HOME
 

Alternative Gas Turbine Technology
 

Fuel reforming with steam (a.k.a. water shift reactions that lead to hydrocarbon conversion into CO and H2, or synthesis gas) provides great potential for improvements in emissions and efficiency for gas turbine (GT) applications. However, the capital cost, maintenance expense and reliability issues of known designs have all deterred implementation. Reforming units require expensive catalysts and involve failure-prone equipment, inconsistent with the reliability requirements for power-generating units. Only one topic – a combustion emissions reduction is considered here as it presented in our US patent No 5,501,162. The other related to GT cycle benefits in the light of our approach to efficiency improvement up to 65+% will not be discussed here due to patent pending issues and confidentiality consideration. Interested parties may contact us directly.

1. Proposed Combustion Method.

The unique feature of this combustor design is the presence of an additional stage for fuel reforming, immediately preceding fuel ignition, therefore eliminating the need for a separate reforming unit. The reactions taking place in this reforming stage are similar to those occurring in steam reforming and partial fuel oxidation. In addition, high-density heat released in the combustor-reactor is utilized directly in the reforming stage, thus sustaining higher temperatures and other advantageous reforming kinetic conditions typically not found in known fuel conversion processes. High-energy radicals released in the process of fuel reforming and fuel-rich combustion increase the rate of reforming reactions. Available data shows that over 90% of fuel/steam conversion will be achieved within the calculated residence time in the reforming zone of nearly 10 milliseconds. By converting parent fuel into CO and H2 prior to ignition, NOx emissions are greatly reduced (~ 2 ppm at 15% O2dry), while the chemical energy losses with CO and VOC fall to below the levels of modern GTs. Therefore, the proposed technology eliminates the need for expensive and unreliable catalysts for either catalytic fuel conversion or post-combustion purification. Moreover, non-catalytic fuel conversion allows the use of any grade of liquid or gaseous fuel. The proposed combustor design is simple. It can easily be retrofitted, requiring only minor modifications to the GT’s combustion chamber, especially for gaseous fuel-fired gensets. This simplicity will make it an attractive alternative to the current low-NOx technology, because of improved economics (both capital and operating costs) and cycle reliability. Burning CO and H2 provides a number of virtues efficiencies, due in part to improved combustion kinetics, but mostly due to not being required to endure efficiency-draining measures to meet environmental regulations. Small power-generating units (1 to 10 MW) will gain the most significant cost reductions, greatly improving the economic prospects for distributed power generation.

Reforming of hydrocarbons is a process employed in a number of industries, including urea production, oil hydro-cracking in refinery processes, hydro-treatment in pharmaceutical industries, metal ore processing, fuel cells and coal gasification in power generation. In gas turbine cycles, fuel reforming has been proposed in the form of “thermo-chemical recuperation” (TCR) and partial fuel oxidation.

2.1. Chemically Recuperated Gas Turbine (CRGT) is an example of the advanced gas turbine cycle that involves fuel reforming is the. The high thermal efficiency of the CRTG is achieved by deployment of an innovative waste heat recuperation system for 1) steam generation, and 2) providing heat duties for endothermic processes of fuel/steam reforming. While capable of outstanding thermal efficiency and satisfying hazardous emissions regulation (NOx and CO), a number of practical obstacles prevent CRGT's acceptance by the power generating industries. First, the temperature level of flue gas leaving the turbine can only support catalytic fuel reforming, bringing with it the associated costs penalties. Secondly, the presence of a reformer substantially increases installation costs. Thirdly, it jeopardizes reliability due to the failure-prone nature of a reformer unit and/or potential catalyst poisoning.

These obstacles have limited the success of the CRGT [2], despite the attractiveness of fuel reforming for its high efficiency and environmental benefits. The technology was actively promoted in the mid-1980s for industrial applications [3] and GT cycles, in particular, [4,5]. Despite these engineering challenges, development efforts continue in the field of fuel reforming.

Exxon [6] recently was awarded the patent for a GT cycle with a heat recovery system involving the reforming of methanol. As in CRGT, the increased cycle-efficiency and reduced emissions are claimed through the deployment of a separate reforming unit, where combustion products exiting the turbine provide the heat required for vaporization, steam generation, and steam reforming. In addition, a steam turbine installation provides about 10% of electrical output complementary to the main genset. Exxon underscores that the available exhaust temperature is insufficient even for reforming of such convenient fuels as methanol, and proposes to use a Cu-based catalyst to achieve a higher degree of reforming.

Partial methane oxidation was evaluated for combined cycle applications (IGCC) [7]. The advantages of thermodynamic efficiency improvement and reduced NOx were emphasized. The absence of a catalyst positively impacts costs. Yet, besides the steam generator and reformer, cycle arrangement includes additional equipment for reformed fuel cooling, a significant amount of piping interconnecting the compressor, gas turbine exhaust, reformer, and multiple steam piping. Again, the drawback is the enormous quantity of auxiliary equipment, restricting its economic feasibility to large-scale power plants only. These drawbacks are similar to both CRGT and STIG cycles, high fixed and operation costs. The absence of a catalyst in this design benefits cycle reliability, but this is offset by the introduction of another failure-prone system element, the reformed fuel cooler.

2.2. The disadvantages associated with traditional thermo-chemical fuel reforming, combined with the understanding of its great potential for environmental benefits, inspired development of the catalytic combustion process . Fuel reforming in this process relies upon an expensive, high temperature catalyst made of noble metals (Pt, Pd, Rh). A high conversion rate of gaseous fuels into CO and H 2 is achieved. Combustion of the fuel, consisting of CO and H 2, leads to the extremely low NOx level of around 2 ppm @ 15% O 2,dry . Its relative simplicity, low emissions rate and the use of a single working media make it similar to the dry-Low-NOx. In catalytic combustion, the disadvantages of the premixed dry-Low-NOx were mitigated and provided higher combustion stability (operation safety), wider turndown range, and consistency in NOx level at all loads. Still, the penalty comprises low thermal efficiency due to inability to recuperate waste heat within the gas cycle and the additional pressure drop in the combustion chamber (reduction of available pressure). Increased fixed and operating expanses arise from the presence of the high temperature catalyst that requires rather frequent replacement.

Another improvement in catalytic combustion was patented by ABB [8]. The benefits result from the use of a separate catalytic “conditioning stage” that reforms some portion of the fuel into hydrogen. Main streams of air and fuel pass an additional catalyst stage, where low-temperature combustion ( ~1000 o C) takes place, before the resultant products are burned in the combustion zone, where ignition is ensured by the start-up burner burning hydrogen produced in the conditioning stage. This stage alone is capable of providing 20% of a total genset output. The final fuel burnout takes place in the separate section downstream of the start-up burner.

The main benefits claimed by ABB are 1) lower temperatures of catalytic combustor operation (reduces catalyst cost and the risk of fowling), 2) smoothing out the temperature profile through the combustor, to prevent the formation of NOx, 3) replacement of the main fuel (natural gas) in the start-up (support burner) with the hydrogen helps to operate in the low-NOx mode at any load.

Yet the complexity of the design still does not present solid ground for consistent and reliable operation. The concerns of this combustor design are low temperature combustion control, which could otherwise damage the catalyst, and reliance on the additional catalyst in the conditioning stage.

In concluding this brief technology review, it should be stressed that the benefits of fuel reforming for efficiency and environmental benefits are recognized by all of the major GT suppliers, despite the fact that no acceptable engineering / economic solutions have been presented to date.

One of the preferred designs of the proposed combustion chamber is presented below. Subsequently, the fundamental principles of fuel reforming are reviewed in conjunction with the proposed method, and include the heat requirements, the kinetics of reforming and partial fuel oxidation, and the kinetics of NOx formation.

Figure 1 below is a drawing of the combustor, where the upper chamber, or rich combustion zone, is lined with ceramic material (5). A concentric re-circulation insert (6), made of ceramic, divides the upper chamber into two distinct regions, the combustion zone and the reforming zone. In this design, the fuel/steam mixture is supplied through the main header (1) into the reforming zone, also protected by a ceramic liner. The header contains multiple injection nozzles (2) directed along the lined chamber wall. The upper chamber is equipped with a start-up fuel injector (3) and a primary air distributor/diffuser (4). Excess of air or air/steam mixture is supplied downstream of the upper combustion zone through a series of nozzles (7), which may have a tangential and azimuth incline to the cylindrical chamber wall.

Stable ignition is achieved under sub-stoichiometric conditions (AFER=1.2-1.7) in the combustion zone (inside ceramic insert 6). The fuel/steam mixture is injected into the reforming zone (outside ceramic insert 6), and flows toward the primary air distributor/diffuser (4). Injection of the fuel/steam in the reforming zone generates momentum sufficient to entrain more than 30% of the incomplete combustion products from the rich combustion zone, containing significant concentrations of CO/CO 2 and H 2 /H 2 O with no oxygen present at temperature around 2200 o F.

Spontaneously, fuel/steam/CO 2 reforming occurs in the reforming zone, due to the high sensible heat content of the re-circulated combustion products and the preheated fuel/steam mixture, combined with the high radiant flux emitted by ceramic insert 6. At the same time, localized micro re-circulation zones between adjacent fuel/steam jets and between the same jets and the outer wall of the chamber create a supply of high-energy radicals promoting high rates of the reforming reactions. The estimated residence time within this reforming section ( 15” to 20” in length) is approximately 10 ms, sufficient for near complete fuel reforming (see explanations below).

The products exiting the reforming zone are forced into contact with the oxidizing air due to momentum produced by the primary air, and are then rapidly mixed and ignited in the pattern dictated by the distributor (4). A low-pressure zone downstream of distributor (4) also stimulates internal re-circulation inside insert 6, where the hydrogen and CO rich mixture reacts quickly with the oxygen of the primary air due to their dominating rates of reaction.

Since decomposition of hydrocarbons takes place in the reforming zone, formation of HCN-radicals is suppressed, thus eliminating the source of the O-atoms production at the fuel ignition zone. Moreover, for parent fuels having fuel-bounded nitrogen, the presence of free N-atoms is markedly reduced, and stable N 2 molecules are formed instead, as fuel reforming experience indicates. These aspects of the proposed combustion regime in the upper rich zone remove the precursors of both prompt and fuel NOx formation. Therefore, for a parent fuel with no or small nitrogen content, zero emissions at the outlet of upper combustion zone will result.

Positive and stable re-circulation flow around the ceramic insert 6 is sustained by fluid momentum generated in opposing directions: by the fuel/steam mixture in the reforming zone flowing outside of the ceramic insert 6, and by the primary air in the direction of the combustion zone exit. There will be some cooling of the ceramic walls of the reforming zone annulus due to the endothermic reactions of fuel reforming, adsorbing heat of the high-temperature combustion products re-circulated from the combustion chamber exit. The wall temperature of header 1 is maintained by the fuel/steam mixture and provides additional preheat, reducing the time required for the fuel reforming.

The number of nozzles 7 and their distribution along the secondary zone provide quick and stable burnout of the zero-emission mixture exiting the upper zone at the lean stoichiometric conditions. A controlled rate of mixing and the high combustion rates of the reformed fuel result in an extremely stable and low-NOx flame. In this stage of combustion, the burnout of primary species such as CO and H 2 takes place at rapid cooling and elevated moisture conditions. Therefore, their high reactivity ensures significantly higher probability of complete burnout vs. species typically present in the second stage of the traditional low-NOx technologies. As a result, combustion losses will be significantly diminished and the chance for any volatile organic compounds (VOC) to survive through both stages of fuel reforming and combustion is practically reduced to nil.

During start-up regime, fuel is supplied into the chamber through start-up fuel injector 3. In this regime, the target temperature near the outlet of the ceramic insert 6 is 1700 o F, whereas steam is constantly supplied for the cooling of the header (1). As soon as the target temperature is achieved, additional fuel is supplied through the header (1). After reaching about 60% of designed load, the fuel rate to the start-up injector is gradually reduced. As this fuel rate reaches 95% of required load, the fuel flow through the start-up injector (3) is turned off.

Veritask Energy Systems, Inc performed the testing of a scaled-down model of the upper chamber. The test section had a diameter of 1.5 inches made of stainless steel (309). No internal ceramics lining was used except for the zone where primary air was introduced. In this test, propane was burned at the rate 4 scfm, and modulation of AFER within 1.1 to 1.3. Testing was conducted using an intermittent regime over a 48-hour period. An approximate 40% NOx reduction versus diffusion mode of combustion was registered in the same rig. The steam/fuel temperature was 240 o F at 16 PSIA, and a propane/steam ratio of approximately 1.3 was maintained during testing. Examination of the combustion and reforming sections, and steam/fuel injection header indicated no presence of soot.

Veritask Energy Systems, Inc also has experience with the combustion process described in burning mid- grade liquid fuel (similar to oil #4) in a 1.3MMBtu/hr burner. Based on observations and measurements, the zone length needed for fuel reforming was significantly shortened when the re-circulation temperature exceeded 900 o C (~1650 o F). Steam and fuel were preheated to ~105 o C (~220 o F). Once the specified operating conditions were achieved, the flame appearance, the level of noise, and emissions level improved dramatically, well beyond that expected.

The proposed method of combustion relies on the presence of steam. It can be used in conjunction with any commercially available gas turbine cycle where water is present as a working media, i.e. steam injection gas turbine cycle and combined cycles. It is also possible to use this method for the HAT cycle, where the desirable excess of water vapor comes with the combustion products re-circulation from the rich-combustion zone into reforming zone (see Figure 1).

Although the use of steam is desirable, it is not a necessity to achieve the proposed low NOx mode. In the “dry” regime, fuel reforming relies on the presence of CO/CO 2 and H 2 /H 2 O in the products of incomplete combustion, re-circulated upon exiting the fuel-rich zone. Yet, fuel preheat in this mode is desirable but will be restricted by the conditions of its pyrolysis.

In addition to providing efficiency improvement, steam performs other functions as well. It ensures reliable operation of combustion equipment. About 1 pound of steam is required per pound of fuel to keep the fuel/steam injectors clean and for the protection of the fuel header. Also, steam provides a higher production of hydrogen, and more efficient waste heat recuperation. Its presence is also essential for the kinetics of NOx formation and resultant combustion temperatures. Therefore, the more energy is recovered, the higher thermodynamic efficiency, and the lower emissions level is achieved (see explanation below).

As described above, the proposed combustor design ensures sufficient residence time for near complete fuel reforming, without any increase in length of the combustion chamber. Therefore, the same enclosure size can be used as with existing combustion chambers. This will ease the implementation of the proposed method for both new design and retrofit.

For small turbines without any heat utilization, a relatively small and inexpensive exhaust heat utilization system can be added. The addition of water treatment should not be a detriment, even for a small unit, as soon as water availability (1 lb per Lb of fuel) is not an issue. Under these conditions and the assumption of a very high price for treated water (up to $10 per ton), the generation cost per kW of energy produced remains unchanged, if only accounting for fuel and water treatment costs (based on 2001 prices). In case where the environmental impact and high cost of SCR (for small generating facility these costs may be as high as $900/kW installed) are taken into consideration, the economic benefits become attractive for any size installations. Yet another benefit is the elimination of potential hazards associated with the use and transportation of ammonia.

Benefits will improve with a more intensive heat recovery system (more than 1 lb per Lb of fuel in steam production capacity). The fact that the recovered heat can be utilized within the cycle itself (unlike the typical cogeneration facility, where heat of exhaust is used for external needs, (such as heating, ventilation, “green house”, etc.) provides a built-in demand for the waste heat recovery.

  1. A method of combustion with internal fuel reforming (inside combustion chamber) is proposed for gas turbine cycles, where a parent hydrocarbon fuel undergoes the conversion into a mixture consisting primarily of the highly reactive CO and H 2 . The introduction of this additional stage, where fuel reforming occurs prior to its reaction with the oxidizer, leads to the lowest level of NOx production, comparable with the best technologies on the market, while avoiding their various and inherent drawbacks. •  High conversion rates of fuel/steam/CO 2 mixture into CO and H 2 are attainable due to a unique, yet simple chamber design that allows a “step change” modification for new GT cycles and opens valuable retrofit options for the existing ones.
  2. In-chamber fuel reforming avoids the need for costly and unreliable equipment, a major obstacle preventing wide implementation of similar cycles with fuel reforming (CRTG, Partial Fuel Reforming). Unlike the existing methods, the proposed method does not rely on expensive reformers, ether catalytic or non-catalytic. However, fuel and/or steam preheat is necessary to achieve thermodynamically and environmentally sound results and stable operation.
  3. The proposed combustion method will improve the emissions performance of the thermally efficient cycles, such as HAT and STIG, eliminating the need for the expensive SCR/SNCR, while providing an additional duty for waste heat recuperation within the cycle (i.e. fuel/steam mixture reheat). When this duty is satisfied, the efficiency of the HAT and STIG cycles improves.
  4. The operation of small and medium sized cycles will benefit from introduction of the proposed technology in many ways, including the elimination of NOx reduction technologies SCR/SNCR at the flue gas exhaust or expensive high-temperature catalytic fuel reformers at the fuel inlet. Since the proposed technology addresses both performance efficiency (independent of any external consumer) and environmental concerns in an economically feasible way, then retrofit will become an attractive option for the enhancement of small generation facilities.
  5. The proposed method of combustion results in the near complete reforming of the parent fuel into CO and H 2 , producing an order less NOx in comparison with the fossil fuel. It makes it possible to achieve levels of NOx comparable with the best available and most environmentally friendly technologies, such as dry-Low-NOx and catalytic combustion systems. At the same time it surpasses these existing technologies in thermal performance, operation and capital costs, and most importantly, in the reliability of its operation.
  6. This combustion method has no fuel limitations. It is capable of burning any grade of gaseous and/or liquid fuel.

6.1. Heat Balance of Reforming and Combustion Reactions
Water shift reactions of fuel reforming and some reactions involved in partial fuel oxidation are endothermic. Therefore, in order to proceed in the direction of CO and H 2 formation, some heat must be supplied to the fuel/steam mixture or fuel/steam/CO 2 mixture. A fuel reaction with steam is typically referred to as wet reforming (WR), while reaction with CO 2 is called dry reforming (DR). Heat required for a complete (100%) fuel reforming accounted by the formation energies (JANAF) of the components and their global reactions is presented below, assuming standard reference conditions (STD), i.e. P= 10 5 Pa (14.695PSIA), T =298 K, (77 o F):


where ( g ) - indicates a gaseous state of the water vapor.

In practice, carbon may appear in the reforming process as an intermediate component, depending on conditions. Therefore reactions (3), (4) and more complex global reactions (1a), (2a) must be considered:

Table 1. Total Heat Effects of Reformed Fuel Combustion
at STD conditions.

Fuel Composition (Combustibles +O2)
See reactions (1) through (4)

Total Heat Effect of Combustion,
106 J/kmol

Parent Fuel
(Net Heat Effect of Combustion,
106 J/kmol)

CO+3H2 + 2O2

-1008.5

Methane (-802.3)

2CO+2H2 + 2O2

-1049.7

Methane (-802.3)

CO + H2 + O2

-524.9

Carbon (-393.5)

2CO + O2

-566.0

Carbon (-393.5)

Still, natural gas reforming can be sufficiently approximated by the correspondent heat effects for methane and carbon. In Table 1, the combustion heat effects for parent fuel and carbon are compared with those for the products of the reforming. [See Eq. (1) through (4)].

As the data indicates, combustion heat effects for parent fuel (methane and carbon) sufficiently yield to the products of reforming. Certainly, combustion heat for products of reforming does not include energy consumed in the process of their formation by reactions (1) through (4). When this heat is accounted for, then the net heat effect of reforming products and the parent fuel becomes equal (as required by the law of energy conservation). However, if heat for reforming is provided by a preheat of parent fuel and evaporation of water in a heat recovery system, then the actual heat effect (release) of combustion will increase in proportion to the heat absorbed by the steam or steam/fuel mixture.

The process of waste heat utilization by means of fuel reforming improves upon air preheat in several important respects. It involves an order less mass supply for a comparable increase in the actual heat effect, therefore requiring less heat transfer surface and power for its transport. Secondly, in the process of heat absorption, the fuel converts into an environmentally friendly one, whereas preheat of air leads to NOx increase. In practice, assuming a reforming reactants preheat of 1000 o F and a resultant yield of about 80%, the increase in the actual combustion heat effect should reach 10% and 17%, for wet and dry reforming, respectively.

As it was emphasized earlier, the proposed technology does not rely on recovered heat to produce the fuel conversion. Here, the degree of fuel reforming is greatly improved by the direct interaction of combustion and reforming reactants occurring in the adjacent volumes separated either mechanically and/or aerodynamically. Yet, heat-recovery that involves the reforming reactants preheat (steam and fuel) is an important mean to augment the reforming kinetics and to improve thermodynamic cycle efficiency.

Material balance of the reactions (1) through (4) shows that the mass of the reformed fuel is higher than it is for the parent fuel. Therefore, for the reforming and combustion reactions taking place within the same closed system, the adiabatic temperature ( Ta ) is lower than for the parent fuel. In turn, preheat of the reforming reactants and degree of fuel reforming affect adiabatic temperature of combustion as shown in Figure 2. Combustion air in these calculations was assumed at STD conditions. As indicated in the figure, the temperature of combustion will remain the same as for natural gas with the preheat around 1000 o F and the completeness of reforming for both dry and wet reactions in excess of 80%.

6.2. Equilibrium and Kinetics of Reforming

Under the assumption of an ideal gas and thermodynamic equilibrium, the estimated yield of the parent fuel conversion by reactions (1) through (4) can be calculated using empirical values for the equilibrium constants obtained in [9] . Equilibrium concentrations for CO and H 2 at different temperature levels and atmospheric pressure are presented in Table 2. With an increase in temperature level, the equilibrium shifts to the right, i.e. in the direction of CO and H 2 production. Data in Table 2 suggests a 95% to 98% yield by weight at temperatures of 1200K (1700 o F) for both methane and carbon. This conclusion is in a good agreement with similar analyses [10]. This study showed that with the increase of a system pressure the equilibrium concentrations of the reforming products decrease as shown in Figure 3. Here, the indices of individual species represent the pressure level in atmospheres. Based on GTs' pressure ration ? £ 30, the achievable equilibrium concentrations exceed 90% yield of reforming at 1400 K.

The time required to reach these equilibrium concentrations depends on the kinetics of the process. The typical residence time required to achieve near equilibrium conditions of methane-steam reforming within range of temperature 1070K to 1330K is measured in minutes. In practice, Ni-based catalysts are often used for this process to reach equilibrium concentrations within seconds.

A significant reduction in time required for conversion is achieved in partial fuel oxidation, where the necessary heat is released through the partial combustion of fuel (or feed-stock) at a fuel-equivalence ratio 1.5 to 3.5. Figure 4 presents results of a study [11] that shows the high and spontaneous yield of hydrogen within 15ms to 20 ms for initial components preheated to T o =1000K, f=2.5 - (fuel-air equivalence ratio), and P=1atm.

Imposition of re-circulation on the “plug” flow (the pattern typical found in the reactor) brings the required residence time down to 10 ms for a uniform distribution. Localization of the re-circulating flow around intake further reduces residence time to about 6ms for a comparable yield level. According to this study, for 15% recirculation rate the residence time reduces to 5 ms, under conditions specified in Figure 4.

It is obvious that the heat release available in any combustion process to assist fuel reforming is not limited as it is in partial oxidation. A recirculation rate in excess of 15% is observed in the typical flames at temperatures considerably higher than the ones found in partial fuel oxidation. It is possible to control the rate of the high temperature re-circulation, while exposing the fuel/steam mixture to the high radiation heat flux from primary combustion, thus accomplishing near complete reforming within 3 ms to 5 ms.. Nevertheless, for the proposed combustor the residence time in the reforming zone is about twice as long (10 ms) to accommodate any imperfection in mixing or possible surface-effects. It should be noted that in comparison with the partial fuel oxidation, the higher hydrogen yield would result from the steam injection used in the combustion method described above (compare H 2 concentrations in Figure 3 and Figure 4). This alone suggests environmental and performance benefits vs. typical partial fuel oxidation. Another benefit of the proposed method relates to an improved reliability, due to endothermic nature of the fuel reforming reactions. They make available a significant heat sink thus providing additional means for protecting the combustors' wall in a desirable location.

 

6.3. Advantages of NO X Formation

NOx are classified by their sources of formation, such as prompt, thermal, and fuel nitrogen oxides. The relative importance of each type of NOx on total emission pertains to the fuel origin and operating conditions. Prompt NOx are formed during the initial stage of fuel thermal decomposition (pyrolysis) in the near flame zone, and in the case of natural gas combustion might contribute up to 20% of the total emissions [12].

???????: CH+N2 óHCN + N,		(6)  C H2 + N2 ó HCN + NH,	(7)  C 2 + N2 ó 2CN,		(8)  The present consensus on ‘prompt' NOx formation is that they are promoted by CN-radicals in the presence of pyrolysis products. The mechanism of CN-radicals formation appears to follow reactions (6), (7), and (8). Moreover, in the near flame zone at fuel-rich conditions, the same hydrocarbon species lead to excessive O-atom concentrations, compared to its partial equilibrium value, and thus supports N-atom oxidation [12, 13, 14].

In the proposed combustion technology, the decomposition of hydrocarbons occurs in the reforming process, without the presence of oxygen. Therefore, N-atoms, when created, have a low probability to oxidize, while the presence of NHi-radicals leads to NO distraction. Furthermore, when reformed fuel reaches the combustion zone to undergo mixing with air, concentrations of CN- and CHi-radicals are markedly diminished or eliminated, therefore suppressing the production of O-atoms. It turn, this provides a mechanism for prompt NOx suppression, a distinct advantage compared to regular staged combustion.

Experimental data [13] supports this conclusion about the suppression of NOx formation, as shown in Figure 5. Under similar thermal and stoichiometric conditions, combustion of fuel comprised of 30% CO and 70% H 2 leads to a NOx reduction of an order of magnitude in comparison with methane.

There have been some indications that the oxidation of N-atoms may occur by direct reaction with OH-radicals. However, some empirical data [14] does not support this fact, while other data indicates that this possibly as a second or third order effect [12].

The mechanism of suppression of thermal NOx in the proposed combustion process relates to the kinetics of reformed fuel combustion and nitrogen oxidation. Because of the much higher reaction speed of CO and H 2 combustion, a shortage of oxygen occurs, suppressing nitrogen oxidation. In addition to a diminished availability of oxygen, nitrogen is also diluted because of the higher specific volume of combustion products generated per unit of reformed fuel.

Based on the kinetics proposed in [12], our calculations show that, due to diminished concentrations of O 2 and N 2 atoms, a 30% to 40% decrease of the thermal and NOx can be expected.

Fuel-bounded nitrogen under fuel reforming/gasification conditions has a high chance of converting into molecular nitrogen. It is important to note that such recombination of the N-atoms into the stable molecular nitrogen increases with temperature [15]. Therefore, the combustion method proposed here will also suppress fuel nitrogen conversion into NOx to a greater extent than technologies used today.

In total, suppression of all of the sources of NOx formation by the proposed technology will result in up to 80% nitrogen oxides reduction for gas turbine burning natural gas and up to 90% for gas turbine operating on liquid fuel.

 
  1. M.J.Moore. NOx Emission control in gas turbines for combined cycle. Proc Instn Mech Eng Vol. 211, pp. 43-49, Part A
  2. State of California Energy Resources Conservation Commission. Docket No: 91 -IGE-1. By I.G.Rice. 1991
  3. D.K.Fleming, M.J.Khinkis. Thermochernical Recuperation System-an Introduction. Annual Toledo Glass and Ceramic Symposium, March 1984
  4. M.J.Khinkis. Low Emission Combustion Thermo-chemical Heat Recovery, Institute of Gas Technology, Presentation at DOE Workshop, April 1991
  5. Thermo-chemically Enhanced Gas Turbines - A High Efficiency, Low Emissions Electricity Generating Technology. Testimony Submitted by The Institute of Gas Technology to California Energy Commission, 1991
  6. High Efficiency Reformed Methanol Gas Turbine Power Plants. US Paten # 5,927,063
  7. G.Lozza, Pchiesa. Partial fuel Decarbonization to Reduce CO2 Emissions From Combined cycles-Part 1: Partial Oxidation. Journal of engineering for Gas Turbines and Power, January 2002, Vol. 124
  8. Method of Operation a Gas Turbine on Reformed Fuel. US Patent 5,729,967
  9. A.Maltsev, B.Maltseva. Osnovnie Kharakteristiki Topliv. Moscow, Energiya, 1983 (Major Characteristics of Fuels)
  10. James L. Johnson. Fundamentals Of Coal Gasification. Institute of Gas Technology, Elliot, 1989
  11. G.A.Karim. The Uncatalyzed Partial oxidation of Methane for production of Hydrogen with recirculation. Fossil Fuel Combustion, Vol. 39, 1992, pp. 77 – 84
  12. C.T.Bowman, Chemistry of Gaseous Pollutants Formation and Distraction. Fossil Fuel Combustion, Edited by. W. Bartok, A.F.Sarofim. 1991 (J.Willey & Son, Inc.)
  13. D.Iverach, N.Kirov, B.S.Haynes. Formation of Nitric Oxide in Fuel Rich Flames. Combustion Science and Technology, 1973, Vol.8, pp. 159-164
  14. C.P.Fenimore. Reaction of Fuel Nitrogen in Rich Flames. Combustion and Flame, 26, 249-256, 1976
  15. Steam. Its Generation and Use, 40 th Edition, Babcock & Wilcox, 1992
 
 
   
51 Shaffer Rd, Bridgewater, NJ 08807 Tel.: 908-419-3996,
Fax: 908-636-2244