Life cycle assessment comparison of technical solutions for CO 2 emissions reduction in power generation

Transcription

1 Energy Conversion and Management 44 (2003) Life cycle assessment comparison of technical solutions for CO 2 emissions reduction in power generation Lidia Lombardi * Dipartimento di Energetica Sergio Stecco, Universita degli Studi di Firenze, Via Santa Marta, 3, 50139Firenze, Italy Received 3 August 2001;accepted 26 December 2001 Abstract An exergetic life cycle assessment (ELCA) and a classical environmental life cycle assessment (LCA) have been performed for three carbon dioxide low emission power cycles. The configuration of the power cycles are: a semi-closed gas turbine combined cycle with CO 2 reduction from the exhausts by means of amine solution chemical absorption;an integrated gasification combined cycle with CO 2 reduction from the synthesis gas by means of amine solution chemical absorption;and O 2 /CO 2 innovative cycle where, burning methane in oxygen, CO 2 becomes the cycle working fluid, and the CO 2 excess, produced in the combustion, is removed in liquid phase without any additional system. The LCA is mainly focussed on CO 2 production during the three phases of construction, operation and dismantling of the plants. This is necessary to verify if the process is effective in terms of CO 2 reduction. Besides the classic LCA, also an ELCA was performed, whose aim is to assess the cost, in terms of exergy losses, of the life cycle of the plants. The CO 2 reduction is effective during the lifetime, and a sort of classification of the studied plant is obtained. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Life cycle assessment;exergetic life cycle assessment;co 2 ;Greenhouse effect;chemical absorption;amines; IGCC;O 2 /CO 2 cycle 1. Introduction The heightened awareness of the global warming issue has increased interest in the development of methods to mitigate greenhouse gases emissions. Further, precise reduction objectives have been determined by international intergovernmental organisations. * Tel.: ;fax: address: lidia@pin.unifi.it (L. Lombardi) /03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S (02)

2 94 L. Lombardi / Energy Conversion and Management 44 (2003) In particular, with reference to power generation, measures to limit carbon dioxide production, which is the major responsible gas for the greenhouse effect, have been studied and are under development world wide. In this frame, considering that power production is the main source of CO 2 in the environment, the aim of this work is to compare different technical possibilities of reducing carbon dioxide emissions on a short-medium term horizon. In fact, the considered options are all based on continuing to use fossil fuels. It is obvious that massive switching to alternative energy sources, as renewables, is not a viable option in a short time and, further, that its self-sufficiency is even more faraway. So, the challenge for the next years is the development of technical solutions which allow continued use of fossil fuels while limiting carbon dioxide emissions. The three possibilities considered in this work are: a natural gas fired combined cycle with partial recirculation of the flue gas and chemical absorption of CO 2 from the exhausts; an integrated coal gasification combined cycle with CO 2 chemical absorption from the syngas; an innovative methane fueled cycle where, due to combustion in pure oxygen, CO 2 is the cycle working fluid. The first two options represent a very close development to the state-of-the-art of modern power plants. In fact, the natural gas combined cycle is today the leading option, offering an efficiency exceeding 60% in the best solutions. The flue gas recirculation, on one side, increases the CO 2 concentration in the exhausts and makes it feasible to attempt its removal. On the other side, it involves a slight decrease in efficiency and the need for gas turbine redesign, at least to obtain improved performances. Integrated gasification combined cycle (IGCC) plants in operation today are only a few and have high investment and operating cost drawbacks. Nevertheless, this is a very promising technology to use coal (also biomass and waste derived fuels) in an efficient and environment friendly way. The basic scheme is to be modified by including shift conversion of the synthesis gas and recovery of CO 2 with chemical absorption. A hydrogen-rich fuel is burnt in the gas turbine, and this is not a completely assessed technology. The CO 2 working fluid cycle has a totally different approach from the other two. Here, a complete redesign of the equipment is required, in particular turbines, because of the high pressure (HP) operating levels. Besides, no kind of additional device is required for CO 2 separation, since it is extracted almost pure at HP in the liquid state. Moreover, the CO 2 reduction in the operating phase of this last cycle is complete, no emission at the stack, while the percentage of removal in the other two cycles depends on how much one is willing to pay in terms of energy requirements for the process and, in the final analysis, in efficiency. For the aim of this work, a CO 2 reduction of 85% was assumed with respect to the original content in the flue gases when no addition of chemical absorption is present. Hence, quite different options have been considered for CO 2 emission mitigation, and there are different criteria to compare them. Of course, efficiency is one of them, but not necessarily the most appropriate one for the purpose of this work. Economic evaluation is another, and it will be needed for assessing the monetary feasibility.

3 L. Lombardi / Energy Conversion and Management 44 (2003) However, in this project, life cycle assessment (LCA) has been selected to be the most appropriate comparison criteria. In fact, considering the entire lifetime of the plants and, hence, total CO 2 emission in the lifetime, it becomes necessary to assess whether they are CO 2 reduction effective when considering not only operation but also construction and dismantling. Further, storage of CO 2 should be included in the assessment. This has not been considered in this study, because for the purpose of comparison, it does not add any useful information. In fact, in the three cases, the separated CO 2 can be thought to undergo the same transportation, assuming the same location for the plants, and final storage processes. 2. Semi-closed gas turbine combined cycle and chemical absorption of CO 2 from the exhausts The semi-closed gas turbine combined cycle (SCGT/CC) concept is one of the possible schemes for carbon dioxide emission reduction and was introduced by several authors [1]. The present attractiveness of this configuration is the increased CO 2 concentration in the exhausts in order to facilitate its removal, both from an energetic and economic point of view, in a downstream separation plant. The basic idea, demonstrated in Fig. 1, is to recirculate the exhaust gases extensively to the compressor inlet, after previous gas cooling to temperatures allowing substantial recovery of water vapour as condensate. The feasibility of combustion requires the inlet of a fresh air flow rate (providing all the necessary oxygen), while a similar flow rate of hot gases is discharged at the stack without cooling, downstream of the combined cycle heat recovery steam generator. In particular, this work refers to simulation, by means of a home developed Fortran code, of the SCGT/CC with a 501F Mitsubishi GT unit (derived from the original Westinghouse design for large heavy duty gas turbines) with a turbine inlet temperature of 1349 C, burning natural gas with a compression ratio of 14. Without considering the energy consumption for the CO 2 load Fig. 1. SCGT/CC simple configuration with steam extraction for CO 2 load solution regeneration.

4 96 L. Lombardi / Energy Conversion and Management 44 (2003) solution regeneration and CO 2 compression, the SCGT/CC efficiency is 51.6% with about 243 MW power output. The non-recirculated exhaust flow rate has to be sent to an appropriate CO 2 removal treatment. In this study, atmospheric pressure chemical absorption of CO 2 with an aqueous solution of blended amines has been considered. In fact, amines absorption is recognised as the present state-of-the-art for carbon dioxide removal from flue gases [2], with the experience coming from the oil industry in gas purification. For this reason, chemical absorption with amines has been selected for the case studies considered in this work (SCGT/CC and IGCC), while particular attention has been paid to consideration of amines blends that are able to reduce the process energy requirements and, hence, the penalties for CO 2 abatement in power generation. The basic scheme was improved by adding devices for heat recovery, control of flow rates and composition and CO 2 purification, leading to the plant scheme reported in Fig. 2, and simulated by means of Aspen Plus [3]. Flue gases enter the absorber from the bottom and are scrubbed by the fresh solution coming from the upside. The purified gases leave the column at the top and the solution, loaded with CO 2, leaves at the bottom. This liquid stream is pre-heated in a heat exchanger, where the regenerated solution transfers heat to the loaded one, which is further heated in the stripper. From the top of the stripper, a gaseous stream is produced, containing essentially water and carbon dioxide, together with a small amount of amines. This is cooled to condense the water and amines that are recycled to the absorption system. The remaining gaseous stream is substantially carbon dioxide that is further cooled and then compressed to a value of about 80 bar, adequate to obtain liquid CO 2. The regenerated solution, after heat recovery to the CO 2 -saturated stream coming from the absorber, is split into two parts. The major part is cooled to the absorber inlet temperature, while a small percentage (0.5%) is extracted, regenerated by means of activated carbon grains and recirculated upstream to the absorber. The extraction of part of the solution is necessary because of Fig. 2. CO 2 chemical absorption plant scheme (DeCO 2 ).

5 L. Lombardi / Energy Conversion and Management 44 (2003) the irreversible accumulation of carbamates, but it should be responsible for the high costs of fresh amines make up and degraded solution disposal. In order to cope with these costs, regeneration with activated carbon seemed to be an adequate solution [4]. In fact, activated carbon filters have the possibility to retain organic compounds as carbamates, allowing recirculation of the clean solution to the absorber. Different blended solutions of amines have been investigated in previous works [5,6] in order to find the one which minimises the energy requirement for regeneration and the unit electricity cost, at a given removal efficiency. The minimum cost, about 4.20 c/kw h referred to 1998 Italian market conditions for fuel and other costs, is achieved by a 25% DEA and 25% MDEA in mass concentration solution [6]. With these conditions, assuming 85% removal efficiency, the energy requirement of the DeCO 2 plant for regeneration of the load solution in the stripper is about 4236 kw/kg of removed CO 2 (3597 kw/kg of CO 2 in the exhausts), and a cycle efficiency equal to 47.71% is achieved. The heat duty required for regeneration of the CO 2 load solution is obtained by means of steam extraction from the LP steam turbine (2 bar). Considering also the separated CO 2 compression up to the pressure of 80 bar, which is adequate for liquefaction, in an intercooled staged compressor with an energy consumption of kj/kg CO 2 [7], the total overall efficiency of the cycle results in 46.04%. With respect to the net power output of the cycle, i.e. decreased by the DeCO 2 consumption, the specific CO 2 emission results in kg of CO 2 per MW h, while the value for the simple SCGT/CC configuration was Integrated coal gasification combined cycle with CO 2 chemical absorption from the syngas In this part of the study, the production of clean coal syngas and its use in a combined cycle has been simulated by means of Aspen Plus. The cycle will be addressed in the following as IGCC with DeCO 2 (Fig. 3). Fig. 3. Schematic of the IGCC with DeCO 2.

6 98 L. Lombardi / Energy Conversion and Management 44 (2003) The goal is to obtain a syngas containing the maximum allowable amount of hydrogen (by loosing the minimum possible energy content) in order to produce as much water as possible during the combustion in the gas turbine. In the gasifier (1400 C and 20 bar), the coal, about 31 kg/s, is converted with the gasifying mediums of oxygen and steam (250 C) to a CO and H 2 rich gas. This raw syngas contains several pollutants, especially carbonyl sulphide (COS), hydrogen sulphide (H 2 S), carbon monoxide (CO) and carbon dioxide (CO 2 ), that need to be removed. First, the COS, if present, is converted into H 2 S and carbon dioxide by the means of COShydrolysis water gas reaction. In this way, the COS sulphur content is more easily removable when it is in the H 2 S form. Then, H 2 S removal takes place by means of 50% mass fraction methyldiethanolammine (MDEA) aqueous solution chemical absorption (MDEA is selective for H 2 S in the presence of CO 2 ). The total mass flow of the solution used in the absorber is set at 0.97 kg solution/kg coal, corresponding to H 2 S removal efficiency of 95%. A catalytic shift reaction follows. This stage is required to obtain a hydrogen-rich syngas from the carbon monoxide-rich raw syngas and also because the obtained high content in CO 2 allows its convenient removal through a subsequent chemical absorption process. Here, the CO reacts with water to produce CO 2 and H 2. In order to enhance the conversion of the CO, this reaction is realised in two reactors at about 450 and 250 C, respectively [8]. In the first reactor, steam at 230 C and 18.6 bar is supplied for the reaction to take place in the ratio of 1.5 moles of water per mole of CO [8]. This steam is obtained by means of steam extraction from the steam turbine. As a result of the catalysts, at the exit of the reactors, the CO conversion rate is 90%, and the CO 2 mass fraction passes from to and to after the water condensation. The CO 2 removal is realised by chemical absorption with aqueous amines solutions (25% DEA þ 25% MDEA) in a system similar to that previously described for the SCGT/CC case. The removal efficiency is 92.3%. This value allows an overall reduction of 85%. In fact, downstream of the syngas combustion, the CO 2 in the exhausts result to be 15% with respect to the exhausts of the same plant without the shift reaction and CO 2 removal system. The clean hydrogen-rich syngas leaves this section in pressurised conditions and is definitely delivered to the combined cycle, where for the steam bottoming cycle, steam is produced, both in a heat recovery steam generator by cooling the exhausts from the GT and by means of heat recovery from the different sections of the plant. The choice of two separate systems for CO 2 and H 2 S removal was imposed by the necessity of obtaining two separate streams, almost pure in CO 2 and H 2 S, respectively. In fact, while the CO 2 is compressed and sent to a disposal treatment, the H 2 S is reused to obtain valuable byproducts by the Claus process. Considering the additional consumption of the air separation unit (ASU) (826 kj/kgo 2 ) plus the O 2 compression from ASU outlet pressure (154 kj/kgo 2 ), assumed to be 5 bar, to the gasification reactor pressure, 20 bar, and the compression of carbon dioxide up to 80 bar (355 kj/ kgco 2 ), the resulting net power is 287,760 kw, and the efficiency is 38.80%. Considering a conventional IGCC, without the shift reaction and CO 2 removal sections, where the syngas leaving the H 2 S removal section is directly sent to the GT, the net power production is 344,439 kw, which, referred to the same coal entering energy, gives an efficiency of %.

7 L. Lombardi / Energy Conversion and Management 44 (2003) Overall, a decrease of 7.64 efficiency points has been estimated, but the specific CO 2 emission passed from 725 to 130 kg/mw h. 4. O 2 /CO 2 cycle Several configurations have been proposed for this kind of cycle, called the MATIANT type, from the names of the authors who first proposed this type of cycle [9,10]. In this study, the Ericsson-like (E-MATIANT) configuration has been deeply investigated [7]. The cycle model has been built by means of the commercial software engineering equation solver. Its temperature entropy (T S) diagram and the corresponding plant layout are shown in Figs. 4 and 5. Starting from point 1, at atmospheric conditions, an almost pure CO 2 stream, a small percentage of water vapour is still present and will be removed in the first intercooler, is compressed, through a three stage intercooled compressor up to 80 bar and 35 C (point 7), where it is in a liquid state. As 80 bar is the supercritical pressure, the working fluid reaches the liquid state without getting into the saturation line. This leads to two main advantages: there are no problems concerning non-condensable gases, and an additional component, the condenser, is not necessary. From point 7, the liquid CO 2 is compressed by a pump to the maximum cycle pressure, about 150 bar, (point 8). After the pump, the fluid enters the regenerative heat exchanger (RHE) (8 9), where it reaches a temperature of about 800 C, recovering the heat of the CO 2 /H 2 O mixture exiting from the low pressure (LP) turbine. Both the first combustion (9 10) and the reheating Fig. 4. O 2 /CO 2 cycle entropy temperature diagram.

8 100 L. Lombardi / Energy Conversion and Management 44 (2003) Fig. 5. O 2 /CO 2 cycle layout. Table 1 Reference working conditions for the quasi-zero CO 2 emission cycle p 1 (bar) 1.1 T 3 ( C) 45 p 2 (bar) 5.5 T 5 ( C) 45 p 4 (bar) 24 T 7 ( C) 35 p 11 (bar) 45 T max ( C) 1300 p max (bar) 150 RHE 0.7 (11 12) bring the gas to the maximum cycle temperature (about 1300 C, considering application of modern gas turbine blade cooling techniques). The fluid expands along and in the HP and LP turbines, producing power. Then, it is cooled by passing through the hot side of the RHE (13 14). The water produced in the two combustion processes is nearly completely separated from the CO 2 in a condensing water separator (14 1) and then removed from the cycle. The excess CO 2, produced in the combustion processes, is removed in liquid state through a valve before entering the pump (point 7). Considering the reference conditions in Table 1 and a CO 2 mass flow rate at the compressor inlet of 10 kg/s, the efficiency results as 43.7%, and the power output is about 10 MW. 5. Life cycle assessment The hypothetical power plants, built on the basis of the investigated cycles, have been studied by means of LCA. This is a quite particular application of LCA, which is performed usually for material products easily identifiable in well-defined objects, while the final product here considered is the energy and power output. However, the different processes to obtain the identified product and all their requirements in terms of materials and energy for the entire life cycle are compared to assess their

9 L. Lombardi / Energy Conversion and Management 44 (2003) environmental impacts and, in particular, their contribution to the greenhouse effect, since the aim of the studied cycles is primarily the reduction of the carbon dioxide emission in power generation. Also, other environmental effects have been considered according to the Eco-indicator 95 method: ozone layer depletion, winter smog, acidification, summer smog, eutrophication, pesticides, heavy metals to air and water, energy, carcinogens and solids. The whole life cycle of the plants has been considered, and hence, the phases of construction, operation and dismantling have been included inside the boundary of the study. This means that the inlet and outlet streams to and from the selected limit consist of the raw material flows and waste/emission flows, respectively. In order to identify the raw materials inlet flows, it is first necessary to size the plants approximately and collect information about the weights, main materials, production processes and scraps of the pieces of equipment to assemble the plants themselves. From the rough calculation of the amount of the employed main materials, it is possible to go back to the raw materials and to their manufacturing processes, i.e. energy consumption, calculating the raw substances use and emission and, hence, the environmental impacts, starting from the resources mining. The same procedure has been used for the operating phase, during which the inlet material flows, mainly fuel, have been led back to resources use and emissions, while the wastes from the plants, mainly stack gases, have been calculated on the basis of the built models. Concerning dismantling of the plants, basically the energy and, hence, the related emissions to disassemble, dispose and/or recycle materials has been calculated, according to the assumption of a waste scenario. In order to pass from the manufactured materials to the raw substances and emissions inventory, SimaPro 4.0 [11] has been used. This is basically a database able to reconstruct the history of several processes and materials and to aggregate the elemental pollutants inventory in order to obtain values for the selected environmental effect indicators. Since the sizes of the compared plants are rather different, as the functional unit for the comparison, the unit energy output has been chosen, in particular 1 MJ. 6. Plants construction The construction phase consists in the assembly of the plant, collecting data in terms of weights, materials, scrap and production processes of the main devices present in the plants. Usually, it is quite difficult to find these detailed data, and it is necessary to scale up or down the weights of known models. Also piping, civil buildings, in particular the concrete and the steel to reinforce it, and the copper for the cabling have been considered. These data are obtained from a report scaling the weights on the basis of the total power of the plant [12], while data about PVC for cabling and insulation of piping were retrieved from Ref. [13]. Transportation of the installed equipment has been considered, assuming an average distance of 1000 km. Moreover, on-site energy consumed has been considered, proportional to the total weight, with reference to Ref. [14], for 85% produced by a diesel engine and for the remainder taken from the grid. The materials considered in the analysis are steel, cast iron, copper, aluminium, plastic, rubber, concrete and asphalt.

10 102 L. Lombardi / Energy Conversion and Management 44 (2003) Plant operation A lifetime of 15 years has been assumed. During the operation of the plants, the main input to be supplied is the fuel, hence, it is necessary to account for its production process. For the SCGT/ CC and for the E-MATIANT cycles natural gas consumption, refinery and delivery have been considered. For the IGCC cycle coal consumption, mining and delivery have been considered. Moreover, for the CO 2 chemical absorption, in the case of the SCGT/CC and IGCC, the amines and activated carbon, for amines regeneration, consumption has been considered [15]. The elemental resources and emissions inventory are calculated by means of SimaPro. During the operation of the SCGT/CC and IGCC, the carbon dioxide due to combustion is vented into the atmosphere, and its total production in the lifetime is calculated on the base of 8760 h of functioning per year (SCGT/CC with DeCO 2 1: kg, IGCC þ DeCO 2 4: kg). Also, the main maintenance and substitution for the plants has been taken into account during the operation [15]. 8. Plants dismantling Dismantling includes disassembly of the plants, transportation an average distance of 1000 km and recycling or disposal of materials. A disposal scenario for dismantled materials has been assumed: steel is recycled for 80% and the rest is land filled;cast iron is recycled for 90% and the rest is land filled, copper is recycled for 100%, aluminium is recycled for 100%, concrete and asphalt are crushed and reused as a low quality landfill, plastic is incinerated, rubber is land filled and steel in reinforced concrete is land filled. Also on-site energy consumption has been considered for dismantling, proportional to the total weight of the installed equipment, with the same reference as above [13] and the same composition. Because of the intensive recycling of some materials, some impact indicator values result in negative figures. In fact, the amount of the recycled materials is taken into account as avoided new materials production and, hence, avoided (i.e. negative) emissions of pollutants. On the other hand, the dismantling/recycling processes are energy consuming and contribute a positive amount of emissions. The total results, for each impact, of the dismantling phase depends on the balance between these positive and avoided emissions. 9. Life cycle assessment results The comparison of the total greenhouse effect, expressed in kg of equivalent CO 2 per MJ, for the three studied cycles, is shown in Fig. 6. The plant for which the score is highest is the IGCC with DeCO 2, followed by the SCGT/CC þ DeCO 2, while the least contribution is due to the E- MATIANT cycle. The fact that the E-MATIANT cycle is the one with the lowest greenhouse effect was quite expected. In fact, no emission of carbon dioxide takes place due to the combustion, greatly reducing the emissions in the maintenance/operation phase, as shown in Fig. 7. In this last diagram, the phases of the life cycle are compared for each cycle. The amount of

11 L. Lombardi / Energy Conversion and Management 44 (2003) Fig. 6. Comparison of the greenhouse effect per functional unit among the three cycles. equivalent CO 2 emitted per functional unit during the construction phases are very similar for the three cycles, with a higher value for the ICGG with DeCO 2 due to the complexity of the plant but, however, of the same order of magnitude. The dismantling phases have a very small contribution, with respect to the total value, and in the cases of the IGCC with DeCO 2 and of the SCGT=CC þ DeCO 2, they have a negative value because of the avoided CO 2 emissions due to the recycle of plant materials. Hence, the contributions which determine the order of magnitude of Fig. 7. Comparison of the greenhouse effect per functional unit among the three cycles for the lifetime phases.

12 104 L. Lombardi / Energy Conversion and Management 44 (2003) Fig. 8. Contribution to the greenhouse effect per functional unit of maintenance and operation for the three cycles. the total values are the maintenance/operation phases, confirming and explaining the classification previously found for the total results. In particular, the values found for the IGCC with DeCO 2 and for the SCGT=CC þ DeCO 2 are of the same order of magnitude, while the E-MATIANT cycle is one order lower. This is basically due to the absence of emissions during the functioning of this last plant. In Fig. 8, the contributions for maintenance, in which, beside the main part substitutions for the major devices of the plant, the fuel production is included, and for the operation are split. The maintenance values for the SCGT=CC þ DeCO 2 and for the E-MATIANT are almost similar, mainly determined by the fuel production process, while for the E-MATIANT cycle, this is the only contribution to this phase. For the SCGT=CC þ DeCO 2, the leading contribution is the combustion CO 2, even when reduced by 85%. In the IGCC with DeCO 2, the maintenance and the operation values are of the same order of magnitude with the former even higher than latter. This is due to the higher amount of coal required, with respect to natural gas, for unit power output and also to the higher CO 2 generated in the coal production process. The greenhouse effect score per one kg of natural gas and coal production and delivery is, respectively, and 0.58 kg of equivalent CO 2 [11]. Moreover, the operation value itself is higher than for the SCGT/CC þ DeCO 2 cycle, because of the lower conversion efficiency of the plant. All these considerations explain the plant classification on the base of greenhouse effect per functional unit. From the results for the other impacts included in the Eco-indicator 95 method [15], it is possible to state, at least in first analysis, that the plant with the lowest greenhouse effect is not affected by a substantial increase in the other indicators, since they are almost of the same order of magnitude for the three plants and confirm the trend obtained for the greenhouse effect [15]. This is a quite important result. In fact, it means that the reduction of greenhouse effect is not accompanied by a transfer of pollution from one sector to another, but it is effective. 10. Exergetic life cycle assessment results Exergetic life cycle assessment (ELCA) is an analysis method based on a life cycle approach in combination with exergy analysis, developed by Cornelissen [13]. Exergy can be viewed as one of

13 L. Lombardi / Energy Conversion and Management 44 (2003) the possible impact indicators in a life cycle and, moreover, the most appropriate parameter for the depletion of natural resources, addressing the life cycle irreversibility. Exergy destruction during the three phases of the plant lifetime has been calculated [16], and the results (in MJ of destroyed exergy per MJ of energy output) showed, as a common trend, that the contribution to the total exergy destruction due to the construction and dismantling phases is almost negligible when compared to operation, where the transformation from fuel chemical exergy into thermal exergy takes place, and maintenance, in which the production process of the fuel itself is taken into account. However, in order to have more detailed information, the comparison among the three cycles is presented for each phase of the life cycle. Concerning the construction phase, the exergy destruction per functional unit is of the same order of magnitude in the three cases (Table 2), but has the higher value for the IGCC with DeCO 2, followed by the SCGT=CC þ DeCO 2, while the E-MATIANT cycle has the lowest value. This is obviously due to the differences in the type of plants. The E-MATIANT cycle is quite simple, very compact, without the addition of any separation device. The SCGT=CC þ DeCO 2 is more complicated because, to the basic GT cycle, the steam bottomer cycle adds notable amounts of construction materials and complication. Moreover, the carbon dioxide separation plant has been added. Finally, the IGCC with DeCO 2 is even more complex for the adding of the gasification section and removal section. These differences in the complexity of the plants are directly reflected in the amount of construction materials and, hence, in the input of exergy for the construction phases. The same argument explains the similar trend for the dismantling phase (Table 2). In fact, a higher exergy investment is required for recycling or disposal for a greater amount of materials. Concerning the exergy use for maintenance of the three plants (Table 2), the exergy destruction per functional unit has a different trend from that of the previous comparisons. The highest value belongs to the SCGT=CC þ DeCO 2, followed by the E-MATIANT cycle and, finally, by the IGCC with DeCO 2 cycle. As already stressed, in this quantity, the exergy invested for the fuel production is the most important part, while the other contributions are almost negligible compared to this. This is also shown in Fig. 9, where it is evident that the terms due to the devices maintenance and to the production/use of the activated carbons and amines when present are negligible. Considering the two plants that use natural gas, it is also possible to see that the exergy investment for the E- MATIANT cycle is higher than for the SCGT=CC þ DeCO 2 due to a greater used amount of natural gas, agreeing with its lower First Law efficiency. On the contrary, considering all the contributions, the exergy investment is higher for the SCGT=CC þ DeCO 2 with respect to the E- MATIANT cycle, mainly because of the use of amines. The IGCC with DeCO 2, that uses coal, Table 2 Summary of the exergy destruction per functional unit (MJ/f.u.) for the life cycle phases of the three compared cycles Life cycle phase SCGT=CC þ DeCO 2 IGCC with DeCO 2 E-MATIANT Exergy destruction (MJ per MJ power output) Construction 5: : : Maintenance 9: : : Operation 1: : : Dismantling 9: : : Total 2: : :

14 106 L. Lombardi / Energy Conversion and Management 44 (2003) Fig. 9. Different contributions to exergy destruction per functional unit for the maintenance of the three compared cycles. shows a lower global exergy investment for maintenance as a consequence of a lower value for the fuel production. Hence, maintenance exergy destruction is completely traceable to the exergy destruction in the production process of the used fuels. Comparing the total exergy destruction per functional unit in the life cycle of the three compared processes, a sort of score is obtained to classify the cycles from the one with the highest value, the IGCC with DeCO 2, through the E-MATIANT cycle, to the SCGT=CC þ DeCO 2 with the lowest score, but with a very slight difference from the previous. 11. Summary and conclusions Three technical solutions for carbon dioxide reduction, considered as candidates for mitigation of the greenhouse effect from power production, have been studied and compared, by means of LCA and ELCA. Two of them are technologies very near to the state-of-the-art of power plants, the SCGT/CC and the IGCC, with the addition of a well-assessed carbon dioxide removal process for which a CO 2 reduction of 85% was assumed with respect to the content in the flue gases when no addition of chemical absorption is present. The third one is an innovative CO 2 working fluid cycle with zero emissions. The thermodynamic simulation of the three cycles showed the following results in terms of First Law efficiency: 46.04% for the SCGT=CC þ DeCO 2, 38.80% for the IGCC with DeCO 2 and 43.70% for the O 2 /CO 2 cycle, and in terms of specific emission: 65 kg of CO 2 per MW h for the

15 L. Lombardi / Energy Conversion and Management 44 (2003) SCGT=CC þ DeCO 2, 130 kg of CO 2 per MW h for the IGCC with DeCO 2 and zero emission per MW h for the O 2 /CO 2 cycle. The LCA, focusing on the indicator greenhouse effect calculated per functional unit, i.e. energy output unit, showed that the highest score is due to the IGCC with DeCO 2, followed by the SCGT=CC þ DeCO 2, while the lowest value applies to the E-MATIANT cycle. A deeper investigation explained that the main contribution to the total life cycle greenhouse effect comes from the operation, while the other phases are almost negligible when compared to it. This also explains the very low score assigned to the E-MATIANT cycle. Considering other environmental indicators, the three cycles show almost similar results. This means that the actions undertaken to reduce carbon dioxide emissions are effective and do not simply move the pollution problem from one environmental sector to another. The classification obtained on the basis of the score in the ELCA, where the considered indicator is the exergy loss per functional unit, sees the SCGT=CC þ DeCO 2 as the one with the lowest exergy destruction in the life cycle, followed by the E-MATIANT cycle and then by the IGCC with DeCO 2. In this case too, operation is the dominant phase, while construction and dismantling have values of the indicator orders of magnitude lower. Hence, the classification is mainly built on the basis of the Second Law efficiency of the considered cycles. Hence, in conclusion, it is possible to state that for products of this kind, i.e. electricity unit, power production cycles, the attention must be focussed on the operation phase, while the other phases, construction and maintenance, are almost negligible when compared to it. This result is valid both for the LCA and ELCA and can be considered exhaustively demonstrated in the present work and, hence, adopted as a reference in the future. Concerning the actual possibility of limiting carbon dioxide emission, from this study, it comes out that the best way is developing dedicated machinery to operate cycles of the E-MATIANT cycle type. However, while such a development is proceeding, the results indicate that the addition of CO 2 chemical absorption, and in general of CO 2 removal technologies, can supply a great advantage with respect to the present state-of-the-art of the power generation technology and, in particular, that the SCGT=CC þ DeCO 2 has a minor value of the greenhouse effect and of the exergy loss in the lifetime with respect to the IGCC with DeCO 2. Of course, the final decision of adopting one or the other solution will be determined by several factors, among which are the fuel availability and economic feasibility. References [1] Facchini B, Fiaschi D, Manfrida G. Semi-closed gas turbine/combined cycle with water recovery IGTI Gas Turbine Conference and Exhibition. [2] Audus H. Leading options for the capture of CO 2 at power stations. Proceedings of the GHGT-5 Cairns, Australia, [3] ASPEN PLUSTM User Guide, Vols. 1 and 2. Release 9.3, [4] Kohl AL, Riesenfeld FC. Gas purification. 4th ed. Houston, Texas (US): Gulf Publishing Company, Book Division;1985. [5] Lombardi L. Studio delle possibilita di utilizzo di ammine per l assorbimento di CO 2 in cicli turbogas semi-chiusi, Graduation Thesis, Faculty of Environmental Engineering, University of Florence (Italy), 1997.

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