What is Biomass Energy?

Biomass is a renewable energy source because the growth of new plants and trees replenishes it. Biomass is the term used for the mass derived from carbonaceous waste of various human and natural activities. It includes agricultural and forest residues, animal waste, household waste and by products of wood and discarded material from food processing plants.

Biomass is basically a hydrocarbon containing carbon, hydrogen and oxygen. Chemical formula for biomass is C_6n(H₂O)_5n.

Solar energy is stored in plants by the process of photosynthesis, absorbing carbon dioxide (CO₂) from the atmosphere and converting into carbohydrates such as sugar, starch and cellulose.

Biomass does not add carbon dioxide (CO₂) to the atmosphere, as it aborbs the same amount of carbon in growing as it releases when consumed as a fuel. So, energy produced from biomass is carbon-cycle neutral.

Biomass can be used in the form of solid, liquid and gaseous fuels. Biomass fuels used in India contribute about 30% of total fuel used at present. It is an important fuel used in over 90% of the rural households and 15% in urban area. The potential of biomass energy in India is over 1,10,000 MW, and a capacity of over 4400 MW has already been commissioned.

TYPES OF BIOMASS AND THEIR APPLICATION

India produces about 55 crore tonnes of agricultural and industrial waste every year. 29 crore cattle population produces about 44 crore tonnes of dung annually. The main types of biomass are described below in brief:

1. Energy crops: Energy crops include fast growing plants which supply wood, vegetable oil and alcohol. These plants are grown on degraded or wasteland. It is called energy farming because the crops are used to produce power.

2. Woody biomass: Woody biomass is obtained from trees. The calorific value of soft wood is 4600 kcal/kg and that of hard wood is 5000 kcal/kg. Woody biomass is mainly used for our energy needs in addition to household and agricultural purposes. In rural areas woody biomass is an important fuel used for cooking and heating purposes. In urban areas, woody biomass is converted into charcoal and used for cooking and other purposes.

3. Crop residues: Crop residues consist of rice husk, wheat straw, corn cobs, cotton sticks, sugar cane bagasse, ground nut and coconut shells. Some of these are converted to briquettes and used as fuel. Bagasse is obtained from sugar mills and used in cogeneration plants. Similarly, rice husk is used in rice mills as cogeneration plants.

4. Animal waste: It is an organic material and is rich source of fuel. Animal dung is mainly used in rural area for cooking and heating purposes in the form of dung cakes. Animal dung is the main raw material of biogas plant and the waste slury obtained from the biogas plant has high nitrogen content and used as manure in agriculture. Biogas has a number of applications like heating, cooking, lighting, engine fuel and power generation.

5. Urban waste: It consists of municipal solid and liquid waste from domestic sewage and effluent from institutional activities. About 4 crore tonnes of solid waste, 600 crore cubic metres of liquid waste are generated in urban area every year. Both the sources are useful for biogass production.

6. Industrial waste: It comprises of paper and pulp industries, starch and glucose industry waste, palm oil industry waste, distillary waste and tannery waste. Industrial waste can be used for power generation by adopting different methods and technology.

ENERGY CONTENT IN BIOMASS

Energy content of biomass depends upon its ingredients. The main ingredients of biomass are carbon (C), hydrogen (H₂), oxygen (0₂). moisture and ash, in addition to some traces of sulphur (S) and nitrogen (N). Carbon is the main source of heat in the biomass. Moisture content of biomass is the quantity of water present in the material and is expressed as percentage of its weight. Moisture content adversely affects the value of biomass as a fuel. Ash is the organic matter left out after complete combustion of biomass. It is also expressed in percentage of its weight.

The energy content of biomass fuel mainly depends upon the carbon, moisture and ash content. High moisture and high ash contents have low energy value and vice versa. For example, woody biomass has low ash content and high energy value in comparison to crop residue which has high ash content and low energy value.

The energy content of wood with 1% ash content and 13% moisture content is 16 MJ/kg. A typical crop residue with 10 % ash and 13% moisture has energy content of 13.5 MJ/kg. Animal dung with 20% ash and 13% moisture has energy content of 14.5 MJ/kg.

BIOMASS BASED FUELS

Biomass is one of the main energy sources, used in the form of solid, liquid and gaseous fuels called biofuels. These are obtained by different conversion processes.

1. Charcoal: It is a smokeless dry solid fuel. It has high energy content.

Charcoal is produced by heating woody biomass at a temperature of 170° to 500°C in the absence of air (pyrolysis).

Charcoal is widely used in domestic as well as industrial applications, The calorific value of charcoal having 30% carbon content is 23 MJ/kg which is obtained at a temperature of PC. At higher temperatures carbon content will be more and will have a higher calorific value.

2. Producer Gas: It is obtained by gasification conversion process of biomass, which takes place at a temperature range of 500 to 1000°C in the presence of air. The main constituents of producer gas are carbon monoxide (CO), hydrogen (H), nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), methane (CH₂). This gaseous fuel is widely used for heating, running engines and power generation purposes. Calorific value of producer gas is 4.35 MJ/ m³ for a fuel content of 35% CO and 65% N₂.

3. Biogas: It is obtained from animal waste, like cattle dung by anaerobic fermentation taken place in a sealed digester at a temperature 35 to 55°C. It is mainly composed of methane (60%) and carbon dioxide (35%) alongwith some traces of other gases. In rural areas it is used for cooking, heating and lighting. Biogass can also be used for power generation purposes.

4. Ethanol (C₂H₅OH): It is a liquid fuel obtained from biomass containing sugar by fermentation process. It can be blended with petrol and diesel which enhances their octane number rating. Also blending of 10% of ethanol with this fuel will save a lot of foreign exchange. The calorific value of ethanol is 30 MJ/kg.

5. Methanol (CH₃OH): The purified producer gas is subjected to liquefaction process over a zine chromium catalyst and is converted into methanol. It is widely used as a liquid fuel having calorific value of 23 M.J/kg. This can be used as biofuel and can be blended with petrol and diesel.

BIOGAS PLANT

Basic Working Principle

Biogas comprises of 60-65% Methane (CH₂), 35-40% Carbon-dioxide (CO₂), 0.5 to 1.0% hydrogen sulphide (H₂S) and rest is water vapours, etc. Biogas is lighter than air. The ignition temperature of biogas is in the range of 650-750°C. It is a colourless and odourless gas, which burns with clear blue flame and has calorific value of 20 Maga Joules (MJ) per cubic meter.

Biogas is produced by anaerobic digestion of animal waste. This conversion process is also known as anaerobic fermentation or biodigestion.

The process takes place in air tight tank called digester in the absence of air. It involves three steps as described below: 

(a) Hydrolysis: In this step of conversion process, all the complex organic matter are broken down into simple water soluble compounds. This process takes about one day at 25°C.

(b) Acid formation: The simple organic materials are decomposed by acetogenic bacteria and converted into acetic acid (CH₃COOH). This process also takes about one day at 25°C. (c) Methane formation: The acetic acid (CH₃COOH) is decomposed into methane (CH₄) and carbon dioxide (CO₂) with the help of methanogenic bacteria.

The whole process of decomposition of biomass into biogass requires many weeks and the total period is called retention period. It depends on feedtstock used and the retention period is 50 days for cowdung; 30 days for night soil and 20 days for pig dung feed stock.

Basic Requirements of a Biogas Plant Site

For selecting a suitable site for a biogas plant following points should taken into consideration: 

(i) The land should be levelled and at a higher elevation than the surrounding to avoid runoff water.

(ii) The soil should not be too loose and should be of good bearing strength. (iii) It should be nearer to place of gas utilization to minimise gas pipeline expenses. 

(iv) It should be near to cattle shed/stable for easy handling of cattle dung.

(v) There should be sufficient supply of water at the plant site. 

(vi) The plant should get clear sunshine during most part of the day.

(vii) The plant should be well ventilated because methane (CH₂) when mixed with oxygen (O₂) is very explosive. (viii) It should be away from any water source used for drinking purposes.

Structure of Biogas Plant

The physical structure of the digester which converts organic matter into biogas is called a biogas plant. They are mainly of two types:

1. Floating Drum Type (Constant pressure, variable volume). 

2. Fixed Dome Type (Constant volume, variable pressure).

(1) Floating Drum Type Biogas Plant

This biogas plant design was approved in the year 1962 by the Khadi and Village Industries Commission (KVIC), and hence is popularly known as KVIC model.

Because of the floating drum structure of the plant constant pressure of the gas is maintained, while the volume varies. In this design the digester chamber is made of brick masonry in cement mortar. A steel or iron drum placed on the top of the digester to collect the biogas produced from the digester. This drum floats over the slurry and moves up when the gas is accumulated in the tank and moves down when the gas is discharged from the tank. There are two separate structures, digester for gas production by anaerobic fermentation of slurry and gas holder for gas collection.

Dung is mixed with water and fed into the digester from the inlet tank. Digester holds the slurry where anaerobic fermentation takes place and biogas is produced. A partition wall is provided in the digester for better circulation and necessary fermentation. Digested slurry is taken out from the digester to an outlet tank.

(2) Fixed Dome Type Biogas Plant

A fixed dome biogas plant consists of an enclosed digester, which is combined with a dome shaped gas holder. It is an economical design and is made of brick, cement and masonary. It has no moving part, so working life of the plan is longer.

Dung and water is mixed in inlet tank, and the slurry is fed into the digester through the inlet gate. The gas is produced in the digester by the inanaerobic fermentation of slurry and is collected in the dome shaped upper part of the digester. When gas is produced pressure in the dome increases. The gas is collected from the gas holder by a gas pipe. Digested slurry flows out to the outlet displacement tank through outlet gate.

Gas pressure increases with the volume of the gas stored therefore the volume of the digester should not exceed 20 m³.

If the gas is required at constant pressure a gas pressure regulator provided with the gas pipe.

ADVANTAGES OF BIOMASS ENERGY

1. It is a renewable source of energy.

2. It is indegenous source.

3. The by-product of biogas production is manure, rich in nitrogen (N), phosphorous (P), potassium (K), which have the capacity to improve the soil fertility, food production and reduce import of chemical fertilizer which currently drains a large amount of foreign exchange (forex).

4. It has inbuilt storage of energy.

5. Biomass is carbon neutral so net air pollution due to carbon dioxide (CO₂) is practically nil, as it releases the same amount of CO₂ as it absorbss during its formation by photosynthesis.

6. Biomass energy also provides economic development in rural areas.

7. There is no problem of waste disposal.

8. Biogas is clean gas which improves health and hygiene.

DISADVANTAGES OF BIOMASS ENERGY

1. The sites for biogas plants are not suitable at all locations.

2. It involves intensive labour and costs for collecting large quantity of biomass required for commercial applications.

3. It has low energy density.

4. It is a dispersed source of energy.

5. Collection of biomass is not reliable.

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What is Fuel Cell?

A fuel cell is a new promising power source, because it has the least environmental impact in comparison to conventional fossil fuel. As an alternative and renewable energy source, it helps in reducing the exhaustion of fossil fuels. Fuel cells have high efficiency, longer operation, so they are widely used for different applications as portable power sources. Basically, fuel cells are electrochemical devices that convert chemical energy from a fuel into electrical energy without a combustion process. They differ from storage batteries in the sense that fuel cells need fuel supply from an external source whereas a battery stores electricity inside it.

In 1839 Sir William Grove discovered fuel cell technology, so he is known as 'father of fuel cell. However, the first fuel cell devices were invented by an engineer Francis Bacon in 1934. Fuel cells found practical application when NASA developed fuel cell technology in 1950 as a power source for space travel and also used fuel cells in its spacecrafts like Gemini, Apollo and Skylab.

In India, Bharat Heavy Electricals Limited is developing fuel cell technology in phosphoric acid fuel cells (PAFC) in capacities of 1, 5, 10, and 50 kW ratings.

TERMINOLOGY USED IN FUEL CELL TECHNOLOGY

Anode - Electrode where oxidation (loss of electrons) takes place. It is the negative terminal in the fuel cell.

Cathode - Electrode where reduction (gain of electrons) takes place. It is the positive terminal in a fuel cell. 

Electrolyte - A chemical compound that conducts ions from one electrode to another.

Stack - A number of individual fuel cells connected in series to increase voltage.

Membrane - Layer separating the anode and cathode and allowing hydrogen ions (H) and oxygen ions (0) to pass through it. It acts as an electrolyte in fuel cells.

Inverter - A device which converts direct current (de) into alternating current (ac).

Ionization - Process in which loss or gain of electrons to form positive or negative ions.

Fuel cell - Device that converts chemical energy of a fuel into electrical energy without combustion. 

PEM - An acronym used for Polymer Electrolyte Membrane; and also used for Proton Exchange Membrane.

MCFC - Molten Carbon Fuel Cell

PAFC - Phosphoric Acid Fuel Cell

SOFC - Solid Oxide Fuel Cell

DMFC - Direct Methanol Fuel Cell 

AFC - Alkaline Fuel Cell

Reformer - Device which extracts pure hydrogen from fuel; also known as processor

Cogeneration - Generation of electricity and useful heat in a single installation

CONSTRUCTION AND WORKING OF A FUEL CELL

A fuel cell consists of two porous electrodes, i.e., an anode and a cathode which are separated by an electrolyte. In PEM fuel cells a proton exchange membrane is used as an electrolyte. The anode is supplied with hydrogen and the cathode is supplied with oxygen. At the anode the hydrogen molecules split into electrons and hydrogen ions(protons).

When the anode and cathode are electrically connected, the protons move to the cathode passing through the electrolyte. The electrons run via an external electrical circuit to the cathode while supplying an electrical load. At the cathode, electrons and protons react to form water with the supplied oxygen.

CHEMICAL REACTIONS IN A FUEL CELL

At anode:

2H₂ → 4H⁺ + 4e^-

The electrons so liberated at the anode flow through the external circuit to the cathode. H+ ions move from anode to cathode through electrolyte.

At cathode:

O₂ + 4e^- → 2O^--

4H⁺ + 2O^-- → 2H₂O

Hydrogen ions combine with oxygen ions and produce water and beat.

e^- → direction of flow of electrons

i → direction of flow of electric current

LOSSES AND EFFICIENCY

There are four types of losses occuring in a fuel cell as described below:

1. Activation losses: These losses are due to slowness of reaction taking place on the surface of the electrodes. So there is a voltage drop due to this loss to drive the chemical reaction that transfers electrons.

2. Ohmic losses: Because of the internal resistance of the fuel cell there is a voltage drop (i.r). An internal resistance (r) in the fuel cell is due to resistance of electrolyte and the contact resistance between the electrode and electrolyte.

3. Concentration losses: These losses are due to two factors:

• slow ion movement in the electrolyte, causing a change in concentration at the electrode.
• slow movement of reactants through porous electrodes.

4. Fuel cross over losses: These losses are due to the wastage of fuel passing through the electrolyte.

Maximum efficiency of an ideal fuel cell:

The efficiency of a fuel cell is given by

η = (Output energy/Input energy) ༝ 100

The efficiency of an ideal hydrogen - oxygen (H2 - O2) fuel cell is calculated as under:

output energy = 237 kJ

losses in the fuel cell = 48.7 kJ

input energy = Output energy + Losses

= 237 + 48.8

= 285.8 kJ

η = (237/285.8) ༝ 100

η = 83%

This is the maximum efficiency that can be achievable by a fuel cell under ideal conditions. However, actual fuel cell produces more heat and hence more losses giving the efficiency lower than 83%.

OUTPUT VOLTAGE OF AN IDEAL FUEL CELL

A hydrogen fuel cell has a maximum theoretical voltage of 1.23 V (which is also the minimum voltage needed for decomposition of water by electrolysis). In actual practice a fuel cell has an operating voltage of 0.6 V to 0.8 V.

For obtaining higher voltages individual cells are connected in series in stacks. Stacks are connected in parallel to get higher currents.

The maximum current which can be produed by a cell is proportional to the surface area of the electrodes.

FUEL CELL TECHNOLOGY

The main objective of fuel cell technology is the development and application of fuel cell at high conversion efficiency. This includes selection of cell materials, stack, operating temperature, etc. and to solve the main problems encountered in fuel cells. Some of these problems are:

1. Slow reaction rate leading to low current and power.

2. Hydrogen as fuel source is not readily available.

To solve these problems, different technologies have been tried on different types of fuel cells, some of which are discussed below:

1. Alkaline fuel cell (AFC): In this type of fuel cell an alkaline solution of potassium hydroxide (KOH) is used as electrolyte. The fuel cell operates at a temperature of about 80°C. The reactants are hydrogen (H₂) and oxygen (O₂) from air. They combine to form water.

Reactions: 

At anode H₂ + 2 (OH) → 2 H₂O + 4e¯

At cathode

O₂ + 2H₂O + 4e¯→ 4 (OH-)

H+ + OH → H₂O

In this fuel cell, hydrogen is applied to anode, which reacts with hydroxide (OH) ion present in the electrolyte and forms water and releases electrons. These released electrons flow through external circuit. Oxygen is applied to cathode where it reacts with water present in the electrolyte and electrons picked up from anode to form (OH-) ions. These (OH) ions combine with (H+) ions to form water.

The efficiency of an alkaline fuel cell is about 70%.

2. Polymer electrolyte membrane fuel cell (PEMFC): The polymer electrolyte membrane fuel cells are also called proton exchange membrane fuel cells. In this type a solid electrolyte in the form of membrane is used. The thickness of the membrane is 0.076 cm or 0.76 mm. The membrane which is kept in between the anode and the cathode allows ions to pass through it and is impermeable for gases.

The operating temperature range is 40°C to 100°C and the efficiency is about 60%. These types of fuel cells are suitable for power output below 1 MW.

3. Phosphoric acid fuel cell (PAFC): The electrolyte used in this fuel cell is phosphoric acid (H₂PO₂). At the anode hydrogen gas is converted into hydrogen ion (H+) and electrons.

H₂ → 2H+ + 2 e

The H+ ions migrate from anode to cathode through electrolyte and electrons flow through the external circuit.

At cathode, H+ ions react with oxygen (0₂) and produce water.

O₂ + 4H+ + 4e¯ → 2H₂O

The operating temperature range is 150-220 °C. Efficiency of the phosphoric acid fuel cell (PAFC) is more than 40% and can be increased to about 80% with cogeneration plant.

4. Direct methanol fuel cell (DMFC): The main difference between PEMFC and DMFC is that of fuel source. H₂ is used in PEMFC whereas methanol is used directly as source of fuel in DMFC.

Reactions involved are:

At anode: CH₂OH + H₂O → CO₂ + 6H+ + 6e

At cathode: 30₂ + 12H+ + 12 e¯ → 6H₂0

The direct methanol fuel cell has efficiency about 40% and operating temperature in the range of 50°C to 120°C. Main advantage is that no reformer or processor is required as methanol is used directly as fuel.

High temperature fuel cells: The main types of fuel cells working at high temperature are described below:

(a) Molten carbonate fuel cell (MCFC): The molten mixture of alkali carbonates are used as electrolyte. Fuel is reformed from other sources into H₂ and CO.

It operates in the temperature range of 600°C to 700°C. The fuel cells are available in the capacity range of 300 kW to 3 MW, mainly used as combined heat and power (CHP) plant. The efficiency is of the order of 50%.

(b) Solid oxide fuel cell (SOFC): Here solid electrolyte of zirconium dioxide called Zirconia is used. The fuel source is a mixture of H₂ and CO obtained by reforming of natural gas.

The fuel cell operates in the range of 700°C to 1000°C. Capacities are available in the range of 1 kW to 2 MW and the efficiency is about 60%. It can also be used as combined heat and power (CHP) plant.

SOURCES OF FUEL CELL

Hydrogen is the main fuel for all fuel cells. As the pure hydrogen gas is not available generally, so in that case it has to be extracted from other sources. These sources are given below:

1. Hydrogen (H₂): It is an important source of fuel cells though it is not available directly in nature. It is to be produced from different raw materials and stored. However, storage of pure hydrogen is very expensive. It is generally reformed (or processed) from raw materials and used in fuel cell.

2. Hydrazine (N₂H₂): It is a liquid fuel, so it is convenient to store. In some types of fuel cells it can be used without transformation.

3. Ammonia (NH3): It is an indirect source of fuel. It is generally available in gaseous form. However, it can be stored in liquid form. It is decomposed into a mixture of hydrogen (H₂) and nitrogen gas (N₂).

This mixture is used as fuel and nitrogen is discharged.

4. Liquid hydrocarbon: Light hydrocarbons also called naphtha are reformed into H₂ and CO. The mixture of this product can be used as fuel.

5. Gaseous hydrocarbons: These are hydrocarbons in gaseous forms like methane, propane, etc. They are decomposed into a mixture of H₂ and CO at high temperature. This mixture is then used as fuel source.

6. Methanol: It is used both as indirect and direct type of fuel source. It is reformed into hydrogen (H₂) and carbon-monoxide (CO) at high temperature (200°C). Methanol can be used directly as fuel source in Methanol Fuel Cell (MFC).

ADVANTAGES OF FUEL CELL

1. There is direct conversion of fuel into electrical energy. So air pollution is practically nil as there is no combustion of fuel.

2. Fuel cells have higher efficiencies than diesel or gas engines. 

3. Quiet operation, therefore no noise pollution, particularly suitable for military applications.

4. Operating time is much longer as compared with batteries. 

5. Fuel cells produce high quality power.

6. No cooling water is required so it can be located at any place.

7. Output of fuel cell power plants can be increased easily by stacking fuel cells. 8. Land requirement for fuel cell power plants is much less as compared to conventional power plants.

9. Maintenance cost is low as there are no moving parts.

10. They produce only water and heat as a by-product.

11. Transmission and distribution cost is practically nil as they can be located near the load.

12. Commercial fuels such as LPG, natural gas, biogas, etc., can be used for reforming (processing). 

13. Fuel cells have cogeneration capabilities.

DISADVANTAGES OF FUEL CELL

1. Production, transportation and storage of hydrogen is difficult.

2. Reforming (processing) of hydrocarbons for producing hydrogen is a technically challenging job. 

3. Size of fuel cells is bigger than batteries.

4. Fuel cell technology is not yet fully developed and only few

products are available.

5. Fuel cells use expensive materials (like platinum as a catalyst).

6. Capital cost of fuel is very high.

7. Fuel cells give the output so for ac power supply additional equipment inverter is required to convert dc into ac.

USES OF FUEL CELLS

Fuel cells can be made into stacks and modules, so they can be used for all kinds of applications from portable devices to power plants. Some of the uses of fuel cells are given below:

1. Military applications: As the fuel cells operate quietly they are very useful at strategic locations. They generate electricity without emission of gasses. So they can be used as electric generators without being detected by the enemy.

2. Space applications: The fuel cells can be used as a power source in space crafts. In fact hydrogen-oxygen (H₂-O₂) alkaline fuel cells had been used in Apollo missions by NASA.

3. Automobile applications: The fuel cells in combination with batteries can be used to power vehicles without refueling. The phosphoric acid fuel cell (PAFC) with methanol as fuel can be suitably developed for transport systems.

4. Electricity generation: Fuel cells can be used for commercial generation of electrical power. The fuel cells can be used as peak load power plants as hydrogen storage could be used to meet additional demands during peak periods.

5. Off-grid power application: They can be used as off-grid power plants for small villages, remote locations and other areas inaccessible by state power grids.

6. Combined heat and power (CHP) applications: Where the fuel to electricity conversion efficiency is low, the fuel energy which is not converted into electrical energy, i.e., wastage can be utilized for heating purposes. This type of plant is also known as cogeneration plant.

7. Portable devices application: The small fuel cells are also called micro fuel cells. They are portable and are used in digital cameras, laptops, mobile phones, etc. They have longer life as compared to lithium batteries.

ENVIRONMENTAL IMPACT OF FUEL CELLS

1. Water and heat are byproducts of fuel cells. They do not cause environmental pollution so their impact on the environment is nil. 

2. No cooling water is required as heat can be used in cogeneration plants or for fuel reforming processes. 

3. Amount of carbon dioxide (CO₂) emission to the atmosphere is the least as compared to other conventional power plants. Other pollutants are also negligible.

4. The storage of hydrogen poses risks as it is highly inflammable. Hydrogen can be liquified and stored in large cryogenic insulated vessels. But liquifying hydrogen is very expensive.

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