Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2001-12-14
2004-02-24
Rinehart, K. B. (Department: 3749)
Furnaces
Process
Treating fuel constituent or combustion product
C110S229000, C110S342000, C110S344000, C110S233000
Reexamination Certificate
active
06694900
ABSTRACT:
BACKGROUND OF THE INVENTION
One of the major problems facing today's society and future generations is the production of air pollution by a variety of combustion systems, such as boilers, furnaces, engines, incinerators and other combustion sources. Air pollutants produced by combustion include particulate emissions, such as fine particles of fly ash from pulverized coal firing, and gas-phase (non-particulate) species, such as oxides of sulfur (SO
x
, principally SO
2
and SO
3
), carbon monoxide, volatile hydrocarbons, volatile metals (i.e., mercury—Hg), and oxides of nitrogen (mainly NO and NO
2
). Both NO and NO
2
are commonly referred to as “NO
x
” because they interconvert, the NO initially formed at higher temperature being readily converted to NO
2
at lower temperatures. The nitrogen oxides are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes.
One other major problem facing society is the ever expanding consumption of and dependence on energy, including specifically fossil fuels. One area of great promise for better, more efficient and environmentally conscious energy usage is in the utilization of waste fuels for energy production. Large quantities of agricultural and other biomass resources are available throughout the world. Biomass is a renewable source of energy, but a lot of this material is being land filled, burned in the open fields, or plowed under and, thus, are not utilized as an energy feedstock. Utilization of biomass for energy production eliminates costs for its disposal, provides a renewable energy resource and decreases CO2 emissions. Currently, due to slagging and fouling of boilers' heat transfer surfaces, biomass boilers cannot use a variety of bio-feedstocks with high alkali content.
Accordingly, two key needs are 1) decreasing NOx emissions from combustion sources and 2) increasing utilization of low-grade waste fuels for energy production.
There are several commercial technologies that are available to control NOx emissions from stationary combustion sources. Combustion modifications such as Low NOx Burners (LNB) and overfire air (OFA) injection provide only modest NOx control, on the level of 30-50%. However, their capital costs are low and, since no reagents are required, their operating costs are near zero. For deeper NOx control, Selective Catalytic Reduction (SCR), reburning, Advanced Reburning (AR) or Selective Non-Catalytic Reduction (SNCR) can be added to LNB and OFA, or they can be installed as stand alone systems.
Currently, SCR is the commercial technology with the highest NOx control efficiency. With SCR, NOx is reduced by reactions with N-agents (ammonia, urea, etc.) on the surface of a catalyst. The SCR systems are typically positioned at a temperature of about 700° F. SCR can relatively easily achieve 80% NOx reduction. However, SCR is far from an ideal solution for NOx control. There are several important considerations, including cost. SCR requires a catalyst in the exhaust stream. Catalysts and related installation and system modifications are expensive. In general, SCR catalyst life is limited. Catalyst deactivation, due to a number of mechanisms, typically limits catalyst life to about four years for coal-fired applications. In addition, catalysts are toxic and pose disposal problems.
Reburning is a method for controlling nitrogen oxides that involves combustion of a fuel in two stages.
FIG. 1
may be referred to in this discussion concerning reburning techniques. As shown in the reburning system
100
of
FIG. 1
, in the main combustion zone
102
80-90% of the fuel is burned with normal amount of air (about 10-15% excess). This corresponds to an Air/Fuel Stoichiometric Ratio (SR) about 1.10-1.15. The combustion process forms a definite amount of NOx. Then, in the second stage, the rest of the fuel (reburning fuel) is added at temperatures of about 2300-3000° F. into the secondary combustion zone
104
, called the reburning zone, to generate a fuel-rich environment. Test results indicate that in a specific range of conditions (equivalence ratio in the reburning zone, temperature and residence time in the reburning zone) the NOx and N2O concentrations can typically be reduced by 50-60%. In the third stage
106
the OFA is injected at a lower temperature to complete combustion. Typically the OFA is injected at 1800° F.-2800° F. to achieve essentially complete combustion.
The flow diagram section, b, of
FIG. 1
illustrates the main reactions in the reburning zone process. Adding the reburning fuel leads to its rapid oxidation by the excess oxygen to form CO and hydrogen. The reburning fuel provides a fuel-rich mixture with certain concentrations of carbon containing radicals
108
, e.g., CH3, CH2, CH, C, and HCCO, which can react with NO. The carbon containing radicals (CHi) formed in the reburning zone are capable of reducing NO concentrations by converting it to various intermediate species with C—N bonds,
110
. These species are reduced in reactions with different radicals into NHi species
112
, e.g., NH2, NH, and N, which react with NO to form N2
114
. N2O is reduced mainly via reaction with H atoms: N2O+H→N2+OH. The OFA added on the last stage of the process oxidizes existing CO, H2, HCN, and NH3.
Typically, reburning fuel is injected at flue gas temperatures of 2300-3000° F. The efficiency of NOx reduction in reburning increases with an increase in injection temperature. This is because at higher temperatures oxidation of the reburning fuel occurs faster, resulting in higher concentrations of carbon containing radicals involved in NOx reduction. Efficiency of NOx reduction also increases with an increase in the amount of the reburning fuel at reburning fuel heat inputs of up to 20-25%. Larger amounts of reburning fuel practically do not increase and sometimes even slightly decrease the efficiency of NOx reduction.
Conventional reburning typically requires 15% to 20% reburning fuel heat input to achieve 40%-60% NOx reduction. In so-called Fuel-Lean Reburning (FLR) the amount of the reburning fuel is controlled to maintain an overall fuel-lean stoichiometry in the upper furnace. Therefore, no additional OFA is required for completing burnout. FLR has shown the potential to achieve about 25-35% reduction in NOx emissions using 7-8% natural gas heat input or less.
Greater levels of NOx control can be achieved using Advanced Reburning (AR) techniques. AR is a synergistic combination of basic reburning and N-agent (ammonia or urea) injection. Initial AR studies focused on N-agent injection into the burnout zone (AR-Lean). It was found that AR-Lean incorporates the chain branching reaction of CO oxidation which promotes the reaction between NO and ammonia. When CO reacts with oxygen, it initiates many free radicals. Experiments and modeling studies have demonstrated that the de-NOx temperature window can be substantially broadened and NO removal efficiency increased, if both CO and the O2 concentrations are controlled to fairly low values (CO at the order of 1000 ppm and O2 at less than 0.5 percent). At the point of air addition, CO and O2 are both at low values because of the close approach to SR=1.0, yielding about 85% NO reduction.
Injection of small amounts of alkali promoter species, such as sodium carbonate, along with ammonia into the reburning zone (AR-Rich) can further improve upon the AR process. These AR improvements are capable of achieving greater than 90% NOx control.
Waste fuels can be very effective for reburning. Tests with several feedstocks (yard waste, furniture manufacturing sawdust, walnut shells, willow wood, waste coal fines and others) demonstrate that advanced waste reburning technologies can achieve higher NOx reduction even than that achieved with natural gas. Efficiency of NOx reduction for most waste fuels increase with an increase in the amount of the reburning fuel.
In one technique, biomass pyrolysis gas serves as the reburning fuel. Pyrolysis-based units produce gas, char and
Lissianski Vitali
Rizeq George
Zamansky Vladimir
General Electric Company
Hunton & Williams LLP
Rinehart K. B.
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