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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Waste Management and Sustainability

Waste management is one of the core themes of sustainability, but achieving sustainable waste management is a challenging and complex task. Despite the fact that an increasing amount of waste has been reused and recycled, landfills still play an important role in the management of wastes. However, waste degradation in landfill produce leachate and harmful gasses viz. carbon dioxide, methane which are considered as greenhouse gases. It has been studied that leachate contribute to 20% emission of greenhouse gases. This can largely risk human health as well as threat to environment. Furthermore, it contains low concentration of gases with heavy aromatic rings, most of them are toxic in nature.

The increasing cost of waste disposal is a cause of major concern in developing nations

Movements of leachate create problem as aquifers need more time for rehabilitation. Leachate can migrate to groundwater or surface water and have potential threat to drinking water. Constructing landfills have adverse effects on aquaculture and habitats by diffusing leachate into surface/groundwater with limited on-site recycling activities. Various studies also claim that residential areas close to landfill areas have low housing values because people don’t prefer to live close to the area enriched with flies, mosquitoes, bacteria and bad odours.

The lower calorific value of wastes lowers the significance of waste-to-energy technologies, such as incineration/gasification, and make waste-to-energy less viable as solution for waste management solution. The low calorific value is an important outcome of waste collection process.

Scavengers often collect in a mixed state with all type of wastes, which include reusable materials, plastic, glass bottles etc. which reduces the calorific value and combustibility of waste. Waste is usually sorted out manually and unfortunately it becomes very difficult to regulate and implement an efficient method. This kind of waste recovery methods is very common in Asian countries e.g. India, Indonesia etc. using improper waste management technique can cause contaminated soil, water and environment.

Water is most easy to contaminate as it dissolves chemicals easily, causing harm to all living organisms including humans. Animal and marine life is most effected with water contamination. It also restricts our use of water for drinking and cooking purposes without cleaning system. The environment is highly harmed because of improper waste management.

Greenhouse gases are generated from decomposition of waste, these gasses are major cause of global warming affecting air precipitation, causing acid rain to severe hailstorms. Moreover humans who live near to garbage dumping area are found to be most significant to risk of health diseases, skin problems, cancer etc.

Olusosun is the largest dumpsite in Nigeria

With proper awareness and teaching methods of efficient waste management we can achieve sustainable solution to waste management. It has been forecasted by Environmental Sanitary Protection Plan that, by 2020 Kamikatsu a city in Japan is going to be 100% free from waste. Although the target of reaching the 100% waste is going to be achieved but the standby waste issue is going to be major hurdle as Kamikatsu have only 34% of land space available.

The lack of availability of standby space for waste is going to be major problem in future because of shortage of space, degraded quality of waste with lower calorific value and formation of leachate. And unfortunately, this issue is not going to be solved very soon.

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.