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Project extension: 3 years The sugar and alcohol industry is facing enormous changes and fast growth all over the world, mainly related to the massive production of ethanol as a transport fuel. High efficiency electricity production is extremely important in this context, as an additional high value by-product, and also considering the future use of bagasse as a raw material for ethanol production and the possibility of additional revenues through the commercialization of carbon credits. Gasification technologies could allow the use of gas turbine combined cycles for cogeneration in sugar alcohol mills (the so called BIG/GT technologies – Biomass Integrated Gasification and Gas Turbines) to double the actual electricity generation specific indices. First theoretical proposals were done by Larsson et al. (1987). Bagasse gasification tests at a laboratory scale were carried out at UNICAMP and UNIFEI in Brazil. Pilot plant scale tests were carried out by the Sweden company TPS at its facilities in Sweden in a GEF Project coordinated by COPERSUCAR Technology Center (project "BRA/96/G31 Energy generation out of biomass: sugar cane bagasse and residues"). After this theoretical and experimental research it is necessary to carry-out a small scale industrial implementation of the BIG/GT technology, which is the proposal of this project. 2- Bibliographic review A
considerable amount of electricity can be generated by using advanced
technologies with a high efficiency of conversion. In this sense the
implementation of the integrated technology of gasification and gas
turbines (BIG/GT) is very attractive for sugar mills with low steam
consumption. The BIG/GT were tested in several demonstration projects in
the USA and Europe. When process steam is needed, an extraction from the steam turbine is enough. So, there would be a BIG/GT system with cogeneration. During gasification, the main goal is the conversion of the biomass into fuel gas through its partial oxidation at high temperatures. This gas, known as poor gas or producer gas, is an intermediate energetic, and it will be able to be further employed on another conversion process in order to generate heat or mechanical power, fitting itself to systems where the solid biomass cannot be used. Basically, the average content of the combustible components in the gas resulting from biomass is: CO between 10 and 15%, H2 between 15 and 20% and CH4 between 3 and 5%.
The fluidized bed gasifiers are considered to be more convenient for high capacity application in BIG/GT systems because of their high flexibility regarding the fuel (the utilization of low density fuels with fine granulometry is allowed, which is the case of most agro-industrial residues), and also due to the facility in using the data obtained in pilot plants for the designing of equipment on an industrial scale. The pressurized systems allow the disposition of more compact installations, even though the biomass feeding system is more complex. The BIG/GT technology has not been implemented in sugar mills yet. Several simulations have been carried out by different authors, and in Australia and in Brazil, the construction of a 3-5 MWe power pilot plant was cogitated in order to help this technology to reach its commercial stage. An interesting progress was the accomplishment of bagasse gasification tests carried out by TPS as part of the project "BRA/96/G31 Biomass power generation: sugarcane bagasse and trash" that was carried out by the Centro de Tecnologia COPERSUCAR.
Hobson and Dixon (1998) carried out a study on the possibility of
implementing BIG/GT systems under Australian conditions. The thermal
scheme that was analyzed is shown in Figure 2. The main conclusions of
this modeling were:
·
For a steam
specific consumption of 520 kg/tc (52 % of steam on sugar cane) the
turbine exhaust gas energy is not enough to generate the process steam.
For this level of steam consumption, 70 % of the bagasse must be
by-passed from the gasifier and feed directly to the steam generators;
Turn (1998) presents the results of a study considering the integration of a BIG/GT system to the Okelele Sugar Company mill in Hawaii with a 120 tc/h milling capacity and a steam consumption of 420 kg/tc. The gas turbine net power is 18.8 MWe corresponding to 4.5 MWe to the steam cycle of 41 bar of pressure. During the off-season period the BIG/GT system operates as a 25.4 MWe power plant and with 28.5 % efficiency using an auxiliary fuel. Another study considers the utilization of Steam Injected Gas Turbines, STIG, and it was accomplished by using the technical data from the Monimusk sugar mill located in Jamaica (Larson et al., 1987). As a result, a 220 kWh/tc surplus electricity generation potential was obtained for a process steam consumption reduction up-to 300 kg/tc.
Figure 2 – Scheme of a BIG/CC system coupled to a sugar mill thermal scheme (Hobson and Dixon, 1998).
Figure 3 - Simulation results of the implementation of a BIG/CC system in an Australian plant of 600 t/h capacity (Hobson and Dixon, 1998). References:
Granastein, D. L., “Case Study on
Waste- Fuelled Gasification Project
Greve
in Chianti- Italy.” Natural Resources
Canada/ CANMET Energy Technology Centre (CETC), 2003. 3-
Objectives Specific
objectives: 4- Justification The necessity of a first industrial pilot scale test facility to progress the further commercial implementation of this technology. The relatively high cost of the first industrial prototype makes international funding and collaboration necessary. High efficiency electricity generation is extremely important in the actual context of a diversified food and energy producing sugar industry, Among future ISBUC members there are research collectives with expertise in bagasse gasification and BIG/GT system modeling. 5- Methodology and action plan
6- Public Target
7- Description of the technical proposal. Simulation of a BIG/GT system using Thermoflex A BIG/GT system consists of the combined operation of a biomass gasifier with a gas turbine. The biogas generated in the gasifier is the fuel of the gas turbine. The simulated case shows a combined cycle that generates a net power of 3 MW. Boundary conditions The ambient air is in ISO conditions (pressure of 101,325 kPa; dry bulb temperature of 15 °C and 60% of Relative Humidity), which results in a wet bulb temperature of 10.82 °C. That is the reason why it is considered that the make-up water in the ambient is found at a temperature close to the temperature of the air wet bulb, that is, 11 °C. The considered biomass is sugarcane bagasse, which is fed into the gasifier at a temperature of 14 °C. The considered sugarcane bagasse has the following composition: 49.6% of Moisture, 1.79% of Ash, 23.58% of Carbon, 3.02% of Hydrogen, 0.1% of Nitrogen, 0.17% of Sulfur and 21.74% of Oxygen. Gas Turbine Modeling The gas turbine to be modeled is a turbine equivalent to the Alstom GT 5, whose design characteristics or parameters are: 15 kg/s of air, 12.2 of Pressure Relation, rotation of 14000 rpm, turbine inlet temperature of 950 °C, 27.3 % electric efficiency and nominal power of 2726 kWe. As the GT 5 is designed to operate with natural gas, whose low calorific value is nearly 50 MJ/kg, the modeling considers that only the GT 5 combustion chamber will be modified in order to burn a gas of low calorific value such as the biogas generated in the biomass gasifier, whose low calorific value does not reach 8 MJ/kg. The same design parameters of the GT 5 will be maintained for the compressor, the turbine and for the electric generator. System Description Operation Scheme and Parameters Figure 4 shows the main screen of Thermoflex, where it is possible to see the scheme used for simulating the BIG/GT system. The gasifier generates gas at 700 °C. The gas is cooled in four stages until it gets into the scrubber at 142 °C, from where it leaves at a temperature of 40 °C and afterwards it is compressed towards the combustion chamber of the gas turbine. The compression is carried out in two stages with intermediate cooling. The heat rejected during the cooling of the gas is used along the four stages, through four heat exchangers to: (1) generate saturated steam; (2) pre-heat the air that will go to the gasifier; (3) pre-heat the gas before it gets into the combustion chamber of the gas turbine; and (4) pre-heat the feed water of the recuperation boiler. The heat rejected in the intermediate cooling of the gas compression is also used for pre-heating the feed water of the recuperation boiler. The recuperation boiler is formed by an economizer, an evaporator and a superheater. The deaerator lies between the economizer and the evaporator. A part of the feed water goes through the economizer of the recuperation boiler and the other part goes directly to the deaerator after it is pre-heated with the heat rejected by the biogas cooling. The deaerator and the economizer operate at 5 bar.
Figure 4: Scheme Used for the Simulation of a BIG-GT System in the Thermoflex software The superheated steam gets into the steam turbine at a temperature of 420 °C and a pressure of 25 bar. The steam for the deaerator comes from a controlled extraction at 5 bar in the steam turbine. The pressure in the condenser is 4.24 kPa. At the condenser outlet, a circulation pump pumps the condensate to the reposition water reservoir that is at atmospheric pressure. At the reposition water reservoir, there is a low pressure pump. The high pressure pump is at the outlet of the deaerator. The heat removed in the condenser is dissipated in a cooling tower. The exhaustion gases leave the gas turbine at 448 °C. At the outlet of the recuperation boiler economizer, the exhaustion gases leave at 226 °C and go to the sugarcane bagasse dryer that is at the gasifier inlet. Consumption, Energy Balance and Performance The system consumes 1.063 kg/s of sugarcane bagasse. The reposition water is 1.061 kg/s due to the evaporation in the cooling tower and in the deaerator, and also due to the drain in the recuperation boiler evaporator. The gas turbine generator produces 2611 kW and the steam turbine generator produces 1125 kW, totaling 3736 gross kW. The gross efficiency of the system is 45.26%. Because of consumption of the auxiliary equipment (pumps, fans and compressors), the net power of the system is 3000 kW, which results in a net efficiency of 36.35 %.
Economic assessment of the
system
Below some real BIG-GT power plant costs:
VARNAMO
Plant in Sweden: (source: Beenackers e
Maniatis, 1996)
ARBRE
Plant in England: (source: Beenackers e
Maniatis, 1996)
GREVE
IN CHIANTI Plant in Italy : (source:
Granastein , 2003) – IEA - TASK
36
SIGAME
Project in Brazil (non executed project) Based on these data we could assume an specific investment cost of 5000,00 US$/kW for the 3 MW BIG-GT pilot plant. Additional resources are asked for additional measuring and control technology regarding the system thermal tests. Using, real cost data and a power to cost relation coefficient of 0,6 the 3 MW BIG-GT power plant cost must be between 24 and 32 million dollars. Lower values corresponding to the ARBRE plant are must recent. Table 2. Results of the economic calculations for the cost of the energy generated by the BIG - GT 3 MW cycle
Figure 5. Cost of Generated Energy for the BIG - GT Technology - 3 MW – for different biomass costs 8- Expected results Technical reports about the project stages related in point 5. 9- Project budget.
Pilot plant including built up
and commissioning (gasifier, gas turbine, recuperative boiler, steam
turbine, heat exchangers and BOP):
US$ 15 106 Total: US$ 27,54. 10 6
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