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In this issue we will explore the new energy and operating cost functionality in BioWin 5.0. Simulations of plant design provides a robust method of comparison and optimization of process performance and a means to identify potential upgrades and/or improvements which would minimize operating expenditures. Three process design alternatives consisting of combinations of physical treatment, aerobic and anoxic biological treatment; chemical addition and energy recovery via a CHP engine will be compared through simulation. We will investigate the effluent quality, energy requirements, and operating cost implications associated with each design.
Background on Power & Costs in BioWin 5.0
BioWin 5.0 enables users to capture a plant-wide inventory of power demand including: Blower Power, Mixing Power, Mechanical Power, Pumping Power, Heating Power, Surface aeration Power, Solid Liquid Separation/Disinfection Power, and Heating Ventilation and Cooling Power. This allows the power requirements to be easily displayed and itemized.
Power is tracked for elements via their Power tabs. Power can be a constant input, such as in mixing power in bioreactors and mechanical power in clarifier elements.
A very important category is blower power requirements. The first step in determining blower power is defining Air Supply groups where you can specify whether aerated elements are part of a group of bioreactors/aerated zones supplied by a common blower. You can choose between three blower power calculation methods (adiabatic/polytropic, linear, and user defined). BioWin will automatically totalize the air delivered by a blower to a group of reactors and calculate blower power requirements, taking into account factors such as inlet air temperature and relative humidity, pressure losses in the air delivery system, etc.
Pumping power may also be tracked, accounting for detailed factors such as pipe material, diameter for dynamic head losses, and pump efficiency.
BioWin calculates the amount of power required to heat anaerobic digesters, accounting for parameters such as boiler efficiency and daily heat loss. Users can also explore onsite power generation and heat recovery via use of a digester’s biogas in a CHP engine and specify the fate of the generated power and heat.
BioWin 5.0 also enables users to track and totalize operating costs associated with energy consumption, consumption of chemicals/consumables and sludge disposal.
BioWin includes the facility to easily implement up to three different daily electricity tariff rates of the day (off-peak, mid-peak, and peak rates), and daily patterns can be different across two seasons (e.g. summer and winter). Monthly peak demand charges and base utility charges can also be factored into the energy cost analysis.
Chemical costs (on a per unit volume basis) are tracked in BioWin on a global project-wide basis for consumables such as methanol or metal salts. You also have the option to include individual elements that may have consumables associated with their operation (e.g. dewatering units, effluent streams) from your flowsheet in the overall project cost tracking via their Costs tab.
A similar interface is offered for each sludge output element to allow for tracking of sludge disposal costs.
With the additional ability to explore onsite power generation and heat recovery via CHP, tracking of other costs including consumables (e.g. methanol and metal salts) and sludge disposal, BioWin 5.0 has expanded capability as a plant management tool. For more detailed information on the new power and cost functionality in BioWin 5.0 please see the Power in BioWin and Operating Costs in BioWin chapters of the BioWin 5.0 Manual. New Developments in BioWin 5.0 are also summarized in the following document and video, respectively:
The BioWin Configurations
The first BioWin configuration (Case #1) we will consider is shown in the figure below.
This configuration provides nitrogen removal via an anoxic/aerobic plug flow activated sludge process with nitrate and MLSS recycle. The plug flow train is followed by a secondary clarifier. The WAS flow is set to maintain a 12.5 day SRT. The WAS stream is thickened and sent to an aerobic digester for further stabilization. Lime is added into the aerobic digester to maintain its pH. Additional physical and operational details for the configuration are provided in the table below.
In addition to the physical and operation parameters, power was also specified in various elements. A mechanical mixing power of 5 W/m3 was input into each of the un-aerated bioreactors (AX-#1, AX-#2, AX-#3). A mechanical power of 3 kW was specified in the ideal clarifier to account for a sludge scraper motor. A power input of 0.09 kW/(m3/d) was specified in the WAS thickening element (DAFT – dewatering unit) while the power input for the digester centrate thickening element (CFG – dewatering unit) was specified as 0.13 kW/(m3/d). The nitrified recycle (NRCY), RAS and WAS pumps had the following characteristics specified for pumping power calculations:
Four air supply groups were specified as follows with the following details for blower power calculations:
Costs were specified for the project as follows. The cost of chlorine disinfection ($0.01/m3 of treated flow) was including in cost calculations for the effluent element. The 3M lime addition was included in chemical costs at a rate of $0.25/L. A sludge disposal cost of $0.07/kg TSS was specified in the WAS Cake element. Electricity costs were specified as seasonal with similar Summer and Winter rates as shown in the screen shot below. The screen shot below also shows the other electricity charges used for the project:
The second BioWin configuration (Case #2) we will consider is shown in the figure below.
The second configuration provides various updates to Case #1. For example, Case #2 includes primary sedimentation of solids. Due to the presence of primary solids in addition to WAS, the aerobic digester element was replaced with an anaerobic digester element. As a result of the solids and BOD loading reductions associated with primary sedimentation, the overall volume of the plug flow activated sludge process in Case #2 was reduced by approximately 48% (from a total of 21,600 m3 to a total of 11,250 m3) which would be included in a comprehensive analysis factoring in capital costs. The SRT was maintained at 12.5 days. Updated elements and element information including power for Case #2 is summarized in the table below:
The third BioWin (Case #3) process flowsheet is identical to Case #2. However, energy recovery via a CHP engine is specified in the anaerobic digester as illustrated in the screen shot below.
Furthermore, all of the CHP power generated will be used on site (with the option to sell any excess back to the electricity grid at a rate of $0.15/kWh if applicable). These choices are selected under Project > Costs/Energy > Combined Heat and Power (CHP) engine.
Setting up the BioWin Album to Track Power and Costs
New output functionality in BioWin 5.0 allows power/energy use and cost outputs to be generated semi-automatically. Power charts include:
- Power Demand Distribution plots
- Pie plot of instantaneous power demand
- Bar plot of instantaneous power demand
- Time Series plots
- Instantaneous power by category
- Total and net instantaneous power
- Energy consumption (Daily)
- Energy consumption (Monthly)
- Energy consumption (Yearly)
- Energy consumption
Power charts can be generated through the following steps:
- In a blank page of the BioWin Album right-click and select Chart
- Select the Power/Energy Use tab
- Select the elements that you want to include in the calculation of power for the chart via each of the power categories.
- For a power demand distribution plot select the Plot and Label Type then click the Instantaneous power distribution button to generate the plot.
- For time series plots select the Axis then click on the button that represents the type of times series you wish to plot.
In addition to these plots, you can also add pre-defined power tables to the album or build your own custom element-specific power table. Adding a pre-defined power table can be completed in 4 simple steps:
- Right click on a blank album page and select Power table…
- Select the elements that you want to include in the calculation of power for the table via each of the power categories.
- Specify any additional options to include power cost, total power and system wide power.
- Click ok to generate the table.
Costing charts include:
- Cost Distribution plots
- Pie plot of instantaneous project costs
- Bar plot of instantaneous project costs
- Time Series plots
- Power/Energy costs
- Instantaneous cost for power and energy taking into account varying daily energy costs
- Running tally of total energy costs to date
- Chemical costs
- Instantaneous and running total costs for chemicals
- Sludge Disposal
- Instantaneous and running total costs for sludge disposal
- Total Project Costs
- Instantaneous and running total costs for the entire facility (including energy, chemicals, and sludge handling).
- Power/Energy costs
In addition to these plots, you can add a pre-defined cost table to the album or build your own custom element specific cost table. Adding a per-defined cost table can be completed in 1 simple step: Right-click on a blank Album page and select Cost table
Results and Discussion
A steady state simulation was run with each of the three configurations and will be used as the basis for comparison. The steady state simulation was run from Seed values. The table below summarizes the steady state effluent, power demand, and costs results.
As shown in the Effluent column of the above table, the effluent data for Case #2 and #3 are identical. This outcome is expected since the only difference between these configurations is that CHP is used in Case #3. In terms of final nitrogen concentrations, Case #1 is able to reach a lower Total Nitrogen concentration since this configuration has no primary settling and therefore has more carbon loading to the activated sludge process for driving denitrification. Effluent pH is higher in Case #1 due to the addition of 3M lime in the process. The concentrations of the other effluent parameters are similar between each of the three scenarios.
As shown in the Power column of the table above, Case #1 has a higher demand in terms of blower power and mixing power. The higher blower power is a result of the additional loading on the activated sludge process arising from the lack of primary sedimentation and additional blower power for the aerobic digester; the higher mixing power is due to larger basin sizes required to maintain design MLSS levels with the higher loading associated with the lack of primary sedimentation. Mechanical power is slightly higher in Case #2 and #3 due to the presence of the primary clarifier in these configurations. The solid/liquid separation/disinfection power is lower in Case #2 and #3 since less WAS sludge is processed by the DAFT dewatering unit as a result of the primary clarification unit sending a fraction of the solids to the digester directly. As proof, if you hover your cursor over the arrowhead of the orange pipe from the WAS pump to the DAFT unit you will notice that the mass loading of TSS into the DAFT unit is 3,763 kg/d for Case #1 and only 1,981 kg/d for Case #2 and #3.
In Case #2 heating the anaerobic digester makes a substantial contribution to the overall power demand. In contrast, Case #3 converts 35% of the digester biogas to heat via the CHP engine, and a portion (due to efficiency loss) of this heat is transferred to the digester input stream. As a result, the heating power is reduced from ~160 kW in Case #2 to ~39 kW in Case #3. In addition, 33% of the digester off-gas in Case #3 is converted to power via the CHP engine. This power produced by the CHP engine appears as a ~177 kW credit (i.e. a negative value is shown since this value represents power produced and not consumed). The option to use this power onsite was specified, so this credit offsets the total power demand for Case #3 and results in a net power of ~141 kW.
Although Case #2 has the highest power consumption and hence highest power costs, the configuration does not require the addition of 3M lime and thus chemical costs are much lower in comparison to Case #1. Despite the fact that all three configurations are run at the same SRT, the sludge disposal costs are larger in Case #1 due to the lack of primary sedimentation and use of aerobic versus anaerobic digestion.
Several steady state charts and tables have been configured in the BioWin Album in addition to the power and costs data including: a chart of the solid mass distribution; ammonia, nitrite and nitrate profiles along the plug flow reactors and in the effluent; a chart of SOTE; a table of aeration parameters; and element info summaries for the effluent element.
In addition to the steady state results, a dynamic simulation was also run for each configuration. A dynamic simulation was run from Current values for 7 days. The results of the dynamic simulation can be viewed in the BioWin Album including plots of: influent flow and load; a state point analysis diagram for the secondary settler; dynamic effluent profiles; dynamic ammonia, nitrite and nitrate profiles along the plug flow reactor; airflow rate, oxygen transfer rate, and SOTE series; instantaneous power demand, total and net power demand, daily and total energy use; dynamic power, sludge, chemical and total costs.
In this edition of the BioWin Advantage, we used three configurations to investigate the power and cost features in BioWin. We also learned how to create power and cost plots and tables in the BioWin Album. In future editions, we will look at more complex energy configurations, and explore using dynamic simulation analysis the impacts of peak energy use and its cost implications.
We trust that you found this technical topic both interesting and informative. Please feel free to contact us at email@example.com (Subject: The BioWin Advantage) with your comments on this article or suggestions for future articles.
Thank you, and good modeling.
The EnviroSim Team
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