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ENERGY FROM

BIOMASS

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Biomass resources fall into three categories Biomass in its traditional solid mass (wood and

agriculture residue) Biomass in non traditional form (converted into

liquid fuels) The frist category is to burn the biomass directly

and get the energy . In the second category , the biomass is converted into ethanol (ethyl alcohol) and methanol (methyl-acohol) to be used as liquid fuels in engines.

The third category is to ferment the biomass anaerobically to obtain a gaseous fuel called bio-gas

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DIGESTION Decomposition of organic matter by anaerobic bacteria in an oxygen-starved environment

Anaerobic digesters compost (or "digest") organic waste in a machine that limits access to oxygen encouraging the generation of methane and carbon dioxide by microbes in the waste. This digester gas is then burned as fuel to make electricity

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Biomass means organic matter and photochemical approach to harness solar energy means harnessing of solar energy by photosynthesis. Solar energy is stored in the form of chemical energy. Hence

Solar energy -Photosynthesis-Biomass-Energy Generation.

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BioFuels The energy stored in dry biomass like

wood and straw is most easily released by direct combustion – although dry materials can also be converted into liquid and gaseous fuels by variety of techniques.

three biofuels – wood , straw and refuse are being burnt on an increasing scale in many countries to provide useful heat.

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Biomass Conversion Technologies

A wide variety of conversion technologies is avaliable for manufacturing premium fuels from biomass.

Some are simple and well understood like digestion and fermentation ; others like gasification have been tested in large pilot plants and are now being commercialised

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Thermochemical conversion

Takes two forms gasification Liquefaction Gasification - take place by heating

the biomass with limited oxygen to produce low heating value gas or by reacting it with steam and oxygen at high pressure and temperature to produce medium heating value gas.

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The later may be used as fuel directly or used in liquefication by converting it to methanol (methyl alcohol CH3OH) or ethnol (ethyl alcohol CH3CH2OH) or it may be converted to high heating value gas.

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Biochemical Conversion Takes two forms anaerobic digestion fermentation Anaerobic digestion involves the

microbial digestion of biomass. (an anarebic is a micro organism that can live and grow without air or oxygen by the decomposition of matter containing it)

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The process take place at low temprature upto 65 degree celsius , and requires a moisture content of at least 80 percent.

It generates a gas consisting mostly of CO2 and methane (CH4) with mimimum impurities such as hydrogen sulfide

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Fermentation is the breakdown of complex molecules in organic compound under the influence of a ferment such as yeast , bacteria, enzymes , etc.

Fermentation is a well established and widely used technology for the conversion of grains and crops into ethnol.

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Wet Processes Anaerobic Digestion Fermentation

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Anaerobic digestion or simply digestion consist broadly of three phases :

Enzymatic hydrolysis : where the fats , starches and proteins contained in cellulosic biomass are broken broken down into simple compound

Acid formation : where the micro organisms of facultative and anaerobic group collectively called as acid farmers, hydrolyse and ferment , are broken to simple compounds into acids and volatile solids.

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As a result complex organic compounds are broken down to short chemical simple organic acids.

Methane formation : when organic acids as formed above are then converted into methane (CH4) and CO2 by the bacteria which are strictly anarobs. These bacteria are called methane fermentors. For efficient digestion these acid formers and methane fermentors must remain in a state of dynamic equilibrium.

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Advantages of anaerobic digestion

Caloric value of gas :one of the main be nefits is the production of a biproduct the biogas which has a calorific value and can therefore, be used as an energy source to produce steam or hot water.

New sludge production : the conversion of organic matter to methane and carbon dioxide results in a similar quantity of excess sludge.

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Low running cost : there is no airation in the anaerobic treatment naturally in this digestion , running costs are quarter of the equivalent aerobic system.

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Dry Processes Pyrolysis – A wide range of energy rich fuels

can be produced by roasting dry woody matter like straw and wood chips.

The materials is fed into reactor vessel or retort in a pulverised or shredded form and heated in the absence of air.

As the temprature rises the cellulose and lignin breakdown to simpler substance which are driven off leaving a char residue behind.

This method has been used for centuries to produce charcoal.

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Liquefication Liquid yeilds are maximized by rapid

heating of the feedstock to comparetively low tempratures.

The vapours are condensed from the gas stream and these seprate into two phase liquor: the aqueous phase contains a soup of water soluable organic materials like acetic acid , acetone and methanol

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The non aqueous phase consists of oils and tars.

These crude oil can be burnt , but it is usually more profitable to up-grade them to premium fuels by conventional refining technique.

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Gasification Pyrolysis of wet biomass produces fuel gas

and very little liquid. An alternative technique for maximum gas

yeilds is to blow small quantities of air or oxygen into reactor vessel and to increase the temprature to over 1000 degree celsius.

This causes part of the feed to burn. Fuel gas from air blown gasifiers has a low calorific values and may contain upto 40% inert nitrogen gas overall yeilds of 80-85% can be expected.

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Steam Gasification Methane is produced directly from

woody matter by treatment at high temprature and pressure with hydrogen gas.

The hydrogen can be added or , more commonly , generated in the reactor vessel from carbon monooxide and steam.

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Recent analysis suggest that steam gasification is the most efficient route to methanol .

Net energy yeilds 55% can be achieved although higher yeilds are likely in the future as the technology is developed.

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Hydrogenation Under less severe conditions of

temperature and pressure (300 – 400 degree Celsius and 100 atmospheres) , carbon monooxide and steam react with cellulose to produce heavy oils which can be seperated and refined to premium fuels.

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Classification of Biogas Plants

Biogas plants are mainly classified as :

Continuous and batch types

The dome and the drum types.

Different variation in the drum type.

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Continuous and Batch Type Continuous plant – here is a single digester in

which raw material are charged regularly and the process goes on without interruption except for repair and cleaning etc.

In this case the raw material is self buffered or otherwise thoroughly mixed with the digesting mass where dilution prevents souring and the biogas production is maintained. The countinous process may be completed in a single stage or separated into two stages

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Single stages process – the entire process of conversion of

complex organic compound into biogas in completed in a single chamber.

This chamber is regularly fed with raw materials while the spent residue keeps moving out.

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Double stage process – The acidogenic stage and methanogenic

stage are physically separeted into two chambers.

Thus the first stage of acid production is carried out in a separate chamber and only the diluted acids are fed into the second chamber where bio-methanation take place and the biogas can be collected from the second chamber.

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The main features of countinous plants are

It will produce gas continously;

It requires small digestion chambers;

It needs lesser periods for digestion;

It has less problems compared to batch type and it is easier in operation.

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Batch Plant The feeding is between intervals, the

plant is emptied once the process of digestion is complete.

In this type, a battery of digesters are charged along with lime ,urea etc and allowed to produce gas 40-50 days. These are charged and emptied one by one in a synchronous manner which maintains a regular supply of the gas through a common gas holder.

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The biogas supply may be utilised after 8-10 days.

Such a plant would be expensive to install and unless operated on large scale it would not be economical.

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Dome and Drum Type There are numerous models of a

biogas plant mainly two main types are usually used :

The floating gas holder plant and other is

Fixed dome digester.

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The floating gas holder digester which is used in india is known as KVIC plant. The fixed dome digester is called the chinese plant.

There are different shapes in both the designs ,cylindrical rectangular , spherical etc.

The floating gas holder digester developed in india is of mansory construction with the gas holder. The gas holder is separated from the digester.

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In the fixed dome digester the gas holder and the digester are combined.

The fixed dome is the best suited for batch process.

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Types of Biogas Plants Biogas plant can be grouped under two

broad heads Floating Gas Holder Fixed dome digestero The family size biogas plants available

today in india are broadly of two types. KVIC model (Khadi Village Industries

Commission) Janta Model

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Circular Digester with Floating gas holder

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Fixed Dome Digester

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The KVIC plant is of steel drum type or floating gas holder design, in which the digestion take place in a masonry well and the drum floats as the gas collects and is taken out from the top

The janta model or fixed dome digester (also called chinese plant) is a drumless type similar in construction to the KVIC model except that the steel drum is replaced by a fixed dome roof of masonry construction.

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The different design variation of family type biogas plants avaliable at present in our country includes:

KVIC (khadi and village industries commission) design

PRAD (Planning , Research and Action division) design (modification of chinese design)

ASTRA (Application of science and technology to rural area) design

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Murugappa chettiar Research centre design Tamil Nadu Agriculture University dome type

design Himachal Pradesh capsule design Kuccha-Pucca Model of Punjab agriculture

University Ludhiana. Deen Bandu Design Plug flow design Roorkee Design Ganesh Model etc…

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Selection of site for a Biogas Plants

Distance : the distance between the plant and the site of gas consumption should be less in order to achieve economy in pumping of gas and minimizing gas leakage. For the plant of capacity 2 cubic meter , the optimum distance is 10m

Minimum gradient – for conveying the gas a minimum gradient of 1% must be made avaliable for the line.

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Open space : the sunlight should fall on the plant as temperature between 15 degree Celsius is essential for gas generation at good rate.

Water table : the plant is normally constructed underground for ease of charging the feed and unloading slurry require less labour.

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Seasonal run off : proper care has to be taken to prevent the interfere of run off water during monsoon.

Distance from wells : the seepage of fermented slurry may pollute the well water. Hence a minimum of 15 m should be maintained from wells.

Space requirement :sufficient space must be avaliable for day to day operation and maintenance. As a guide line 10 to 12 cubic meter is needed per cubic meter of the gas.

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