EPSRC THERMAL MANAGEMENT OF INDUSTRIAL PROCESSES

Transcription

1 EPSRC THERMAL MANAGEMENT OF INDUSTRIAL PROCESSES Case Study: Sheffield District Heating (December 2010) Report Prepared by: SUWIC, Sheffield University Researcher: Dr Q. Chen Investigators: Professor Jim Swithenbank Professor Vida N Sharifi Sheffield University Waste Incineration Centre (SUWIC) Department of Chemical and Biological Engineering Sheffield University 1

2 Executive Summary Energy sustainability and greenhouse gas emissions have become critical international social issues. Among all the state-of-the-art technologies, combined heat and power (CHP) can help address both issues with efficiency better than separate heating and electrical generating plants. Sheffield district heating network is one of the largest and most successful CHP schemes operating in the UK. By harnessing the energy from this local energy recovery facility, the district heating system provides an economical, low carbon and environmentally friendly heat source to businesses; householders and local authority in Sheffield. In accordance with our EPSRC grant proposal, Sheffield University has carried out a techno-economic feasibility study of Sheffield District Heating system. Two key parameters were investigated: i) energy efficiency and ii) reduction in CO2 emissions. Some main findings from this study are as follows: 1. By reclaiming a certain amount of the by-product heat for heating purposes, combined heat and power technology can achieve the energy thermal efficiency higher than 75%. Recovery of energy from MSW for heat and power generation can reduce the CO 2 emissions by around 70,000 tonnes per year. 2. The recovery of low grade latent heat from water vapour in the flue gas can greatly increase the thermal efficiency of the CHP plant from 75% to 93%. It is possible to achieve higher efficiencies if the temperature of return water temperature in the system is lowered to about 30C. Condensation of flue gas not only helps to recover certain amount of low grade latent heat, but also saves up to 90,000 tonnes of CO2 emissions per year. 3. Solid Recovered Fuel (SRF) and Refuse Derived Fuel (RDF) have much better fuel qualities (e.g. calorific value) when compared to Municipal Solid Waste (MSW). The electrical and thermal efficiencies of SRF-fired CHP systems are higher than those of MSW-fired plants. The net CO 2 emission reduction by SRF (80,000tonnes/year) is greater than those by MSW in a CHP system. 4. A simplified cost analysis is performed to evaluate potential benefits for an MSW fired CHP/DH system using SRF as a fuel. The capital cost for the proposed MBT facility is around 57.3 million. Given no time value of money, the payback period for this replacement is approximately 13 years. 2

3 List of Contents 1. Introduction CHP and District Heating Solid Recovered Fuel Sheffield District Heating System Energy-from-Waste System District Heating Scheme Emissions Scenario Analysis Base Cases: Fossil Fuel Fired Power Generation and Heating Systems Case I-A: Coal-fired Power Plant Case I-B: Gas-fired Condensing Boiler for Residential Heating Case II: MSW-fired CHP System Case III: SRF-fired CHP System Efficiencies of Energy Conversion and Utilisation Environmental Impacts Reduction in CO 2 Emission (Energy Recovery from MSW) Reduction in CO 2 Emission (Energy Recovery from SRF) Influence of Flue Gas Condensation on CO 2 Emissions Impacts on Other Flue Gas Emissions Economic Analysis Capital Cost of MBT Facility for SRF Production OPEX and CAPEX for SRF Production Benefits References

4 1. Introduction Energy sustainability and greenhouse gas emissions have become critical international social issues. Energy demand across the world continues to grow in long term. The effect of energy production and usage on the global environment has triggered increasing concerns worldwide. To address these issues, alternative energy sources and technologies have gradually attracted more and more attention. Governments target for reductions in carbon dioxide emissions and an increase in the percentage of electricity generated by renewables. For example, the UK government has published its Low Carbon Transition Plan, which sets out how the UK will cut CO 2 emissions by 34% of 1990 levels by 2020 and at least 80% by 2050 (HM Government, 2009). In this plan, the government expects 40% of the power used in 2020 to come from low carbon sources 30% from renewables, the rest from nuclear (including new build) and clean coal. For homes and communities, around 15% of the yearly emission cuts between now and 2020 will be achieved by making homes more efficient and supporting small scale renewable energy. Among all the state-of-the-art technologies, combined heat and power (CHP) can achieve both targets with efficiency better than separate heating and electrical generating plant. District heating supplied by a CHP plant increases the overall thermal efficiency from approximately 50% for the best electricity generating plants (CCGTs) to approximately 85% for a CHP plant. This can potentially reduce CO 2 emission by 30%. On the other hand, municipal solid waste (MSW) generally represents almost 20% of the total energy needs of a city. Therefore, when MSW is used as fuel for a CHP plant, the combination can contribute a major reduction to net CO 2 emissions. 1.1 CHP and District Heating The EU Cogeneration Directive (EU 2004) defines CHP as delivering minimum levels of primary energy savings, with savings of 10% required for most CHP capacity. This legal requirement, which must be met to qualify for most forms of public support, is enacted in the UK through the CHP Quality Assurance (CHPQA) programme (CHPA 2010). Generally, CHP systems are based predominantly upon existing, proven power generation technologies: steam turbines, gas turbines and reciprocating engines used the world over to generate energy. This use and adaptation of existing technology not only contributes to the relatively low cost of CHP, but also ensures that it is a proven and reliable technology, capable of delivering an immediate impact in transforming our energy system. Connected to a district heating network, CHP can provide heat and power to multiple customers in city centres, towns, villages, industrial zones and other built environments with a dense heat load, this being a high concentrated demand for heat. Up to 2007, over 4,000 CHP and district heating utilities are operating in towns and cities across Europe. 1

5 District heating is a convenient way to heating space and tap water. In many processes, for example when electricity is generated or waste is burned, large parts of the energy are set free in the form of surplus heat. The fundamental idea behind modern district heating is to recycle this surplus heat which otherwise would be wasted- from electricity production, from fuel and biofuel-refining, and from different industrial processes. A district heating system is essentially composed of a network of insulated pipes used to deliver heat, in the form of hot water or steam, from the point of generation (source) to an end user. The network provides a means to transport heat efficiently. The carrier pipe system is mainly of steel but other materials such as plastics are used. The distance a network can reach is easily extended by simply adding more providers of heat, or heat sources, along the way (CHPA 2010). In addition to energy from fossil fuels, heat sources for district heating scheme include, Waste heat from power generation. Heat produced by the incineration of municipal waste. Reject heat from process industries. Landfill gas extraction. Geothermal (hot rocks) and thermal springs. Biomass - Agricultural and Animal waste products. Heat pumps. Fuel cells and solar thermal arrays The ability to integrate diverse energy sources implies that end users are independent upon a single source of supply. This helps guarantee service reliability and continuity of the system. District heating networks also have the ability to balance the supply and generation of heat, across location and over time. Over the course of the day, heat demand shifts between residential consumers to commercial, industrial and public buildings and back again. A heat network can match and manage these flows, whilst maximising the utilisation of the plant providing the heat. Demand can also be managed across seasons, with networks supporting the operation of distributed absorption cooling plants in the summer providing cooling on a significant scale (CHPA 2010). Heat sources can be either directly connected to the distribution system or indirectly connected through a heat exchanger. The direct system is limited to use where water is the distribution medium and where the water quality and pressure requirements are the same for the heat source and the building s internal distribution system. The indirect connection allows the heat source and the distribution system to be operated as separate systems with different temperature and pressures, allowing more design flexibility for both systems. The district heating medium (steam or hot water) is distributed from the heat source through supply pipes to the end users interface and is returned after heat has been 2

6 extracted. Delivery is accomplished by circulating pumps which create a pressure differential between the supply and return pipes. Pumps are selected to overcome the flow resistance in the supply and return pipes and also the pressure differential in the customer installation at the end of the system or the index point. The use of variable speed drives to control the pumps ensures that consumed power is minimised. Figure 1 illustrates the principle of variable pumping (Skagestad and Mildenstein 1999). In areas where the ground level varies dramatically, it is important to ensure that a minimum pressure is maintained in the return pipe to avoid evaporation and cavitation in equipment such as pumps and valves. Direct district heating systems typically operate with flow and return temperatures of 85/65 C and pressures of below 6 bar, and indirect systems with temperatures of 110/65 C and pressures of below 16 bar. The greater the temperature-difference between the flow and return, the lower the flow rate required. Figure 1 District heating network pressure diagram Figure 2 Pipe diameters in relation to temperature difference 3

7 In Figure 2, difference in pipe sizes is compared when operating with alternative temperature differences for two heating capacities. The district heating operator will seek to ensure that the secondary return water temperature is as low as possible to minimise pumping. It is common practice to compensate the district heating medium supply temperature. When the heating demand decreases, its supply temperature should be decreased to reduce energy losses from the pipe system. By this adjustment, the energy efficiency using low-grade heat sources can be increased. Figure 3 presents a typical example showing the variations in the supply temperature depending on the outdoor temperature. Figure 3 District heating compensation curve of the supply temperature For the pipe networks, there are a number of different types of pipe material available on the market. The vast majority of systems are based on pre-insulated steel pipes. In smaller dimensions, the media pipe may be made of stiff or flexible plastic pipes. The district heating medium supply temperature is often limited by the type of pipework used. A common supply temperature range is 85 to 120 C. The low end of the range is normally the temperature required to meet domestic hot water needs during the summer. Pressures can go up to 25 bar but the majority operates with a maximum pressure of 16 bar, while 25 bar is common in transmission systems. By reducing the normal operating temperature and by reducing the effects of pressure fluctuations, the life of the pipework can increase dramatically. Figure 4 illustrates this phenomenon. 4

8 Figure 4 Relationship between expected life of pipe and continuous operating temperature District heating can serve residential, public and commercial buildings as well as meeting industrial demands for low-temperature heat. Building systems may be connected directly or indirectly to a district heating distribution system. With a direct connection, the heating medium is distributed within the building to directly provide heat to terminal equipment such as radiators, unit heaters, etc. An indirect connection uses a heat exchanger in the building to transfer the energy from the district heating distribution system (primary system) to the building distribution system (secondary system). The heat exchanger serves as an interface between the district heating network and the building s own radiator and hot water system. There s no boiler, no burning flame needed in the house and maintenance is taken care of by professionals. Thus, compared to owning and operating an on-site boiler, conversion to district heating can benefit the end users through increased reliability, greater comfort, reduced investment, operating cost savings, increased energy efficiency and greater fuel flexibility. 1.2 Solid Recovered Fuel A major drawback to combustion of waste is that the fuel is likely to be 5

9 non-homogenous, damp and it will come in large fragments. The water content will lower the recoverable energy content per unit mass of fuel. The lack of homogeneity will make for inconsistent combustion which will cause fluctuations in emissions adding to the difficulty of cleanup, and it impedes designing for maximum efficiency. In some cases, the combustion process often neglects materials recycling which comes higher in the waste hierarchy, and so is not an ideal sustainable solution (WMAA, 2003). Turning waste into refuse derived fuel (RDF) or solid recovered fuels (SRF) is one of the options available for waste treatment that can both reduce the volumes of waste sent to landfill and simultaneously recover embodied energy from the waste material (Arias-Garcia and Gleeson, 2009). SRF and RDF are fuels produced from non-hazardous municipal solid waste (MSW) and commercial and industrial (C&I) wastes. The term SRF is commonly used in place of RDF. SRF is a refined form of RDF, intended for use in energy recovery facilities, which has been produced to meet a standard published by the European Committee for Standardization (CEN) standard, CEN/TS The specifications and classes are shown in Table 1. Table 1 CEN/TS Solid recovered fuels specifications and classes The input waste can be production specific waste, municipal solid waste, industrial waste, commercial waste, construction and demolition waste and sewage sludge. SRF represents an interesting route to the development of CHP infrastructure. Some of the benefits of SRF fired CHP are (Arias-Garcia and Gleeson, 2009): the facilitation of an easier planning process, increased overall energy efficiency, flexibility of location, volume, security of supply, price and 6

10 the possible development of supply chain for waste wood disposal and co-firing. During SRF production, plants take bulk waste and remove recyclable or non-combustible materials, the remainder then being dried and shredded or processed into a uniform fuel. This fuel has a calorific value much higher than that for municipal waste. SRF can take various forms including a loose or flock material, which has been size-reduced or further densified to produce a fuel pellet, the final form of SRF is dependent on the mode of energy recovery. Consequently, there are many methods for producing SRF. These may include some or all of the following processing systems: screening, air classification, dry, pelletising, magnetic recovery (Wilen 2004, Chen et al. 2008). Figure 5 Flow diagram of SRF production from household waste and commercial waste (Wilen 2004) Figure 5 presents a flow diagram of a typical SRF production plant. Roughly source separated household waste passes through a fairly complicated production process including operations like crushing, magnetic separators, screening, eddy-current for non-magnetic materials, pneumatic separation and optic sorting. The purpose is to separate the impurities (typically biowaste, glass, metals, aluminium, PVC) as much as possible and to produce good quality SRF to be used in energy recovery plants. As commercial waste generally contains little biowaste or fine impurities, the sieving of the pre-crushed waste is usually bypassed (Wilen 2004). 7

11 In general, SRF or RDF will burn in an incinerator cleaner and hotter. The process will therefore be slightly more efficient and will need less of a cleanup operation. RDF incineration is considered a more environmentally sound option for MSW incineration, a life cycle analysis described by Ferrer et al. (2005) outlines RDF incineration over mass burn as more favourable option: Life cycle analysis favours RDF combustion over mass burning because of the better environmental performance. The potential growth in RDF incineration is illustrated by future plans for Finland, the National Waste Management Plan shown abandonment of mass burn method and unsorted MSW to Landfill, instead focusing on source separation, resource recovery and the utilisation of RDF (Strafford 2006). 8

12 2. Sheffield District Heating System Sheffield district heating network is one of the largest and most successful CHP schemes operating in the UK. It has been developed around a municipal solid waste (MSW) incinerator located close to the city centre since By harnessing the energy from this local energy recovery facility, the district heating system provides an economical, low carbon and environmentally friendly heat source to businesses; householders and local authority in Sheffield. It is built using the latest technology and is designed to maximise the efficient generation of combined heat and power for the city s residents (Veolia, 2010). On average, Sheffield residents produce over 240,000 tonnes of waste every year. Non-recycled waste collected in Sheffield is taken to the energy recovery facility (i.e. the MSW incinerator) of the district heating system where it is burnt at temperatures of over 850 C in a specially controlled environment. A network of pressurised hot water pipelines under the city is integrated with the incinerator to recover heat from household waste. Owing to this innovation, the city sends a relatively low level of waste to landfill compared to most other regions in the UK. In 2001, Veolia Environmental Services singed a 35-year waste management contract with Sheffield City Council and is responsible for maintaining the waste collection and plant operation and services (Veolia, 2010). 2.1 Energy-from-Waste System In Sheffield CHP system, the heat from MSW incineration is converted to steam and used to generate electricity and for districting heating. The incinerator is designed to handle 225,000 tonnes of municipal solid waste a year (Veolia, 2010). Figure 6 shows the process flow diagram of the energy recovery system. Waste (1) from households, local authority services and some local businesses is brought to the energy recovery facility. It is tipped into a waste storage bunker (2). From the bunker the waste is lifted into a feed hopper (3) by an overhead crane (fully automatic grab loading crane) at a rate of 28 tonnes per hour. The hopper feeds the waste into a single incineration unit where it is burned at temperatures in excess of 850 C. Gas fired auxiliary burners are used to ensure that the correct temperature of 850 C is reached before any waste can be fed into the incinerator (Veolia, 2010). Above the incinerator, a large CNIM 4-pass vertical boiler (5) produces superheated steam at 400 C. A condensing steam turbine (10) uses this 40bar steam to generate electricity for the National Grid and to produce hot water (11) for the district energy network. Pressure take-offs from the condensing steam turbine allow a variety of combinations to be used to optimise the use of energy between heat and electricity. Air cooled condensers, sized for full load rejection, allow the thermal cycle to be completed with the minimum environmental impact (Veolia, 2010). 9

13 (a) (b) Figure 6 Process flow diagram of the energy from waste facility (Veolia, 2010) Urea (4) is introduced to the furnace to treat NOx (Oxides of Nitrogen) emissions. Lime and activated carbon (6) is introduced to neutralise the acidity of the flue gas and adsorb other pollutants. The cooled flue gases pass through a filter house (7) where the particulate (dust) is captured by 1760 filters. Particulate collected in this process is then stored in a silo for separate disposal later. Cleaned gases (8) are then released through the chimney. These gases are continuously monitored to ensure they meet strict environmental regulations. An electromagnetic overband separator (12) removes metal from the ash. The metal is delivered to a local company for recycling. Ash (13) from the incineration process goes into a bunker. Particulates removed from the filtering process are taken to a process plant for treatment and then safe disposal. Table 2 summarises some technical data for the MSW incineration plant. 10

14 Table 2 Technical summary of the energy-from-waste system (Veolia, 2010) Total plant capacity 225,000 tonne of MSW Bunker storage capacity 2,700 tonne of MSW Plant throughput rate MJ/kg 72 MW Grate Martin reciprocating, 5 rows, 13 steps Steam flow rate 86 tonne/hr Steam pressure 40 bar Steam temperature 400 C Maximum electrical output 19 MW Maximum thermal output 60 MW Chimney height 75 m Gas fired auxiliary burners 2 20 MW 2.2 District Heating Scheme At Sheffield CHP system, the plant generates up to 19MW of electricity for the national grid, enough to power up to 22,600 homes. Up to 60MW of heat is supplied to over 140 buildings connected to the district heating network so far. Currently, these include 3 university campuses, 4 swimming pools, 3 theatres, 3 art galleries, 2 cinemas, 1 radio station, 1 glasshouse, 25 hotels, 21 private developments, other local authority housing and corporate buildings, etc. Over 2,800 dwellings have benefited from district energy in the Sheffield area. The flow diagram of the CHP system is shown in Figure 7. Figure 7 Flow diagram of the Sheffield CHP system (Veolia, 2010) The district heating system provides buildings in Sheffield City centre and the 11

15 surrounding areas with a low carbon energy source that is generated from MSW in a central location, converted to hot water and pumped through a network of underground pipes and delivered to a heat exchanger in buildings of all sizes and types. There are currently 44km of pipeline installed across the city centre through two networks. The system is supported by back-up facilities with 3 pre-heated stand-by/peaking boiler stations ready to come on line at a moments notice with 84.6 MW of capacity. These back-up boiler stations consists of 5 gas and 4 oil-fired boilers in total. Figure 8 shows schematically the process diagram of the district heating system. In a typical year around 120,000 MWh of heat is delivered to buildings in Sheffield City Centre and the surrounding areas. Table 3 summarises the technical data for the district heating system (Veolia, 2010). Figure 8 Districting heating scheme (Veolia, 2010) Table 3 Technical data for the Sheffield district heating system (Veolia, 2010) Hot water temperature 120 C Water pressure 16 bar Pumps in the distributed pipework system 15 Capacity of backup boilers 87 MW 2.3 Emissions The MSW incinerator is operating under the regulation of EU Waste Incineration Directive and the Pollution Prevention and Control (PPC) regime (Defra 2009). Table 4 presents the air emission limit values for the Sheffield CHP plant to meet. The reference conditions are: temperature 0 C, pressure 101.3kPa and 11% oxygen 12

16 dry gas. Figure 9 presents the monthly-averaged daily emission values from the MSW incinerator. As can be seen, the missions are well below the limit values. Table 4 Daily average emission limit values for the incinerator (Defra 2009) Pollutant Emission limits Units Dust 10 mg/m 3 Total organic carbon (TOC) 10 mg/m 3 HCl 10 mg/m 3 CO 50 mg/m 3 SO 2 50 mg/m 3 NOx 200 mg/m % of Emission Limit Dust TOC HCl CO SO2 NOx /09 03/09 05/09 07/09 09/09 11/09 01/10 03/10 05/10 07/10 09/10 Figure 9 Air emissions from the Sheffield CHP plant in recent years (Veolia, 2010) 13

17 3. Scenario Analysis Sheffield University has carried out a series of calculations (using Sheffield CHP plant as a case study) in order to investigate the merits of CHP-DH (combined heat and power generation with district heating). The application of low grade heat recovery and use of SRF as a fuel were also considered. Some primarily economical analysis was also conducted. Table 5 presents brief descriptions of all 3 cases. Table 5 Summary of the case studies Case Category Description Based on Drax Base case A Coal-fired power generation system Case I Power Plant Base case B Residential gas-fired condensing boiler 30kW A MSW-fired CHP system for power generation only B MSW-fired CHP system for heat supply only Case II MSW-fired CHP system for district Based on C heating Sheffield CHP MSW-fired CHP system for district Plant D heating with lower return water temperature Case III SRF-fired CHP system for district heating 14

18 Figure 10 CO 2 emissions from SRF preparation, transportation and combustion (Gaillarde 2008) These cases can be compared with each other in terms of their contribution to the greenhouse gas emissions from the plant. Only an assessment encompassing the whole energy cycle from conversion to delivery (thus including transportation) can give a realistic picture (Euroheat & Power 2007). However, as shown in Figure 10, CO 2 emissions from the preparation and transportation of SRF are minor when compared to emissions from SRF combustion. Therefore, in this work, only the emissions from SRF combustion and related efficiencies were considered. 3.1 Base Cases: Fossil Fuel Fired Power Generation and Heating Systems As shown in Table 5, these two base cases are based on a coal-fired power plant and a natural gas-fired residential heating boiler. The base cases correspond to separate power generation and heating plants that are widely used throughout the world. Energy efficiencies and CO 2 emission factors were calculated as reference in this case study Case I-A: Coal-fired Power Plant This case is based on Drax Power Plant, the largest coal-fired power station in the UK (Drax 2010). It consists of 6 power generating units. Each unit has a capacity of 660MW, giving a total capacity of 3960MW. Although it is capable of co-firing biomass and petcoke, coal is assumed to be the only fuel in this case study for the 15

19 sake of simplicity. Drax Power Station generates 7% of electrical power required by the UK. At full output, it consumes around 36,000 tonnes of coal a day (Drax 2010). The coal fuel comes from a mixture of both domestic and international sources, with domestic coal coming from mines in Yorkshire, the Midlands and Scotland, and foreign supplies coming from Australia, Colombia, Poland, Russia and South Africa. Each of the six Babcock Power boilers supply superheated steam (16.6MPa and 563 C) to a steam turbine set. Each steam turbine consists of one high pressure (HP) turbine, one intermediate pressure (IP) turbine and three low pressure (LP) turbines. One HP turbine generates 140MW of electricity. Exhaust steam (4.2MPa and 360 C) from HP turbines is fed back to the boiler and reheated (4.02MPa and 565 C), then fed to the 250MW IP turbines and finally passes through the 90MW LP turbines. Table 6 presents assumptions used in the calculations. Table 7 gives the proximate and ultimate analysis results (BCURA 2002) for the Daw Mill coal used in the plant. The calculation results show that the electricity output is approx. 4000MWe and the net electrical efficiency of the plant is around 33% correspondingly. As a result, the CO 2 emission factor for the power station is approx. 0.26kg/MJ (or 0.938kg/kWh). In 2007, Drax produced 26.66TWh electricity in total (Drax 2007). Thus, based on our calculations, the estimated CO 2 emission from the plant was around 25 million tonnes in This value is slightly higher than that reported by Drax, which was 22,160,000 tonnes in 2007 (Drax 2007). This overestimation could be due to the difference in fuel composition and slight underestimation of the electrical efficiency in the calculations. Table 6 Main assumptions used in the Case I-A Parameter Unit Coal feed rate 1500 t/hr Excess air 20 % Thermal energy loss in the combustor 5 % Main steam pressure 166 bar Main steam temperature 563 C Turbine efficiency 85 % Steam condenser pressure 0.05 bar Temperature of flue gas exiting the boilers 140 C Table 7 Properties of the coal supplied by Daw Mill Proximate Moisture 12.0 analysis Volatile Matter 33.5 (%ar) Ash 4.1 Fixed Carbon 50.4 Ultimate C 81.3 H

20 analysis O 11.5 (%, dry ash N 1.3 free) S 1.1 NCV (MJ/kg) Case I-B: Gas-fired Condensing Boiler for Residential Heating Condensing boilers have now replaced most of conventional designs in powering domestic central heating systems in Europe. In the UK, since 2005 all new gas central-heating boilers fitted in England and Wales must be high-efficiency condensing boilers. Condensing boilers are designed to capture a fraction of the latent heat, i.e., the energy released by condensing water vapour in the flue gas. By extracting this latent heat in the condensing boiler, the whole system can achieve higher efficiency levels than non-condensing boilers. Typical models of condensing boilers offer efficiencies around 90% (based on the lower heating value of fuels). Case I-B is based on a 30kW domestic condensing boiler for residential heating. Table 8 lists the assumptions made in the calculations. The fuel is natural gas with its properties as shown in Table 9. In this case, the dew point of the flue gas is approximately 56 C. The return water temperature is well below this dew point and a portion of the water vapour latent heat can be recovered. Table 8 Main assumptions used in the Case I-A Parameter Unit Heat input 30 kw Fuel feed rate 3.25 m 3 /hr Excess air 10 % Thermal energy loss in the combustor 2 % Return water temperature C Temperature of flue gas exiting the boilers 50 C Table 9 Assumed composition of natural gas in Case I-B (Uniongas 2010) Component Range Assumed value Methane, vol% Ethane, vol% Nitrogen, vol% Net calorific value, MJ/kg 44.7 Gross calorific value, MJ/kg 51.0 For this gas fired condensing boiler, the output heat capacity is 28.4kW. The thermal efficiency thus reaches 94.9% (based on lower heating value) or 83.1% (based on higher heating value of the fuel). The CO 2 emission factor is 17

21 approximately 0.06kg/MJ (or 0.23kg/kWh). 3.2 Case II: MSW-fired CHP System In this case, a series of thermodynamic calculations were carried out to obtain mass flow rates, temperatures and enthalpies for all the streams of the Sheffield CHP System using heat and mass balances. Table 10 lists the assumptions made for the calculations. Typical composition of British MSW was used to represent the properties of MSW for Sheffield CHP plant, as presented in Table 11. In the calculations, it was assumed that Sheffield CHP plant had a net energy input of 72MW th. Calculations were conducted for four different scenarios, namely, Case II-A: for electricity production only Case II-B: for district heating only Case II-C: combined heat and power for district heating Case II-D: combined heat and power for district heating with low return water temperature Table 10 Some assumptions used in the Case II Scenario A B C D Energy Input (based on LHV), MW MSW feed rate, t/hr Excess air, % Main steam pressure, bar Main steam temperature, C Turbine efficiency, % Pressure of steam condenser or take-off for heating, bar Steam temperature under the above pressure Temperature of flue gas exiting the boilers, C Pressure of hot water for district heating, bar Temperature of hot water for district heating, C Return water temperature, C Using the data presented in Tables 10 and 11, the electricity and/or thermal outputs of the plant can be calculated for each of the four scenarios. In Case II-A where all MSW is used to produce electricity, the total output is 19.0MW e with a net electrical efficiency of 26.4%. In Case II-B where all MSW is burned to produce heat for district heating, the thermal energy is approximately 60.3MW th with a thermal efficiency of 83.7%. These outputs are identical to those listed in Table 2. In the cases where both electricity and heat are produced, the electricity outputs are 18

22 both 7.94MW e (Cases II-C and II-D) and the electrical efficiencies are 11.0%. In Case II-C, the thermal output for district heating is 47.5MW th. The thermal efficiency is approximately 65.9%. As MSW has the moisture content as high as 31%, the latent heat of water vapour in the flue gas is quite high. This portion of low grade heat can be recovered using flue gas condenser as additional energy source for district heating. However, the operation of the flue gas condenser requires return water with low temperature below the dew point of the flue gas (55.7 C). Therefore, in Case II-D, the temperature of return water from the district heating system is only 30 C lower than in Case II-C. The flue gas from the incinerator (or Energy-from-Waste system) is used to preheat the return water. Thus, some latent heat in the flue gas can be recovered in this case. As a result, the thermal output in Case II-D is 59.2MWth with the thermal efficiency of 82.2%. As MSW consists of hydrocarbons, combustion of MSW is also a source of CO 2 emission. In all the four scenarios of Case II, the MSW feed rates are assumed to be the same (30t/hr). Consequently, the emission rates of CO 2 are identical (i.e. 24.3tonnes/hr ) for all the scenarios. In Case II-A where all MSW is used to produce electricity, the CO 2 emission factor is 0.355kg/MJ (or 1.28kg/kWh). In Case II-B where all MSW is burned to produce heat for district heating, the CO 2 emission factor is 0.112kg/MJ (or 0.40kg/kWh). 19

23 Table 11 Typical composition of British MSW (Optimat 2001) 3.3 Case III: SRF-fired CHP System In this case, the fuel for Sheffield CHP System is assumed to be SRF. Table 12 presents the fuel properties of RDF and SRF samples. As shown in Table 12, RDF and SRF have much lower moisture content and ash content than MSW. The carbon content and net calorific value of RDF and SRF are higher than MSW. As SRF is a refined form of RDF, the moisture and ash contents of SRF are slightly lower than RDF. Table 13 lists the conditions for the calculation in this case. Due to the low moisture content in the SRF fuel, the water vapour fraction in the flue gas after SRF combustion is also small. As the dew point of the flue gas is around 36 C, the latent heat of water vapour in the flue gas is very difficult to recover to preheat the return water from district heating system. 20

24 Table 12 Typical properties of RDF and SRF (Hernandez-Atonal et al. 2007; Dunnu et al. 2009) Sample (as received) RDF SRF Moisture (%ar) Volatile Matter (%ar) Ash (%ar) Fixed carbon (%ar) C (%daf) H (%daf) O (%daf) N (%daf) S (%daf) Cl (% daf) NCV, MJ/kg Table 13 Some assumptions used in the Case III Energy Input (based on LHV), MW 72 SRF feed rate, t/hr 10.1 Excess air, % 120 Main steam pressure, bar 40 Main steam temperature, C 400 Turbine efficiency, % 85 Pressure of steam condenser or take-off for heating, bar 0.5 Steam temperature under the above pressure Temperature of flue gas exiting the boilers, C 120 Pressure of hot water for district heating, bar 16 Temperature of hot water for district heating, C 120 Return water temperature, C 65 Based on mass and energy balances, the electricity output in this case is approximately 8.4MW e, corresponding to an electrical efficiency of 11.7%. The heat output for district heating is 50.4MW th and the thermal efficiency is thus 69.9%. In this case, the CO 2 emission rate is around 19.9t/hr. 3.4 Efficiencies of Energy Conversion and Utilisation Figure 11 summarises the electrical and thermal efficiencies calculated for all cases mentioned above. Cases I-B and II-B are two cases for heat production only. The energy losses are mainly due to the flue gas released to the atmosphere. Hence the thermal efficiencies for these two cases are well above 80%. As the gas-fired boiler in Case I-B is working with flue gas condensing, the efficiency is extremely high (around 93% based on LHV). 21

25 In Case I-A, as the parameters of superheated steam (temperature and pressure) are higher than those in Cases II and III, the electrical efficiency is therefore the highest among all the cases. In the cases of combined heat and power generation, steam is taken off from the turbine under 5bar in order to reject heat at a fairly high temperature to enable district heating. This lowers the overall plant electrical efficiency to 11% in the cases Cases II-C and II-D from 26% in Case II-A where the plant operates for electricity only. However, in cases where only electric power is generated, a large amount of heat is wasted and released to the atmosphere through cooling towers and flue gases. Thus, a maximum 26-35% is achieved for the overall plant (electrical) efficiency in these cases. By contrast, combined heat and power technology captures a certain amount of by-product heat for heating purposes. Although the electrical efficiency is inevitably reduced, the thermal efficiency is thus greatly increased. The overall energy efficiencies of the plant in Cases II-C, II-D and III are above 75% Thermal efficiency Electrical efficiency 70 Efficiency, % Case I-A Case I-B Case II-A Case II-B Case II-C Case II-D Case III Figure 11 Comparison of electrical and thermal efficiencies for all the cases It should be noted that the recovery of low grade latent heat from water vapour in the flue gas can greatly improve the thermal efficiency of the plant if the fuel has fairly high moisture content. As shown in Case II-D, the total plant energy efficiency is close to that from the domestic gas-fired condensing boiler. The achievement of this high efficiency requires sufficient low temperature of return water from district heating system. 22

26 3.5 Environmental Impacts The CO 2 emission rates and emission factors for all the cases are calculated and summarised in Table 14, together with the electricity and heat outputs. It should be noted that Case I is based on the combustion of fossil fuels whereas Cases II and III are based on waste combustion. Based on the data from Case I, some environmental impacts of MSW/SRF fired CHP are briefly discussed. The influence of low grade latent heat recovery on CO 2 emission reduction is also analysed. Table 14 Summary of the calculation results for all the cases Electricity, MW e Heat, MW th CO 2, t/hr Emission factor, kg/mj Base case A Case I Base case B A B Case II 24.3 C D Case III Savings in CO 2 Emission (Energy Recovery from MSW) As shown in Table 14, the emission factors (Cases II-A and B) from MSW-fired power and heat generation appear to be higher than those from the coal-fired power plant (Case I-A) and a gas-fired condensing boiler (Case I-B). However, various assessments have shown that about 20-40% (depending strongly on the degree of separate collection of paper and organic waste) of the carbon in MSW is derived from fossil sources, e.g., plastics (as shown in Figure 12). The remainder is derived from biomass and can be considered a renewable resource (IEA Bioenergy 2003). Consequently, the non-renewable amount of CO 2 emissions from MSW-fired power generation (Case II-A) is approximately 0.14kg/MJ or 0.52kg/kWh (i.e. 40% of the emission of 0.36 kg/mj). Similarly, the non-renewable amount of CO 2 emissions from MSW-fired heat production (Case II-B) is about 0.04 kg/mj or 0.16kg/kWh. These values are thus less than the CO 2 emission factors of the coal-fired power plant (Case I-A) and the gas-fired condensing boiler (Case I-B). Therefore, recovery of energy from MSW for power generation or heat production produces a net reduction in greenhouse gas emissions. In Case II-A, the total CO 2 emission reduction is 2.28kg/s, or over 70,000 tonnes per year. In Case II-B, the saving in CO 2 emission is thus 0.96kg/s, equivalent to approximately 30,000 tonnes per year. In Case II-C where both power and heat are produced from MSW, the total CO 2 emission saving is 2.21kg/s, which equals around 69,000 tonnes/y. 23

27 20% 20% 60% Renewable (biomass derived) carbon - 60% Variable fraction - 20% Non-renewable (fossil derived) carbon - 20% Figure 12 Sources of carbon content in MSW The calculated values of CO 2 emission reduction by no means take into account the emissions from MSW landfill. As the energy in MSW is recovered for power and heat generation, the emissions from traditional landfill are avoided. If the MSW was consigned to landfill then about 70kg of methane (actual range kg) could be released for each tonne of waste. Given the higher global warming potential of methane, this is equivalent to 1610kg CO 2 per tonne of MSW. In modern landfills about half of the methane can be extracted and used for energy production, therefore reducing the overall emissions (IEA Bioenergy 2003) Savings in CO 2 Emission (Energy Recovery from SRF) Given the same energy input for the plant, SRF gives less CO 2 emission rate than MSW, as shown in Table 14. The renewable carbon content in SRF is about 50-55% (Zucchelli 2009). Thus, the non-renewable CO 2 emission in Case III is approximately 9.55tonnes/hr. Consequently, the CO 2 emission saving is 2.55kg/s and the annual CO 2 emission reduction is around 80,000tonnes Influence of Flue Gas Condensation on CO 2 Emissions As shown in Figure 11, low grade latent heat recovery in Case II-D has an advantage in improving the overall thermal efficiency of the CHP system. This therefore results in greater CO 2 emission reduction than Case II-C. In Case II-D, the saving in CO 2 emission is 2.92kg/s. Thus, the annual CO 2 reduction is approximately 91,000tonnes. Figure 13 summarises all the calculated savings in CO 2 emissions for Cases II and III. As shown, the net CO 2 emission reduction by SRF is greater than those by 24

28 MSW in a CHP system. As flue gas condensation can recover certain amount of low grade latent heat, it leads to a significant reduction in CO 2 emissions of a system which recovers energy from MSW Avoided CO 2 emissions, 10 3 tonnes Case II-A Case II-B Case II-C Case II-D Case III Figure 13 Comparison of avoided CO 2 emissions among Cases II and III Impacts on Other Flue Gas Emissions In addition to the reduction of CO 2 emission, another major benefit associated with energy recovery from MSW is the reduction in emission of other gaseous pollutants. Table 15 compares the ELVs of some key pollutants from large-scale power stations with those from Waste Incineration Directive (WID). As can be seen, the ELVs for MSW incinerators are more stringent than those for coal-fired power stations. Using the best available techniques (BAT), the waste incineration industry has reduced its emissions over the last ten years by a factor of 10 or more due to enhanced legislative environmental controls (Last 2010). In particular, dioxin emissions have been reduced to well below those of other combustion process under the regulation of the WID. Table 15 Emission limit values of some pollutants from large-scale coal fired power plants (O 2 reference concentration: 6%) Pollutant Large-scale coal fired power plants* Derived ELVs for incinerators** Dust, mg/m Total organic carbon (TOC), mg/m

29 HCl, mg/m 3-15 CO, mg/m 3-75 SO 2, mg/m NOx, mg/m (200 after 2016) 300 * (EU 2001) ** Calculated from Table 4 from O 2 ref. concentration of 11% to 6% However, the Best Available Techniques (BAT) for flue gas treatment installed in most recent industrial units built in Europe have emissions that are often significantly lower than those imposed by law. Therefore, assuming that the emission factors must be equal to emission limits appears to be too optimistic (Consonni et al 2005). An up-to-date evaluation of environmental impacts can be achieved based on direct measurements carried out on state-of-the-art combustors. Tables 16 and 17 compare the emission factors from an Energy-from-Waste system with those from a coal-fired power plant and a gas-fired boiler. As shown, the emission factors of some pollutants, such as PM 10, NOx, NMVOC, SO 2 and HCl, from MSW combustion are lower than those from coal combustion. Table 16 Emission factors for a coal-fired power plant and a gas-fired boiler for domestic heating (Giugliano et al. 2008) Coal-fired power plant Gas-fired boiler for domestic heating CO 2 g/kwh CO mg/kwh PM 10 mg/kwh NO x (as NO 2 ) mg/kwh SO x (as SO 2 ) mg/kwh N 2 O mg/kwh HCl mg/kwh HF mg/kwh 38 - Cd µg/kwh Hg µg/kwh Pb µg/kwh 62 - NMVOC mg/kwh Dioxin (I-TEQ) pg/kwh 41 - Table 17 Emission factors (EF) for Energy-from-Waste systems in Italy and calculated EF for Cases II-A and B EF from combustion of 1tonne MSW* Calculated EF based on Case II-A** Calculated EF based on Case II-B** CO 2 (fossil) 425 kg/t MSW 671 g/kwh 334 g/kwh 26

30 SOx(as SO 2 ) 49 g/t MSW 77 mg/kwh 38 mg/kwh NMVOC 20 g/t MSW 32 mg/kwh 16 mg/kwh NOx(as NO 2 ) 855 g/t MSW 1350 mg/kwh 672 mg/kwh PM g/t MSW 19 mg/kwh 9 mg/kwh Dioxin(I-TEQ) 310 ng/t MSW 489 pg/kwh 244 pg/kwh Cd 61 mg/t MSW 96 µg/kwh 48 µg/kwh Hg 61 mg/t MSW 96 µg/kwh 48 µg/kwh Pb 610 mg/t MSW 963 µg/kwh 479 µg/kwh HF 4.3 g/t MSW 7 mg/kwh 3 mg/kwh Ammonia 12 g/t MSW 19 mg/kwh 9 mg/kwh HCl 43 g/t MSW 68 mg/kwh 34 mg/kwh N 2 O 100 g/t MSW 158 mg/kwh 79 mg/kwh CO 61 g/t MSW 96 mg/kwh 48 mg/kwh * (Consonni et al 2005) ** Calculated from the 2 nd column based on the outputs from Cases II-A and B 3.6 Economic Analysis Although the fuel quality of SRF or RDF is improved and makes for better more efficient combustion, the cost of the process is a major drawback (Rudder et al. 2005). It is a capitally intensive process which has to be done on a grand scale if it is ever to pay off its costs. The process of sorting, drying and pelletising of MSW is costly and time consuming. The use of RDF entails significant additional costs which can only be commercially viable in plants of over 1,000 tonnes per day (Optimat 2001). The sale of recyclable materials will go some way to generate revenue, but the current state of the recycling market does not bode well for this. Often a RDF/SRF plant operator will not have the revenue (or even expertise) to build an accompanying incineration plant so will have to develop a business partnership in order to sell the fuel. The payback period is long as costs are high; therefore revenue from sales has to be guaranteed. This necessitates a guaranteed partnership with an incineration company for an extended period in order for the plant to make profit (Cox et al. 2008). A simplified cost analysis is performed in order to evaluate potential benefits for an MSW fired CHP/DH system using SRF as a fuel. The cost analysis compares financial cost entailed in purchasing necessary equipment for SRF production versus financial benefits of recovering energy from SRF incineration. This is based on the assumption that existing MSW collection, transportation and incinerator system do not need to be upgraded or amended and thus no other financial costs are entailed. The costs associated with this fuel replacement include initial purchase price of machinery, operating and maintenance costs. As there is negligible difference in the renewable biomass content between MSW and SRF, the loss of Renewable Obligation 27

31 Certificates (ROCs) can be neglected Capital Cost of MBT Facility for SRF Production The principle of the MBT (Mechanical Biological Treatment) plant is to stabilise and separate the residual waste stream into less harmful and / or more beneficial output streams. MBT is a generic term for an integration of several processes (Last 2010). The processes are designed to handle raw black bag municipal waste (after any source segregated recycling and composting has taken place) and tend to involve a recycle recovery element (typically metals and glass) and drying/partial composting of the remaining waste to produce a more stabilised residue. The recyclable component may be extracted either prior to or post stabilization. The remainder of the waste is screened/sorted and homogenised to produce either a feedstock for another treatment process (e.g. RDF/SRF for energy recovery in a gasification, co-incineration, or Energy from Waste plant) or may be sent to landfill as a partially stabilised residue. The capital cost (C S ) for a whole MBT plant with MSW treatment capacity of 88,000tpa is approximately 27 million (Monson et al. 2007). The capital cost (C L ) of the MBT facility treating 240,000 tonnes of waste can be scaled with capacity (Cp) power by an exponent 0.75 (Consonni et al. 2005), i.e., C L = C S Cp Cp L S 0.75 Hence, for the proposed MBT facility, its capital cost is around 57.3 million. Generally, the conversion rate for MSW to SRF is about 50%. Thus the throughput of SRF is 120,000 tpa. For such a scale of facility the land requirement is approximately 3-4Ha (Last 2010) OPEX and CAPEX for SRF Production The operating and maintenance costs reported in literature appear to vary widely. Monson et al. (2007) estimated the operating costs at 35-55/tonne SRF, approximately half was spent on exhaust air treatment based on a case study in Germany. Some technology suppliers figures show the operational cost to be 10-35/tonne SRF. In addition, /tonne SRF of CAPEX should also be considered for the MBT facility (Arias-Garcia and Gleeson, 2009) Benefits Potentially, the gate fee to a MBT facility for SRF production is different from that to an incinerator. According to Waste & Resources Action Programme (2010), the gate fees for existing incineration facilities range from 32 to 79 per tonne with a 28