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1. Selective Catalytic Reduction (SCR):
One Piece of the Tier 4 Emissions Compliance Puzzle
Introduction. Environmental Protection Agency Tier 4 Stage 4 emission compliance standards and
regulations are a complex of policies, rules, and guidelines scheduled for full implementation in the
manufacture and production of “clean diesel” engines for all power unit applications to be deployed in
the United States.
Europe, Canada and Japan have collaborated on emissions compliance standards and regulations in
order to ensure emissions reduction targets for all diesel power are met by all makes and models
across borders, including off-highway (non-road) applications.
As leading manufacturers and marketers of diesel power equipment for their own industrial and
agricultural markets, and as exporters of such equipment, the United States, Europe, Canada and
Japan have made determined investment in research and development programs to bring clean diesel
technology to 21st century markets.
The primary clean diesel technology innovations for emissions control are EGR and SCR systems. Both
systems have emerged as high degree refinements and improvements to traditional EGR and SCR
applications from different sectors of the wide range of commercial diesel applications. Some of the
more prominent commercial EGR applications come from the agricultural, construction, and mining
sectors. SCR applications have been developed over decades of reliable usage in utilities power
generation, locomotive and marine power, and small to mid-sized boiler applications.
The contrast between both systems is an important part of this presentation because it highlights the
development possibilities going forward beyond the fundamentals of meeting emissions standards. As
a first wave challenge to each participating lead country, the emissions compliance efforts have yielded
two effective technologies for controlling particulate and NOx emissions to near-zero levels.
Now comes the ongoing challenge to manufacturers research and development to optimize clean diesel
efficiency in producing near-zero emissions power across the board in all applications deployed at all
levels of the power output ratings (horsepower) curve, both on and off-highway.
Both technologies present challenges unique to their own pedigrees. EGR and SCR technologies are
being adapted to fit new generation power units from as small as a single horsepower to the
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2. gargantuan 750 plus horsepower off-highway engine applications. Each level brings its own design
challenges to ensure operational efficiency and emissions control while incorporating the most current
characteristics of EGR and SCR technologies.
The Purpose of the Report. The purpose of this report is to articulate the scientific, technological,
and applied mechanical concepts of Selective Catalytic Reduction technologies as applied in off-
highway clean diesel power units, in order to contribute to an understanding of Tier 4 compliant
emission control technologies, and the emergent issues surrounding this specific technology relative to
competing compliant technology, the various commercial fields of operation where competing
technologies compete for a) market share, and b) adoption-diffusion primacy as the norm in
environmentally compliant clean diesel technologies.
Scope of the Report. This report makes no claims to present any original research or findings about
SCR technologies, their worth, value or effectiveness, as applied to the off-highway clean diesel
market. This report will use an aggregate approach to the presentation of facts and, where necessary,
expert opinions, about SCR technology as these pertain to off-highway applications.
Conceptual Overview
Nitrogen Oxide as Pollutant. The reduction of nitrogen oxides (NOx) as a diesel emissions by-
product is the benchmark measurement of emissions reduction technology effectiveness with good
reason.
Nitrogen oxides are progressively lethal pollutants. NOx is a byproduct of diesel combustion. The net
effects of NOx pollution of the surrounding environment center mainly on soil beds and groundwater
deposits across the country.
The general term for NOx pollution is acidification. Acidification of soil has far reaching environmental
impacts. Acidification by NOx saturation disrupts normal soil pH levels.
The disruption of soil pH causes further imbalances. Nutrients for plant, tree and fauna life is disrupted
because the chemical changes in soil alter the availability of nutrients needed for healthy plant life,
both wild and domesticated species. The alteration of the nutrient bed affects animal health. Plants,
trees and fauna upon which animals feed, both wild and domestic, have less nutritional value.
Animal metabolisms adapt to the nutritional degradation over time due to reduced vitamin and mineral
content in the plants they consume, and which are converted to feedstock. Where feed stocks are
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3. concerned, additional supplementation requirements become necessary to sustain livestock
productivity but have residual impacts on human consumers of such livestock.
Where rain water percolates over NOx saturated soils, the effect is a chemical change in organic
mineral compounds. The organic mineral compounds become free metallic compounds. These free
metallic compounds are highly toxic to plant, animal and human life. The compounds leach into
groundwater supplies or remain suspended in soil.
Selective Catalytic Reduction and NOx Pollution Control. Selective catalytic reduction (SCR) has
been proven to reduce diesel engine emissions of NOx to near-zero levels. The findings from an
extensive testing program in the United States, Europe, Japan and Canada have all validated SCR’s
effectiveness as a driver of reduced N Ox emissions worldwide. Findings have been generally indicative
of high performance in this regard at all levels of the horsepower rating continuum from single
horsepower off-highway power units through the top horsepower range above 700 horsepower.
SCR is based on principles of applied chemistry. A gas is mixed with a compatible reductant creating a
mixture that physically contacts a catalyst. The original gas is transformed into a different compound.
Whereas the original compound contained harmful pollutants before catalysis, post-catalysis the
compound is an organic compound of zero toxicity to plants, animals and humans.
SCR converts NOx found in untreated diesel emissions or exhaust gases. To convert NOx to its
constituent compounds, a chemical catalytic process is required. The exhaust gases which carry the N
Ox out of the engine must be altered by mixing the compounds with a gaseous reductant. The
reductant reacts with NOx when mixed in precise, metered, gaseous p roportions. The NOx is thus
prepared to bond with the catalyst when it comes into contact.
Diesel exhaust undergoes a chemical alteration as the exhaust stream is infused with the gaseous
reductant as the exhaust is discharged from the combustion chamber. Certain exhaust temperatures
are optimal for the alteration to be effective. The NOx rich exhaust gas, once infused with reductant,
can be absorbed in contact with the catalyst.
The gaseous reductant compound used in new generation diesel engines with SCR emission
technologies is commonly known as “urea.” Urea is a form of nitrate that is a staple of agricultural
fertility compounds which improve, artificially, soil nitrogen levels for crop production.
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4. When urea is suspended in a 32.6% aqueous solution, then injected into the NOx rich exhaust gas
stream as an atomized compound, the new gaseous mixture flows into contact with the catalytic
apparatus. The by-product of the instantaneous reaction is carbon dioxide.
The catalytic reduction process known as SCR transforms NOx compounds into diatomic nitrogen and
water vapor. The catalytic reduction process produces a non-toxic compound from toxic diesel
emissions.
Diesel Particulates. For the purposes of this report the main focus on SCR is N Ox emission
reductions. Off-highway clean diesel power units are held to the same high standard for reducing
particulate emissions. EGR and SCR power units rely on electronically regulated, heat controlled,
absorbent-adsorbent filtration technologies to reduce particulates emitted into the atmosphere. The
particulate filtration systems are part of the after treatment systems in EGR and SCR emission models.
Regeneration systems have been designed to keep filtration clog-free with the increased demands
placed on filters to scrub exhaust gas of particulates to near zero levels. Diesel oxidation catalyst
systems filter carbon and THC emissions by a full 90 per cent.
SCR After Treatment System Components. The SCR after treatment system features an
engineering model involving various components brought together to catalyze toxic engine out N Ox
emissions to near zero levels and to scrub exhaust of suspended particulates to gradually achieve
industry compliance standards by 2015, across the board for all horsepower classification, with Tier 4
Stage 4 emission standards set by the EPA.
Focusing on the NOx emission reductions process, this aspect of the after treatment system is of
paramount focus in this report. The system components developed by Mack Engines are commercially
viable here in the United States, and the diesel off-highway markets of collaborating countries
mentioned throughout this report. These components provide an economical means for describing
how the theory behind SCR is actually applied in a working off-highway power unit.
The components include:
• Diesel Exhaust Fluid Tank
• Diesel Exhaust Fluid Pump
• Diesel Exhaust Fluid Injector
• Selective Reduction Catalyst
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5. Diesel Exhaust Fluid (DEF) Tank. The tank holds the urea solution that will be mixed with the
exhaust stream. In cold temperatures the DEF can gel or wax. The tank is heated, when needed, to
maintain a baseline temperature so as not to effect the optimized exhaust gas temperature and offset
the NOx-urea mixing process. The DEF tank is mounted on the opposite end of the power unit frame as
the fuel tank.
DEF Fluid Pump. The DEF transporting mechanism from tank to injection point is the DEF fluid pump.
This is a low pressure pump optimized to deliver a steady supply of DEF to the DEF injection pump. The
pump is located in close proximity to the DEF tank, with a hose that is laterally connected to the DEF
injector. When the engine is shut down, the pump returns in-line DEF back to the tank.
DEF injector. Engine controls monitor NOx concentration in the exhaust stream. Sensors signal the
DEF injector to send a metered amount of DEF, atomized under pressure, into the exhaust stream.
The heated DEF NOx decomposes the urea into ammonia creating an N3 NOx mix in the time it takes
for the gaseous mixture to travel 2 to 3 meters in the exhaust line.
Selective Reduction Catalyst. On contact with the substrates inside the catalyst unit, the ammonia-
NOx gas mixture is immediately broken down to diatomic nitrogen, N2 and water vapor.
SCR Limitations. SCR also relies on micro-porous materials for its catalytic converters. These
materials can become easily clogged or degraded by compounds such as silicon, ammonia bisulfate,
sulphur compounds and fine particulates. There are also compounds that can actually eat away the
catalytic surface, affecting its porosity.
SCR technology relies upon temperature to ensure proper operation of the gas-based after treatment
system. The system requires tuning in order to ensure the instability of the urea mixture atomizes
properly once it leaves the tank at a cooler temperature than the exhaust environment into which it will
be injected. Inconsistencies will allow NOx emissions to escape unprocessed through the catalytic
converter or the exhaust compounds can clog the converter. There is also the cooler exhaust
temperature on a cold start-up, causing high NOx output.
Competition from Exhaust Gas Recirculation Technology. Proponents of EGR technology see
advantages over SCR systems. First, the EGR system requires far less maintenance because the after
treatment systems do not require 4 to 5 additional components mounted on the basic power unit.
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6. Second, EGT does not require as many temperature sensitive relationships across the system to
manage N Ox emissions. There is no dependence on a “second” fluid to store for engine operation, nor
is there a need to manage a mobile supply of the liquid urea fluid on off-highway sites.
Across the continental United States, there is a readily available and qualified corps of mechanics and
technicians who are trained and familiar with basic EGR technology. Service is readily available,
especially for mobile off-highway applications. It is still not clear as to how the different types and
grades of diesel fuel with their divergent NOx levels will respond across the board to SGR technology.
SGR has traditionally been a marine, locomotive and stationary power source.
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