How the City of Lebanon TN Implemented Gasification for Biosolids Disposal and Power Generation

Transcription

1 How the City of Lebanon TN Implemented Gasification for Biosolids Disposal and Power Generation Introduction The City of Lebanon, TN has completed construction of a waste-to-energy system sited at the wastewater treatment plant (WWTP). This project will not only create electricity, but will also utilize existing wood waste, WWTP sludge, and the county s discarded tires as fuel creating an ideal situation for reducing waste and producing renewable energy. The designed system is the integration of three well-established commercial technologies: 1) a biomass gasifier, 2) a thermal oxidizer/hot oil heat exchanger, and 3) Organic Rankine Cycle (ORC) power generator. The plant is designed for a 25-year operating life. The system design is fundamentally different from incineration since it uses gasification to reduce NOx, SOx, and fly ash. As such, the project design further supports the environmental goals of the Tennessee Department of Environment and Conservation since it first converts the biomass into a clean combustible gas and then combusts it in a thermal oxidizer yielding stack emissions similar to natural gas unlike traditional incineration-to-energy projects. Project revenues are derived from the avoided cost of purchased electricity and anaerobic digestion using ATAD and tip fees for processing wood waste and tires. As a result, the project is expected to provide the city a net savings of $300,000 annually, and $6 million during its operational lifetime through avoided costs and operational efficiencies while simultaneously meeting all selection criteria of the TDEC Energy Program and its broader goals. The City of Lebanon s WWTP is supplied electricity by the Middle Tennessee Electric Cooperative (MTEC), a municipal distribution utility serving residential and commercial customers within the corporate limits of the City of Lebanon. MTEC is a separate entity and is not affiliated with the Lebanon WWTP. Lebanon WWTP is currently supplied electricity at an average cost of $.10/kW. Equally important however, are the costs in managing and disposing of existing waste streams via traditional waste disposal methods. The City of Lebanon collects and surrounding industries contribute more than 27 tons of woody biomass per day that currently must be disposed of in a landfill. In addition, the city must also dispose of over 3 tons of biosolids per day from the wastewater treatment facility via land application. At current volumes, over 1840 trips a year are required to dispose of the material. As is explained below, the implementation of this project will consolidate these waste streams at the WWTP and eliminate the majority of disposal costs while adding substantial energy and environmental benefits that will contribute to the long term viability and continued growth of the City of Lebanon. PROPOSED SOLUTION: PHG Energy, a Tennessee based company, has proposed to design, engineer, install and commission a downdraft gasification-to-energy system that can convert approximately 32 tons per day of the wood, tires and sludge waste collected in the city into carbon neutral electricity for use at the WWTP. Gasifiers, thermal oxidizers and ORC s are well-established commercial technologies individually. This project combines a state-of-the-art biomass gasification system with thermal oxidation equipment with ORC technologies to produce reliable electrical power with emission levels comparable to natural gas combustion. ORC power generators are often preferred over traditional steam cycle or internal-combustion engine (ICE) power generators because they can offer much lower operating and maintenance costs while operating without the need for dedicated attendance by an operator. Using gasification instead of incineration uses a fundamentally different thermochemical 365

2 process to cleanly convert the city s waste streams into a combustible gas, called producer gas, limiting the formation of NOx, SOx, and fly ash emissions inherent to an incineration process. Gasification offers the added benefit of feedstock flexibility; the same equipment can handle a wide variety of feedstock giving the city flexibility in handling its waste streams over the 25-year life of the plant. The PHG gasifier, from Antioch, TN-based PHG Energy, LLC, is an industrial-grade downdraft gasifier that converts biomass into a clean combustible producer gas. PHG gasifiers have over 45,000 hours of combined operation and run with relatively little need for operator interaction. The downdraft-style gasifier is well known to be the most efficient method of gasification and produces the cleanest gas from woody biomass. 1 The simplicity of the design enables the entire gasification process to occur with very few moving parts and relatively low parasitic loads. Thermal oxidizers are the most common and most efficient technology employed today to destroy hydrocarbons in the form of Hazardous Air Pollutants (HAPs) and Volatile Organic Compounds (VOCs) and have been in use for more than 70 years. The principal behind thermal oxidation of HAPs and VOCs is that when combusted, the hydrocarbon pollutants will convert to CO2 and H2O in the outlet stream. The Thermal Oxidizer proposed for this project will include an attached hot water heater that will transfer the energy from the thermal oxidizer s exhaust flue stream into a closed loop circulating stream of hot water. The hot water stream will carry the heat energy into the ORC generator, where it will be used to drive the ORC s closed-loop power generation. Hundreds of ORC s are operating in the field today from various manufacturers including General Electric and Pratt and Whitney. An ORC operates similarly to a steam-turbine generator, however, the ORC uses a working fluid inside an enclosed system that evaporates at a much lower temperature than water and is non-corroding to valves, tubing and turbine blades that drive the generator. Since the system is enclosed, there is no make-up fluid in the generator itself, and it does not require a boiler-certified operator to function. The ORC is simple to start and stop, operates very quietly and requires minimum operation and maintenance. 3 Overall, the proposed system offers a combination of commercially proven technologies that are both robust in its design and flexible in their operation. The complete system will use landfill bound waste to provide a continuous stream of domestically produced energy that is clean, carbon-neutral, and renewable providing the city with both economic and long-term environmental benefits. ENERGY EFFICIENCY This project will produce measureable energy savings through onsite reductions in electrical energy purchases and offsite reductions in diesel fuel consumption through avoided trips to landfills. A generation meter and monitoring system will provide real time and accurate measurements of the amount of electrical energy specifically produced by the system. Since the system will be connected directly to the common electrical grid, it is assumed that the metered amount of electricity produced will result in a corresponding decrease in the amount of electricity purchased from the City of Lebanon s electrical utility supplier. Currently, industrial and manufacturing waste within the city is hauled to a landfill located approximately 22 miles away from the source of the material, resulting in round trip distances of approximately 44 miles per trip. At current volumes, over 1680 round trips are needed annually to dispose of existing waste streams. By providing a central location within the corporate city limits for this material to be transported and converted into energy, thousands of truck miles will be saved. The effect of this project activity will be measurable through the identification of avoided individual tipping tickets at the currently utilized landfills. 366

3 Savings Energy savings can be calculated and discussed in terms of both energy amount and dollar value. Assuming a conservative availability, over 748MW-hrs can be generated annually from the facility. This yields approximately $75,000/year in electrical energy savings. In addition, taking the ATAD offline will net an additional $175,000 annually in electricity savings. Energy savings resulting from reduced transportation and combustion of diesel fuel needed to transport the waste streams to the gasification site is realized. Savings are calculated based on the transportation distance and the number of trips needed per year. Project implementation avoids approximately 1680 trips and more than 104,000 gallons of diesel fuel annually. Finally, avoiding landfill tipping fees associated with the disposal of waste wood and sludge combined with the labor and fuel savings will save the approximately $248,000 per year. When combined with the electric savings, over $300,000 per year will be saved from all aspects of the project after expenses. In addition to the saving realized above the City will be developing markets for the Biochar that will provide additional revenues for the project. Currently the City is able to give the Biochar away for free. GENERAL PUBLIC BENEFIT Immediate Results & Project Readiness The site for the project is owned by the City of Lebanon and has been reviewed by the city council. Existing waste streams (fuel supplies) are already part of the City of Lebanon s existing services and infrastructure so there is no need to engage in lengthy and extensive fuel procurement negotiations. As the required technology is readily available and no long lead times are expected, the project can be fully operational within 9 to 12 months of grant approval. Once operational the project will produce immediate and meaningful beneficial results. Long-Term Savings The project will be designed, constructed and maintained for at least 25 years. The project can provide net savings to the citizens of Lebanon of more than $6 million over the life of the project. This estimate is considered conservative and is calculated as the sum of electrical savings and avoided costs associated with the transportation and disposal of existing waste. Other Public Benefits Since the wastewater treatment plant sludge will be gasified at the site of the Lebanon treatment facility, the need to transport the sludge or tires over highly traveled local highways will be eliminated. As a result, the risk of a spill will be greatly diminished ensuring that no environmental pollution due to an accident will occur. The same is true of the current wood waste and tire disposal methods. 367

4 Biochar is also an inert byproduct of the gasification system and can be used as a soil additive or mixed with compost to further enhance its economic value. Also independent companies seek and purchase biochar for reuse. This further eliminates any additional materials being sent to a landfill and opens opportunities for additional revenue streams. Renewable Resources Biomass comes in many forms and there is significant debate as to which forms should be considered renewable. Virgin timber and the use of materials from forestlands harvested unsustainably face considerable scrutiny when being evaluated as renewable. However, since the fuel to be used for the Lebanon project is derived from waste streams, it can be deemed renewable. As the city provides a service and maintains its streets and right of ways, the wood waste will continue to be produced without impacting carbon storage or contributing to deforestation. Wastewater sludge also may not normally be thought of as renewable, but as this stream will also to continue to be created everyday it too can be deemed as a renewable resource. The result is 100% renewable fueled power generation. The only fossil fuels used are to start up the thermal oxidizer and to maintain a pilot flame, which in totality are negligible and insignificant to the overall project implementation. No additional energy is needed for preparation of discarded tires as the nylon cords and steel belts do not have to be removed in order for the tire to be used as feedstock as they become part of the biochar. PROTECTION OF ENVIRONMENT RESOURCES Utilizing Existing Resources The wood and wastewater sludge waste are expensive to dispose of and are not good for the air quality as presently disposed because it can convert to methane. The utilization of waste products for energy represents the highest value for otherwise low or no value materials. Innovation and utilization of existing resources has been a hallmark of Lebanon leadership and is consistent with their vision and future of their community. While it is true that many landfills currently utilize methane collection, capture and flare or energy generation technologies, the US EPA estimates that 60-90% of the methane is collected. This wasted energy represents a significant weakness in existing resource utilization strategies and presents a unique opportunity for forward-looking communities to control their own environmental and energy futures. Discarded tires are currently picked up and reused or recycled. Use of tires in the feedstock will further remove transport vehicles, labor costs and its emissions from the environment. Conservation Gasifying the wood waste, tires and sludge inside the city limits of Lebanon will result in a significant reduction in the use of coal and petroleum based products. Best practice energy and environmental strategies recommended by experts in the field consistently call for the use of efficiency and conservation strategies as the first line in forming a comprehensive plan for sustainable communities. The City of Lebanon hopes to become a leader and demonstrate in a concrete fashion to other communities in Tennessee and the nation how a conservation strategy aimed at utilizing existing resources can ultimately turn a liability into an asset. 368

5 Reduction in Carbon Intensity Biomass, when gasified and used consistent with the proposed project design, will also emit CO2. However, wood waste, tires and WWTP sludge are considered renewable and are accounted for as a short term biogenic carbon emissions. This proposal presents a significant opportunity to reduce the carbon intensity associated with the Lebanon WWTP. Reduction in the usage of diesel fuel used in the transportation of waste material to landfill and disposal sites also reduces the Lebanon WWTP s carbon intensity. AIR QUALITY Overall Emissions Reduction The City of Lebanon is located within the Tennessee Valley Authority electric generation grid, specifically Sub region: SERC Tennessee Valley. This project will reduce the need for electricity to be provided from power plants located within the TVA service area through Lebanon Electric. Since TVA will not have to generate the 748,880 kwh provided locally by the Lebanon Waste Biomass Gasification to Energy Project, there will be a reduction of 9809 pounds of sulfur dioxide, 3719 pounds of NOx, and 2717 tons of carbon. Non-Attainment Area Targeted The most direct reduction in emissions from the offset of electric energy is assumed to occur in Lebanon/Wilson County. At present the wastewater sludge is disposed of by local land application. Wood waste is taken to a landfill in Rutherford County. Discarded tires are taken out of state. The elimination of transportation of these waste streams would further reduce GHG, Sulfur dioxide, VOC's, Nitrogen oxides and HAP's in Wilson and Rutherford counties. ABILITY TO LEVERAGE (MATCH) FUNDING TO ENHANCE OVERALL PROJECT OBJECTIVES The City of Lebanon has been granted a Qualified Energy Conservation Bond for this project. QEBC will pay 70% of the interest, a savings of $122,000 per year. Qualified Energy Conservation Bonds were first authorized by Congress in October At that time, Congress allowed a maximum of $800 million in QECB volume cap nationwide. In February 2009, through the American Recovery and Reinvestment Act, Congress increased the volume cap to $3.2 billion. These bonds may be issued by state, local and tribal governments to finance qualified energy conservation projects. A maximum of 30% of the aggregate bonds may be used to finance private activity projects

6 ESTIMATE OF REDUCTIONS AND/OR AIR EMISSIONS This project is projected to result in over 2717 tons of CO2, 3719 pounds of NOx, and 9809 pounds of SO2 reduction since the fuel source is carbon neutral biomass using gasification and a thermal oxidizer to convert the biomass to heat. In addition, over 124 MW-hrs. of electricity from line losses will be saved since the power will be generated on-site. 370