Alternative Energy Technologies

Transcription

1 Alternative Energy Technologies High Tech Solutions for Urban Carbon Reduction Written by: William Caruso Danielle Sorenson Ashley Mossa In conjunction with: London Borough of Merton Council i

2 Table of Contents Chapter 1: Introduction... 1 Chapter 2: District Heat and Power DHP Case Study Advantages and Disadvantages...3 Chapter 3: Combined Heat and Power CHP: Case Study Advantages and Disadvantages...5 Chapter 4: Hydrogen Fuel Cells How it Works Types of Fuel Cells Polymer Electrolyte Membrane Phosphoric Acid Fuel Cell Molten Carbonate Fuel Cells Solid Oxide Fuel Cells Case Study: CHP Hydrogen Fuel Cell...12 Chapter 5: Pyrolysis Pyrolysis Plants and the Pyrolysis Process Pyrolysis of Municipal Solid Waste Advantages and Disadvantages...16 Chapter 6: Anaerobic Digestion The Anaerobic Digestion Process Anaerobic Digestion Systems Types of Anaerobic Digestion Systems Anaerobic Digestion and CHP Advantages and Disadvantages...19 Bibliography...21 i

3 List of Figures Figure 1: DHP to Destination...2 Figure 2: Combined Heat and Power...4 Figure 3: Schematic of Woking Hydrogen Fuel Cell...13 Figure 4: Schematic Diagram of the Inputs and Outputs of the Pyrolysis Process Figure 5: General Layout of Anaerobic Digestion System...18 List of Tables Table 1: DHP Advantages and Disadvantages...3 Table 2: CHP Advantages and Disadvantages...5 Table 3: Fuel Cell Comparison... 8 Table 4: PEM Advantages and Disadvantages...9 Table 5: PAFC Advantages and Disadvantages...10 Table 6: MCFC Advantages and Disadvantages Table 7: SOFC Advantages and Disadvantages...12 Table 8: Projected Advantages and Disadvantages of Pyrolysis...16 Table 9: Projected Advantages and Disadvantages of Anaerobic Digestion ii

4 Chapter 1: Introduction Fossil fuel consumption has increased over the past century, becoming a primary source of energy for many countries around the world and accounting for over 85% of the global energy produced. In combination with a rise in energy demands, this dependence on fossil fuels is leading to high carbon emissions resulting in climate change problems. In addition, because fossil fuels are a finite source of energy, energy prices are continuing to rise due to the depletion of fossil fuel reserves. International and national initiatives such as the Kyoto Protocol and the UK Renewables Obligation have been implemented to reduce carbon emissions by encouraging the use of more renewables and the implementation of alternative energy saving technologies. Unfortunately, these efforts have had little impact on the current state of energy consumption and carbon emissions in the UK, which are still above target levels. These problems are becoming particularly evident in London where carbon emissions are beginning to increase due to growing energy consumption and population increase. As a result, the Mayor of London, Ken Livingstone, has proposed the Mayor s Energy Strategy, an attempt to focus initiatives on a local level in order to combat the problems associated with carbon emissions and energy consumption. As part of the Mayor s Energy Strategy, the London Borough of Merton has been chosen as one of four London Energy Action Areas for set carbon and energy reductions, as well as serving as a replicable model for other boroughs. Under this strategy, LBM has focused their efforts in Mitcham, a small town in South East Merton. One option for Merton is to implement a district heat and power network run by a series of combined heat and power units as well as integrating renewables into the energy supply. Merton also intends to implement hydrogen fuel cells within the next years, and is strongly considering the use of waste-to-energy technologies such as pyrolysis and anaerobic digestion within the next 5 years, both of which will simultaneously produce fuel for energy generation in CHP as well as reduce waste disposal in landfills, which is also a growing problem in Merton and greater London. This report is designed as an educational resource for the Borough of Merton and the surrounding community. It includes detailed information and depth on those technologies which Merton has indicated are of high interest including district heat and power, combined heat and power, hydrogen fuel cells, pyrolysis, and anaerobic digestion. Of the number of new and developing alternatives which Merton could potentially implement to meet their goals of reductions in carbon emissions and energy consumption, these technologies are the most promising options for an urban area such as the Borough of Merton. This report is the result of extensive research and is intended to provide a detailed background on developing alternative technologies and provide a list of resources for future self-directed research studies. 1

5 Chapter 2: District Heat and Power District heat and power (DHP) is the simultaneous delivery of heat, power, and in some cases, cooling, through a network of underground pipes. Unlike district heating, which can be as simple as generating heat from a boiler and pumping it to a certain location, DHP takes both electricity and waste heat from a power plant and transmits and distributes it to a variety of locations including residents, hospitals and municipal offices. DHP is most effective at locations where energy, heating and cooling are in high demand. Error! Reference source not found. illustrates a typical DHP network. Figure 1: DHP to Destination (District Energy St. Paul, 2006) According to Figure 1, multiple fuel sources can be used to power a plant. From the plant, electricity is shipped off through an energy grid, and thermal energy is sent into thermal storage, or to commercial, industrial, or residential buildings for heating. 2.1 DHP Case Study St. Paul, Minnesota, United States has a widely developed district heating and power system which provides heating and electricity service to twenty-nine million square feet of building space including 170 buildings and 300 homes. Year round heat and hot water are provided by the non-profit company, District Energy St. Paul. Their heating system, which is made up of 97,600 feet of threequarter inch pipes each way, provides heat at temperatures of 190 F to 250 F at a pressure of 180 psi. Once circulated, the heat is returned for reprocessing at an approximate temperature of 150 F. An added bonus is the cooling system which cools 14.5 million square feet. Cooling water comes in at 42 F and 150 psi, and is returned at 56 F. The heating and cooling systems operate at high reliability rates of 99.99% and 100% respectively (District Energy St. Paul, 2006). Among the 2

6 benefits of higher efficiency and reliability District Energy St. Paul offers customers cleaner air and lower energy prices. Thermal energy for the St. Paul DHP system can be generated from a number of fuel sources, ranging from oil and coal to wood and natural gas. In 2003, a 25 MW wood waste fired facility was constructed to supply threequarters of the thermal energy required by the district heating system (Pawlisch, Nelson, and Schoenrich, 2003). The urban St. Paul region will receive wood from downed trees, tree trimmings and branches around the twin cities area (Pawlisch, Nelson, and Schoenrich, 2003). The facility was designed to consume about 280,000 metric tons of wood, which is roughly half of the annually collected 600,000 metric tons of wood waste, and it is expected to help reduce the importation of fossil fuels and cut sulfur dioxide emissions by 600 tons per year. 2.2 Advantages and Disadvantages Although most major cities have some form of district heating, the possibility of supplying both electricity and heat through a DHP network is becoming a more promising alternative for urban environments such as London and other major cities in the UK that are trying to reduce energy consumption and carbon emissions. Table 1 outlines some of the advantages and disadvantages associated with DHP. District Heat and Power Advantages Disadvantages Highly efficient method Eliminates need and usage of fuel and refrigerant on location Hook-up is direct thus making maintenance and fuel delivery obsolete Reliability to five nines ( percent) (International District Energy Association, 2001) On-demand heat or cooling Lower capital costs to companies due to necessity of heating/ cooling schemes DHP pipes are expensive to buy DHP systems are expensive to install High demand necessary for full utilization Force city/borough/district to produce more heat and power, opposed to some offices and factories Table 1: DHP Advantages and Disadvantages (International District Energy Association, 2001) 3

7 Chapter 3: Combined Heat and Power Combined heat and power (CHP) is the simultaneous generation of heat and power from one source. Conventional centralized power plants are, at best, 40% efficient and only generate electricity, much of which is lost during the transmission and distribution of electricity through national grid lines. CHP is a more promising alternative to these conventional plants as it localizes energy generation (resulting in less transmission losses), is very efficient, and yields both electricity and heat. Moreover, CHP has the ability to utilize a number of different fuel sources including natural gas, biomass, coal, biogas/bio-oil, wood chips, and fuel oil (U.S. Environmental Protection Agency Combined Heat and Power Partnership, 2005). Using one process to combine heat and power generation results in lower consumer energy bills and less fuel consumed during production. The extra heat collected from energy production can be used to heat buildings which constantly demand energy such as hospitals or super markets. The power generated can be distributed at the location of the CHP site, through a private wire network or sold to the national grid. Figure 2 compares the overall efficiencies of a CHP system to conventional power plants. Figure 2: Combined Heat and Power (Pawlisch, Nelson, and Schoenrich, 2003) As Figure 2 illustrates, combined heat and power plants are more efficient than traditional power plants. Although it states 80% efficiency rates, CHP plants in operation can be as efficient as 90% with large systems producing 40% electrical energy and 50% thermal energy (Pilavachi, 2000). Long term European goals aim to expand into tri-generation systems which of fuel generate electricity, heating and cooling. 4

8 3.1 CHP: Case Study Combined heat and power is an attractive option for many cities because of its reduction in fuel consumption, promotion of a cleaner environment and use of highly efficient systems. Despite the large upfront costs, CHP and trigeneration is projected to decrease overall fuel costs in the long run, eventually paying for itself. Several districts in London, such as Ashfield, already have working CHP units. A 19,500 grant from the Residential CHP Programme was used to convert a twenty-five year old heating system into an electricity and heat producing powerhouse. In addition to providing one-hundred and thirty residents with heat, the system uses excess thermal energy to keep the leisure center and swimming pool warm (Combined Heat and Power Agency, 2005). Initial start up costs for the product totaled 112,000, with the largest portion being the CHP plant ( 75,000). However, despite these expenses, Ashfield currently saves 31,500 and 1,260,000 kilowatt-hours (kwh) annually with the upgrade. 3.2 Advantages and Disadvantages Although the ideal of combined heat and power is a promising concept for the future of technology, it is still relatively new and characterized by high capital and infrastructure costs. Table 2 outlines some of the advantages and disadvantages associated with combined heat and power. Combined Heat and Power Advantages High efficiency Increased reliability/ security of supply Eliminates need for boilers in individual buildings Reduction in fuel consumption Reduction of emission Disadvantages Electricity and heat demand must be high and simultaneous Potential for high maintenance costs High capital cost o CO 2 o Greenhouse gases No grid distribution charges mean lower cost to consumers Can incorporate green renewable sources Table 2: CHP Advantages and Disadvantages (Jimison, 2004; Bluestein, 2003; Alberto, Martin, McFadden, n.d.) 5

9 Chapter 4: Hydrogen Fuel Cells There are several different methods of combined heat and power generation, from reciprocating engines that capture exhaust heat to microturbines that capture waste heat. Some of the most promising technology for the future of power generation is fuel cells. There is currently large scale research and development in many countries for reliable and efficient operation of fuel cells for stationary combined heat and power generation. The following section will provide an overview of fuel cell heat and power production technology including: Basic operation Potential renewable fuel sources Case study: Borough of Woking Advantages and Disadvantages Fuel cells, like batteries, produce electrical energy from a chemical reaction. However the difference between fuel cells and batteries is that fuel cells take a direct and constant input of a fuel to produce electrical energy process to produce water and energy. The hydrogen and oxygen reaction is represented by the following equation: The idea to use fuel cells to harness this energy and convert it into electricity is not new. The first fuel cell was built in 1859 by British engineer Francis Thomas Bacon (Wikimedia, 2006). Now more than 145 years later, over $15 billion has been invested globally in fuel cell research and development (Romm, 2004). Almost every major car manufacturer has a working model fuel cell powered automobile and more companies are turning to fuel cells for utility power generation. Fuel cells are attractive for power generation for a variety of reasons: Clean Fuel cells produce far less, sometimes zero, greenhouse gas emissions compared to combustion-based methods Efficient Particular types of fuel cells can operate at efficiencies as high as 80% if used with a CHP system Fuel Versatility A large variety of fuel sources can be used in fuel cells from fossil fuels, to biomass to pure hydrogen Application Versatility Fuel cells can be designed to power anything from a large utility power station to a laptop computer (US Department of Energy [DOE], 2005) Though fuel cells are very attractive for clean and efficient energy production, there are still issues associated with the technology such as high manufacture costs, part lifetime issues and high cost of fuel production and transportation. 6

10 4.1 How it Works There are several different types of hydrogen fuel cells, but they all share a similar design layout. All hydrogen fuel cells have three basic components involved during operation: a catalyst coated anode, a catalyst coated cathode and an electrolyte. While each different type incorporates different combinations of components, all fuel cells operate on a similar principal: 1. Hydrogen or a fuel rich in hydrogen flows into the anode area where it is stripped of its electrons (ionized) by the catalyst. 2. Simultaneously, oxygen is ionized at the cathode. 3. The stripped electrons from the hydrogen molecules cause a buildup of negative charge on the anode (Rocky Mountain Institute [RMI], 2005). 4. The electrons follow a path of least resistance, which is provided by an external electric circuit connecting the anode and cathode. Through this circuit the electrons flow to make the usable electricity (RMI, 2005) 5. The combining of electrons and hydrogen and oxygen molecules completes the electrochemical process and occurs by two different mechanisms in the different types of fuel cells. a. Positively ionized hydrogen atoms (protons) from the anode move through the electrolyte to the cathode and combine with the oxygen atoms and electrons to form water and heat. This occurs in Polymer Electrolyte Membrane and phosphoric acid fuel cells (US DOE, 2005) b. Negatively ionized OH- ions travel from the cathode through the electrolyte to the anode to produce water and heat. This occurs in the alkaline, molten carbonate and solid oxide fuel cells (US DOE, 2005). The entire process occurs in a single fuel cell and yields consistent low voltage DC electrical power. To meet high demands, multiple fuel cells can be bridged together to form fuel cell stacks. The size of individual fuel cells and the amount of cells in a stack can vary to adhere to almost any demand, leading to great versatility in size and power output. The primary advantage shared by all fuel cells is low greenhouse gas emission. The only by-product emission of a pure hydrogen fuel cell is water and heat. Later in this section, the challenges of producing pure hydrogen as a fuel source will be discussed. Since fuel cells have no mechanical processes involved during operation, power generation is quiet. Likewise, the electrochemical process is capable of producing a reliable and consistent flow of electricity. These qualities attract companies in demand of high quality power without worries of power failure. Hewlett-Packard is an example of one company with such demands. The company estimated that 15 minutes of power failure at a single chip plant could result in a company loss of over $30 million dollars (RMI, 2005). There are several different types of fuel cell technology that are under current research and development. Each one incorporates different use of materials and operates under a variety of different conditions. For the purpose of 7

11 this proposal, stationary fuel cells that can efficiently operate in co-generation (CHP) schemes will be focused upon. 4.2 Types of Fuel Cells Each different fuel cell offers a variety of different material used, operating temperatures, efficiencies, production prices, power outputs and stage of development. The following section will provide some detail into stationary fuel cell options and basic specifications associated with efficiency, cost, environmental impact and feasibility as a CHP unit. Table 3 gives a basic comparative overview of the fuel cells to be discussed. Type Polymer Electrolyte Membrane Anode Gas Pure Hydrogen Cathode Gas Electrolyte Catalyst Oxygen (pure or atmospheric) Op. Temp. (C) Efficiency (%) Price ($/kw) Solid polymer membrane Platinum % <$3000 Output range kw Cogen? Yes (hot water) Phosphoric Acid Molten Carbonate Pure Hydrogen Pure Hydrogen Atmospheric Oxygen Atmospheric Oxygen Phosphoric Acid Platinum % Carbonate $4000- $4500 Variety nonprecious metals % ~$ kW 250kW- 10MW Solid Oxide Pure Hydrogen Atmospheric Oxygen Ceramic Oxide Nonprecious % goals of <$400 1kW- 10MW Table 3: Fuel Cell Comparison (RMI, 2005; US DOE 2005; Wikimedia, 2006; California Energy Commission, 2003) Yes (hot water) Yes (HP & LP steam) Yes (HP & LP steam) Polymer Electrolyte Membrane Polymer Electrolyte Membrane (PEM) fuel cells, also known as proton exchange membrane fuel cells, are one of the most promising fuel cells for widespread production (RMI, 2005). The low operating temperature does not require long periods of warm-up time and does not cause as much wear on component materials, extending overall life of the cell. Furthermore, because the electrolyte is non-corrosive and solid by nature, the orientation of the fuel cell is of little significance in the efficiency and wear on the components. Though there are some uses for stationary medium to large scale power generation, these characteristics lend themselves to making the PEM a prime candidate for the transportation industry. The main problem with the PEM fuel cell is its use of a noble-metal catalyst, which is usually made from platinum. The platinum is not only expensive, but very sensitive to carbon monoxide (CO) poisoning. This limits the fuel cell to use only pure hydrogen, which requires an external fuel reformer (usually natural gas is used) and may require a fuel purifier, both of which can add to overall expense. Table 4 shows the advantages and disadvantages associated with this fuel cell technology. 8

12 PEM Advantages High Power Density, efficiencies of 50%+ Need only hydrogen, atmospheric O2 and water to operate No corrosive materials Low temperature, short start up times Compact design Disadvantages Need pure hydrogen, requires external reformer High manufacture cost Sensitive to CO poisoning and other impurities Requires extra equipment for fuel purity Problems associated with freezing temperatures Applications Commercial Status Transportation: cars, trucks, trains, etc. Small scale application and portable power production Medium to large scale utility power generation Most widely developed Several power generation plants in operation Technology advancement heavily funded Table 4: PEM Advantages and Disadvantages (RMI, 2005; US DOE, 2005; Wikimedia, 2006; California Energy Commission, 2003) Phosphoric Acid Fuel Cell Phosphoric acid fuel cells (PAFC) are the most mature of all the fuel cell technologies with more than 200 in current use since the commercial introduction in According to the U.S. Department of Energy, PAFC s are most suitable for stationary use because of their large size and heavy weight. They are particularly attractive for stationary use because of the high efficiency of cogeneration (up to 85%). They are also relatively resistant to carbon monoxide poisoning that the PEM fuel cells are. However, due to the large size and weight, PAFC have an inferior power to weight ratio relative to other fuel cells. PAFC are also expensive, costing up to 4500 USD per kilowatt to operate (US DOE, 2005). Table 5 shows the advantages and disadvantages associated with this fuel cell technology. 9

13 Phosphoric Acid Fuel Cells Advantages Mature technology Usually used in stationary power generation More tolerant than PEM of poisoning Up to 85% efficiency when used for co-generation Applications Stationary power generation Back-up or primary CHP generation Over 300 units in operation globally Disadvantages Expensive up to $4500/kW Low electric generation efficiency Expensive platinum catalyst required Commercial Status Over 200 units in operation globally 1.1 billion kwh and 7 million hours total operation Primary manufacturer is UTC Power which is a division of United Technologies Table 5: PAFC Advantages and Disadvantages (RMI, 2005; US DOE, 2005; Wikimedia, 2006; California Energy Commission, 2003; UTC, 2006) Molten Carbonate Fuel Cells Molten Carbonate Fuel Cells (MCFC s) are most often used for stationary power generation and are currently being developed for large scale power generation in existing conventional combustion plants. MCFC are large, high temperature fuel cells that are attractive for co-generation because of 600 C operating temperatures. MCFC s are cost effective relative to PAFC s because of the flexibility of the cathode (non-precious metals may be used) and because of high electrical efficiencies of over 60%. If co-generation is used, energy efficiencies of over 85% are possible. Because of the high operating temperatures, MCFC s are capable of reforming fuels internally, eliminating need for external fuel processing. They are also not as susceptible to CO and CO2 poisoning, which could yield possibility for internal reforming of fossil fuels as well as using pure hydrogen. It should be noted that if fossil fuels are used for fuel, the fuel reforming emits carbon dioxide and other greenhouse gasses. Because of high temperature and corrosive electrolyte, MCFC s can potentially suffer from issues of durability, resulting in shorter cell life (US DOE, 2005). MCFC s are designed to run continuously because of the high temperature, thus application of an MCFC requires continuous energy demand. Table 6 shows the advantages and disadvantages associated with this fuel cell technology. 10

14 Molten Carbonate Fuel Cells Advantages Can be used with fossil fuels High temperature can be used for co-generation Don t require external hydrogen reformer Can use non-precious metals for catalyst Up to 85% efficiencies with co-generation Applications High Capacity stationary power generation Back-up or primary CHP generation Disadvantages Low durability due to high temperature and corrosive materials Some thermal protection needed Commercial Status Some commercial power generation in place ranging from 250kW- 2MW Units available from MTU CFC Solutions for trigeneration large scale applications MTU Boasts hours of MCFC successful operation Table 6: MCFC Advantages and Disadvantages (RMI, 2005; US DOE, 2005; Wikimedia, 2006; California Energy Commission, 2003; MTU, 2003) Solid Oxide Fuel Cells Solid oxide fuel cells (SOFC s) have similar advantages and disadvantages associated with the molten carbonate fuel cell. The solid oxide fuel cell operates at a very high temperature (1000 C), which makes it a prime candidate for cogeneration. Furthermore, high temperature eliminates the need for precious catalyst and allows for internal fuel reforming. SOFC s are also resistant to CO and CO2 poisoning, more so than the MCFC (US DOE, 2005). The use of a solid electrolyte yields more stability than the liquid electrolyte in MCFC s. Electrical and co-generation efficiencies are comparable to MCFC, at up 60% and 85% respectively (US DOE. 2005). However, the SOFC experiences downfalls because of the high temperatures and therefore, requires significant thermal shielding, has a slow start up time, and also suffers from durability issues. The key to widespread use is to develop high durability, low-cost materials. SOFC s are designed for continuous operation. Table 7 shows the advantages and disadvantages associated with this fuel cell technology. 11

15 Solid Oxide Fuel Cells Advantages Can be used with fossil fuels High electric production efficiency Up to 85% efficiency with co-generation Internal fuel reforming, don t require external hydrogen reformer Highly resistant to poisoning Can use non-precious metals for catalyst Disadvantages Low durability due to corrosive materials and high temperature Slow start-up time Significant thermal protection needed Commercial Status Applications High Capacity stationary power generation Back-up or primary CHP generation Some commercial power generation in place ranging from 1kW- 100kW Units available from SOFCo-EFS Holdings LLC for small scale 50kW modules Units being tested from Siemens Westinghouse for large scale up to 1MW operations Table 7: SOFC Advantages and Disadvantages (RMI, 2005; US DOE, 2005; Wikimedia, 2006; California Energy Commission, 2003; SOFCo-EFS Holdings LLC, 2005; Siemens, 2006) Each different type of fuel cell offers its share of advantages and disadvantages in terms of the operation and physical nature of the actual cell. Some of the main problems are cost, durability, large size or contamination due to impurities (in PEM and PAFC). 4.3 Case Study: CHP Hydrogen Fuel Cell The London Borough of Woking 1 uses a hydrogen fuel cell combined heat and power unit to provide heat and electricity to their leisure center and pool. The unit installed is a PC25 PAFC installed and manufactured by UTC, which is currently the only combined heat and power fuel cell unit commercially available (UK DTI, 2005). It uses pure hydrogen chemically reformed from natural gas and pure oxygen directly extracted from the surrounding air. The overall performance of the unit has been monitored by the UK s Department of Trade and Industry in an ongoing five year project with the first year end report released in September The report is readily available for detailed feedback and made several noteworthy conclusions: The high cost of the initial implementation of the unit is due not only to the ticket price, but also because of significant computer control necessary for monitoring operation of the fuel cell and interface of fuel cell and CHP plant. Operation reliability has been in the 90% range. Maintenance costs have been higher than expected, though may be due to first year settling of unit. 1 It should be noted that the Borough of Woking (population 89,840) is about half of the size of the Borough of Merton (population 187,908) (National Statistics, 2006). 12

16 Because not all thermal load has been utilized, the facility has been operating at an average of 57% efficiency, though up to 85% efficiencies are possible. Expected electrical efficiencies of 37% have been realized The environmental impact in terms of greenhouse gas emissions has been satisfactory. The environmental impact, along with other past energy initiatives, have helped Woking reduce energy consumption by 142,000 MWh and 78,000 tonnes of carbon emission in only 10 years (Oakley, 2005). The facility will be continually monitored to give feedback in order to benefit the community. The ambitious implementation of this new technology is a learning experience and has potential to set a positive example for surrounding communities. Error! Reference source not found. shows the schematic of the London Borough of Woking s fuel cell. Figure 3: Schematic of Woking Hydrogen Fuel Cell (UK DTI, 2005) 13

17 Chapter 5: Pyrolysis Pyrolysis is a rapidly developing thermal conversion technology in the United Kingdom under the Renewables Obligation. The thermal process involves the superheated degradation of carbon-rich organic matter in the absence of oxygen in order to produce a mixture of char (ash), pyrolysis oil (bio-fuel), and synthetic gas (syngas), a gas mixture of carbon monoxide, hydrogen, carbon dioxide, and methane (Friends of Earth, 2002). Diesel engines, gas and steam turbines, or boilers can be used directly to generate electricity and heat in combined heat and power systems using syngas and pyrolysis oil. Syngas may also be used as a basic chemical in petrochemical and refining industries (Friends of the Earth, 2002). 5.1 Pyrolysis Plants and the Pyrolysis Process Pyrolysis plants are quickly developing around the world. As of 2001, there are 100 small-scale pilot plants globally processing more than 4000 tonnes of residue per year. Many plants are designed for specific processes such as separation and recovery services, and specific organic materials such as industrial by-products or residuals (Gale, 2001). Several projects have been spearheaded across the United Kingdom. The ARBRE project in Yorkshire, United Kingdom is an interesting example of a pyrolysis plant. A goal of the project was to harvest rapidly growing crop to maximize fuel intake; however the plant remained operational for eight days and shut down due to fuel incompatibility (Brennand, 2004). Pyrolysis plants are typically used to degrade carbon-rich organic materials such as biomass, household and commercial waste, residues from materials recycling, and more recently, municipal solid waste (Juniper Consultancy Services Ltd, 2003). Upon arriving on site, these organic materials must first be sorted and pre-treated. Organic matter is sterilized and inorganic materials, such as metal and ceramics, are removed, leaving behind the desired pyrolysis feedstock. The remaining feedstock is shredded and the moisture content is lowered to ~1% to reduce any wasted energy (Gale, 2001). Rotary kilns, rotary hearth furnaces, and fluidized bed furnaces are commonly used as waste pyrolysis vessels (Federal Remediation Technology Roundtable). Material is supplied to these vessels which are then heated to temperatures as high as o C. Various proportions of char (ash), pyrolysis oil, and syngas are produced based on such factors as organic composition and reaction parameters, including temperature, pressure, and time (Veringa, 2005). Syngas is cleaned to remove particulates, hydrocarbons, and soluble matter, and then combusted to generate electricity (Friends of the Earth, 2002). In some cases, gas may further be sterilized by processes such as gasification. Pyrolysis oil may also be used as liquid fuel for diesel engines and gas turbines to generate electricity as well. Figure 4 gives a general schematic of the inputs and outputs of the pyrolysis process and the potential environmental impact of each product. 14

18 Figure 4: Schematic Diagram of the Inputs and Outputs of the Pyrolysis Process (Eunomia Research and Consulting, n.d.) 5.2 Pyrolysis of Municipal Solid Waste In the past, municipal waste in the UK has been transported to landfills and left untreated (Friends of the Earth, 2002). This is becoming an increasingly expensive process for LBM and other urban areas. It currently costs 48 a ton to truck municipal waste away. In order to reduce the amount of waste transferred to landfills and decrease the amount of greenhouse gas emissions produced from landfills, new technologies such as pyrolysis are being explored. Pyrolysis provides an alternative to current methods of municipal waste disposal such as anaerobic digestion, landfill storage, and more specifically incineration. Incineration diverts municipal waste from landfill storage by burning it to ashes. This process causes detrimental effects to the environment by destroying natural resources, adding to climate change, and causing pollution from air emissions and toxic ash. Pyrolysis provides an option which uses less oxygen and puts waste to direct use (Friends of the Earth, 2002). There are a number of plants in the UK and around the world today which have incorporated the pyrolysis process to specifically degrade municipal waste. WasteGen UK is a promising company which implements Materials and Energy Recovery Plants (MERPS) that combine pyrolysis with recycling and composting (WasteGen UK, Ltd., n.d.). In a typical MERP, of the 200,000 tonnes of municipal waste annually input, 118,000 tonnes is prepared for energy recovery and the remaining 82,000 tonnes are set aside for recycling and composting. Approximately 18.3 MW of electricity is generated. In Burgau, Germany a 15

19 WasteGen UK MERP was implemented 17 years ago. Since then, 36,000 tonnes of municipal solid waste has been converted to gas and burnt for electricity generation (WasteGen UK, Ltd., n.d.). Similarly, in Avonmouth, Bristol, UK, Compact Power has recently developed and built a fully operating plant that uses pyrolysis, gasification, and high temperature oxidation to process municipal waste and generate heat, electricity, and other materials. The plant has a capacity for 8,000 to 32,000 tonnes of municipal waste per annum. The company expects that the plant will reduce climate change, meet the requirements for sustainable waste management, and provide a solution for the processing of difficult and industrial waste (Compact Power, n.d.). 5.3 Advantages and Disadvantages Many of the advantages and disadvantages associated with pyrolysis are based on small scale pilot plants and experimental research. Information in regards to capital and operating costs for current pyrolysis plants is scarce, and many projects must be considered on a case-by-case basis to determine their economic feasibility (Juniper Consultancy Services Ltd., 2003). Table 8 outlines the projected advantages and disadvantages of the overall pyrolysis process. Advantages Produces few air emissions due to limited use of oxygen Contamination of air emissions is easy to control because syngas is cleaned after production to rid it of any contaminants Replaces coal and natural gas as viable fuel sources, causing a reduction in climate change Produces useful products for multiple applications Can be easily implemented in CHP systems More efficient than incineration (70% vs. 40%) Pyrolysis plants are flexible and easy to operate because they are modular. They are made up of small units which can be added to or taken away when the mass or volume of organic matter changes Pyrolysis Disadvantages Generates possible toxic residues such as inert mineral ash, inorganic compounds, and unreformed carbon Potential to produce a number of possible toxic air emission such as acid gases, dioxins and furans, nitrogen oxides, sulfur dioxide, particulates, etc. Pyrolysis plants require a certain amount of materials to work effectively Table 8: Projected Advantages and Disadvantages of Pyrolysis (Friends of the Earth, 2002; Fortuna, Cornacchia, Mincarini, & Sharm, 1997) Although pyrolysis has much potential for the future, certain factors must be kept in mind for the implementation of such a process in an urban environment including the availability of particular biomass and municipal waste resources, and the economic feasibility of a pyrolysis plant. 16

20 Chapter 6: Anaerobic Digestion Anaerobic digestion (AD) is a growing technology in the UK and around Europe for the treatment of waste and biomass. Anaerobic digestion (AD) is the natural bacterial decomposition of organic matter in the absence of oxygen. It is primarily used for wet organic materials such as green agricultural crops and residues, animal slurries, sewage sludge, and municipal solid waste (Maunder, 1995). The AD process generates three by-products including biogas, bio liquid or liquid digestate, and fibre digestate. Biogas is a gaseous mixture composed of 60% methane (CH4) and about 40% carbon dioxide (CO2), with trace amounts of hydrogen sulfide and ammonia (Duerr, 2005). Biogas can be used in combustion engines or micro-turbines in combined heat and power (CHP) plants to generate heat and electricity. In addition, liquid and fibre digestate can be used as a fertilizer or compost to improve soils (Friends of the Earth [FOE], 2002). 6.1 The Anaerobic Digestion Process Anaerobic digestion occurs naturally in environments of high organic composition and little to no oxygen. In industry, the AD process is induced in anaerobic digesters, large enclosed tanks that are heated and depleted of all oxygen to enhance the growth of microbes and the active digestion of matter (Spiegel and Preston, 2000). AD occurs in three phases: hydrolysis/liquefaction, acetogenesis, and methanogenesis (Verma, 2002). During the first phase of hydrolysis/liquefaction, fermentative bacteria secrete enzymes which break down complex organic matter into simple molecules such as sugars, amino acids, and fatty acids. These monomers are further converted by acetogenic bacteria, or acid formers, during acetogenesis into carbon dioxide, oxygen, and simple organic acids. In methanogenesis, the final phase of the AD process, bacteria known as methane formers, or methanogens, produce methane gas by cleaving acetic acid molecules into carbon dioxide and methane or reducing carbon dioxide molecules with hydrogen (Verma, 2002). 6.2 Anaerobic Digestion Systems The AD process is just a small part of an industrial anaerobic digestion system. AD systems are composed of four stages: pre-treatment, digestion, gas recovery, and residue treatment. Before organic matter, or feedstock, can be fed into an anaerobic digester, it must first be pre-treated. Source or mechanical separation is used to remove any recyclables or non-digestable material such as glass or metal from the feedstock. The feedstock is then shredded and fed into the digester (Verma, 2002). Digestion occurs in an anaerobic digester at one of two temperature ranges: the mesophilic range which is between 20 o C-40 o C, or the thermophilic range which is between 50 o C-65 o C. Inside the digester the feedstock is diluted with water and mixed to ensure uniformity throughout the sample. The feedstock is left inside the digester for a designated retention time to allow 17

21 microbes to grow and digest all available organic matter. The amount of byproduct generated from this process depends on specific reaction parameters including feedstock composition, ph level, temperature, carbon to nitrogen ratio, total solids content, retention time, and mixing (Verma, 2002). Biogas recovered from the digester is cleaned to rid it of any contaminants and burnt to generate electricity and/or heat. Additionally, liquid and fibre digestates recovered from the digester are used as soil enhancers or disposed of depending on factors such as their level of contamination or how biodegradable the incoming feedstock was (FOE, 2004). The liquid digestate may also be reused as a water source to dilute incoming feedstock (Verma, 2002). Error! Reference source not found. gives the general layout for a typical anaerobic digestion system. Figure 5: General Layout of Anaerobic Digestion System (Duerr, 2005) Types of Anaerobic Digestion Systems There are a number of different types of anaerobic digestion systems available for the treatment of waste and other organic matter. Optimum systems depend on such factors as the type of feedstock being supplied, land availability, energy supply, available technology and cost. These systems can be classified in one of two ways: according to the total solids (TS) content of feedstock in the digester reactor or by the number of reactors used in the digestion process. AD systems classified according to total solids (TS) content of feedstock can be separated into two categories: low solid systems and high solid systems. Low solid systems are used to treat feedstock with a total solids content of less than 10%, while high solid systems are used to treat feedstock with a total solids content of 22%-40% (Verma, 2002). AD systems are classified by the number of reactors used in the digestion process may also be separated into two categories: single-stage processes and multi-stage processes. In a single-stage process, one digester reactor is used for all three phases of anaerobic digestion. The processes are separated by time and occur in logical order one stage at a time. In a multistage process, two or more reactors are used for AD in order to separate the acetogenesis and methanogenesis phases. Either of these two processes can be incorporated into a batch reactor which digests feedstock and re-circulates material between a system of vessels (Verma, 2002). 18

22 6.3 Anaerobic Digestion and CHP AD has been used in industry in the UK to generate power as early as the 1890s when biogas was taken from wastewater treatment plants and used to fuel street lamps in England (Hinrichs, 2006). Since then, AD has been increasingly used on farms to treat animal slurry and in wastewater treatment plants to pretreat sewer sludge. The ability of biogas to fuel power generation has enabled industries and farmers to freely recycle the AD by-product in order to reheat digesters, generate electricity for AD plants, and more recently, power combined heat and power (CHP) plants. In October 1994, an anaerobic digestion system incorporating an Enviropower CHP facility was implemented in Walford College Farm in the UK as a part of a three year project. Pig slurry was used to feed the digester. Liquid and fibre digestates generated by AD were stored and later used as compost for farm land. Biogas, however, was used to fuel the CHP unit. The CHP unit rated at 35 kw output for electricity and 57 kw output for heat. About 30 kw of heat was harnessed from the CHP engine s coolants and exhaust system to heat the digester (VTT Energy, 2001). Current projects are also being done to test fuel cell operation powered by biogas from AD. In 1999, the EPA created a project to assess a fuel cell energy recovery system in conjunction with an anaerobic digester wastewater treatment plant. A 200-kW fuel cell power plant was constructed in Yonkers, NY near a wastewater treatment (WWT) plant. Biogas produced from the anaerobic digester in the WWT plant was initially pretreated to rid of contaminants such as sulfur and halide. The dilute gas was fed to the fuel cell. The power generated from the fuel cell was used to power the Yonkers plant. According to an assessment of the plant, it operated successfully on biogas for several thousand hours and attained an average electrical output of 150 kw. Measured air emissions produced by the fuel cell were similar to those emitted by a natural gas powered fuel cell (Spiegel and Preston, 2000). 6.4 Advantages and Disadvantages Many of the advantages and disadvantages associated with AD are based on agricultural and wastewater treatment plants that have incorporated AD technology for centuries. Many of these plants are small-scale plants that generate small amounts of power and are used primarily for the treatment of animal slurry, agricultural residue, and sewer sludge. Table 9 outlines the projected advantages for the AD process: 19

23 Anaerobic Digestion Advantages Can process a variety of biomass materials Produces practical by-products which can easily be captured and used for soil fertilization and the generation of heat and/or electricity Produces the least amount of air and solid emissions in comparison to typical waste management processes such as incineration, pyrolysis and gasification; AD plants can be small and unobtrusive, which makes them suitable for location within towns Digestion of sewage waste via AD results in 10% reduction in carbon dioxide emissions AD with CHP produces net reductions in pollutant emissions Disadvantages If AD does not completely digest all the waste, the resulting digestate may not meet Government standards Poor feedstock used in the AD process can result in the production of unusable by-products Depending on the feedstock, AD may create contaminated digestates that are high in metals such as mercury Combustion of biogas produces nitrogen oxides, which are associated with lung problems and allergies Biogas is composed of high concentrations of methane and carbon dioxide which are toxic greenhouse gases associated with climate change AD plants generate lots of waste water high in nitrites AD plants may cause environmental problems such as odour, dust and pollutants due to the burning of methane for power generation Table 9: Projected Advantages and Disadvantages of Anaerobic Digestion (FOE, 2004) Anaerobic digestion has potential for the future of power generation, but certain factors must be kept in mind for the implantation of AD in an urban environment including economic feasibility, the availability of biomaterials and the environmental impact of the AD process. 20

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