Pulverized Pellet-fired Heat Plant for Rapid Load Control of CHP Networks

Transcription

1 Pulverized Pellet-fired Heat Plant for Rapid Load Control of CHP Networks Tero Joronen Boilers and Combustion Manager Metso - MW Power *) Timo Heinonen Development Manager Tampereen TET **) *) Lentokentänkatu 11, P.O. Box 109, FIN Tampere, Finland first.lastname@metso.com **) Voimakatu 15, Tampere first.lastname@sahkolaitos.fi

2 ABSTRACT: The purpose of this paper is to introduce an application of pulverized wood pelletfiring in district heating. The application offers an alternative for fossil fuel based peak and backup heat generation and it can be utilized instead of the more commonly used oil- and gas-fired solutions. The starting, ramp up and shut down times are comparable to these traditional solutions. Wood pellets offer low CO 2 alternative and additional fuel in the portfolio increases flexibility and offers cost saving in the fuel management. The Tampere case is especially interesting due to the large-scale utilization of pellets, and rapid load changes. The Tampere case is a wood pellet-fired district heating plant for Tampereen Energiantuotanto Oy. The delivery comprised a full-scope turn-key boiler plant solution, including all the necessary equipment, and commissioning. The heating plant is used as a peak load and backup plant and it will replace some of the existing oil- and gas-fired capacity. Replacing old boilers with a modern pellet-fired boiler will significantly reduce the CO 2 emissions resulting from the production of district heating. The plant s thermal input is 33 MW with pellets, and it started the heat production in the end of year The burner has a light oil firing possibility with 47 MW firing rate. Pulverized fuel allows clean, energy-efficient and flexible heat generation during start-ups, and quick load changes during the production. The plants control capability fulfills the requirement of the district heating network of the Tampere. Flue gas emissions requirements are met; wood pellet is fired with low CO and NO x emission. The plant efficiency and the availability are high. Due to its beneficial properties, pulverized pellet combustion is an expanding technology in the field of biomass-based energy generation. Wood pellet Concern of the CO 2 emission boosts renewable energy production. The biomass combustion offers an alternative that is totally independent of weather conditions at all hours. Pellet-firing offers a high energy density and tidy alternative to fossil fuels that can be utilized in urban surroundings. Pellets are mainly utilized in small-scale central heating application, co-fired in pulverized coal combustion or special pellet-fired burners (Obernberger & Thek, 2010). The pellet-fired burner application offers flexibility and capacity comparable to the oil and gas fired applications. Figure 1. The wood pellet is a high-quality fuel with a constant quality and properties. Pellets are high-quality biomass fuel that provides a consistent alternative to conventional fuels like coal, oil and natural gas. Pellets are a good alternative as they are low in moisture, high in energy density and homogenous in size and shape (Obernberger & Thek, 2010). The characteristics of wood pellets compared to wood chips are significantly better; higher calorific power, lower moisture content, apparent density and ash content (see Table 1). The energetic content per unit volume is about four times better for pellet compared to wood chip. This makes the application of wood pellets feasible also in urban environments as high amounts of thermal energy can be stored

3 in relatively small volumes compared to wood chips. The pellets are delivered by trucks that can offload pneumatically to enclosed silo, thus there are no dust emissions to the environment from the fuel handling. Table 1. Main characteristics of wood pellets and wood chips (Giacomo & Taglieri, 2009) Wood pellets Wood chips Calorific power 17.0 GJ/t 4.7 kwh/kg 3080 kwh/m³ 13.4 GJ/t 3.7 kwh/kg 750 kwh/m³ Water 8% 25% (Finland 45 %) Apparent density 650 kg/m³ 200 kg/m³ Ashes content 0.5% 1% The pellets are available globally for a competitive price and simple and sufficient fuel logistics. For instance, the English markets have developed to enable the larger-scale utilization of wood pellets and provide increasing potential for greenhouse gas reduction (Hansen;Jein;Hayes;& Betaman, 2009). In addition, on the Mediterranean region, Giacomo and Taglieri found the potential of woody pellets to be significant and the applications would be economically and environmentally feasible (Giacomo & Taglieri, 2009). Pellet-fired burner applications The advantages of entrained pulverized pellet-fired combustion are the availability of low NOx combustion, good load control and the possibility of fast alternation of load. The disadvantages are the relatively higher variable and fixed costs due to limited particle size and the requirement of a start-up burner (Swithenbank;Chen;Zhang;& Sharifi, 2011). In the normal implementation the pellet firing is integrated into a multi-fuel burner that starts as an oil-fired burner and after heating up converts to pellet firing (Forsberg, 2012). The fuel flexibility decreases the dependence of the single fuel. One major reason to invest in a pellet-fired heating plant is to reduce the utilization of fossil fuels in peak, back-up and industrial boiler plants. The retrofitting existing oil & gas application to the multi-fuel solution offers a feasible potential. The pellet firing has a low environmental impact, as the pellet-fired plants are almost CO 2 emission free. Naturally some CO 2 production results from the harvesting, production and transport of the pellets, but these emissions are far lower compared to oil combustion. The possibility to implement fast load changes and start-ups makes it possible to apply pellet-fired boilers almost as flexibly as oil- or gas-fired boilers. Some limitations come with the lower energy density compared to oil. Larger storage volumes are required and stocks need to be refilled quite frequently. However, no dust or odor emissions emanate from the plant as the fuel handling is completely closed system. The pellet-fired burner is very reliable, the final pellet feeding is achieved by using pressurized air and it is practically as reliable as the applications in pulverized coal combustion. The operations of the burner are fully comparable to those of oil burners. For instance, the ignition of the burner is

4 fully automatic, as well the operation of the whole plant. The plant is remotely controlled and is unmanned. The utilization of pellets is feasible if the price of alternative peak load fuel (typically oil or natural gas) is high enough and the gained CO 2 reduction has additional value though CO 2 trading. The price level of the pellet, due to higher taxation of fossil fuels, for instance, in Sweden the cost per MWh being approximately close to 40 % of that of oil (Lehtinen, Marjaana, 2012). The pellet market has extra production capacity available. The Tampere heat plant Metso owned MW Power has delivered pellet-fired burners for environmentally friendly applications for several years already. A green solution was developed based on market demand. Renewable energy from biomass is obviously the way of the future. Pellet firing is somewhere between oil and wet biofuel with regards to complexity, cost, fuel variation, and other factors. The delivery includes complete process equipment with all the major components, including the fuel handling, fuel mill, burner, boiler, exhaust filters and control system. Figure 2. Basic process arrangement in a pellet-fired heating plant In the solution, the pellet fuel is stored in separated storage silos. The pellets are pulverized in a separate grinding mill that blows the pulverized pellets into a smaller dust silo. From the dust silo the wood dust is metered by a dosing device into a pressurized air feeding line (Figure 2). Pulverized and consistent fuel quality allows complete combustion with limited CO emissions. The pellet-fired burner has a turning rate of approximately 1:4. The high boiler efficiency is possible as the water content of the fuel is low. The low variation of the fuel characteristics makes it possible to design the burner for flexible heat generation. This makes it possible to control the boiler load in a wide range at 10% per min, thus the time from the minimum to the maximum load is only 7.5 minutes. The load change rate is in many cases network-limited as the temperature and flow can t be changed as fast as the rate of change for the burner and the boiler.

5 The boiler and flue gas cleaning is specially designed for wood dust combustion. Furnace heat loading and arrangement of heat exchange surfaces are take care of combustion circumstances to ensure high availability, stable combustion, low emission, and fouling in the boiler. A special combustion model was developed to simulate the combustion and boiler performance. The technology partner in the combustion technology is World Thermal Service Ab (WTS) from Sweden. WTS has proven reliable and solid performance in combustion power and flue gas emission requirements. Applied powder burners are available for a power range of 2 50 MW th. The particle size of all of the pellet dust must be below 1 mm, and 70% below 0.5 mm, and moisture content below %. The reference lists the flue gas emission ranges as follows; NO x emissions are mg/nm³ and for CO in the range of 0 50 mg/nm³ (Forsberg, 2012). Whenever handling dry pulverized wood, there is possibility certain fire risk. This issue is taken carefully into consideration within the concept. There are several special combustion detectors situated in critical locations within the pulverized pellet supply system. The detectors automatically activate the water mist spray to limit the temperature to prevent a fire from developing. An effective means of protection is to neutralize the small amounts of energy (ignition sources) before they manage to ignite the fine particles (See for instance; Firefly, ). The Tampere plant is a wood pellet-fired district heating plant to Tampereen Energiantuotanto Oy. The company is a subsidiary of Tampere Power Utility (Tampereen Sähkölaitos, TKS) and responsible for the group s electricity and district heating production and maintenance of the systems. The pellet-fired plant is a good addition to their long-term heating plant palette. Increasing the proportion of renewable energy sources is part of their strategy. For example, in recent years, they have continuously increased the use of biomass in their Naistenlahti 2 BFB fired power plant unit (Lehtinen, Marjaana, 2012). The new heating plant will utilize MW Power s pellet firing solution, district heating and plant delivery expertise (See Figure 2). Advantages for district heat with pellet fired plant District heat networks are built in city areas where heat demand is dense enough for economical business. In smaller networks, heat production units are used due to smaller investment cost. In bigger networks, combined heat and power (CHP) production units with high energy efficiency are economical. In this case, the heat plants are applied as peak load capacity and reserve. Traditionally in Finland heat plants are oil, gas, wood chips or peat fired. Nowadays wood pellets are also available globally and offer a feasible alternative. The economical trends affect the heat plant market, for instance, Europe is now in recession. In the recession, extra electricity production capacity is available while the use of it decreases bringing the price of it down. The heat production becomes more feasible as the production of the electricity is not as profitable. When times are tight, application of local work and materials decreases the variable cost. There are also affect to the local economy, money rotates in homeland, bringing jobs and welfare. In the city of Tampere, the district heat is gas-based. The Naistenlahti CHP plant uses quite a lot of woodchips and peat. Heat is product mainly on gas turbines and in the heat recovery boilers connected to them. In general, the price of gas follows the oil price and keeps on rising. In Finland,

6 the tax of the heat production depends on applied fuels. Due to rising gas price and tax and decreasing electricity price often running the gas-fired CHP plants is not profitable. This is dependent of the heat load, and it might be high, and this makes the pellet-fired heat only plant easily feasible. Applying wood pellet increases the use of sustainable energy and helps fulfilling the renewable energy requirements. Thus wood pellets have no extra tax on heat production. The total price of pellet-based heat is lower than gas-based, but higher than wood chip-based. When the annual operating hours of the heat plant are limited the investment on more expensive woodchip boiler is not feasible. Pellet is an economical alternative when annual operating hours are between 500 and The lower limit comes from the comparison to the heavy oil firing, and above the higher limit wood chip-fired boiler becomes cost-effective. Having several optional fuels in the production portfolio increases the economical flexibility when the fuel prices go up and down. Picture 1. An screen shot of the plant automation, flue management The delivery comprised a full-scope turn-key boiler plant solution, including all the necessary equipment, tailored Metso DNA automation system, fully automatic control application, and commissioning. The heating plant is used as a peak load and backup plant and it will replace some of the existing oil- and gas-fired capacity. Replacing old boilers with a modern pellet-fired boiler will significantly reduce the CO 2 emissions resulting from the production of district heating. Some technical parameters are listed in Table 2.

7 Picture 2. The view of the plant from the gate

8 Table 2. The technical data of the Tampere pellet-fired district heating plant Thermal output Annual production Start-up of heat production Load control 33 MWth (pellet) and 47 MW (light oil) 28,500 MWh (estimation) By the end of 2012 Modulating operation Peak load and back up boiler plant Turn down ratio 1:4 Load change rate 10%/min for the burner and the boiler Fuel storages Pellet silos 2x500 m³ Pellet dust 50 m³ Boiler efficiency over 92% Flue gas cleaning Electrostatic precipitator Figure 3. The side view of the Tampere pellet-fired boiler Operational data The plant has been in operation since the first pellet fire 16th December The plant has been over taken after the tuning and testing period. The flue gas emissions are meeting the emission limits and contract. Table 3. Some operation data Variable Plant load Flue Gas CO Flue Gas O2 Value and unit 35 MW th 50 ppm 4,5 % The plant operation has been easy and is done remotely from the Naistenlahti Power Plant control room, 10 kilometers away. The plant master controller (district heating power or boiler water temperature) alters the boiler master that controls the fuel feed and combustion air. Table 3 shows an operational display of the plant automation.

9 Conclusions Pulverized pellet firing offers a sustainable, high availability, safe and fully automatic alternative for district heating in larger district heating plants. The plant can be used to compensate the heat load requirements. Compared to other combustion technologies it offers fast load variation with moderate variable and fixed costs. High energy density and easy fuel handling makes it possible to use pellets also in urban surroundings. The Tampere plant shows an economical and technically feasible example of the very latest development in the field of renewal bioenergy. The plant solution was feasible for 500 to 3000 annual operating hours on full load. References Firefly. (July 1, 2012). Viewed in 2012 at Changing the World of Fire Protection. Forsberg, B. (2012). Biomass powder burners. Technical datasheet. Nyköping, Sweden: WTS Ab. Giacomo, G. D. & Taglieri, L. (2009). Renewable energy benefits with conversion of woody residues to pellets. Energy, Hansen, M. T.; Jein, A. R.; Hayes, S. & Betaman, P. (2009). Pelletatlas, English handbook for wood pellet combustion. Knowlhill: FORCE Technology/ EUBIA/ National Energy Foundation. Klimstra J. and Hotakainen M. Smart Power Generation The Future of Electricity Production. Wärtsilä Finland. Avain Publishers, Helsinki. 4th Improved edition. ISBN Printed in Arkmedia, Vaasa 2011 Koskela T., Orhanen J. and Heinonen T., Metso toimittaa Suomen suurimman pellettilämpölaitoksen Tampereen Energiantuotanto Oy:lle (In Finnish, Metso deliver the biggest pellet fired heat plant for Tampereen Energiatuotanto Inc.), Kraft and Drift, ISSN , 2/2012 p. 8, MIKTOR Helsinki, Finland Lehtinen, Marjaana. (2012). A Pellet-fired Heating Plant. Results Power, 1, Metso & Lehtinen. (1, 2012). A pellet-fired heating plant. results/power, Metso's customer magazine for power business line, pp Obernberger, I. & Thek, G. (2010). The pellet handbook. London: Earthscan. Swithenbank, J.; Chen, Q.; Zhang, X. & Sharifi, V. (2011). Wood would burn. Biomass & Bioenergy 35,