21 Biomass As An Energy Source

Dr. Dhanya M.S M.S and Dr. Shiv Prasad Prasad

1. Introduction

Biomass is produced by means of photosynthesis which is light dependent process and chemosynthesis which is light independent process. Majority of the biomass consists of green plants that harness solar energy and converted into chemical energy by means of photosynthesis and is stored as biomass energy. The biomass serve as a primary fuel as well as secondary fuel. The processing of biomass ad its further utilization makes it a secondary fuel. Many countries, including India, are exploring biomass as an alternative energy sources. Since the discovery of fire, bioenergy has been one of the most widely used forms of renewable sources of energy worldwide. Biomass provides fuel flexibility to match a broad range of energy demands. It can be stored and has a benefit over other sets of renewable energies. Presently, all types of biomass collectively provide nearly 14% of the global primary energy supplies and represent almost 80% of the world renewable energy share. In some of the developing countries, the major share is from bioenergy (around 90% of energy supply), with the use of traditional biomass for cooking and heating (UNEP, 2016).

2. Energy efficiency in photosynthesis

The photosynthesis reaction is described by Calvin cycle

3. Biomass resources

The biomass is produced in land and in water. The biomass produced in water is marine biomass and fresh water biomass (mainly aquatic plants and algae). The terrestrial biomass mainly includes woody biomass, agricultural residues, energy crops or plantations, municipal or industrial wastes.

The biomass resources for bioenergy production are grouped into

Energy crops

Woody biomass/Forestry

Forestry waste: Logs, woods, chips, barks, leaves are the major forestry waste. Sawdust produced during processing of timber also adds to forest waste.

Agricultural residues

The biomass produced as a by-product of processing, harvesting of agricultural crops are known as agricultural residue. It is classified as

• Primary residue – The residue produced after harvesting the yield of crop in the field. Eg. rice straw, wheat straw, maize straw, sugarcane trash, etc. It is mainly used as animal feed and so less available for energy production in India
• Secondary residue – The residue generated during processing. Eg: rice husk, bagasse. These are potential energy sources.

The biomass from crop residues is either of dry or wet biomass form. The calorific value, moisture content, fixed carbon content, ash content are important properties in energy generation from the biomass.

Agro-industrial waste

Paper mills, molasses, pulp waste from food processing industries, textile fibre waste adds to agro industrial waste

Municipal waste

The waste collected from household consist mainly the organic portion that can be utilized for energy recovery. Food and kitchen waste, green waste, other biodegradable portion of waste constitutes municipal solid waste. Sewage and animal manure from households also has the energy potential.

Industrial wastes

The waste and waste water from the industries like paper and pulp industry, dairy industry, breweries, vegetable packaging industry, confectionary industry can also be used as energy resources. The food industry wastes from hotel, restaurants and community kitchens are also a potential bioenergy source. The baggase, a byproduct residue from sugar mills after the juice extraction is used largely for cogeneration to produce electricity.

4. Classification of Energy crops

Many countries depend upon natural vegetation or crop residues to meet their energy demand. The bioenergy crops are categorized into

4.1 First generation bioenergy crops (FGECs): The food crops which are used as energy crops come under this category. It includes corn, sugarcane, oil palm and rapeseed. The fermentation or transesterification processes are used for energy conversion.

4.2 Second generation bioenergy crops (SGECs): These are non-food feedstocks. These mainly include lignocellulosic crops having high energy content than FGECs by means of fermentation or thermochemical processes. The examples are perennial forage crops like Switchgrass (Panicum virgatum), Reed canary grass (Phalaris arundinacea L.), Lucerne or alfalfa (Medicago sativa L.), Napier grass (Pennisetum purpureum Schumach.), and Elephant grass (a hybrid, Miscanthus giganteus). The non-edible oilcrops like Jatropha curcas L. (30-50% oil) and soapnut (Sapindus mukorossi and S. trifoliatus, 52% oil) are also comes under SGECs. It can also include residues from forest products and field crops.

4.3 Third generation bioenergy crops (TGECs): This is based on specially engineered energy crops and algae.

4.4 Dedicated bioenergy crops (DECs)

The plant species grown purposefully for energy without compromising on food is known as dedicated energy crops. The energy crops include the following cellulosic crops

Eucalyptus (Eucalyptus spp.), Poplar (Populus spp.),

Willow (Salix spp.), Birch (Betula spp.),

Giant reed (Arundo donax),

Reed canary grass (Phalaris arundinacea), Switchgrass (Panicum virgatum),

Elephant grass (Miscanthus x gigantus), Johnson grass (Sorghum halepense),

Sweet sorghum (Sorghum bicolor)

Castor bean (Ricinus communis), Physic nut (Jatropha curcas),

Pongamia (Pongamia pinnata).

5. Characteristics of Bioenergy Crops

The bioenergy crops should have the following characteristics

High growth rate and fast growth

The growth of biomass crops should be higher and are of short duration.

High biomass and energy yield High calorific content

Adaptable to marginal lands,

Low requirements of inputs for growth like water, fertilizer, pesticides, etc. The energy input to those crops should be less.

Tolerance to biotic and abiotic stresses,

Plant should have capability to minimize plant-to-plant competition and competition with weeds,

Plant should have ability to increase radiation interception, radiation, WUE and NUE and hence improving biomass and energy production.

Plant species should have capability to adjust to thermal time sensitivity

Plant species should have high competitive capability for light utilization, canopies should be of low extinction co-efficient, leave characteristics should be for improved light absorption with optimum leaf area index and specific leaf area.

Growing C4 or CAM plants as energy crops to improve WUE Cost of production of crop should be less

Biochemical composition of the plant

6. Components of agricultural waste biomass

The portions of cellulose, hemicellulose, and lignin in agricultural residues and wastes are presented in Table 1.

Table 1: Cellulose, hemicellulose and lignin content in residues and wastes.

7. Bioenergy routes from biomass

The conversion technologies from biomass are based on type of biomass (different physical nature and chemical composition of the biomass), and required energy form (heat, power, transport fuel). The conversion of biomass to energy is mainly by means of Thermo-chemical technologies Bio-chemical technologies Chemical technology

7.1 Thermo-chemical conversion

The thermal conversion processes consist of pyrolysis, biomass gasification, combustion and liquefaction.

Combustion: It is the process of biomass oxidation in air. During burning process chemical energy in the biomass is converted to heat energy. This is further used to generate power and electricity with the help of boilers and steam turbines. This technology is suitable for biomass with less moisture content. Otherwise pre drying is required before combustion. The efficiency ranges from 20% to 40%.

Gasification: The process where biomass is converted into a combustible gas mixture by the partial oxidation of biomass at high temperatures is known as gasification. The main product is syngas.

Pyrolysis: The process of converting biomass mainly to liquid fuel (bio-oil or biocrude) in the absence of air is known as pyrolysis. It has an efficiency of 80 percent.

7.2 Bio-chemical conversion

The fermentation and an-aerobic digestion are the two types of biochemical conversion technologies for biomass to energy.

Fermentation: The technology where sugar or starch is converted to liquid fuel with the help of microbes is known as fermentation. The main products of fermentation are bioethanol, biobutanol, etc. The biomass used for fermentation is sugar crops or starch crops or even lignocellulosic materials.

Anaerobic digestion: In this process the organic matter is converted to mixture of gas with manure by microbes under anaerobic condition. Biogas and biohydrogen are major products. This technology is suitable for biomass with high moisture content (80-90%).

7.3 Trans-esterification

The process in which oil or fat (triglyceride) reacts with alcohol to produce fatty acid ester (biodiesel) is known as transesterification.

The Fig1 gives the schematic view of different products produced through different conversion routes from biomass.

7.4 Pelletization:

It is the process of compressing the biomass into pellet form. This densified biofuel have homogeneous size that improves the thermal energy production in comparison to traditional domestic wood. The steps involved in pelletization include feedstock storage, removal of undesirable impurities, size reduction, transportation by screw conveyors, biomass drying, mixing and conditioning, pellet production and finally the cooling ad moisture removal from pellets.

7.6 Torrefaction: It is the thermal pretreatment process in which the biomass is converted to densified coal like solid biofuel. In this process slow heating of biomass in an inert environment at a temperature of 200-300oC in the absence of oxygen.

8. Factors determining conversion of biomass into energy

Moisture content: It represents the intrinsic water present in the biomass Calorific value: The latent heat of water vapour reduces high heating value (HHV) or Gross calorific values (GCV) to Lower heating value (LHV or Net calorific value (NCV)

Proportion of fixed carbon vs volatiles Ash /residue content

Silica content

Cellulose-lignin ratio 

9. Scope of biomass as an energy source

The biomass is considered as a sustainable energy source because of abundancy and availability of biomass in comparison to dwindling in petroleum prices, depletion of fossil fuel reserves.

• – Biomass is the abundantly available raw material on the Earth.

• – Biomass potential of waste to energy

• – Agricultural biomass includes the broad range of the importantly wasted energy source.

After China, India is the world’s biggest producer of paddy. India now produces 98 million tons of paddy with roughly 130 million tons of straw of which only nearly half is used for fodder. India also produces about 350,000 tons of cane that will yield about 50 million tons of cane trash that are also an attractive biomass fuel. Biomass with high silica content has no commercial use and is therefore almost burned. Beside this agro-residues from the processing of maize, cotton, millets, pulse, sunflower and other stalks, bull rushes, groundnut shells, coconut trash, etc., all produce good biofuels. environmental advantages provided by the modern biomass energy.

It decrease air pollution and diminish atmospheric CO2 accumulation. This is in contribution to the reduction of GHGs and other environmental benefits. Miscanthus sp. is capable of carbon fixation (CO2) of 5.2 to7.2 tC/ha/yr.

Bioenergy Sustainability

– The use of forest residues could lead to either a positive or negative effect. The rate of carbon sequestration into biomass or soil through the decomposition of residues is slower than the rates of forest waste combustion. Harvesting of forest remains may result in the buildup of CO2 in the atmosphere. Through increased use of forest residues via thinning and sustainable management strategies, forest growth can be greaterand fires also could be prevented, thus reducing overall CO2 emissions.

– Energy crops could also contribute to emission reductions if they were cultivated sustainably on waste or surplus land.

advancements in bioconversion technology and biomass production have made bioenergy competitive with fossil fuel based energy generation in some situations.

enhancement of energy security and diversity of energy supply. Employment generation and rural development, and restoration of degraded lands as a result of plantation and possibility of the increase in biodiversity.

The transfer of modern bioenergy technologies to developing countries will be further encouraged by the Clean Development Mechanism CDM) of the Kyoto Protocol.

High energy yield from biomass- The energy yield of hybrid poplar is around 6.15 MJ/m2/yr, switchgrass is 5.8 MJ/m2/yr and reed canary grass is 4.9 MJ/m2/yr.

10. Challenges in Bioenergy utilization

The most economical option is to focus on better utilization biomass waste through improved collection of agro-residues and cattle dung, the better use of waste from sugar mills and wood processing units, and enlarging waste product use (e.g. briquette of sawdust).

Sustained supply of biomass will require enhanced production of energy crops (e.g. wood fuel plantations, sugar cane as feedstock for ethanol).

The potential competition for land and biomass uses must be judiciously managed.

The use of conventional crops for power consumption can also be utilized with careful consideration of the availability of land and food demand.

In the medium term, bioethanol from lignocellulosic biomass could be produced on marginal, degraded and surplus lands, which may provide large biomass stocks. In the long term, algal biomass could also make a significant contribution.

The productivity of food and biomass for bioenergy needs to be adequately addressed.

Transforming forest land into agricultural land for growing bioenergy crop, which would store less carbon, or just combusting beyond excess forest growth levels, would result in a substantialvolume of additional CO2 emissions.

Conversion of bioenergy feedstocks into final products.

Conversion technologies of ligno-cellulosic biomass to bioenergy are at the developmental stage. So far, only a limited capacity is commercialized, and capital costs are still high compared to conventional biofuel generation plants especially first generation biofuels, i.e., molasses to ethanol.

Economic viability poses a challenge for existing production capacity of biofuels as well.

However, the production of food and fuel from the same piece of land is more promising. Considering the tremendous potential of this second generation biofuel, the developed nations are investing in a big way in bioenergy research and development through biotechnological approaches.

Not all residues are available for bioenergy production because they are needed for livestock feed and litter and to maintain soil fertility.

Biomass burning to air pollution- The farmers with time constraints to their crop cycles have to some burn large quantities of biomass that contributes to pollution and responsible for global warming.

Bioenergy standards for the sustainable market growth

Quality control of bioenergy identified as a key factor for the sustainable market growth of these fuels and can lead to many issues. ISO 13065:2015 applies to criteria, specifies principles, and indicators for bioenergy supply chain to assess environmental, social and economic aspects of sustainability. It applies to the whole supply chain, or parts of a supply chain or a single process in the supply chain. The ISO 13065:2015 applies to various bioenergy forms, irrespective of raw material, geographic location, and technology or end use. The ISO 13065:2015 is intended to facilitate comparability of various bioenergy processes or products. It can also be used to promote comparability of bioenergy and other energy options.

11. Bioenergy consumption pattern in the world

The biomass supplies nearly 50 EJ of energy at the global level, which represents 10% of worldwide annual primary energy consumption. The share of renewables in the world energy production is around 13% of this 77 % accounts for bioenergy primary raw material which is wood biomass (87%), agricultural crops and byproducts (9%) and municpal and industrial wastes (4%). The biomass is primarily used for heating and cooking purposes in most of the countries. Brazil and USA together produce more than 80% of the total global energy production from biomass. Many countries of the world such as India, Indonesia, China, Argentina, Australia, Canada, Colombia, Mexico, Senegal, South Africa, Zambia, etc. introduced the biofuel friendly policy.

12. Biomass energy in India

In India, the contribution of biomass is around 32% of total primary energy consumption. The contribution of biomass power is 18.63 percent, cogeneration from bagasse is 5.31 percent and waste to energy is 2.88 per cent. The Ministry of New and Renewable Energy promotes Biomass Gasifier based power plants for electricity generation from locally available biomass sources like crop residues. The crop residues like bagasse, rice husk, straw, cotton stalk, coconut shells, soya husk, de-oiled cakes, coffee waste, jute wastes, groundnut shells, saw dust etc. The surplus biomass availability of biomass in India is estimated to 120 – 150 million metric tones per annum of agricultural and forestry residues. The estimated potential from these residues is around 18,000 MW and an additional 7000 MW power is produced through bagasse cogeneration. Punjab has highest potential for bio energy followed by Maharashtra. The other leading states for biomass power projects are Chhattisgarh, Uttar Pradesh, Andhra Pradesh and Tamil Nadu. The states which have taken position of leadership of baggase cogeneration projects are Andhra Pradesh, Karnataka, Maharashtra, Tamil Nadu and Uttar Pradesh.

Conclusion

Bioenergy must become gradually competitive with other energy sources. Because of its renewable and sustainable nature, abundance and availability the surplus biomass residues is used in energy production. The thermochemical and biochemical conversion routes of biomass are used for energy generation. There is an urgent need for more technological advances, leading to the most efficient conversion of a diverse range of biomass feedstocks to bioenergy.