PowerGen Europe - June 9-11, 2015 Amsterdam, The Netherlands

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1 DESIGN OF A HIGH EFFICIENCY WASTE TO ENERGY PLANT WITH HYBRID COMBINED CYCLE AND CONSUMING A LIMITED AMOUNT OF NATURAL GAS H. SIOEN 1,* and S. GUERREIRO RIBEIRO 2 1 WATERLEAU Group N.V., Wespelaar, Belgium. 2 WTERT-Brasil, Rio de Janeiro, Brazil. *Corresponding author: herman.sioen@waterleau.com, Keywords: waste-to-energy, high efficiency, optimized combined cycle, OCC, natural gas, hybrid cycles Abstract Several power plants use hybrid cycles where gas turbines or gas engines exhaust heat is used in steam bottoming cycles burning biomass or MSW [1, 2, 4, 5]. One of the largest of this kind is the Zabalgarbi plant in Bilbao, Spain, where the clean exhaust gases of a GE LM6000 gas turbine are used to superheat the steam produced in the MSW boiler. This has many advantages such as reducing corrosion in boiler superheaters and increasing thermodynamic efficiency. The drawback however is that the natural gas share, a fossil fuel, is high. In Bilbao only 22% of the exported power comes from MSW. This also poses an operational problem when the large gas turbine is down for maintenance or due to malfunction, or when the gas price is too high to allow economic operation. In this work we present a way, named Optimized Combined Cycle (OCC), to reduce the natural gas share to less than 25% by introducing a small gas turbine, or gas engine, providing approximately the plant s own power consumption. The exhaust of such a small gas machine does not have the energy necessary to superheat all the steam produced in the MSW boiler, in general, below 420 C to avoid corrosion. To solve this problem the gas machine exhaust flow is artificially increased with pre-heated fresh air and using a natural gas duct burner (DB) to reach the required steam superheating. Natural gas consumption in the DB is reduced by preheating the air supply to the duct burner using the hot flue gas exiting the external steam superheater (SH). In OCC the optimum configuration is not the highest thermodynamic efficiency solution, in general leading to higher prices, but the one that gives the best economic results for a given amount of MSW to be treated. OCC is an efficient tool to improve economic feasibility of WTE plants. It has the additional advantage that we can replace natural gas with landfill gas or biogas from digestion of biomass or of the organic fraction of MSW (OFM). 1 - INTRODUCTION Waste management in developing markets is biased towards landfilling after highly informal recycling. Since there are no requirements similar to European Landfill Directive 1999/31/CE their cost is much lower than waste-to-energy combustion technologies such as mass-burning or RDF (or SRF) incinerators. Also improper regulation leads to landfills that would not be licensed in Europe and the USA. Therefore modern waste-to energy facilities, environmentally superior to landfills, are not, in general, economically feasible in countries with low (often zero) tipping fees and cheap landfills. The only source of income in this case is the sales of power. One way to improve that is to produce more energy per ton of waste. 1

2 But also in developed markets there is a continuous drive to improve the plant efficiency. The R1 criterion pushes plant design to higher electrical output per ton especially in cases where no heat can be recovered and sold. When striving for higher efficiency, higher steam pressure and temperature are the first and foremost logical process choice. However, to avoid corrosion in the superheaters of MSW boilers steam parameters are limited to 40bar/400 C which leads to relatively low thermodynamic efficiency. To overcome this problem several waste-to-energy (WTE) plants use hybrid cycles where gas turbines or gas engines exhaust heat is used in steam bottoming cycles burning biomass or MSW [1,2,4,5]. In the Zabalgarbi power plant in Bilbao, Spain, shown in Figure 1 [3], the clean exhaust gases of a GE LM6000 gas turbine are used to superheat the steam produced in the boiler. This has many advantages such as reducing corrosion in boiler superheaters and higher thermodynamic efficiency. The drawback however is that the natural gas share, a fossil fuel, is high. In Bilbao only 22% of the exported power comes from MSW. This also poses an operational problem when the large gas turbine is down for maintenance or due to malfunction [11]. Besides Bilbao several WTE plants use hybrid cycles such as Sakai, Linköping, Vantaa, Heringen [2,9,10] and others but either the natural gas share is high or its efficiency is low. Figure 1 Zabalgarbi Plant Concept in Bilbao, Spain [3] This work presents a high efficiency waste-to-energy plant configuration, named Optimized Combined Cycle (OCC), combining a small amount of natural gas and MSW that can be economically feasible for conditions found in many emerging countries: low tipping fees, high electricity and natural gas prices, landfills wasting biogas energy in flares and medium LHV waste. Of course in countries with better gate fees and lower natural gas cost like USA the OCC method can also be extremely attractive from a financial point of view. 2

3 2 OPTIMIZED COMBINED CYCLE OCC The OCC concept can be illustrated with an example as shown in Figure 2. First the steam pressure in the MSW boiler is increased to 90 bar which requires additional superheating far beyond 400 C to avoid excessive moisture at the steam turbine exit. Instead of the large gas turbine we use a small one, providing approximately the plant s own power consumption or slightly more. The exhaust of such a small gas machine does not have the energy necessary to superheat all the steam produced in the MSW boiler at a temperature of (normally) below 420 C to avoid corrosion. To solve this problem the gas machine exhaust flow is artificially increased with pre-heated fresh air and using a natural gas duct burner (DB) to reach the required steam superheating. Natural gas consumption in the DB is greatly reduced by preheating the air supply to the duct burner using the flue gas exiting the external steam superheater (SH), similar to what is done in SCR De-NOx systems [7] for flue gas cleaning. To further increase the steam bottoming cycle efficiency the boiler flue gas stack temperature was decreased to 70 C, this way reducing the stack loss. The proposed OCC concept greatly reduces the amount of natural gas needed to increase the efficiency of MSW combustion [5]. With OCC 75% or more of the net energy comes from MSW, allowing the natural gas to be replaced by fuels not commonly available in large amounts, including landfill gas or biogas from anaerobic digestion. The efficiency of the MSW can reach values of more than 30% and the natural gas efficiencies are similar to those attained when large gas turbines are used in stand-alone combined cycles without MSW. The natural gas efficiency approaches 50%, even for small gas turbines around 5 MWe or gas engines smaller than 3 MWe. The OCC concept has other advantages such as the plant being able to startup without external power, no attemperation needed for live steam temperature control, increased range of choices for the natural gas machine to match a given amount of MSW to be combusted, reduced amount of startup auxiliary fuel both in design and operational phases. 3 DESCRIPTION OF THE PROCESS There are several possible configurations sharing the same OCC hybrid cycle concept and all leading to low natural gas consumption while keeping high efficiency. First we consider the gas turbine (GT) case as shown in Figure 2 where high temperature GT exhaust is used to superheat the steam generated in the MSW boiler. Power is generated by one small GT and a high pressure, 85bar/500 C, steam turbine. Steam is superheated in the MSW boiler up to a corrosion-safe temperature, say 420 C, and to a higher temperature, 500 C, in the external superheater (SH). After the external superheater (SH) the flue gas is at temperature T2, above 420 C, and can be used in an air pre-heater (APH) to preheat the extra air that will be mixed with the GT exhaust before the duct burner (DB). This has three effects: it reduces the amount of natural gas in the duct burner, increases the O2 content of the gas turbine exhaust and avoids the temperature after the DB from being too high. 3

4 Figure 2 OCC Scheme Using a Small Gas Turbine After the air pre-heater (APH), the flue gases have a high O2 content, in some cases above 15%, and can be used as part of the combustion air to the MSW boiler. This presents other advantages: keeping the energy in the system, reducing NOx formation in the boiler and forcing the GT exhaust to pass through the air pollution control (APC) of the boiler. To increase the steam bottoming cycle efficiency the stack temperature is lowered using condensing heat exchangers (CHX) for condensate and additional combustion air pre-heating. Now consider the gas engine (GE) case as shown in Figure 3 where medium temperature GE exhaust is used as part of the combustion air in the MSW boiler. Since the GE exhaust temperature is much lower than the GT it is better to use it as partial combustion air to the boiler and use a little more natural gas in the DB to superheat the steam in SH. The low O2 content of the GE exhaust, around 9%, does not have an impact on total O2 for MSW combustion because the mass flow is less than 10% of total combustion air. Figure 3 OCC Scheme Using a Small Gas Engine As the open cycle efficiency of the GE is higher than that of the GT the overall natural gas efficiency does not change much when the amount of natural gas in the burner is increased. There is however a quite remarkable distinction between the two configurations, i.e., with GE 4

5 the total natural gas share is much lower than the GT case. The optimal solution will depend on the economics represented by the internal return rate (IRR) of the investment for example. Finally consider the case with just the duct burner and no gas internal combustion machine (ICM) shown in Figure 4. Although the natural gas efficiency is lower, because it is the same of the Rankine cycle, its share is lower than in the two previous cases, 7.45%. Again economics will govern the optimum solution as shown in Figure 5. Figure 4 OCC Scheme without Internal Combustion Machine All three configurations share in common the presence of the duct burner, external superheater and air pre-heater that compose the core of OCC allowing high superheating with low natural gas consumption. This can be even lower using reheat steam cycles. Korobitsyn [4] has defined the natural gas share (FNG) efficiency, ηcc, as the efficiency of a stand-alone combined cycle consuming the same amount of natural gas as in the hybrid cycle configuration which he considered to be 52%. By his definition the MSW share (FMSW) efficiency would be: (1) In this work the natural gas share is extremely small and a standalone combined cycle would not be feasible or its efficiency would be much lower than 52%. We decided to define the efficiencies considering all the heat rejected by the gas ICM as input to the steam cycle together with the MSW plus the natural gas burner thermal energies. Therefore the MSW share (FMSW) and the natural gas share (FNG) LHV efficiencies would be, respectively: Where, PTOTAL total electric energy produced by the plant, MWe WST total electric energy produced by steam turbine, MWe WGT total electric energy produced by gas turbine, MWe (2) (3) 5

6 FMSW FNGT FNGB FNG = ηgt thermal energy from MSW, MWth thermal energy from natural gas consumed by gas turbine, MWth thermal energy from natural gas consumed by gas burner, MWth (FNGT + FNGB) total thermal energy from natural gas, MWth open cycle gas turbine efficiency In all calculations above the total energy loss in MSW boiler was conservatively considered 5.39% of thermal input. This includes thermal losses, un-burnt fraction, leakages, dirty tubes heat transfer effects, LHV variation and others. In practice these losses should be less than the assumed value leading to somewhat higher efficiencies and better economic results. 3 - RESULTS AND DISCUSSION In Figure 5 we compare the IRR for the three cases above (Gas Turbine, Gas Engine and No ICM) versus the cost of natural gas considering two prices for the electricity to be sold by the plant, EUR 60 and EUR 70 per MWhe. Natural gas varies from EUR 10 to EUR 40 per MWhth. For comparison we have included the conventional, NO OCC, WTE configuration using 40 bar/400 C live steam parameters leading to a net efficiency around 22%. The IRR for the conventional WTE plant configuration are 7.75% and 13.2%, respectively, for the two electricity prices considered. In the financial analysis we used a different CAPEX for each case ranging from EUR 90 million, for the NO OCC case, to EUR 103 million for the GT configuration. In Brazil NG prices for power generation are close to EUR 25/MWhth (USD 10/MMbtu) while in USA this drops to under EUR 10/MWhth (USD 4/MMbtu). For an electricity price of EUR 70/MWhe, the GT case is better than GE for NG cost from EUR 10 up to EUR 27 presenting no advantage over conventional plants when NG is higher than EUR 32. For GE this goes up to EUR 35 and for NO ICM case, i.e. with just the DB, it is always better to have OCC regardless the NG cost. For the electricity price of EUR 60, the GT case is better than GE for NG cost from EUR 10 up to EUR 22 presenting no advantage over conventional plants when NG is higher than EUR 27. For GE this goes up to EUR 30 and for NO ICM case it is always better to have OCC regardless the NG cost. It is interesting to note that the GE case will always stay in between GT and NO ICM case but the GT case presents a sharp dependency on NG cost. This dependency is graphically shown in Figure 5 by the negative slope of the straight lines which in the limit become horizontal for the NO OCC case since natural gas is not used. For Brazilian NG costs the choice seems to be limited between GE and NO ICM cases and the suggestion is to design the plant with GE and in case natural gas prices have a sharp increase we can turn it off since the superheating does not depend on the GE exhaust. Of course combustion air pre-heating would be done with additional steam turbine extraction. To further illustrate the advantages of OCC we can introduce a reheat steam cycle similar to what is done in the Amsterdam WTE plant [8] using steam drum extraction to reheat the steam leaving the HP steam turbine plus OCC duct burner and GE as shown in Figure 6. 6

7 Figure 5 IRR Comparison between GE, GT and no ICM OCC Solutions Figure 6 OCC with Reheat Steam Cycle using Drum Extraction [8] OCC is also ideally suited to optimize an integrated waste management plant, i.e. a site where all kinds of waste are processed in an integrated way, profiting as much as possible from synergies between the different units. In areas without source separation a sorting plant may be used to recover the recyclable fractions (paper, glass, plastics, etc.). Otherwise the MSW is directly deposited in the bunker. Organic matter, source separated and/or from the sorting plant, is anaerobically digested. The biogas from that plant is used to supply the GE or GT in the OCC 7

8 setup, as above. Residual heat may be used for sludge drying or other applications, further improving the overall plant efficiency. INTEGRATED WASTE MANAGEMENT PLANT heat Sludge Sludge dryer WWTP Water City waste Pretreatment AD Digestate BIOGAS Kitchen & Restaurant Waste Pretreatment Impurities, oil OCC Power, heat MSW Bunker WtE Fe metals Bottom ash FGC residues option Sorting plant Recyclables Central odour treatment plant WTP Figure 7 How OCC fits in a total waste management complex 4 CONCLUSIONS In this work it was shown how a very simple modification in the steam cycle of WTE power plants can bring outstanding improvements on economics using the Optimized Combined Cycle (OCC) concept. The introduction of a small quantity of natural gas, biogas or landfill gas, when available, can turn projects originally economically unfeasible into a profitable one. Using one of many possible configurations based on the OCC concept it is always advantageous over conventional design no matter the cost of natural gas. It should be noted that besides a pressure increase in the MSW boiler, desirable but not essential, the method does not require any changes in the original conventional MSW boiler design using moving grate technology. The natural gas share can be as low as 4% and never higher than 25% with the same efficiencies found in the modern plant of Zabalgarbi in Bilbao, Spain where 78% of the energy comes from natural gas. The low dependency on natural gas, an OCC feature, is a key factor when making the decision to use or not use NG in WTE plants. In order to decide to apply the scheme the first step should be to determine the amount and the cost of the natural gas at the site and then make a configuration choice among the options shown: gas turbine (GT), gas engine (GE), duct burner only (DB) in single pressure steam cycles; or GE, DB only in reheat steam cycles. OCC provides a design tool in a WTE project that allows us looking at the problem to find the solution that satisfies some pre-determined boundary conditions instead of trying to fit the problem into an established solution. 8

9 REFERENCES [1] Petrov, P. M., Biomass and Natural Gas Hybrid Combined Cycles Licentiate Thesis 2003 Department of Energy Technology Division of Heat and Power Technology Royal Institute of Technology, Stockholm, Sweden. [2] Branchini, L., Advanced Waste-To-Energy Cycles Research Doctorate Università di Bologna, Final exam [3] Martin, J., Global Use and Future Prospects of Waste-to-Energy Technologies Fall Meeting Columbia University, Oct.7-8, [4] Korobitsyn, M.A., New and Advanced Energy Conversion Technologies. Analysis of Cogeneration, Combined and Integrated Cycles Laboratory of Thermal Engineering of the University of Twente [5] Bohórquez, W.O.I., Análise Termoenergética, Econômica e Ambiental da Repotenciação e Conversão de UTEs com Ciclo Rankine para Ciclo Combinado Utilizando Turbinas a Gás - Tese de Mestrado em Ciências em Engenharia da Energia Universidade Federal de Itajubá MG, [6] Guerreiro Ribeiro, S., Kimberly, T., High Efficiency Waste to Energy Power Plants Combining Municipal Solid Waste and Natural Gas or Ethanol - NAWTEC-18 Orlando, Fl 2010 [7] Kamuk, B. Is De-Nox by SCR to Be the Future in Us? Technology and Tendencies within APC-equipment - NAWTEC-17 Chantilly, Virginia 2009 [8] Simões, P., Amsterdam s Vision on the 4th-generation Waste-2-Energy - CEWEP Congress - Bordeaux, June 12 th 2008 [9] Main, A., Maghon, T., Concepts and Experiences for Higher Plant Efficiency with Modern Advanced Boiler and Incineration Technology - NAWTEC-18 Orlando, Fl 2010 [10] Nishioka, T., Introduction of Waste Power Generation with Natural Gas Repowering System Joint Meeting of IEA Bioenergy Task 32, 33 and 36 Tokyo, 28 - Oct, 2003 [11] Consonni, S., Silva, P., Off-design Performance of Integrated Waste-to-Energy, Combined Cycle Plants Applied Thermal Engineering 27 (2007)