Municipal Wastewater Biogas Cogeneration

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Municipal Wastewater Biogas Cogeneration
4N04 Group 6 Final Presentation
Simon Burrell Jingjing Li Yaru Lu Shashank Pai
Alvin Pilobello Joshua VanRavenswaay
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Outline

• Background and Overview
• Engineering Principles
• Profitability Analysis
• Unit Operability
• Safety Analysis
• Problem Solving Methodology

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Background

• Hamilton wastewater treatment plant (WWTP)
• Biogas by-product 60 methane
• Historically Flared
• Cogeneration energy-efficient, cost-effective
• solution to meet environmental requirements?

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Overview

• Hamilton WWTP Biogas Cogeneration Process
• Cogeneration Heat Electricity
• Methane CH4 2O2 ? CO2 2H2O energy
• Available energy
• Electrical 32 ? 1.6 MW
• Thermal 48

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Overview
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Overview
Heat exchanger - Exhaust gases
Generating Engine (V20)
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Engineering Principles

• Biogas Generation in Anaerobic Digester
• Anaerobic digestion is used effectively in
  wastewater treatment process
• Organic wastes are broken down by micro-organism
  strictly in the absence of oxygen
• Operation Condition
• - Temperature range 15 to 45 oC
• - Residence time 2 to 60 days
• Digestion Process
• - Hydrolysis
• - Acidogenesis
• - Acetogenesis
• - Methanogenesis

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Engineering Principles

• Biogas Treatment
• Table shows the composition of biogas after it
  exits the digester
• Biogas leaves the digester and is saturated with
  water
• A condenser is facilitated to dry biogas and
  knock out all the impurities which dissolves in
  water
• Hydrogen Sulfide needs to be removed by iron salt

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Engineering Principles

• Electricity Generation by An Engine-Generator
• In the cogen system, electricity generator is
  powered by a reciprocating engine burning biogas
• A Reciprocating engine is a type of internal
  combustion engine
• Hot gas drives the engine piston
• An electrical generator converts the mechanical
  energy to electrical energy by electromagnetic
  induction

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Engineering Principles

• Heat Transfer of Combustion Exhaust
• In a cogen system, combustion exhaust (mainly hot
  CO2 at 600C) is used to generate steam from
  water (around 90C) and utilize it for plant
  heating application
• Although water is used as heat transfer medium,
  fouling is still possible

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Engineering Principles

• Environmental Impact
• Question
• What is the main environmental benefit we can
  get from cogeneration system?

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Profitability Analysis

• Government Incentives
• EcoENERGY program offers a 1 cent/ kWh subsidy
  for renewable energy for the first 10 years of a
  project.
• Government of Canada offers tax incentives which
  allow the capital cost of renewable energy
  projects to be depreciated using a 30 declining
  balance method over the life of the project.

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Profitability Analysis

• Project Cost Overview
• Capital cost 4 million dollars
• Project life is 20 years
• Estimated 200,000 per year for operating and
  maintenance costs

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Profitability Analysis

• Revenues from Electricity generation
• Average market rate of 4.87 cents per kWh over
  the past 12 months
• Generates 13,200 MWh per year (95 of 1600kW
  capacity)
• Subsidy of 1cent/kWh for the first ten years
• Revenue of 775,000/yr for the first ten years
  and 640,000/yr for the remaining 10 years

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Profitability Analysis

• Revenue from heat generation
• Combustion of 17000m3 of biogas per day at 21230
  Btu/m3
• Heats at 48 efficiency
• Offsets a total of 63,200 mmBtu/yr of Natural Gas
  heating
• Average market rate of 9.20/mmBtu over the past
  12 months
• Revenue estimated at 580,000/yr

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Economics

• Profitability Analysis
• Using a tax rate of 35 and a IRR of 15
• 30 declining balance depreciation as per tax
  incentive
• NPV 1,522,000
• Compared to NPV of 588,000 unsubsidized
• Subsidized DCFRR of 22.7 and 17.6 unsubsidized

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Operability

• Operating Window
• Controllability
• Reliability

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Operability
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Operating Window
Hard Constraint
Maximum Hot Water Temperature Constraint
Generator Constraint
Refrigeration Constraint
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Controllability
Combustion Engine (V-20)
Electricity
Hot Exhaust gases
Heat Exchanger 2 Water ? Steam
Cool water
Steam
Cooled Exhaust gases
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Reliability

• Process Inventory feed storage tank

• Existing biogas storage tank provides
• Uninterrupted fuel supply.
• Emergency fuel storage capacity.
• Feed composition buffering.

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Safety Analysis

• Hazards Within the Cogen Plant

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Safety Analysis

• Identified Nodes

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Safety Analysis
Risk Matrix
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Safety Analysis

• Node 1V20 Internal Combustion Engine

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Safety Analysis
Node 2 Shell Tube Exhaust Gas Heat Exchanger
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Trouble Shooting

• Overall Disturbances to the process
• Electrical grid
• Waste Water Treatment Plant disorder
• Mechanical problems in cogen facility

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Trouble Shooting Case

• There is a new plant operator on his first day of
  work
• The flow rate of the steam leaving the heat
  exchanger is lower than normal (approximately
  20)
• The cogeneration facility just finished a 1 day
  maintenance shutdown

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Trouble shooting Fish bone Diagram
Incomplete combustion
Power loss
cavitation
Fouling
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Conclusion

• Efficient energy recovery
• Cost-effective alternative to utilities
• Environmental benefits
• Offsets electricity generated from other sources
• Less natural gas dependence