Biomass gasification power systems have followed two divergent pathways, which are a function of the scale of operations. At sizes much less than 1MW, the preferred technology combination today is a moving bed gasifier and ICE combination, while at scales much larger than 10 MW, the combination is of a fluidized bed gasifier and a gas turbine.
Larger scale units than 25 MW would justify the use of a combined cycle, as is the practice with natural gas fired gas turbine stations. In the future it is anticipated that extremely efficient gasification based power systems would be based on a combined cycle that incorporates a fuel cell, gas turbine and possibly a Rankine bottoming cycle.
Integrated Gasification Combined Cycle
The most attractive means of utilising a biomass gasifier for power generation is to integrate the gasification process into a gas turbine combined cycle power plant. This will normally require a gasifier capable of producing a gas with heat content close to 19 MJ/Nm3. A close integration of the two parts of the plant can lead to significant efficiency gains.
The gas from the gasifier must first be cleaned to remove impurities such as alkali metals that might damage the gas turbine. The clean gas is fed into the combustor of the gas turbine where it is burned, generating a flow of hot gas which drives the turbine, generating electricity.
Hot exhaust gases from the turbine are then utilised to generate steam in a heat recovery steam generator. The steam drives a steam turbine, producing more power. Low grade waste heat from the steam generator exhaust can be used within the plant, to dry the biomass fuel before it is fed into the gasifier or to preheat the fuel before entry into the gasifier reactor vessel.
Schematic of integrated biomass gasification combined cycle
The gas-fired combined cycle power plant has become one of the most popular configurations for power generation in regions of the world where natural gas is available. The integration of a combined cycle power plant with a coal gasifier is now considered a potentially attractive means of burning coal cleanly in the future.
Biomass Fuel Cell Power Plant
Another potential use for the combustible gas from a biomass gasification plant is as fuel for a fuel cell power plant. Modern high temperature fuel cells are capable of operating with hydrogen, methane and carbon monoxide. Thus product gas from a biomass gasifier could become a suitable fuel.
As with the integrated biomass gasification combined cycle plant, a fuel cell plant would offer high efficiency. A future high temperature fuel cell burning biomass might be able to achieve greater than 50% efficiency.
Cogeneration of bagasse is one of the most attractive and successful biomass 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.
According to World Alliance for Decentralized Energy (WADE) report on Bagasse Cogeneration, bagasse-based cogeneration could deliver up to 25% of current power demand requirements in the world’s main cane producing countries. The overall potential share in the world’s major developing country producers exceeds 7%. There is abundant opportunity for the wider use of bagasse-based cogeneration in sugarcane-producing countries. It is especially great in the world’s main cane producing countries like Brazil, India, Thailand, Pakistan, Mexico, Cuba, Colombia, Philippines and Vietnam. Yet this potential remains by and large unexploited.
Using bagasse to generate power represents an opportunity to generate significant revenue through the sale of electricity and carbon credits. Additionally, cogeneration of heat and power allows sugar producers to meet their internal energy requirements and drastically reduce their operational costs, in many cases by as much as 25%. Burning bagasse also removes a waste product through its use as a feedstock for the electrical generators and steam turbines.
Most sugarcane mills around the globe have achieved energy self-sufficiency for the manufacture of raw sugar and can also generate a small amount of exportable electricity. However, using traditional equipment such as low-pressure boilers and counter-pressure turbo alternators, the level and reliability of electricity production is not sufficient to change the energy balance and attract interest for export to the electric power grid.
On the other hand, revamping the boiler house of sugar mills with high pressure boilers and condensing extraction steam turbine can substantially increase the level of exportable electricity. This experience has been witnessed in Mauritius, where, following major changes in the processing configurations, the exportable electricity from its sugar factory increased from around 30-40 kWh to around 100–140 kWh per ton cane crushed. In Brazil, the world’s largest cane producer, most of the sugar mills are upgrading their boiler configurations to 42 bars or even higher pressure of up to 67 bars.
The prime technology for sugar mill cogeneration is the conventional steam-Rankine cycle design for conversion of fuel into electricity. A combination of stored and fresh bagasse is usually fed to a specially designed furnace to generate steam in a boiler at typical pressures and temperatures of usually more than 40 bars and 440°C respectively. The high pressure steam is then expanded either in a back pressure or single extraction back pressure or single extraction condensing or double extraction cum condensing type turbo generator operating at similar inlet steam conditions.
Due to high pressure and temperature, as well as extraction and condensing modes of the turbine, higher quantum of power gets generated in the turbine–generator set, over and above the power required for sugar process, other by-products, and cogeneration plant auxiliaries. The excess power generated in the turbine generator set is then stepped up to extra high voltage of 66/110/220 kV, depending on the nearby substation configuration and fed into the nearby utility grid. As the sugar industry operates seasonally, the boilers are normally designed for multi-fuel operations, so as to utilize mill bagasse, procured Bagasse/biomass, coal and fossil fuel, so as to ensure year round operation of the power plant for export to the grid.
Modern power plants use higher pressures, up to 87 bars or more. The higher pressure normally generates more power with the same quantity of Bagasse or biomass fuel. Thus, a higher pressure and temperature configuration is a key in increasing exportable surplus electricity.
In general, 67 bars pressure and 495°C temperature configurations for sugar mill cogeneration plants are well-established in many sugar mills in India. Extra high pressure at 87 bars and 510°C, configuration comparable to those in Mauritius, is the current trend and there are about several projects commissioned and operating in India and Brazil. The average increase of power export from 40 bars to 60 bars to 80 bars stages is usually in the range of 7-10%.
A promising alternative to steam turbines are gas turbines fuelled by gas produced by thermochemical conversion of biomass. The exhaust is used to raise steam in heat recovery systems used in any of the following ways: heating process needs in a cogeneration system, for injecting back into gas turbine to raise power output and efficiency in a steam-injected gas turbine cycle (STIG) or expanding through a steam turbine to boost power output and efficiency in a gas turbine/steam turbine combined cycle (GTCC). Gas turbines, unlike steam turbines, are characterized by lower unit capital costs at modest scale, and the most efficient cycles are considerably more efficient than comparably sized steam turbines.
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