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FAIR-CT95-0291
Indirectly fired gas turbine (IFGT) for rural electricity production from biomass |
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Proposal No: | FAIR-CT95-0291 |
| Date Prepared: | .September 1999 | |
| Source: |
Final technical report |
Summary
Introduction Based on preliminary design studies, a biomass indirectly-fired gas turbine (BIFGT) system was identified as a potentially attractive option for rural energy production at scales of around 200 kWe. Based on these promising studies, this project was set up to investigate the components required for an indirectly heated gas turbine cycle operating on biomass feedstock for on-farm generation of combined heat and power (CHP). The aim of the project was to determine the techno-economic feasibility of a cost-effective system for on-farm combined heat and power (CHP) generation based on an indirectly heated gas turbine cycle operating on biomass fuels.
The specific objectives of the project are:
Most of the components for the proposed system are available but required some modification and development work. The turbine, compressor, combustor and low temperature recuperator are derived from existing sources. However, there was a need for some development work on combustion systems (to reduce potential fouling of the heat exchangers), on higher temperature heat exchangers and on turbo-machinery (especially the turbine side).
This was accomplished through completion of a number of tasks, as indicated below. The task Dynamic Modelling was originally not included in the workplan but considered by the partnership to be important to the outcome, and was therefore added. For each task an end-of-task-report has been prepared.
Results
A market survey of the demand for the BIFGT has been made by participants for their own countries, and for selected areas worldwide. Results of the market survey indicated a large potential for small-scale on-farm biomass energy production with the IFGT cycle. A more accurate market assessment could be made after more data on investment and operating costs were available. The integrated design that is currently available, shows that the IFGT-cycle can compete with alternative biomass-based energy generation plants at the projected scale.
Various BIFGT cycles were evaluated for their suitability for use in a biomass-fired CHP plant. In the thermodynamic studies, the so-called HAT (humid air turbine) cycle and the simple cycle were thoroughly evaluated. The simple cycle was chosen as the most promising. With this 20% electric efficiency can be obtained while the investment costs are relatively low.
In the preliminary economic study, the investment costs were calculated at 2434 ECU/kWe. After the final design was completed the investment costs emerged slightly higher at 2837 ECU/kWe for a BIFGT with an electric efficiency of 19% and a total efficiency (power plus heat) of 74%. This was mainly due to the high costs of the heat exchanger; approximately 45% of the total investment. An electric efficiency of 20% is also achievable, but at increased investment cost. In is believed, however, that the former represents a better trade-off between efficiency and investment cost.
Economic analysis showed that under certain conditions the IFGT cycle has a better economic performance in comparison to fossil fuel fired alternatives. Calculations of the electricity production costs as function of the electric efficiency and biomass fuel prices showed that the electricity production costs decreases slightly with lower electric efficiency. The decrease is caused by:
Of course this is only applicable in cases where all the extra heat produced can be sold. Therefore, for each application the optimum electric efficiency should be determined based on local conditions. Four case studies were conducted to find this optimal trade-off. This evaluation showed that the cycle is very attractive for small-scale sewage sludge combustion in Denmark, mainly because of the very high heat prices and taxation of sludge disposal.
Various combustion systems have been reviewed. Based on the selection criteria, the two-stage combustor is chosen as the most promising system for the IFGT cycle. An important advantage of the two-stage combustor is the possibility to regulate the combustion temperature regime. Experiments showed that the alkali emissions are minimal which is important since no flue-gas cleaning will be installed. The most critical component of the IFGT cycle is the heat exchanger. To make a proper heat exchanger selection a theoretical cycle analysis was conducted to determine heat exchanger constraints, e.g. cost, thermal efficiency, maintainability, etc. A number of heat exchanger types were analysed with respect to these requirements which resulted in the selection of a metallic shell-and-tube heat exchanger as the preferred type.
A detailed review was also made concerning corrosion aspects of metals exposed to flue-gas from biomass based power plants. In this review the critical aspects of material fouling and corrosion were investigated, e.g. biomass minerals, ash deposit analysis in combustion systems, high temperature alloys and corrosion, etc. A quantitative relationship has been determined to describe the condensation of alkali salts on heat exchanger, tubes.
Based on this analysis five different heat exchanger materials were selected for laboratory tests to determine the corrosion behaviour under maximum temperatures. The results of these first experiments were used to make a further selection of the heat exchanger material for thorough tests. The following three materials were selected: HR-120, HR-160 and SS-310. During the extended two-week tests, each material was exposed to biomass flue gases across the whole temperature range of interest, i.e. 450 to 850 ºC. Based on these tests, SS 310 was selected for further examination.
In order to judge whether SS 310 was sufficiently resistant to high temperature corrosion over prolonged periods, a duration test of 3.5 weeks was conducted with wood and a two-week duration test with Miscanthus. Based on the observed corrosion, the estimated metal loss after 15 years would be one mm if a parabolic rate is assumed. With Miscanthus, containing a higher alkali content, no major differences were found on fouling and corrosion.
Results of the duration tests were used to establish maintenance schemes for heat exchanger cleaning, and the selection of a suitable cleaning method. A manual cleaning method using a flexible rotating shaft with stainless steel brush was selected for cleaning the inner-side of the tubes. Cleaning - which takes about 5 to 7 hours - is required every 3 to 5 months (depending on the type of fuel). The shell side requires no cleaning as it is exposed to clean filtered ambient air only.
Since the cycle considered incorporates a gas to gas heat exchanger, the overall heat transfer coefficient will be low, thus requiring a large heat transfer area. Design calculations on the heat exchanger have been made for a one-pass shell and tube heat exchanger with one stationary head and one floating head. 385 tubes of 8.4 meters are required for a 242,5 kW electric output. The outside tube diameter is calculated at 31.8 mm and the wall thickness 2.5 mm. Internal refractory insulation will be required to avoid thermal stress of the shell, and to optimise the efficiency.
The integrated system incorporates a heat recovery heat exchanger. In this supplementary heat exchanger flue gases from the main heat exchanger are cooled with water. Transferred heat can be used elsewhere.
Marine turbochargers and gas turbines have been evaluated for their suitability to the BIFGT. Marine turbochargers were selected to meet the requirement of developing a low-cost CHP plant. Detailed design and thermodynamic data were obtained and it was agreed to operate at a pressure ratio of 3.5. A brushless type alternator was selected for direct coupling to the turbocharger. As the design of the gas turbine requires an inlet temperature of 837 ºC for optimum performance, any deviation below that temperature will impose significantly lower efficiency and power output.
The BIFGT includes a topping combustor that is fossil fuel-fired. This topping combustor is required for the start-up of the CHP plant. Moreover, it will increase the power of the cycle at no extra costs resulting in a decrease of the specific investment costs, and it will increase the efficiency of the cycle. It is envisaged that 10% of the fuel input will be fossil fuel. During normal operation the fossil fuel supply can be turned off. This will result in a 2.4% lower efficiency and approximately 20% lower power output.
An optimal control strategy for the total system has been designed resulting in a stable power production with voltage and frequency within the limits allowed for the grid. Since the BIFGT will be grid-connected there is no load-following operation. Also, the gas turbine rotational speed is controlled by the grid, resulting in a relatively simple and non-expensive control system. During normal operation the IFGT can work unmanned.
For the design of the BIFGT control and instrumentation, a dynamic model was developed and step responses calculated. This model was also used to obtain an indication of the system performance at start-up, and in case of turbine trip. The integrated design includes all required safety measures, controls and protocols.
Discussion and Conclusions
In this feasibility study, a basic design and lay-out of a 250 kW biomass indirectly-fired gas turbine cycle (BIFGT) has been developed. The concept seems to be technically feasible. Economic feasibility occurs only under certain conditions. If the uncertainties can be minimised, a commercial prototype can be engineered and built to demonstrate the BIFGT concept under actual conditions. Regarding the uncertainties a distinction can be made between technical, economical and environmental aspects.
The main issues here are as follows.
Aspects of the financial viability must be determined.
The BIFGT, cycle is developed for clean type of biomass feedstock like wood chips, straw pellets and the like. The influence of different types of feedstock on the technical performance, heat-exchanger corrosion, heat-exchanger cleaning frequencies and heat-exchanger emissions is subject to further investigation.
Once positive results are obtained with additional duration tests, it is recommended to construct and operate a commercial prototype to demonstrate its technical and economic feasibility under actual conditions. Moreover, a pilot plant is needed to convince potential clients that the BIFGT cycle is a promising technology for their specific application.
The demonstration project should be accompanied with a development programme consisting of an engineering phase, a commissioning phase, an operation optimisation phase, and a test phase for different types of fuels. This should result in a marketable product, i.e. a proven technology which is financially and economically viable.
© Copyright 2006 Policy Statements
Updated
by CPL Press:
03/07/2007
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