The document discusses how using oxygen to enhance the combustion of alternative fuels in cement kilns enables increased fuel substitution rates and clinker production. It provides examples from cement plants in the US where oxygen injection allowed fuel rates to increase by 38-51% while boosting clinker output by 5-13%. Oxygen improves burning zone control and stability, reducing emissions and enabling more consistent burning even with lower quality alternative fuels.

1. ALTERNATIVE FUELS
I Enhanced combustion
by Greg Ambrogi,
Julian Inskip, Larry Farmer,
Alternative fuel substitution can provide an effective means to significantly
reduce plant operating costs but often at the expense of production.
Air Products and Chemicals Inc The use of oxygen to improve the combustion of alternative fuels has
& Tim Menke, Buzzi Unicem, been shown to reduce or eliminate emission concerns and increase
USA flame temperature. The resulting improved burning zone control enables
operators to maintain feed and burn at a more consistent rate. Over time,
improved kiln stability translates into more production, increased fuel
substitution rates, and lower operating costs. Actual operating results and
data from several cement plants are presented in this article.
Figure 1: Air Products makes
a delivery to Buzzi Unicem
T
he use of alternative fuels is the associated environmental impacts. enhance the overall economics of fuel
beneficial to the cement industry as Waste products fired in cement kilns are a substitution. As a result, a wide variety of
well as to our environment. Firing viable fuel source, which would otherwise wastes, such as tyres, animal meal, waste
alternative fuels reduces primary fuel be subject to incineration or landfill. oil, plastics, waste wood, and waste paper
consumption, typically coal, and lowers European cement producers have been products or ‘fluff’, etc, are routinely fired
operating costs for the cement plant. leaders in the utilisation of alternative in cement kilns. In the United States,
From an environmental standpoint, the fuels as they can significantly reduce their tyres and petcoke are more commonly
reduction of the primary fuel decreases demand for expensive coal. High landfill employed with many of the older, less
the use of non-renewable fossil fuel and and disposal costs in Europe further efficient wet kilns firing hazardous waste
SEPTEMBER 2007 ICR 29

2. ALTERNATIVE FUELS
fuels (waste derived fuel). Since landfill on the fuel ‘quality’ and production to ensure complete combustion and
costs are less expensive in the United demands of the kiln. The fuel ‘quality’ or destruction of the wastes. Sufficient
States, the economics of using these heating value determines the potential ‘excess air’ is often required to ensure
wastes depends heavily on transportation substitution rate. Low quality fuels possess complete combustion. In some cases,
costs. a lower heating value either due to higher too much air through the kiln becomes
Another key difference between the moisture content, lower carbon content, detrimental as it will change the kiln
United States and Europe has been vastly or lower volatility of the fuel component. temperature profile and possibly impact
different market conditions. While there When firing alternative fuels, generating combustion efficiency due to the higher
has been an excess of cement capacity sufficient heat in the kiln can be difficult. kiln velocities. In order to operate within
in Europe, US cement producers have The flame temperature of alternative regulated emission limits, plants must
experienced very high demand levels since fuels is routinely below the primary fuel carefully monitor emissions and make
the mid 1990’s. Until the recent housing flame temperature. This ‘colder’ flame necessary adjustments to fuel substitution
slump in the United States, many of the necessitates a reduction in feed rate to and production rates.
US cement producers were operating at maintain burning zone temperature and
extremely high capacity levels to keep clinker quality. Oxygen impact
up with demand. Most could not afford Additionally, variations in the Oxygen is required for any combustion
to implement changes in their processes composition (chemical and physical) process. Although air is the most common
that would negatively impact production. of alternative fuels make it difficult to source of oxygen, it is not the most
Therefore, substitution of their primary maintain stable operating conditions. effective, since it also contains about 79
fuel with lower heating value waste Variations in the fuel quality and per cent nitrogen. Nitrogen in air takes
products has not been an economically composition lead to incomplete up volume, absorbs heat, and lowers
viable option. combustion and fluctuations in kiln flame temperature. Adding pure oxygen
emissions, specifically carbon monoxide (oxygen enrichment) improves the
Kiln operation (CO) and total hydrocarbons (THC). overall combustion process by increasing
Utilisation of alternative fuels can range Stringent emission regulations for kilns the flame temperature and the amount
from five to 100 per cent, depending burning certain alternative fuels exist of available heat. In the kiln, oxygen
Figure 2: oxygen supply system at Buzzi’s Chattanooga plant
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3. ALTERNATIVE FUELS
Air Products then installed a similar
Table 1: oxygen injection into wet kiln firing waste derived injection technique at a second plant
fuels (solvents, chemicals, solids) that was already using oxygen and firing
waste derived fuels (see Table 2). Not only
Case study A Base Operation with % change were increases in fuel substitution made
operation oxygen possible, but an overall reduction in total
Clinker (tpd) 1000 1080 +8% oxygen consumption was also achieved.
Waste Derived Fuel (kg/min) 100 148 +48% Common to both of these locations
Coal Replacement 38.5% 58% +51% were fuel substitution rate increases with
Waste Fuel Cut-Offs (per day) 3.4 1.8 -47% slight increases in production. Prior to
implementing oxygen, this combination of
All data is normalised benefits had not been possible.
Recent experiences at
Buzzi Unicem
Table 2: modified oxygen injection into wet kiln firing waste During the past two years, Buzzi
derived fuels and using oxygen Unicem USA has implemented oxygen
at three locations (see Table 3). The
Case study B Original Operation with % change first of these was a precalciner that
oxygen injection did not fire alternative fuels. Oxygen
Clinker (tpd) 1000 1050 +5% was implemented for the sole purpose
Waste Derived Fuel (kg/min) 100 138 +38% of gaining an incremental production
Coal Replacement 37% 51% +38% increase. After observing the positive
Oxygen Flow Rate (Nm3/hr) 1200 800 -33% impact of oxygen on kiln performance and
burning zone control, Buzzi sought to use
All data is normalised oxygen at one of their locations that fired
a combination of alternative fuels.
enhances burning zone control and immediate – a reduced level of THC Chattanooga, TN
improves kiln stability. emissions was achieved, enabling the Buzzi Unicem’s Chattanooga facility is a
The use of oxygen to improve the plant to significantly increase their fuel modern five-stage precalciner dry process
combustion of alternative fuels has substitution rate while gaining a slight with a 2475tpd baseline clinker capacity
been shown to reduce or eliminate production increase. This plant has been burning petcoke as the primary fuel.
emission excursions and increase flame using oxygen effectively since 2000, often The plant also burns oil for heat up of a
temperature. The resulting improved achieving net operating costs similar to a cold kiln. Air Products and Buzzi Unicem
burning zone control enables operators modern precalciner due to the increased implemented oxygen enrichment in July
to maintain feed and burn at a more revenue from the waste fuels processed 2005 primarily for clinker production
consistent rate. Over time, improved kiln and reduced coal costs. Normalised data increase. The oxygen enabled Buzzi
stability translates into more production, from the first few months of operation is Unicem to fire additional fuel and increase
increased fuel substitution rates, and presented in Table 1. plant production over 13 per cent during
lower operating costs. Depending on the
value of the alternative fuels, increased
substitution can lead to a net negative fuel Table 3: oxygen enrichment results at Buzzi’s facilities
cost (including the cost of oxygen).
Base Operation with % change
Initial use of oxygen to operation oxygen
increase alternative fuels Chattanooga Clinker (tpd) 2475 2796 +13%
Oxygen was implemented at a customer’s Fuel (% petcoke) 100% 100% –
wet kiln to increase the rate of waste
derived fuel (WDF) consisting of non- Cape Girardeau Clinker (tpd) 3787 4014 +6%
disposable chemicals, solvents, etc. The WDF (kg/min) 152 164 +8%
kiln was experiencing THC concentration
spikes, which in turn would require the Greencastle Clinker (tpd) 3719 3904 +5%
cessation of firing with waste derived SNHW (tph) 1.36 1.36 –
fuel. To optimise kiln operation, constant WDF (kg/min) 272 272 –
fuel firing needed to be achieved. Air
Products’ kiln combustion specialists SNHW – Shredded Non-Hazardous Waste
devised a solution employing oxygen WDF – Waste Derived Fuel
injection. The positive results were
SEPTEMBER 2007 ICR 31

4. ALTERNATIVE FUELS
the trial, while maintaining 100 per increase in liquid waste derived fuels of Summary
cent petcoke firing. Further, during kiln eight per cent. Recent implementation of oxygen
heat up, the oil flame was unstable, enrichment at several Buzzi Unicem USA
typically requiring another fuel source to Greencastle, IN locations raised clinker production while
stabilise the heat up flame. Oxygen was Following the success of oxygen maintaining or increasing WDF rates. In
implemented to stabilise the oil flame enrichment at the Chattanooga and Cape total for these three plants, over 700tpd
resulting in much shorter kiln heat up Girardeau locations, Air Products and of incremental clinker capacity has been
times and eliminating the need for the Buzzi Unicem implemented an oxygen added through the use of oxygen. Other
secondary fuel. trial at Greencastle, IN during December plants have substantially increased their
2006. The Greencastle plant is a single- utilisation of alternative fuels especially
Cape Girardeau, MO stage precalciner semi-dry kiln with a where emission limits (CO, THC) have
Based on the positive results at 3719tpd baseline clinker capacity. The been the rate limiting factor.
Chattanooga, Buzzi Unicem implemented plant burns coal, shredded non-hazardous Employing oxygen injection to enhance
oxygen at its Cape Girardeau, MO plant. solids and a very high rate of liquid the combustion of alternative fuels is
This facility has a four-stage precalciner waste derived fuels in both the kiln and technically and economically viable. With
dry kiln process with a 3787tpd baseline precalciner. Oxygen was implemented minimal capital investment, plants can
clinker capacity. The kiln burns liquid primarily to increase clinker production implement oxygen enrichment to increase
waste derived fuels (solvents, chemicals, and potentially to increase the burning of fuel substitution rates and increase clinker
etc) and coal. Air Products and Buzzi liquid waste derived fuels and solid waste. production. Oxygen also enables the use
Unicem implemented oxygen in April Clinker production increased five per cent of lower quality (or higher value) waste
2006 primarily to increase clinker while maintaining the maximum firing fuels, further enhancing economics. Each
production, while increasing the burning rate of liquid waste derived fuels. More cement plant must evaluate their specific
of liquid waste derived fuels. With testing is planned to determine the impact economics and operating requirements
oxygen, the kiln’s clinker production of oxygen on the firing of additional to determine the optimal utilisation of
increased about six per cent with an shredded non-hazardous solids. oxygen enrichment. _________________I
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34 ICR SEPTEMBER 2007