Energy Potential of Bagasse

Sugarcane is one of the most promising agricultural sources of biomass energy in the world. Sugarcane produces mainly two types of biomass, Cane Trash and Bagasse. Cane Trash is the field residue remaining after harvesting the Cane stalk while bagasse is the fibrous residue left over after milling of the Cane, with 45-50% moisture content and consisting of a mixture of hard fibre, with soft and smooth parenchymatous (pith) tissue with high hygroscopic property. Bagasse contains mainly cellulose, hemi cellulose, pentosans, lignin, Sugars, wax, and minerals. The quantity obtained varies from 22 to 36% on Cane and is mainly due to the fibre portion in Cane and the cleanliness of Cane supplied, which, in turn, depends on harvesting practices.

The composition of Bagasse depends on the variety and maturity of Sugarcane as well as harvesting methods applied and efficiency of the Sugar processing. Bagasse is usually combusted in furnaces to produce steam for power generation. Bagasse is also emerging as an attractive feedstock for bioethanol production. It is also utilized as the raw material for production of paper and as feedstock for cattle. The value of Bagasse as a fuel depends largely on its calorific value, which in turn is affected by its composition, especially with respect to its water content and to the calorific value of the Sugarcane crop, which depends mainly on its sucrose content.

Moisture contents is the main determinant of calorific value i.e. the lower the moisture content, the higher the calorific value. A good milling process will result in low moisture of 45% whereas 52% moisture would indicate poor milling efficiency. Most mills produce Bagasse of 48% moisture content, and most boilers are designed to burn Bagasse at around 50% moisture. Bagasse also contains approximately equal proportion of fibre (cellulose), the components of which are carbon, hydrogen and oxygen, some sucrose (1-2 %), and ash originating from extraneous matter. Extraneous matter content is higher with mechanical harvesting and subsequently results in lower calorific value.

For every 100 tons of Sugarcane crushed, a Sugar factory produces nearly 30 tons of wet Bagasse. Bagasse is often used as a primary fuel source for Sugar mills; when burned in quantity, it produces sufficient heat and electrical energy to supply all the needs of a typical Sugar mill, with energy to spare. The resulting CO2 emissions are equal to the amount of CO2 that the Sugarcane plant absorbed from the atmosphere during its growing phase, which makes the process of cogeneration greenhouse gas-neutral.

35MW Bagasse and Coal CHP Plant in Mauritius

Cogeneration of Bagasse is one of the most attractive and successful energy projects that have already been demonstrated in many Sugarcane producing countries such as Mauritius, Reunion Island, India and Brazil. Combined heat and power from Sugarcane in the form of power generation offers renewable energy options that promote sustainable development, take advantage of domestic resources, increase profitability and competitiveness in the industry, and cost-effectively address climate mitigation and other environmental goals.

Biomass Gasification Process

Biomass gasification involves burning of biomass in a limited supply of air to give a combustible gas consisting of carbon monoxide, carbon dioxide, hydrogen, methane, water, nitrogen, along with contaminants like small char particles, ash and tars. The gas is cleaned to make it suitable for use in boilers, engines and turbines to produce heat and power (CHP).

Biomass gasification provides a means of deriving more diverse forms of energy from the thermochemical conversion of biomass than conventional combustion. The basic gasification process involves devolatization, combustion and reduction.

During devolatization, methane and other hydrocarbons are produced from the biomass by the action of heat which leaves a reactive char.

During combustion, the volatiles and char are partially burned in air or oxygen to generate heat and carbon dioxide. In the reduction phase, carbon dioxide absorbs heat and reacts with the remaining char to produce carbon monoxide (producer gas). The presence of water vapour in a gasifier results in the production of hydrogen as a secondary fuel component.

There are two main types of gasifier that can be used to carry out this conversion, fixed bed gasifiers and fluidized bed gasifiers. The conversion of biomass into a combustible gas involves a two-stage process. The first, which is called pyrolysis, takes place below 600°C, when volatile components contained within the biomass are released. These may include organic compounds, hydrogen, carbon monoxide, tars and water vapour.

Pyrolysis leaves a solid residue called char. In the second stage of the gasification process, this char is reacted with steam or burnt in a restricted quantity of air or oxygen to produce further combustible gas. Depending on the precise design of gasifier chosen, the product gas may have a heating value of 6 – 19 MJ/Nm3.

Layout of a Typical Biomass Gasification Plant

The products of gasification are a mixture of carbon monoxide, carbon dioxide, methane, hydrogen and various hydrocarbons, which can then be used directly in gas turbines, and boilers, or used as precursors for synthesising a wide range of other chemicals.

In addition there are a number of methods that can be used to produce higher quality product gases, including indirect heating, oxygen blowing, and pressurisation. After appropriate treatment, the resulting gases can be burned directly for cooking or heat supply, or used in secondary conversion devices, such as internal combustion engines or gas turbines, for producing electricity or shaft power (where it also has the potential for CHP applications).

 

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