How Green is That Product? An Introduction to Life Cycle Environmental Assessment

Homework #2 Solutions

Correct Answers in Red

Goals: In this assignment, youll do the following:

practice scaling of mass flows across unit processes in a life-cycle system; and

analyze and interpret primary and energy carrier data for the U.S. electric power system.

Instructions: The questions below can be answered offline. When you complete the assignment, return

to the Week 2 course page on the Coursera website. Click on the Submit Homework Assignment

Answers button, which will allow you to enter your answers into a web form for automated grading.

Grading: This assignment is worth 100 points. The point values of each answer are listed below. You

can submit a maximum of 30 attempts. The highest scoring attempt that is submitted before the

deadline will count toward your official grade. Scores for each attempt will be available immediately

after submission of your answers.

Numbers: In this assignment, and throughout this course, numbers will be expressed using the U.S.

numeric convention where commas separate thousands and the dot (or decimal point) is the decimal

separator. For example, the number one thousand two hundred and one-tenth is written 1,200.1.

Normalizing inventories and scaling of mass flows (10 points for each correct answer)

Consider a fictitious product, which well call a widget. A simplified unit process system for

manufacturing widgets looks like this:

Table 1 contains selected mass flow data for manufacturing plants representative of those that would be

needed in the supply chain for widgets, expressed in units of metric tons (t). Data have been collected

from each plant for an entire year.

Table 1: Annual operations data for four different manufacturing plants (t/year)

Manufacturing plant Raw material input Manufactured product output Steel production 1,425,000 t iron ore 367,000 t raw steel Shape forming 525,000 t raw steel 515,000 t steel bars Widget cutting 50,000 t steel bars 35,000 t rough widgets Widget grinding 35,000 t rough widgets 34,300 t finished widgets

Note: Numbers expressed using U.S. convention where commas separate thousands and the dot

is the decimal separator.

Your job is to normalize the mass flow data that were collected from each plant and then relate and

scale the data such that you can answer the following questions. Important: When entering your

answers online, do not enter the units in the answer boxes (i.e., do not enter t). Do not include

commas or dots or spaces to separate thousands; for example, the number 1 thousand should be

entered as 1000 and the number 1 million should be entered as 1000000.

Question 1: How many metric tons of iron ore are required to ultimately produce 1000 metric tons of

finished widgets? 5733 tons of iron ore

Question 2: How many metric tons of raw steel are required to ultimately produce 1000 metric tons of

finished widgets? 1488 tons of raw steel

Question 3: How many metric tons of steel bars are required to ultimately produce 1000 metric tons of

finished widgets? 1459 tons of steel bars

Question 4: How many metric tons of rough widgets are required to ultimately produce 1000 metric

tons of finished widgets? 1020 tons of rough widgets

First, remember that when constructing unit process inventories its helpful to express all product

outputs in multipliers of 1 for easy scaling. While this isnt required from a purely mathematical

perspective, it allows for easier data interpretation and model construction in LCA. Here are the process

input requirements expressed per metric ton (t) of product output:

Steel production: (1,425,000 t iron ore)/(367,000 t raw steel) = 3.88 (t iron ore/t raw steel)

Shape forming: (525,000 t raw steel)/(515,000 t steel bars) = 1.02 (t raw steel/t steel bars)

Widget cutting: (50,000 t steel bars)/(35,000 t rough widgets) = 1.43 (t steel bars/t rough

widgets)

Widget grinding: (35,000 t rough widgets)/(34,300 t finished widgets) = 1.02 (t rough widgets/t

finished widgets)

Based on these values, we can easily calculate the mass requirements as follows:

1,000 t finished widgets * 1.02 (t rough widgets/t finished widgets) = 1,020 t rough widgets

1,020 t rough widgets * 1.43 (t steel bars/t rough widgets) = 1,459 t steel bars

1,459 t steel bars * 1.02 (t raw steel/t steel bars) = 1,488 t raw steel

1,488 t raw steel * 3.88 (t iron ore/t raw steel) = 5,773 t iron ore

Fuel inputs required for electricity generation (10 points for each correct answer)

Table 2a contains data on fuel inputs for four different types of fossil fuel-fired electrical power plants in

the United States in 2012. These data are in physical units for each type of fuel. Table 2b contains data

on the corresponding amount of electricity that was generated from each type of fuel in 2012. Table 2c

contains data on the average calorific value (i.e., energy content) of each type of fuel in 2012.

Note: the data in Table 2a are presented exactly as obtained from the U.S. Department of Energys

Electricity Data Browser (http://www.eia.gov/electricity/data/browser/), so they are indicative of the

non-SI units used in energy statistics in the United States. You may encounter such non-SI units when

using U.S. LCA data sources. However, youll easily convert to SI units for your answers using the data in

Table 2c.

Table 2a: 2012 U.S. power plant fossil fuel consumption and units Quantity

Consumption for electricity generation using coal (thousand short tons) 616,501

Consumption for electricity generation using petroleum liquids (thousand barrels) 17,759

Consumption for electricity generation using petroleum coke (thousand short tons) 2,112

Consumption for electricity generation using natural gas (thousand Mcf) 4,115,509

Notes: Mcf equals the volume of 1,000 cubic feet (cf) of natural gas. Numbers expressed using U.S. convention where commas separate thousands and the dot is the

decimal separator.

Table 2b: 2012 net U.S. power plant generation by fuel (thousand megawatt-hours) Quantity

Coal 1,147,861

Petroleum liquids 9,990

Petroleum coke 5,680

Natural gas 507,801

Note: Numbers expressed using U.S. convention where commas separate thousands and the dot is the decimal

separator.

Table 2c: 2012 average calorific value of each fuel (HHV) Energy content

Coal 20.6 GJ/short ton

Petroleum liquids 6.3 GJ/barrel

Petroleum coke 30.5 GJ/short ton

Natural gas 1.1 GJ/Mcf

Note: Numbers expressed using U.S. convention where commas separate thousands and the dot is the decimal

separator. Calorific values are expressed on a higher heating value (HHV) basis.

Using the data in Tables 2a, 2b, and 2c, and your own calculations, answer the following questions.

When entering your answers online, do not enter the units in the answer box (i.e., do not enter

MJ/kWh). Use two decimal places after the decimal separator (e.g., X.XX).

Question 5: How much primary energy is necessary to generate one kilowatt-hour of electricity from

coal? 11.06 MJ coal per kWh generated

Question 6: How much primary energy is necessary to generate one kilowatt-hour of electricity from

petroleum liquids 11.20 MJ petroleum liquids per kWh generated

Question 7: How much primary energy is necessary to generate one kilowatt-hour of electricity from

petroleum coke? 11.34 MJ petroleum coke per kWh generated

Question 8: How much primary energy is necessary to generate one kilowatt-hour of electricity from

natural gas? 8.92 MJ natural gas per kWh generated

Here we must first determine the physical quantities of fuel associated with electricity generation from

each type of fuel, which is readily available from the data in Table 2a and 2b:

Coal: (616,501,000 short tons)/(1,147,861,000 MWh) = 0.537 short tons coal/MWh

Petroleum liquids: (17,759,000 barrels)/(9,990,000 MWh) = 1.778 barrels petroleum

liquids/MWh

Petroleum coke: (2,112,000 short tons)/(5,680,000 MWh) = 0.372 short tons petroleum

coke/MWh

Natural gas: (4,115,509,000 Mcf)/(507,801,000 MWh) = 8.10 Mcf natural gas/MWh

Next, we can calculate the energy content of the physical quantities of each fuel required per kWh of

electricity generated using the data in Table 2c as follows:

Coal: (0.537 short tons coal/MWh)*(20.6 GJ/short ton)*(1000 MJ/GJ)/(1000 kWh/MWh) = 11.06

MJ/kWh

Petroleum liquids: (1.778 barrels petroleum liquids/MWh)*(6.3 GJ/barrel)*(1000 MJ/GJ)/(1000

kWh/MWh) = 11.20 MJ/kWh

Petroleum coke: (0.372 short tons petroleum coke/MWh) *(30.5 GJ/short ton)*(1000

MJ/GJ)/(1000 kWh/MWh) = 11.34 MJ/kWh

Natural gas: (8.10 Mcf natural gas/MWh)*(1.1 GJ/Mcf)*(1000 MJ/GJ)/(1000 kWh/MWh) = 8.92

MJ/kWh

Question 9: Which fossil fuel was associated with the MOST efficient electricity generation from fossil

fuels in the United States in 2012? Hint: to answer this question, youll need to calculate the net power

plant efficiency for each fossil fuel. To do this, you need to recognize that 1 kWh of power plant output

= 3.6 MJ of electricity.

To solve this problem, use the primary energy results (MJ/kWh) you obtained for each fuel in Questions

5-8. Note that the calculation procedure below follows directly the calculation procedure demonstrated

in the Lecture 5 Supplement video for net power plant efficiency:

Coal [3.6 (MJ/kWh)/11.06 (MJ/kWh)] = 32.6%

Petroleum liquids [3.6 (MJ/kWh)/11.20 (MJ/kWh)] = 32.1%

Petroleum coke [3.6 (MJ/kWh)/11.34 (MJ/kWh)] = 31.7%

Natural gas [3.6 (MJ/kWh)/8.92 (MJ/kWh)] = 40.4%

Question 10: What was the weighted average net power plant efficiency of all fossil fuel generation in

the United States in 2012? To answer this question, youll need to use the net power plant efficiencies

you calculated for each fossil fuel in Question 9 and observe the data in Table 2b. Enter your answer

online as a fraction using two decimal places after the decimal separator (e.g., 0.XX). For example, a

net power plant efficiency of 25% would be entered as 0.25.

First, calculate the % of total 2012 generation from each fossil fuel in Table 2c:

Table 2b: 2012 net U.S. power plant generation by fuel (thousand megawatt-hours) Quantity

% of total generation

Coal 1,147,861 68.7%

Petroleum liquids 9,990 0.6%

Petroleum coke 5,680 0.3%

Natural gas 507,801 30.4%

All fossil fuels 1,671,332 100%

Next, multiply the net power plant efficiency of each fuel (from Question 9) by its corresponding percent

contribution to 2012 total generation (from the table above) and sum to arrive at the weighted average

net power plant efficiency:

Fossil fuel Net power plant efficiency (A)

% of total generation (B) A*B

Coal 32.6% 68.7% 22.4%

Petroleum liquids 32.1% 0.6% 0.2%

Petroleum coke 31.7% 0.3% 0.1%

Natural gas 40.4% 30.4% 12.3%

Weighted average net power plant efficiency = SUM(A*B) 35.0%