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| Increasing crude processing intensity and energy intensity with worsening oil quality. OQ: Crude feed oil quality. PI: Crude processing intensity. EI: Refinery energy intensity. Observations are annual weighted averages for districts 1 (yellow), 2 (blue), 3 (orange), and 5 (black) in 1999-2008. Diagonal lines bound the 95% confidence of prediction for observations. Credit: ACS, Karras et al. Click to enlarge. |
Switching to heavy crude oil and synthetic crude oil derived from oil sands bitumen in refineries could double
or triple refinery emissions and add 1.6-3.7 gigatons of carbon
dioxide to the atmosphere annually from fuel combustion to
process the oil, according to a new study by Greg Karras at Communities for a Better Environment (CBE), published in the ACS journal Environmental Science & Technology. This would increase total petroleum fuel cycle emissions by
14-33%. Extraction emissions would add to these percentages, Karras noted.
Karras compared refinery crude feed, processing, yield, and fuel data
from four regions (PADD I, II, III and V, PADD=Petroleum Administration for Defense Districts) accounting for 97% of US refining capacity
from 1999 to 2008 for effects on processing and energy consumption predicted by the processing characteristics of heavier, higher-sulfur oils. (Smaller, landlocked PADD IV, the Rocky Mountain states, refined non-diverse crude feeds.)
Preliminary estimates from fuel cycle analyses suggest that a switch to heavy oil and tar sands could increase the greenhouse gas
emission intensity of petroleum energy by as much as 17-40%, with oil extraction and processing rather than tailpipe emissions accounting for the increment. This raises the possibility that a switch to these oils might impede or foreclose the total reduction in emissions from all sources
that is needed to avoid severe climate disruption. Accurate
prediction of emissions from substitutes for conventional
petroleum is therefore critical for climate protection. However,
estimates of the emissions from processing lower quality
oils have not been verified by observations from operating
refineries.
Crude oils are extremely complex, widely ranging mixtures of hydrocarbons and organic compounds of heteroatoms
and metals. Refiners use many distinct yet interconnected
processes to separate crude into multiple streams,
convert the heavier streams into lighter products, remove
contaminants, improve product quality, and make multiple
different products in varying amounts from crude of varying
quality. Factors that affect emissions from refinery
process energy consumption include crude feed quality,
product slates, process capacity utilization, fuels burned for
process energy, and, in some cases, preprocessing of refinery
feeds near oil extraction sites.
Estimates that construct
process-by-process allocations of emissions among these
factors have not been verified by observations from operating
refineries in part because publicly reported data are limited
for refinery-specific crude feeds and unavailable for process-level
material and energy inputs and outputs. Research
reported here distinguishes effects of crude feed quality on
processing from those of the other factors using refinery-level
data from multiple operating plants to estimate and
predict the process energy consumption and resultant fuel
combustion emissions from refining lower quality oil.—Karras 2010
The density of crude oils is proportional to the fraction of higher molecular weight, higher boiling point, larger hydrocarbon compounds in the oils that are distilled in a vacuum, then cracked into fuel-size
compounds to make light hydrocarbon fuels, Karras wrote. The larger
hydrocarbons have lower hydrogen/carbon ratios that require
hydrogen addition to improve product quality and higher
concentrations of sulfur and other catalyst poisons that are
freed by cracking and bonded with hydrogen to remove them
from the oil and protect process catalysts.
The hydrocracking and hydrotreating of gas oil and residua uses
several times more hydrogen than does hydrotreating of
lighter streams such as naphtha. These processing
characteristics require increased capacity for vacuum distillation,
cracking, and hydroprocessing of gas oil and residua
in refineries designed to make light liquid products from
heavier, higher sulfur crude oils.
Karras found that crude feed density and sulfur content could predict 94% of processing intensity, 90% of energy intensity, and 85% of carbon
dioxide emission intensity differences among regions and
years and drove a 39% increase in emissions across regions
and years.
The fuel combustion energy for processing increased by
approximately 61 MJ/m3 crude feed for each 1 kg/m3 sulfur and 44MJ/m3 for each 1kg/m3 density of crude refined. He also found that differences
in products, capacity utilized, and fuels burned were not
confounding factors.
This prediction applies to average CO2 emissions from
large, multiplant refinery groups with diverse, well-mixed
crude feeds and appears robust for that application. However,
the method used here should be validated for other applications.
If it is applied to different circumstances, the
potential for significantly different product slates, poorly
mixed crude feeds, synthetic crude oil impacts on refining,
and effects on fuel mix emission intensity and hydrotreating
resulting from choices among carbon rejection and hydrogen
addition technologies should be examined.—Karras 2010
Resources
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Greg Karras (2010) Combustion Emissions from Refining Lower Quality Oil: What Is the Global Warming Potential? Environmental Science & Technology 44 (24), 9584-9589 doi: 10.1021/es1019965