GENERATING BIOMASS ENERGY
May 31 - June 6, 2010
The current human activities depend on energy annual consumption of which is equivalent to 10 billion tonnes of crude oil. Most of the energy sources are fossil fuels, which are exhaustible. Suitable measures must be implanted to prevent the occurrence of acid rain and emission of particulate matter (PM) which is considerable to be generated by the exhaustible energy sources. Biomass resources are attracting attention as a solution for energy and environmental problems.
ENERGY FROM BIOMASS
Biomass energy or bio-energy refers to the use of a wide range of organic materials as fuels. These are produced by biological processes, and include forest products, agricultural residues, herbaceous and aquatic plants, and also municipal wastes.
In principle, biomass is inexhaustible and renewable. Biomass can either be burned to produce heat or electricity, or transformed into liquid fuels such as ethanol, methanol, or bio-diesel.
Biomass has been predominant source of fuel since 19th century. Dried plants, plant oil, animal fat or dried dung have been used for lighting and cooking needs. The most common biomass source has been wood.
Biomass dominance was since then progressively replaced by fossil fuels, first coal and then oil and gas during the 19th and 20th centuries. Today biomass supplies only about 11 per cent of the world's primary energy consumption.
Many poor countries continue to be the most important source of energy for heating and cooking purposes, whereas in developed nations only a small fraction of energy needs are covered by biomass. A wide range of biomass fuels such as wood, charcoal, wood waste (branches, roots, bark, saw dust) as well agricultural residues- maize cobs, coconut shells, cereal straws, rice husks -can be used as fuel for biomass gasification.
Theoretically, almost all kinds of biomass with moisture content of 5-30 per cent can be gasified. However, not every biomass fuel leads to the successful gasification. Most of the development work is carried out with common fuels such as coal, charcoal and wood. Key to a successful design of gasifier is to understand properties and thermal behavior of fuel as fed to the gasifier.
Energy content of fuel is obtained in most cases in an adiabatic, constant volume bomb calorimeter. The values obtained are higher heating values which include the heat of condensation from water formed in the combustion of fuel. The heating values are also reported on moisture and ash basis. Fuel with higher energy content is always better for gasification. The most of the biomass fuels (wood, straw) has heating value in the range of 10-16 MJ/kg, whereas liquid fuel (diesel, gasoline) posses higher heating value.
The moisture content of the most biomass fuel depends on the type of fuel. It is treated before it is used for gasification. Moisture content of the fuel is usually referred to inherent moisture plus surface moisture. The moisture content below 15 per cent by weight is desirable for trouble free and economical operation of the gasifier. Higher moisture contents reduce the thermal efficiency of gasifier and results in low gas heating values. Igniting the fuel with higher moisture content becomes increasingly difficult, and the gas quality and the yield are also poor.
BULK DENSITY OF BIOMASS FUEL
The fuel size affects the pressure drop across the gasifier and power that must be supplied to draw the air and gas through gasifier. Large pressure drops will lead to reduction of the gas load in downdraft gasifier, resulting in low temperature and tar production. Excessively large sizes of particles give rise to reduced reactivity of fuel, causing startup problem and poor gas quality.
Acceptable fuel sizes depend to certain extent on the design of gasifier. In general, wood gasifier work well on wood blocks and wood chips.
Bulk density is defined as the weight per unit volume of loosely tipped fuel. Bulk density varies significantly with moisture content and particle size of fuel.
Volume occupied by stored fuel depends on not only the bulk density of fuel, but also on the manner in which fuel is piled. It is also recognised that bulk density has considerable impact on gas quality, as it influences the fuel residence time in the fire box, fuel velocity and gas flow rate.
Fuel with high volatile matter content produces more tar, causing problems to internal combustion engine. Volatile matters in the fuel determine the design of gasifier for removal of tar. Compared to other biomass materials:
* crop residue : 63-80 per cent
* Wood : 72-78 per cent
* Peat : 70 per cent
* Coal: up to 40 per cent
* Charcoal contains least percentage of volatile matter: 3-30 per cent
ASH CONTENT OF FUEL
Mineral contents of fuel which remains in oxidized form after combustion of fuel is called ash. In practice, ash also contains some unburned fuel. Ash content and ash composition have impact on smooth running of gasifier. Melting and agglomeration of ashes in reactor causes clinker formation. If no measures are taken, clinker formation leads to excessive tar formation or complete blocking of reactor. In general, no clinker formation occurs with fuel having ash content below 5 per cent. Ash content varies fuel to fuel. Wood chips contain 0.1 per cent ash, while rice hust contains high amount of ash (16-23 per cent)
FUELS AND THEIR ASH CONTENT
FUEL ASH CONTENT % WEIGHT FUEL ASH CONTENT % WEIGHT Alfalfa seed straw 6 peanut husks 0.9 Barley straw 10.3 Rice hulls 16-23 Charcoal 2-5 Safflower straw 6 Coffee hulls 1.3 Walnut shell 1.1 Coal 5-17 Wheat stalks 7.4 Cotton grin thrash 17.2 Wood chips 0.1
ELECTRICITY FROM BIOMASS
The cheapest, most used, and simplest way of using biomass to generate energy is to burn it. On a commercial scale, this is done in a process similar to the one burning coal to produce electricity or heat. In these applications, wood, wood waste and municipal solid waste are the most utilised fuels.
BIOMASS IS CLASSIFIED INTO TWO TYPES OF BIO-ENERGY RESOURCES.
1. Product Biomass (energy plantation biomass)
2. Unused Biomass (wasted biomass)
CLASSIFICATION ITEM EXAMPLE OF BIOMASS RESOURCES Energy plantation biomass On Land under water Sugar cane, beet, corn, and colza, etc. Sea weed, microorganisms etc. Unused biomass Agriculture Livestock Forestry Fishery Municipal waste Rice straw, chaff, straw, bagasse, & vegi-waste Livestock excreta, Residue of meat factory, etc. Waste Woods, Saw Mill Waste, Agri-waste, etc. Fishery processing residue etc. Household garbage, Sewage Sludge, etc.
BIOMASS AVAILABLE IN PAKISTAN
* 50,000 tons garbage collected in major cities.
* Millions of gallons of wastewater produced daily.
* Rice Husk, Cotton Stalks and baggasse.
The process of generating biogas is simple and yields a gaseous product consisting of 55-60 per cent methane and the remaining carbon-dioxide gas.
Biogas of this quality can be used to generate electricity and may also be used as fuel for steam boilers, space heaters and refrigeration equipment. Biogas is also combustible which can be used in cooking.
The buffalo manure emits large quantities of methane, carbon-dioxide and hydrogen sulfide. This pollutes the air and atmosphere around Cattle Colony where pollution level is high.
Pakistan has one of the world's largest animal populations. According to an estimate, around 1 million cattle are present in three different locations of Karachi. Brief data analysis shows that a 38MW power plant can be installed by using cow dungs.
Several small scale biogas plants have been installed in villages to provide gas to the residents of the villages for their cooking needs. Due to lack of maintenance, most of these biogas plants are not operating on full capacity.
Pakistan is producing enormous quantity of biomass through agricultural residues and different solid wastes and government should take measures to harness biomass potential for generating electricity which is now recognised as a clean and reliable renewable source of energy.
In Pakistan the energy demand is expected to increase threefold by the year 2040. The currently used resources such as natural gas and oil will not be able to meet its future energy needs as the fossil fuel reserves are on the decline while the natural gas is likely to go to alarming low levels by 2020. In this scenario, biomass offers an attractive and cheap alternative source of energy.
Biomass like rice husk, cotton stalks, jute waste and other crop residues are produced in thousands of tons in Pakistan that should be used for power generation.
Moreover, every city of Pakistan produces thousands of tons of solid municipal waste as well as millions of gallons of wastewater which can be converted into energy and organic fertiliser.
Many countries utilising many waste-to-energy technologies produce clean energy through the combustion of biomass like agricultural residues and municipal solid waste in specially designed power plants equipped with the most modern pollution control equipment to clean emissions.
Installation of biomass energy plants will also contribute positively to the country's economy by providing jobs apart from generating electricity, conserving fossil fuels, preserving precious foreign exchange and saving the space required for land filling.
China, Nepal and Sri Lanka: agricultural residues are predominantly used in the domestic sector. India, Pakistan & Philippines: 50 per cent agricultural residues (mostly bagasse) are used in the industrial sector. 90 per cent total biomass energy is consumed in domestic cooking. Most traditional cooking stoves have low efficiencies leading to considerable waste of biomass energy. Substitution of all traditional stoves by improved stoves has a total biomass saving potential of 277 million tonnes per year (35.5 per cent of total consumption.
For an energy self-sustained plant (i.e. no other external energy supply other than the feedstock) 49 per cent of the energy content of the dry feedstock can be converted to methanol and another 12 - 21 per cent be recovered as hot water for district heating.
Conversion of process feedstock to methanol is 57 per cent but needs auxiliary fuel (electricity) to make the plant energy self-sustained. However, it should be possible to augment this figure to about 55 per cent by improved technology and power/heat management.