tightness.

ANAEROBIC DIGESTION• Types of Biogas Plants– Advantages of floating drum type.

Simple operation Provide gas at a constant pressure Stored gas-volume is immediately recognizable by the

position of the

– Disadvantages of floating drum type. The steel drum is relatively expensive and maintenance-

intensive. Removing rust and painting has to be carried out regularly. The life-time of the drum is short (up to 15 years; in tropical

coastal regions about five years).

ANAEROBIC DIGESTION• Biogas Yield– Biogas yield of a biomass material depends on the

organic fraction of dry matter in the material and the waste management system associated with it.

– The dry matter (DM) of the waste is the matter left after removal of its moisture content. It may be obtained as the weight loss on heating to a temperature of 105 C.

– Whereas, Volatile Solids (VS) are defined as the organic fraction of dry matter in waste.

– Around 50-60% of the initial energy content in the organic material can be converted to biogas in a properly operated digester.

ANAEROBIC DIGESTION• Biogas Yield

– Biogas production after addition of substrate

ANAEROBIC DIGESTION• Biogas Yield– The resultant gas mixture consists of about 50 - 70%,

CH4 and the rest is CO2 with small amounts of water vapours, H2S, NH3, and some organics that give bad odour.

– Methodology of Estimation:• Amount of biogas that can be potentially produced from

recoverable wastes= Amount of dry matter recoverable (kg DM/year) Volatile solids fraction in dry matter (kg VS/kg DM) Biogas yield (m3/kg VS)

• Energy potential of the biogas recoverable (MJ /year) = Amount of biogas recoverable (m3/year) Heating

value of biogas (MJ /m3)

ANAEROBIC DIGESTION• Biogas Yield– The heating value of the biogas depends on its

composition, especially the amount of methane. – HHV of methane is about 35.8 MJ/m3 and therefore

biogas with 60% methane, HHV could be taken as 20 MJ/m3.

– Selected values for waste characteristicsAnimal Type Fraction of Volatile

Solid (VS/DM)Biogas Yield(m3/kg of VS)

Cattle 0.8 0.20 – 0.3

Poultry 0.8 0.35 – 0.60

Pigs 0.7 – 0.8 0.25 – 0.50

Straw 0.8 – 0.9 0.15 – 0.35

Grass 0.9 0.55

Fruit Waste 0.75 0.25 – 0.50

Garden Waste 0.9 0.20 – 0.50

ANAEROBIC DIGESTION• Biogas Yield– Benchmarks for specific methane yields

The End

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