WEF Residuals and Biosolids Conference 2017

Transcription

1 Triple Bottom Line Analysis of Energy Recovery from Thermal Oxidation of Wastewater Solids Compared to Coal Anna J. Munson 1*, Webster F. Hoener 1, Robert P. Dominak 2, James E. Welp 1 1 Black & Veatch. 2 Friar Consulting. * MunsonA@bv.com ABSTRACT Many water resource recovery facilities (WRRFs) use thermal oxidation (incineration) to manage solids produced by the wastewater treatment process. While this management option has typically been used to achieve volume/mass reduction and sterilize the solids, energy recovery from thermal oxidation is of increased interest as a component of approaches to attain energy neutrality. Because of the increased interest in energy recovery, the Water Environment & Reuse Foundation (WE&RF) has undertaken a state of science review, with the objectives to: Evaluate the potential for energy and heat recovery from the thermal oxidation of wastewater solids based on the latest generation of thermal oxidation technology. Compare the value of energy recovered from wastewater solids by thermal oxidation with that from coal, based on a triple bottom line (TBL) approach, evaluating economic, environmental, and social criteria. Estimate the quantity of renewable energy available from thermal oxidation of wastewater solids and residuals from domestic wastewater and associated feedstocks, such as fats, oil, and grease (FOG), scum, and imported biomass. This paper summarizes the findings from one of the major research activities; a TBL life cycle assessment comparing thermal oxidation of wastewater solids with coal. Documentation of the potential recoverable energy from wastewater solids and residuals and a state of technology summary reviewing the different systems available for recovering energy from incinerators were included in previous papers. KEYWORDS: Thermal oxidation of wastewater solids, energy and heat recovery. INTRODUCTION Many water resource recovery facilities (WRRFs) use thermal oxidation (incineration) to manage solids produced by the wastewater treatment process. While this treatment option has typically been used to achieve volume/mass reduction and sterilize the solids, energy recovery from thermal oxidation is of increased interest as a component of approaches to attain energy neutrality. Trends in anaerobic digestion, such as co-digestion of FOG or imported organic wastes to increase energy production, raise the question if similar enhancement of energy recovery could be accomplished for thermal oxidation with the co-combustion of similar 401

2 alternative feedstocks. Understanding the role energy recovery from solids through thermal oxidation could perform is an important factor in determining ways for publicly owned treatment works (POTWs) that practice incineration to reduce operating costs. Because of the increased interest in energy recovery from thermal oxidation of wastewater solids, the Water Environment & Reuse Foundation (WE&RF) has undertaken a state of science review, supported by Target Collaborative Research funding, identified as project ENER13T14. The objectives of the research were: Evaluate the potential for energy and heat recovery from the thermal oxidation of wastewater solids based on the latest generation of thermal oxidation technology with combined heat and power (CHP) systems. Compare the value of energy recovered from wastewater solids by thermal oxidation with that from coal, based on a triple bottom line (TBL) approach, evaluating economic, environmental, and social criteria. Estimate the quantity of renewable energy available from thermal oxidation of wastewater solids and residuals from domestic wastewater and associated feedstocks, such as FOG, scum, and biomass at existing incineration facilities to assist WE&RF in informing the Department of Energy, regulators, and lawmakers. The findings from two of the major research activities were presented previously. The evaluation of potential energy and heat recovery from thermal oxidation of wastewater solids with the latest generation of thermal oxidation technology was presented at WEF s 2016 Residuals & Biosolids Conference. A summary of the thermal oxidation technologies available and industry experience with the technologies was presented at WEF s 2016 WEFTEC Conference. This paper summarizes the findings from the remaining major research activity; a TBL life cycle assessment comparing the energy value from thermal oxidation of wastewater solids to the energy value from coal. TRIPLE BOTTOM LINE (TBL) EVALUATION OF THE ENERGY VALUE FROM WASTEWATER SOLIDS COMPARED TO COAL Sustainability is an important goal for the water resource recovery field. WRRF managers are advancing the sustainability of their operations through energy efficiency measures, renewable energy systems, and resource recovery projects. Sustainability is widely defined in business, industry, and the public sector as standing on three pillars: economic, environmental, and social. These are the three components of the TBL analysis; a relative simple concept to grasp, which is used widely by corporations and government entities worldwide. As part of the research to achieve energy neutrality at WRRFs, WE&RF developed a TBL evaluation tool (Elenbaas, 2014) to help WRRF decision-makers assess solids management technology options and process configurations based on which can yield the most sustainable outcomes. The TBL evaluation tool was used to assess the sustainability of energy harnessed from thermal oxidation of wastewater solids compared with energy produced at a coal-fired power plant. Coal was selected for comparison because the combustion and energy recovery 402

3 equipment used is similar to that for wastewater solids and is one of the most common energy sources for producing electrical power in the United States. Energy Alternatives The alternative for energy recovery from thermal oxidation of wastewater solids (Wastewater Solids Alternative) is based on new heat recovery and power production improvements to an existing fluidized bed incineration facility to produce electricity. The system configuration of an FBI co-fired with FOG and wastewater solids was selected as the basis for the TBL analysis as most representative of current technology for thermal oxidation systems in the United States using the most effective feedstocks for renewable energy production. The facility is based on a capacity of 90 dry metric tonnes (dmt) (100 dry tons (dt)) of wastewater solids per day and 378 megaliters (ML) (100 million gallons (MG)) of wastewater per day. The energy recovery system includes: waste heat boiler steam turbine generator steam system electricity transmission system building Energy recovery from the system was estimated at 631 kwh per dry metric tonne of wastewater solids processed. For the purposes of evaluation, only capital and operating costs for the new energy recovery improvements were evaluated, as the solids thermal oxidation systems would be existing with a primary purpose of processing wastewater solids and operating costs for treatment would occur whether energy was recovered or not. The alternative is based on sufficient land being available at the facility for construction of new energy recovery systems. The alternative for electrical power production from a coal-fired power plant (Coal Alternative) is based on a super critical (SC) pulverized coal (PC) plant with a single reheat cycle and a CO 2 capture system. A SC PC plant was selected to represent current technology for efficient power production from coal combustion. Use of a CO 2 capture system was selected for compliance with future 2030 Clean Power Plan emission requirements. The facility is based on power production capacity of 550 MW. The SC PC plant includes: coal storage and feed systems PC boiler generator with high-, intermediate-, and low-pressure turbine sections baghouse emission control system, including selective catalytic reduction, flue gas desulfurization, and carbon dioxide recovery systems steam system cooling water system electricity transmission system stack building 403

4 The Coal Alternative was based on coal supplied from existing mines, but mining activities were considered when evaluating social and environmental impacts. Similar to the energy recovery alternative, the cost for land acquisition was not included in evaluation of the Coal Alternative. Criteria regarding sizing, energy balance, emissions, and costs for the SC PC plant are based on Case 12 of the National Energy Technology Laboratory report Cost and Performance Baseline for Fossil Energy Plants (NELT, 2013). TBL Categories and Scoring The criteria selected for evaluating the Wastewater Solids and Coal Alternatives followed the general criteria established in the TBL evaluation tool (Elenbaas, 2014). Criteria and weightings from that tool were adapted for the scope and objectives of this research using the following considerations: Determine the value of each criterion to the decision-making process and eliminate criteria that result in similar scores among all options. Ensure that the criteria address all key elements of sustainability. Develop metrics for each criterion, using measurements that are appropriate and commonly understood by stakeholders. Table 1 lists the criteria and weightings used in the TBL evaluation. Additional air quality criteria and workplace conditions were added to assess elements that were important to the stakeholders in this evaluation. Each prime category of economic, environmental, and social impacts were given equal weight to reflect the importance of each of these categories. The research team adjusted the weightings for the sub-categories and criteria to reflect the study focus of energy resource recovery. The economic category included lifecycle cost assessments and engineering/technical criteria of simplicity, flexibility, and state of technology. Lifecycle cost expresses the total cost of construction and operation of a given alternative. The net present value (NPV) of operations and maintenance (O&M) costs for 20 years was included in the lifecycle cost assessment for each alternative. The lifecycle cost was a major consideration for this evaluation and was weighted at 85% of the total economic category score. Engineering and technical considerations comprised the remaining 15% of the economic category score. Within this subcategory simplicity, flexibility and the state of technology were weighted equally important. The simplicity criterion is a measure of how simple the system is to operate, maintain, and repair. Flexibility is a measure of how easily the alternative can be modified to support future process changes and capacity expansion. The state of technology is a measure of how well established an alternative is within the industry. Industry experience lowers the economic risk to maintain and optimize the well-established technology. 404

5 Table 1. TBL criteria and weighting for Wastewater Solids and Coal Alternatives. Subcategory Category Criteria TBL Category & Criteria Weight Weight Weight % of TBL Score Economic 33.3% Net Present Value 85% 28.3% Engineering/Technical 15% 5% Simplicity 33.3% 1.7% Flexibility 33.3% 1.7% State of Technology 33.3% 1.7% Environmental 33.3% Conservation of Resources 50% 16.7% Ash Use 15% 2.5% Fixed Carbon - Energy Recovery 35% 5.8% Fixed Carbon - Greenhouse Gases 35% 5.8% Water Conservation 15% 2.5% Air Quality Impact 25% 8.3% NOx 17% 1.4% SO2 17% 1.4% Particulate Matter 17% 1.4% Lead 17% 1.4% Mercury 17% 1.4% HCl 17% 1.4% Regulatory 25% 8.3% Social 33.3% Nuisance Issues 50% 16.7% Dust 33% 5.5% Traffic 33% 5.5% Visual 33% 5.5% Workplace Conditions 50% 16.7% Workplace Environment 50% 8.4% Illness/Injury 25% 4.2% Fatality 25% 4.2% The research team considered conservation/optimization of resources more important than the other sub-categories in this particular evaluation because the study focused on energy resource recovery. This sub-category was weighted as 50% of the environmental category score. The conservation/optimization of resources sub-category reflects the number of ways in which ash remaining from combustion of wastewater solids and coal can be reused and how efficiently each alternative uses carbon-based fuel and water. Ash is a common byproduct of both alternatives and was considered a resource for this analysis. In some cases, ash is used for products such as cement add-mixtures, road base aggregate, and roofing shingle production. The ash criterion was assessed based on the volume of ash produced 405

6 and its potential for reuse. The research team assigned ash production a weighting of 15% of the conservation sub-category score. Similarly, conservation of water criterion was assigned a weighting of 15% of the conservation sub-category score. Each alternative was assessed based on the amount of water consumed per unit of electricity produced. The analysis considered only raw water used from external sources and not water recycled within the process itself. Whether energy was from a renewable biogenic source or non-renewable fossil fuel source was evaluated in the Fixed Carbon - Energy Recovery sub-category and assigned a weighting of 35%. The greenhouse gas criterion reflects the relative amounts of greenhouse gases produced by each alternative. The research team calculated greenhouse gas emissions for each alternative based on carbon dioxide equivalent emissions from existing FBIs and the emissions identified for coal power plants with carbon (CO 2 ) capture technology in Cost and Performance Baseline for Fossil Energy Plants (NETL, 2013). The research team assigned the greenhouse gas criterion a weighting of 35% of the conservation/optimization of resources sub-category score. The sub-category of air quality was weighted as 25% of the environmental category score. Pollutant emissions from each alternative were evaluated based on the maximum amount of pollutant release allowed by regulatory requirements for each technology. For coal power plants, emissions must meet the Utility Maximum Achievable Control Technology (MACT) standards. Facilities using thermal oxidation of wastewater solids must meet the Sewage Sludge Incinerator (SSI) MACT standards. Both MACT standards contain emission limits for the following compounds: oxides of nitrogen (NOx) sulfur dioxide (SO2) particulate matter hydrogen chloride (HCl) mercury (Hg) lead (Pb) The TBL air quality impact sub-category assessed the amount of each compound that could be released by the alternative per unit of energy produced. The remaining 25% of the environmental category score was a measure of the alternative s flexibility to adjust to new environmental regulations. Both thermal oxidation of wastewater solids and coal power generation are subject to extensive regulations. For this TBL assessment it was assumed that future environmental regulations for combustion facilities will increasingly restrict exhaust gas emissions and byproduct use or disposal. This study assessed how each alternative could respond to more restrictive environmental regulations. The social category represents the impact of each alternative on the community and was based on two sub-categories: nuisance issues and workplace conditions. The sub-categories were 406

7 assigned equal weighting for the total social category score. Nuisance issues were evaluated based on criteria of dust, visual impact, and traffic. Workplace conditions were evaluated qualitatively on workplace environment, such as how comfortable the facilities are for employees, and quantitatively on workplace safety incidents. Workplace environment scores were assigned a workplace condition weight of 50%. The workplace safety analysis compared illnesses/injuries and fatalities reported to the U.S. Bureau of Labor Statistics from for coal mining, coal power plants and wastewater treatment plant operations. The illness/injury scores and the fatality scores were equally weighted for the workplace conditions sub-category. Economic Category Scores The Coal Alternative received an economic category score of 4.8 whereas the Wastewater Solids Alternative scored The criteria analyzed to determine the scores were the lifecycle cost of both alternatives and the engineering/technical considerations. The lifecycle cost assessment included the cost of construction, operation and maintenance for each alternative. Both alternatives were to be constructed on property already owned by the facility. Construction cost included the equipment, building, instrumentation, electrical and all design and construction services. Operation and maintenance costs included electricity, chemicals, fuel, maintenance materials, water, waste disposal and labor. The lifecycle cost for the Coal Alternative was much higher than for the Wastewater Solids Alternative due to the difference in size of each facility. The coal power plant was based on an electric production capacity of approximately 550,000 kw, whereas the wastewater solids energy recovery facility was based on an electric production capacity of 1,900 kw. Costs for both alternatives were normalized on a unit of energy basis ($/kwh). Normalized capital costs for the Coal Alternative were significantly lower than the Wastewater Solids Alternative, reflecting the economies of scale for the larger production facility. Normalized operating costs for the Wastewater Solids Alternative were significantly lower than the Coal Alternative, reflecting the relatively low incremental operating costs of operating energy recovery for an existing process facility. The costs are summarized in Table 2. Table 2. Wastewater Solids and Coal Alternative costs. Cost Parameter Wastewater Solids Coal Capital Cost $16,794,000 $1,867,567,000 Net Present Value (NPV) Operations and Maintenance (O&M) Cost $3,088,000 $3,233,151,000 Total NPV Cost $19,882,000 $5,100,718,000 Net Power Output (kw) ,970 Capital Cost ($/kw) $8,839 $3,396 O&M Costs ($/kw) $1,625 $5,879 Lifecycle Cost ($/kw) $10,464 $9,

8 The Coal Alternative received a score of 5.0 in this subcategory reflecting that it had the lowest lifecycle cost, while the Wastewater Solids Alternative received a score of 4.4. The engineering/technical sub-category was scored qualitatively as shown in Table 3. Table 3. Engineering/technical criteria scores. Engineering/Technical Criteria Criteria Weights Wastewater Solids Score Simplicity 33.3% 4 4 Flexibility 33.3% 3 3 State of Technology 33.3% 4 4 Weighted Average Score Coal Score The lifecycle cost and engineering/technical sub-category weights of 85% and 15%, respectively, were applied to the criteria scores to generate the economic category scores depicted in Figure 1. The Coal Alternative scored slightly higher than the Wastewater Solids Alternative because of the economies of scale associated with the large coal-fired power plant Wastewater Solids Figure 1. Category weighted economic results. Environmental Category Scores The Environmental Category evaluated each alternative for the conservation of resources and ability to adjust to regulatory changes as well as the air quality impacts of each alternative. The Wastewater Solids Alternative received an environmental score of 3.8 whereas the Coal Alternative received an environmental score of 2.1. Coal NPV Engineering/Technical

9 Conservation/optimization of Resources The resources evaluated for this assessment were carbon, water and ash. The energy recovery portion of the wastewater solids thermal oxidation process does not consume or produce any of these resources. However, energy cannot be recovered without the operation of the incineration process, and the environmental impacts were evaluated for the entire thermal oxidation process to consider how this process compared overall with the energy produced from coal-fired power plants. The scores for this sub-category are provided in Table 4. Table 4. Conservation/optimization of resources scores. Conservation of Resources Criteria Wastewater Weight Solids Score Coal Score Ash Use 15% 3 4 Fixed Carbon Energy Recovery 35% 5 0 Fixed Carbon Greenhouse Gases 35% Water Conservation 15% 5 0 Weighted Average Score Air Quality Impact The impact of each energy production alternative on air quality was considered in the environmental category assessment. The compounds evaluated were selected as representative of key pollutant impacts from the emissions regulated by the MACT standards for SSIs and utilities. The wastewater solids thermal oxidation system emissions were based on the emission limits of the MACT standards for existing fluid bed incinerators. The emissions from coal-fired power plants were based on Utility MACT standards emission limits and values were obtained from the 2013 NETL report. Emissions were normalized on an energy unit basis for comparison of alternatives. Table 5 shows the calculated emissions for each alternative. Table 5. Air pollutant emissions. Wastewater Coal, Air Pollutant Solids, g/mwh g/mwh HCl NO x SO Hg Pb Particulate Matter Each pollutant was weighted equally to determine the weighted average of the air emission criterion. The Wastewater Solids Alternative received a weighted score of 1.83, whereas the Coal Alternative received a weighted score of

10 Regulatory Flexibility The flexibility of each alternative to adapt to regulatory changes was assessed and scored qualitatively. The weighted score of each criterion were multiplied by the sub-category weight and summed to calculate the environmental category score for each alternative. The environmental category scores are depicted in Figure Wastewater Solids Coal Conservation of Resources Air Quality Impact Regulatory Figure 2. Category weighted environmental results. Social Category Scores The Social Category included nuisance issues that impact the general public and workplace conditions that impact plant personnel. Nuisance issues considered were dust, visual impacts, and traffic. The Wastewater Solids Alternative would generate little dust outside of the facility. An energy recovery facility would look very similar to other buildings at the WRRF and would have minimal visual impact. Only minor increases in truck traffic would be expected with construction of an energy recovery facility. The nuisance issues for coal power plants are more prominent than for the wastewater solids incineration facilities. Coal mining was included in this assessment because it is an integral activity of power production from coal. A coal power plant with coal mining operations produces more dust and traffic than a wastewater solids energy recovery facility. The visual impacts on the community are about the same for the respective alternative energy generation facilities, but the Coal Alternative scored lower in this category because a coal mine has negative visual impacts. The scores for the nuisance issues subcategory are shown in Table

11 Table 6. Nuisance issues scores. Nuisance Issues Criteria Weight Wastewater solids Score Coal Score Dust 33.3% 5 0 Visual 33.3% 4 2 Traffic 33.3% 4 1 Weighted Average Score The workplace conditions sub-category included the criteria of workplace environment and workplace safety. The workplace environment assessment was qualitative. Workplace safety scores were based on incident data available from the U.S. Bureau of Labor Statics for 2012 to Data from the categories for Mining/Coal Mining and Utilities/Fossil Fuel Electric Power Generation was used for the Coal Alternative. Data from the Utilities/Sewage Treatment Facilities was used for the Wastewater Solids Alternative. The inverse of average incidents was normalized to the percent of the maximum score and then scores were adjusted down so that the lowest score is 0. Table 7. Workplace injuries and fatalities calculations and scoring. Workplace Safety Illness & Injury Average Incidents Per Year Inverse Normalized Score Sewage Treatment Facilities 284, E Coal Mining and Power Generation Fatalities 1,047, E Sewage Treatment Facilities Coal Mining and Power Generation Adjusted Score The workplace conditions score was calculated by applying a weighting factor of 50% to the workplace environment score, 25% to the illness/injury score and 25% to the fatalities score as shown in Table 8. The Wastewater Solids Alternative received a workplace conditions score of 4.03 and the Coal Alternative received a score of

12 Table 8. Workplace conditions subcategory scores. Criteria Wastewater Workplace Conditions Coal Score Weight Solids Score Workplace Environment 50% Injuries 25% Fatalities 25% Weighted Average Score The nuisance issues score and workplace conditions scores were each weighted at 50% of the Social Category score. Figure 3 depicts the weighted scores that were generated Wastewater Solids Coal Nuisance issues Workplace Safety Public Engagement Figure 3. Social category results. 412

13 Overall TBL Evaluation An overall weighting of the category scores resulted in a 4.1 rating for the Wastewater Solids Alternative and a 2.6 rating for the Coal Alternative. The overall TBL ratings demonstrate that based on overall sustainability considerations, energy recovery from wastewater solids has advantages for generation of electricity compared with coal-fired power plants. Figure 4 shows the Category Weighted TBL results Wastewater Solids Coal Economic Environmental Social Figure 4. Category weighted TBL results for energy recovery from thermal oxidation of wastewater solids and coal-fired power production. CONCLUSIONS FROM THE TBL ANALYSIS The primary conclusion from the TBL analysis of power produced from energy recovered from thermal oxidation of wastewater solids compared to energy generated from coal is that the Wastewater Solids Alternative is more sustainable. This is mainly a result of the higher score that the Wastewater Solids Alternative achieves when environmental and social impacts are accounted for. Additional conclusions from the TBL analysis include: Lifecycle costs for the Wastewater Solids and Coal Alternatives are relatively similar, with lower costs for electricity produced from coal-fired power plants attributed to economies of scale. Conservation of resources and regulatory flexibility were significantly more favorable for the Wastewater Solids Alternative than the Coal Alternative, resulting in a higher environmental category score for the Wastewater Solids Alternative. Although a coal-fired power plant itself was not rated to be a major nuisance, the mining and transportation of the coal significantly decreased the social sustainability score for this alternative. 413

14 ACKNOWLEDGEMENTS The authors thank WE&RF for its support of this research. In addition, the authors thank the many personnel at facilities that provided information and numerous hours of in-kind contributions and the ENER13T14 PSC team for their valuable guidance and oversight. REFERENCES Elenbaas, M., A. Patil, S. Tarallo, N. Beecher, A. Carpenter, and S. Brown Triple Bottom Line Evaluation of Biosolids Management Options. Report No. ENER1C12a. Alexandria, VA: Water Environment Research Foundation /New York State Energy Research and Development Authority. Dominak, R., W. Hoener, J. Welp, and G. Queiroz WEFTEC Conference Presentation: Energy Recovery from Thermal Oxidation of Wastewater Solids: State of Science Review. Water Environment Research Foundation. Hoener, W., R. Dominak, J. Welp, and G. Queiroz Residuals & Biosolids Conference Presentation: Evaluation of Energy Potential and Heat Recovery from Thermal Oxidation of Wastewater Solids with the Latest Generation of Thermal Oxidation Technology. Water Environment Research Foundation. National Energy Technology Laboratory Cost and Performance Baseline for Fossil Energy Plants Volume 1: Bituminous Coal and Natural Gas to Electricity, Revision 2a; U.S. Department of Energy. U.S. Department of Labor, Bureau of Labor Statistics, Census of Fatal Occupational Injuries (CFOI), Industry by private sector, government workers and self-employed workers. Summary Tables (2012, 2013, 2014). (accessed Aug 2016). U.S. Department of Labor, Bureau of Labor Statistics. Incidence rate and number of nonfatal occupational injuries by industry and ownership. Summary Tables (2012, 2013, 2014). (accessed Aug 2016). 414