POTENTIALS OF POLYGENERATION IN DISTRICT HEATING SYSTEMS

Transcription

1 POTENTIALS OF POLYGENERATION IN DISTRICT HEATING SYSTEMS Nguyen Le Truong Linnaeus University Växjö, Sweden Leif Gustavsson Linnaeus University Växjö, Sweden ABSTRACT Operating capacity of district heat production depends on heat load demand which varies throughout the year. In this paper we analyze polygeneration of district heat, electricity, wood pellet and motor fuels. Our analysis is based on the district heating system in Östersund, Sweden. The annual measured heat load is used as the reference heat load duration curve. We calculate the production costs and primary energy use of generated products for different polygeneration concepts. Furthermore, we investigate how the scale of district heating systems influences the production costs of polygeneration. Costs are calculated under the Swedish taxation scheme. The optimal production system comprises different production units that meet the district heat demand following the reference heat load duration curve and at the same time minimize the district heat production cost. We optimize and integrate different production units into the district heating system step-bystep and evaluate the value of generated refined fuels. We scale the capacity of the system up and down to examine the influence of scale on the production costs. 1. INTRODUCTION Polygeneration of different energy carriers by integration of different energy conversion units has been proven to give primary energy, environmental, and cost benefits [1]. The application of such integration exists in various systems such as combined heat and power (CHP) units in district heating plants, steam and power production in industry. District heating (DH) with integrated units is a primary energy efficient system [2]. DH can contribute to a sustainable energy development through improved energy system efficiency and replacement of fossil fuels by renewable energy sources such as biomass. In Sweden, DH is the main source of heat for multi-dwelling buildings, non-residential and public buildings [3]. The total amount of heat delivered in district heating systems (DHS) in 28 was close to 5 TWh [4]. District heat production in Sweden has shifted from fossil-based to biomass-based fuels during the last three decades [3]. In 28, biomass covered 48% of the total fuels for heat production [5] and the share is expected to further increase especially with the application of combined heat and power (CHP) plants [6]. The present gross effective power-to-heat ratio of the Swedish DHS is low, only.13, compared to the actual potential [7]. The role of DH in Sweden may increase [8] and various studies have suggested different ways to increase its performance. Knutsson et al. [8] analysed the potential expantion of CHP generation in DHS. Wetterlund and Soderstrom [9] and Difs et al. [1] showed that the introduction of biomass gasification to produce motor fuels in district-heat production brings economic and CO 2 benefits due to the ability to increase the production of high value products. Marbe and Harvey [11] suggested the integration of biofuel gasifiers in natural gas combined heat-and-power plants The integration of biomotor fuel with district-heat production can be a suitable combination as the production process of bio-motor fuels requires heat and electricity while a certain amount of low temperature heat is discharged that can be used for district-heat supply [12]. This may lead to an overall costefficient system [13]. Wood pellet production may also be integrated with district heat production. The Swedish market supply of wood pellets, a suitable solid fuel for small-scale heating applications, is increasing [14]. The production of wood pellets requires electricity for processing and a significant amount of heat for drying. Therefore, the integration of pellet production with processes with a surplus of heat may improve the overall energy efficiency of such systems and reduce its production cost. The production of 1

2 biomass-based motor fuels and wood pellet integrated with district heat production can be a multi-benefit solution due to: i) the longer process utilization time of district heat production units; ii) the improvement of the system efficiency; iii) the operation flexibility during the year; iv) the diversification of generated products; and v) the reduction of fossil fuel dependence due to the production of fossil-fuel substitutes. In this study, we analyze the potential to integrate the production of district heat, electricity, wood pellets and bio-motor fuels under the Swedish taxation schemes. We consider production costs and primary energy use. Furthermore, we investigate how the scale of district heating systems influences the production costs. 2. METHOD AND ASSUMPTION Our analysis is based on the district heating system in Östersund, Sweden. The annual measured heat load (Fig. 1) is used as the reference heat load duration curve. Based on the types of production units (Table 1) we design a least-cost district heat production for the given heat load duration curve [15]. Capacity (MW) Day Fig. 1: Reference heat load duration curve We consider, step-by-step, the production of district heat, wood pellets and motor fuels along with co/polygenerated electricity. The selection of district heat production units is based on the utilization time and minimum production cost of each considered products. The optimal production cost of the previous optimization is then used for the later optimization. We calculate the production costs and primary energy use considering fuel cycle energy input of generated products from different production concepts. TABLE 1. DISTRICT HEAT PRODUCTION UNITS AND REFERENCE CONDENSING POWER PLANTS Technology Reference Investment Fixed O&M Variable Efficiency Scale scale costs costs O&M costs heat electricity fuel factor Heat-only boilers (MW heat ) ( /kw heat ) ( /kw heat ) ( /MWh fuel ) - Biomass a Coal b Fuel oil a Wood powder c CHP units - BST b BIGCC a CST b NGCC a Stand-alone power plant (MW elect ) ( /kw elect ) ( /kw elect ) ( /MWh fuel ) - BIGCC a BST d CST b CST with CCS b NGCC b Motor-fuel production (MW fuel ) ( /kw fuel ) ( /kw fuel ) ( /MWh fuel ) - DME e Methane e Wood pellet production f Note: BIGCC: Biomass integrated gasification combined-cycle; BST: Biomass-based steam turbine; CST: Coal-based steam turbine; CCS: Carbon capture and storage; NGCC: Natural gas combined-cycle. a [16] with adjustment for the difference in investment cost between [17] and [16]; b [17]; c [18] with 17% adjustment; d Estimated from [16]; e [19] and [16]; f [12]. The analyses are done in three steps. First, we optimize the production cost of a district heat production system based on a CHP plant. The value of cogenerated electricity is equal to the least cost of producing electricity in condensing power plants based on technologies presented in Table 1. The optimal production system comprises different production units that meet the district heat demand following the reference heat load duration curve and at the same time minimize the district heat production cost. Next, we cost optimize a district 2

3 heat production system that polygenerate district heat, electricity and wood pellets based on calculated district heat production cost and value of cogenerated electricity in the first step of the analysis. This allows us to calculate the pellet production cost for an optimized system in the second phase. In the third step, we cost optimize a district heat production systems that polygenerate district heat, electricity, wood pellets and motor fuel, either dimethylether (DME) or methane, based on the calculated district heat and pellet production cost as well as the calculated value of cogenerated electricity in the first and second steps of the analysis. This allows us to calculate the motor fuel cost for the optimized systems in the third phase. Data of considered production units is presented in Table 1. Costs of fuels are based on the year 29 Swedish prices and including energy, carbon dioxide and sulphur taxes. Table 2 shows the used fuel costs. We also consider the average value of green electricity certificate (GEC) of 26./MWh [3] in the analysis. This GEC is applicable for electricity produced from renewable energy sources for a maximum period of 15 years [2]. TABLE 2. FUEL COSTS ( 21 /MWh) [3, 14] Fuel type /MWh - Coal 59.6 / 26.5 * / 18.8 ** - Diesel oil Forest fuel Gasoline Heating fuel oil Natural gas 63.3 / 44.1 * / 39.6 ** - Wood powder 27.8 *** * For CHP plants ** For electricity production *** Estimated. Equivalent to 1% of forest fuel, based on [14] & [18] We use the subtraction method to estimate the primary energy dedicated for each polygenerated product [21]. Primary energy use for co/polygenerated electricity is assumed to be equivalent to the primary energy use from stand-alone power plant. In the first step of the analyses, the left-over primary energy after subtracting the amount for cogenerated electricity is credited for district heat. Primary energy use of wood pellets and motor fuels is estimated after take into account the primary energy use for coproduced electricity and district heat. Production cost of district heat depends on system scale. The minimum yearly economic district heat production is about 12 GWh, for a system with a CHP unit [5], a fifth of the reference district heat production. A large district heat production system in Sweden deliver annual about 38 GWh of district heat [5], which is 6 times larger than the considered reference system. We consider the influence of scales to the production cost of district heat by assuming that the size of reference heat load is five times smaller and 6 times larger, respectively, of the reference system scale. We assume that all the production units are scalable and their energy efficiencies are the same as for the reference scale. Investment costs of production units are scaled according to their scales using the given scale factors. For all the production units, we assume a discount rate of 6%, an economic plant life of 25 years and a maximum operating period of 72 hours per year. In our calculations, exchange rates of EUR/SEK = 9.6 and USD/SEK= 7.2 are used. 3. RESULTS 3.1. Reference stand-alone power plant Using the year 29 Swedish fuel prices and taxes and the benefit of GEC, a BST condensing power plant is the cheapest option among the considered condensing power technologies and is selected as the reference power plant. The electricity production cost from this condensing power plant is 42.2/MWh Cost-optimal district heat production system The production cost of a district heat unit depends on the utilization time. Fig. 2 presents the production cost of district heat from considered heat-only boilers and CHP units. The figure shows that CHP-BIGCC has the lowest district heat production cost when the utilization time is more than 2 days a year. Whereas, for short time operation of less than 1 day/year, the oil boiler give the lowest production cost and wood powder and biomass boilers for a medium operating period. Heat production cost ( /MWh) Boiler - Fuel oil Boiler - Wood powder CHP - CST CHP - BST Boiler - Coal Boiler - Biomass CHP - NGCC CHP - BIGCC Utilization time (day/yr) Fig. 2. District heat production cost of different production units as a function of the utilization time The cost optimal district heating system comprises four different units, a CHP-BIGCC unit for the base load, a biomass boiler and a wood powder boiler for the medium load and an oil boiler for the peak load. Fig. 3 shows how these units produce the yearly district heat for the reference district heat load. 3

4 Table 3 presents the characteristic of this cost-optimal district heat production system. The power to heat ratio of this cost-optimal system reaches.78 due to the BIGCC technology. Oil boiler cover 13.7% of the total installed capacity but the heat production is only.3% of total heat production. The cost-optimal system has an annual average district heat production cost of 23.8/MWh. TABLE 3. CHARACTERISTICS OF THE COST-OPTIMAL DISTRICT HEAT PRODUCTION Production unit Installed capacity Utilization time Production (GWh) Primary energy use (MW heat ) (hrs/year) Heat Electricity (GWh fuel ) Boiler-Oil Boiler-Wood powder Boiler-Biomass CHP-BIGCC District heat capacity (MW) Boiler - Fuel oil Boiler - Wood powder Boiler - Biomass CHP - BIGCC Day Fig. 3. Contribution of production units in a cost-optimal district heat production system The integration of a wood pellet production unit changes the district heat production system. Table 4 presents the cost-optimal district heat production system with wood pellet production. The installed capacity of CHP-BIGCC unit increased 4.5% whereas that of wood powder boiler decreases 14.8% compared to the cost-optimal system without wood pellet production. At a given district heat production cost of 23.8/MWh, the cost for produced pellets is 25.4/MWh. A stand-alone wood pellet production system with internal heat production boiler gives a production cost of 28.4/MWh of wood pellet. This is 12% higher than that for wood pellet produced in an optimal district heat production system. TABLE 4. COST-OPTIMAL DISTRICT HEAT PRODUCTION SYSTEM WITH WOOD PELLET PRODUCTION Production unit Installed capacity Utilization time Production (GWh) Primary energy (MW heat ) (hrs/year) Heat Electricity Wood pellet use (GWh fuel ) Boiler-Oil Boiler-Wood powder Boiler-Biomass CHP-BIGCC Wood pellets * 3. (24.5) * Number in parentheses show the capacity in MW fuel The motor fuel production requires that the CHP unit is replaced as base-load unit. Table 5 shows the cost-optimal system with different motor fuel production options. DME and methane production options require external electricity but motor fuel-to-heat ratios of the corresponding district heating system reach 3.69 and 2.3, respectively. TABLE 5. COST-OPTIMAL DISTRICT HEAT PRODUCTION SYSTEMS WITH MOTOR FUEL PRODUCTION Production unit Installed capacity Utilization time Production (GWh) Primary energy (MW heat ) (hrs/year) Heat Electricity Motor fuel use (GWh fuel ) DME production Boiler-Oil Boiler-Wood powder Boiler-Biomass Motor fuel Methane production Boiler-Oil Boiler-Wood powder Boiler-Biomass Motor fuel

5 The pellet production prolongs the utilization time of the motor fuel production units and therefore increases the amount of produced motor fuels. Also production cost of motor fuels is slightly reduced, as shown in Table 6. If DME and methane are used to substitute diesel oil and natural gas respectively, it is feasible to polygenerate these bio-motor fuels in DHS. Production costs of DME and methane are 47.7% and 77.3% compared to their fossil-based counterparts. TABLE 6. PRODUCTION COST OF MOTOR FUELS ( /MWh) Motor fuel type Without wood pellet production With wood pellet production DME Methane Primary energy cost of polygenerated products Fig.4 presents the primary energy use per unit of polygenerated products (MWh of fuel per MWh of products). District heat has a low primary energy use per produced unit. This is because the cogenerated electricity substitutes electricity in a stand-alone power plant. Of the two considered motor fuels, the methane requires less primary energy per produced unit than the other. Primary energy use (MWh fuel /MWh) Electricity* District heat Wood pellets DME Methane Product Fig. 4. Primary energy use of polygenerated products 3.4. Influences of system scale Fig. 5 shows the changes in production cost of different polygenerated products for different scales of the district heat production. Production cost ( /MWh) DME Methane Wood pellets District heat x.2 x.5 Reference x 3 x 6 Scale of district heating system Fig. 5. Production cost of polygenerated products The district heat production cost is reduced by -34.3% and increased by +37.6% compared to the reference scale, whereas the cost variation for pellet production is more marginal, -3.8% and +1.5%. The corresponding figures for produced motor fuels are -6.2% and +11.2%. 4. DISCUSSION AND CONCLUSIONS In this study, we have explored the potential of producing district heat, electricity, wood pellets and bio-motor fuels in district heat production systems. Large quantity of polygenerated products can be produced from district heat production system. Composition of a cost-optimal district heat production depends on several factors such as the available technologies and price of fuels. Under the Swedish taxation scheme, a cost-optimal district heat production based on CHP plants is mostly based on biomass. Fossil oil covers only.3% of the total heat production via peak load boiler even though the installed capacity covers 13.8% of total production capacity exclusive reserve units. The potential to cogenerate electricity is large. A cost-optimal district heat production may have a power-to heat ratio of.78, which is 6 times higher than the current average value of Swedish district heat production. Primary energy dedicated for district heat is low when the cogenerated electricity replaces electricity produced in stand-alone plants. This gives a primary energy efficient supply system that helps to reduce primary energy use. Methane and DME production units have high potential to be integrated in district heat production systems. These motor fuels have low production cost and have large produced amount. The integrated production of wood pellet helps to reduce its production cost compared to stand-alone production and increase the amount of polygenerated products. Production cost of polygenerated products depends on system scale and in particular the district heat production cost. The study showed that biomass-based district heating system could contribute to improved system energy efficiency and at the same time generate fossil fuel substitutes. REFERENCES (1) Serra, L.M., et al., Polygeneration and efficient use of natural resources. Energy, (5): p (2) Gustavsson, L. and B. Johansson, Cogeneration: One way to use biomass efficiently. Heat Recovery Systems and CHP, (2): p (3) Swedish Energy Agency, Energy in Sweden 29, 21, Swedish Energy Agency, Sweden. p Web accessed at on May, 21 5

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