Advanced Steam Parameters in a Large Scale CFB Application Operating on REF and Biofuels background and experiences

Transcription

1 Advanced Steam Parameters in a Large Scale CFB Application Operating on REF and Biofuels background and experiences Matts Strömberg Soderenergi AB, Sweden Presented at Power Gen Europe 2011 Milan, Italy June 7 9, 2011 TP_CFB_11_05

2 Advanced steam parameters in a large scale CFB application operating on REF and biofuels background and experiences Mats Strömberg, Söderenergi AB, Sweden 2

3 ABSTRACT The Swedish municipality owned utility company Söderenergi AB signed in June 2007 the supply contracts for their new CHP block (IKV) based on a 240 MWth, 540 ºC, 90 bar cocombustion CFB boiler. The plant was taken over in December 2009 and after the first operating season the experiences are now summarized for a first feedback check keeping the investment goals in mind. The main driver for the investment was to add electricity production to secure the long-term competitiveness of district heat deliveries, for economical feasibility but also for environmental focus. Important criterias to address during the development/permitting period were a very wide fuel flexibility with co-firing possibility, high steam data giving a high electrical efficiency, low emission levels, high boiler efficiency as well as a high availability. The chosen solution is based on a CFB concept with the final superheating in sand lock super heaters allowing the high selected steam data, considering the difficult design fuels. Other features decreasing the risk of high temperature corrosion are an empty pass before the convective super heaters and furnace feeding of sulphur granulate. The total plant output is ~73 MW electricity (net), ~151 MW district heating from the turbine condenser and ~58 MW district heating from the flue gas condenser, giving approximately 110 % total plant efficiency, calculated from fuel LHV and over 90% calculated from fuel HHV. The boiler is designed to fire a fuel mixture including maximum 25% REF pellets and 75% biofuels, as well as firing up to 70% demolition wood together with 30% biofuels. The delivery of fuels to the production site are handled mainly by ship and trucks. To optimize the fuel access a buffer station, reachable by train, is built. 3

4 1. INTRODUCTION Söderenergi produces district heating for some people, offices and industries in the southern Stockholm area and with the new CHP, called IKV, in full operation from 2010 enough electricity to power homes. Söderenergi is owned by the municipalities of Södertälje, Huddinge and Botkyrka. Heat production totals approximately GWh a year. Of this, GWh is delivered to Telge Nät and Södertörns Fjärrvärme AB, which sell and distribute heating to end-customers in Huddinge, Botkyrka and Södertälje. The remainder is delivered to Fortum Värme for distribution in central Stockholm, with which Söderenergi has a production cooperation agreement. Under this agreement, Söderenergi supply heat to areas of Stockholm during the winter and get heat when the Igelsta plant is shut down for maintenance in the summer. Söderenergi produces both power and heat in the Igelsta CHP plant and only heat in four other facilities: the Igelsta district heating plant in Södertälje, the Fittja plant in Botkyrka, Huddinge Maskincentral in Huddinge and Geneta Panncentral in Södertälje. Figure 1. DH production with the new CHP, IKV, in operation. The figure doesn t include export of heat to Fortum which is up to 800 GWh a year. 4

5 The main production units are situated at Igelsta, close to Södertälje, where this new CHP have been built. Figure 2. Igelsta plant areas sketch, including the new CHP plant (IKV). The fuel is stored in three storage silos at the yard, of which one is used for REF pellets and the two others are used for demolition wood and biomass fuels. REF pellets and biomass/demolition wood is mixed before being fed to the fuel day silo in the boiler house. The district heating network is the third largest conterminous network in Sweden and the new Igelsta combined heat and power plant is Sweden s largest bio-fuelled co-generation plant. The plant produces 200 MW of heat including heat from the flue gas condensing plant and 73 MW of net electricity output. The basis for making the investment in a new CHP, was to produce electricity to secure the long-term competitiveness of district heat and to do this in a way that was both economically feasible and environmentally responsible. 5

6 2. INVESTEMENT BACKGROUND Söderenergi had studied the economic viability of a new thermal power plant based on biofuels since the early 1990's. The electricity price was too low to have an adequate income from electricity generation. A survey conducted in 2004/2005 showed a promising feasibility because of the introduction of the Swedish electricity certificate system for renewable fuels but also because of higher electricity prices. The Söderenergi owners therefore took a decision in 2005 to continue the preparations for an investment in 2006/2007.The final decision was taken in May 2007 andthe plant was ready for commercial operation from the end of the year Söderenergi s assessment of future income from power generation was with the assessment of future fuel prices the most important parameters in the decision of investment. 3. FUELS During the 1990`s, for their DH production, Söderenergi switched from coal and oil to firing mainly bio-fuels and fuels derived from recovered waste materials. Thus, Söderenergi has a long experience of heat production with these types of fuels. An important main target of the CHP-project was to combine the goal of high fuel flexibility with regard to fuel economy and supply security with a high share of electricity generation. The choice of the main fuels with regard to availability, price and transportation-related issues, for the boiler inquiry, was: wood residues such as branches and tops with moisture %, recovered fuels such as pelletized REF (source selected industrial waste consisting of paper, plastic and wood) with moisture 3-12% and demolition wood with moisture 5-40%. 4. PLANT CONCEPT Söderenergi developed the plant concept together with the boiler supplier Foster Wheeler, who provided input primarily in the areas of the firing concept selection, the feed water temperature and the steam parameters in relation to various combinations of challenging fuels Boiler with auxiliary equipment In this application, to allow the selected advanced steam data in firing the difficult design fuels, the chosen concept is based on circulating fluidized bed technology, CFB. To further 6

7 adopt for the waste and recycled fuel fractions and minimize the corrosion risk Foster Wheeler has used several systems from their concept of waste firing boilers, such as empty pass, horizontal pass for the convective super heaters and fully retractable steam blowers in high temperature areas. The boiler key parameters are: Thermal output 240 MWth Steam data Steam flow Fuel LHV range 90 bar / 540 C 92 kg/s 6-16 MJ/kg Fuel moisture 14-60% The most important features in the IKV CFB boiler concept are the described in the following chapters Söderenergi AB, Igelsta IKV 240 MWth, 92 kg/s, 90 bar, 540 C Figure 3. Boiler cross section. 7

8 Intrex TM sand lock super heater The use of a CFB concept with the final superheating in sand lock super heaters (Intrex TM ) is maximizing the fuel flexibility and will minimize the risk of high temperature corrosion and fouling of the convective super heaters. The Intrex TM super heaters are located in the fluidized chamber between the furnace and separator. This environment is non-corrosive and the efficient heating increases the steam temperature from 420 C up to 540 C Furnace and separators The CFB concept with two integrated separators provides a high combustion efficiency and due to the turbulence in the furnace and the separators the flue gases are effectively mixed with oxygen. With a low combustion temperature together with a long residence time the emission levels, such as NOx, CO, TOC and UBC are controlled and kept at low levels. The base level of NO X in a CFB is low but due to the emission trade program in Sweden there is an economic incentive to further reduce the NO X emissions. Because of that an SNCR system is in operation. Considering the difficult recycled fuels the furnace bottom is constructed as a step grid with sex parallel bottom ash outlets for an optimized removal of unfluidized material Empty pass The empty pass upstream the convective pass reduces the flue gas temperature down to 700 C to further reduce the corrosion risk for the super heaters. This pass is a part of the water cooled cycle and is neither refractory lined nor overlay welded. To save the super heaters from fouling particles there is an ash hopper by the end of the empty pass Horizontal convective super heater pass The convective super heater packages are located in a separate horizontal pass, with two individual ash hoppers below for separation of fouling particles. This design also admits an easy exchange of the super heaters packages if needed. The flue gas entering the horizontal pass has been cooled down to around 700 C. Hence, as the final super heating is performed in the sand lock super heaters, the steam temperature in the horizontal pass is modest and less than 420 C and thus there is a reduced risk of corrosion. 8

9 Sulphur feeeding To further increase the corrosion protection in the IKV boiler a system has been added for feeding sulphur granulate into the furnace. The sulphur reacts with alkalis, preventing the formation of alkali chlorides, which are known as the major substances causing high temperature corrosion in convective super heaters Overall plant concept Plant concept In order to recover heat from the boiler flue gases a flue gas condensation plant (FGC) is installed downstream the ID fan. The facility consists of two lines each containing a flue gas condenser and a humidifier. In the FGC cooled flue gas and water vapor from the moist biofuels condenses and the heat transferred to the district heating water. The saturated gas is then cooled further with combustion air in the rotating humidifier of Ljungström type. Condensate is transferred in humidifier for combustion air which gives a higher water content in the flue gas, which ultimately results in higher heat output in the flue gas condenser. For operation without the FGC plant the flue gas ducting is equipped with a by-pass.. The FGC plant supplier was Radscan Intervex AB, Sweden. The turbine plant consists of a 84 MW el axial turbine with dual steam inlet. The turbine has a two stage district heating condenser, one low pressure pre-heater and one high pressure preheater. With a rotation speed of 3000 rpm, the turbine has no gearbox, which provides higher efficiency and less noise.. The supplier of the turbine package was Siemens AG, Germany. The main plant process is outlined in the following figure. 9

10 Figure 4. IKV, CHP process flow sheet Overall plant output The overall plant output, when firing the design fuel mix at 100% MCR, corresponding to 240 MWth, is summarized as per following: - Net electric output, considering auxiliary consumption: 73 MW - District heating from the turbine condensers: 151 MW - District heating from FGC plant with humidifier in operation: 58 MW - Totally 282 MW plant energy output. This high energy output is possible to reach using the flue gas condensing and humidified combustion air as well as further maximized by recovering heat from cooling of blow down, bottom ash and water/steam sampling to the return DH water IKV emission levels Based on the design selections, the plant shall contractually fulfill the following emission levels: Particles 10 mg/nm 3 SO 2 75 mg/ nm 3 NO X 35 mg/mj CO 50 mg / nm 3 NH ppm TOC 10 mg/ nm 3 HCl 10 mg/ nm 3 HF 1 mg/ nm 3 Cd + Tl 0.05 mg/ nm 3 Hg 0.05 mg/ nm 3 Heavy metals 0.5 mg/ nm 3 Dioxines + Furanes. 0.1 ng/ nm 3

11 Fuel quality management An important factor for an operation with high availability is a proper fuel quality management. Control and testing of the fuels are performed both in the fuel preparation process as well as at the receiving station. Follow-up s based on combustion process measurements, such as HCl content and ash analysis, are continuously carried out. 5. EXPERIENCES 5.1. Project The project execution went very well and the objectives were met as for both time schedule and budget. The start-up went better than expected particularly with regard to how quickly the boiler reached full capacity, performance and fuel flexibility. The coordinated safety and environmental work at the construction site was planned and verified in collaboration between Foster Wheeler and Söderenergi. No serious accidents were reported and in view of the high number of workers (600), a lot of work at height, handling of heavy lifting and adjacent operating activities at the existing plant, the safety work was a success. The performance test was conducted also with good margins with reference to the contractual guarantees for both capacity, energy and environmental performance Performance experience The thermal efficiency of the boiler has been around 91%. The annual averages of emissions in 2010: - NOx about 62 mg/nm3 which corresponds to about 22 mg / MJ input fuel energy. - SO2 around 7 mg/nm3. - CO 5 mg/nm3. During the current operating season 2010/2011 an average of the fuel mix is summarized to 72% of wood residues, 23 % demolition wood and 5 % pelletized REF (industrial waste). 11

12 5.3. Availability Availability calculated, according to the requested output and with planned stops excluded, were from January 2010 to April %. Two serious disturbances have occurred: - 7 days shutdown in September 2010 because of tube leakage in left cyclone. The cause was a poor seam in the refractory which led to the sand-blasted holes in the tube. - 8 days shutdown in April/May 2011 due to a tube leakage in the pass to the right cyclone. The reason was erosion of the refractory. No corrosion/erosion of the super heaters and furnace were detected during the inspections at the summer shutdown Concluding comment In view of that the goals for time schedule and budget were met, despite the extreme economic boom during the execution, the project is considered as very successful. The expectations of Söderenergi have by far been met, exemplified also by the plant performance and the good cooperation with the suppliers. REFERENCES Slotte M, Large scale co-firing of REF and biofuels in a CFB with advanced steam parameters and very high plant efficiency, PowerGen Europe