Quimica Verde

December
2007

www.che.com

Engineering
What we can do
to support
its goals
Miguel (Mike) Mendez, P.E.,
Aspen Technology

G

reen engineering, as explained
by chemists and bio-scientists,
includes production of energy
and materials from “green” feedstocks, such as biomass and microorganisms, with the intent of reducing energy,
waste, greenhouse gases (GHGs) and
waterconsumption. By extension, other
contributions that reduce GHG emissions also support green engineering.
The obvious solution to creeping global
GHG inventories is to cut back extraction
of fossil carbon from below the ground. It
is unlikely we will stop using fossil fuels,
but it is reasonable to expect we will
curb GHG emissions to rates sustainable by the earth’s natural sequestration
systems.Efficient use of fossil fuels and
wide spread use of renewable energy are
required to meet this goal.
Everyone can make their personal
contribution to reduce GHG emissions
by driving fuel-efficient vehicles and
judiciously adjusting the thermostat
at home. For engineers, however, the
responsibility is greater since the decisions we make can have orders-ofmagnitude higher impact.

GHGmitigation presents opportunities for chemical engineers to devise,
quantify, and implement innovative
green solutions. By identifying and
evaluating options and resources useful to support green engineering, significant and invaluable improvements
can be gained.

Top sources of GHG emissions
Power. Today hundreds of aged power
plants release large volumes of GHGs
while supplying electricityfor the U.S.

These seldom top 38% thermal efficiency even though technologies exist
that can better 50%. A 1% efficiency
improvement out of 26 quadrillion Btu
conversion losses from U.S. power production (see Figure 1) would result in
savings of 260 trillion Btu, an equivalent of GHG emissions from 3.5-million
passenger automobiles.*
*Note. Equivalent emissions from passenger automobilesprovides an idea of the impact in terms
most of us can relate to. The basis is 7.50x107
Btu/yr per automobile, 12,000 mi/yr and 20 mi/
gal mileage.

FIGURE 1. The national (U.S.) energy-to-electricity balance for 2006 (Source: Energy Information Admin., DOE/EIA-0384, June 2007

Integrated gasification, combinedcycle (IGCC) leads the list of solutions
to this problem. IGCC combines twothermodynamic cycles: a gas combustion cycle and a steam cycle, each with
its own turbine and generator. Natural
gas or coal gasification provides energy
for the first cycle. Heat from the flue
of the first cycle is used to generate superheated steam to drive the second set
of turbines. Larger temperature differences between the hot and cold ends of
the combined cycle allow higher thermalefficiency relative to single cycles,
netting benefits of 20% less GHG and
20–40% lower water usage.
IGCC capacity planned for 2014
is 14.8 GW with 27 projects in 16
states. Worldwide, nearly 4 GW of
IGCC currently operate and 50 new
projects totaling 27 GW have been
announced [1].
Ocean and terrestrial (vegetation
and soils) CO2 sequestration are being
investigated. The environmental impactof these methods is unknown at
this time. CO2 storage in soils as magnesium carbonates or as CO2 clathrate
are promising as safe, solid materials
offering compact storage with potential
commercial value [2].
2

Transportation fuels. Transportation fossil fuels release the second largest volume of GHGs. Renewable fuels
(bioethanol and biodiesel) are leading
solutions reducing GHGs from 7to
90% per gallon, compared to gasoline,
depending on feedstock and process
type, according to Argonne National
Laboratory [3]. Applying the low end
of this range to the 160-billion gal/yr
of gasoline consumed in the U.S., 133million tons of CO2 emissions would be
prevented. For companies interested in
biofuels production, an excellent repository of reports and models is accessible at...