# Presentation: 394473

## Overall Energy Balance for the Corn Stover to Ethanol Process

- Brianna G. Atherton, Mark F. Ruth,
- John L. Jechura, Kelly N. Ibsen
- 24th Symposium on Biotechnology For Fuels and ChemicalsApril 28 — May 1, 2002

**Operated for the U.S. Department of Energy by Midwest Research Institute • Battelle • Bechtel**

## Methodology

- Aspen Plus® energy balance basis converted to heat of combustion basis
- Ethanol energy ultimately released via combustion
- All energy flows put on a common basis for comparison
- Higher heating value (HHV) used for combustion heat
- Liquid water has negligible contribution to HHV
- Results
- Energy available in inlet & outlet streams
- Ethanol energy yield

## Enthalpy Calculation Scheme

- Ideal State Enthalpy
- Constant of integration at reference temperature
- Typically set to match enthalpy of formation
- Residual Correction
- Convert “ideal” state to real state
- Phase change latent heat effects
- Non-ideal mixture effects

## Heat of Combustion

- Lower heating value (LHV) — water in gas state
- Higher heating value (HHV) — water in liquid state

## Energy Balance Equations

**Energy Balance Equations**

- No chemical reaction, constant of integration term disappears
*n**i,in** – n**i,out* = 0

## Heat of Combustion Related to Heat of Formation

**Heat of Combustion ****Related to Heat of Formation**

## Alternate Ways to Convert Results to Heat of Combustion Basis

- 1. Calculate the pieces
- Heat of combustion at reference temperature (25°C)
- Sensible heat effect for temperatures other than 25°C
- Use Aspen Plus calculated overall heat capacity for each stream
- Latent heat effect for liquid components in gas phase
- Use heat of vaporization for each component at 25°C
- 2. Directly adjust the Aspen Plus calculated enthalpy
- Avoids potential problems of appropriateness of overall heat capacity
- Does not split out the different effects

## Feedstock Potential

## Process Energy Balance (2,000 tonne/day dry feed)(69 MM gal/yr ethanol)

## Process Energy Balance

**Process Energy Balance**

- Not all streams shown in diagram.  Individual sections may not appear to balance.

## Section Energy Balance

- Lignin residue provides majority of the energy
- Steam to process gives an overall high energy recovery
- Flue gas energy content primarily due to condensables in gas phase

## Energy Balance Observations

- Over 50% of the available energy exits process as ethanol product or exported electricity
- Flue gas energy due to latent heat effects
- Must remain hot enough to keep any acidic liquids from condensing
- More heat integration could potentially reduce cooling tower losses

## Other Process:Integrated Gasification Combined-Cycle Power System

- “Cost and Performance Analysis of Biomass-Based Integrated Gasification Combined-Cycle (BIGCC) Power Systems,” K.R. Craig & M.K. Mann, NREL/TP-430-21657, October 1996

## Comparison With Other Processes(Basis: 100 kcal Biomass Feed)

- “Renewable Energy Technology Characterizations,” TR-109496, Topical Report, prepared by the Office of Utility Technologies, U.S. Department of Energy, & EPRI, December 1997.

- “Determining the Cost of Producing Ethanol From Corn Starch and Lignocellulosic Feedstocks,” Kelly Ibsen, Andy McAloon, Frank Taylor, Robert Wooley, Winnie Yee, NREL/TP-580-28893, October 5, 2000.

- “Comparison of USDA and Pimentel Energy Balances,” Michael S. Grabowski, National Corn Growers Association Position Paper, August 22, 2001.

**Notes:**
Net Energy Efficiency is the ratio of all (Energy Out) divided by all (Energy In).

For the Biomass to EtOH process with energy generation this will be:
	 Net Energy Efficiency = (49 + 5)/100 = 0.54

For the Biomass to EtOH process with imported steam & power this will be:
	 Net Energy Efficiency = (49 + 48)/(100 + 3 + 32) = 0.72

For the Corn Dry Mill this will be:
	 Net Energy Efficiency = (57 + 35)/(100 + 2 + 26) = 0.72

## Acknowledgments

- Biochemical Conversion Element of the
- Office of Fuels Development of the
- U.S. Department of Energy