The complete combustion of which of the following substances produces carbon dioxide and water?

2.3.2 Oxy-combustion

Complete combustion of hydrocarbons (without impurities) in the presence of enough oxygen produces water vapor and carbon dioxide. Separation of water vapor from the gas stream is simple; condensation can provide the suitable separation. In precombustion technologies, combustion is made using pure oxygen (up to 97% purity); however, a small amount of nitrogen and argon remains. A big part of the exhaust gases is recycled in the boiler to maintain the usual temperature levels of pulverized coal (PC) without capture. This avoids full resizing of the boiler and its associated exchangers. The flow of uncycled CO2 still contains water vapor, impurities (particularly some NOx and SOx amounts that remain in the exhaust gas of scrubbing systems), and incondensable gases such as oxygen, nitrogen, and argon. The latter originates from the ASU (air separation unit) and parasite air entries due to the lack of a seal of the PC cycle (combustion chamber, deduster, gas-gas heater, etc.). The content of CO2 in these exhaust gases is about 75% (wet basis). The next step is to condense the water and purify the CO2 until it is 99% pure so that it can be transported in the supercritical state. A simple schematic of the oxy-combustion system is shown in Fig. 23.7 (Sadati et al., 2015; Kanniche et al., 2010).

Example.

A natural gas sample consists of 67 percent by volume methane and 33 percent by volume ethane. Determine the amount of oxygen and air needed for complete combustion.

Benefit of Condensate Return.

This can be seen in the following sample calculation:

Steam rate: 25,000 kg/hr

Feedwater temperature: 15°C

Condensate temperature: 50°C

Average specific heat of water: 4,180 J/kg°C.

Case 1: No condensate return (assume 1 hour operation):

E=m˙CpΔTt=(25.103)(4,180)( 100−15)(1)=8.9GJ.

This is the energy input required for feedwater heating in the absence of condensate return.

Case 2: 100 percent condensate return:

E=m˙CpΔTt=(25.103)(4,180)(100−50)(1)=5.2GJ.

The savings from condensate return is 3.7 GJ/hr. Now suppose the fuel is oil rated at 6 GJ/bbl; this is a savings of more than one barrel of oil per hour (boiler thermal efficiency of 70 percent).

Benefit of Automatic Fuel Controls.

The proper ratio of air and fuel can be maintained by an operator if care is taken. However, in some cases a fuel and air metering system can automatically maintain efficient operation. For a moderately sized steam plant (25,000 kg/hr), such a system could pay for itself in a year or two by a fuel savings of only 5 percent.

Steam Generation and Distribution.

Improvements in steam systems fall into two broad categories. The first applies to the steam system itself, while the second applies to the uses to which the steam is put. For steam systems, consider:

Steam leaks from lines and valves

Defective steam traps

Proper sizing and maintenance of distribution systems, including insulation

Proper management of condensate return

Proper maintenance of steam tracing systems

Most of these EMOs are self-evident and do not require discussion. Small steam leaks resulting from defective traps or valves can lead to surprisingly large energy waste (fig. 9.7). As steam loads change over time, the distribution system may be used for purposes other than those for which it was originally designed. If the lines are too small, pressure drops may be excessive. If they are too large — i.e., supplying small loads — the losses may be disproportionately large. Condensate return saves energy in a number of ways. Not only is less energy needed to heat feedwater, but less energy will be expended in pumping and chemically treating makeup water. Steam tracing systems (used to heat pipes, tanks, etc.) can waste energy if not maintained properly. An obvious caution is to turn them off when not needed.

Steam is used for heating purposes, for operating steam driven equipment, or for heating buildings. Some EMOs include:

Supply steam at the lowest pressure possible

Review steam uses to see if more efficient alternatives exist

Apply the cascade principal to steam uses

Steam usually will be supplied at the pressure of the highest load. In industrial operations, this may be 1 to 3 MN/m2 if steam turbine driven equipment is used. Steam is also used to transfer fluids by means of steam jets. For heating purposes, pressures are more typically in the range 0.1 to 0.3 MN/m2. If most of the loads are at lower pressures, steam should not be supplied at high pressure. Instead, a substitute energy source could be found for the high-pressure load.

In some cases, electric motors or some alternative drive system will be more efficient than steam turbines. This would be true in cases where the steam loads are small, or are distant from the steam plant. Electricity might even be better for heating in a particular case, if line losses and the effect of more precise temperature control are included.

Sometimes steam must be provided at high pressure. Instead of using pressure-reducing valves, look for opportunities to cascade steam use, within acceptable limits on steam pressure, temperatures, and quality. For example, high pressure steam can first be expanded through a noncondensing turbine to do useful work, and then the turbine exhaust steam used for process or building heating.

The importance of proper steam line insulation cannot be overemphasized. Figure 9.8 illustrates the magnitude of heat loss from uninsulated lines. The benefit of insulation can be seen from the following example.

Example.

A 25 cm inner diameter steel pipe of wall thickness 9.5 mm has steam at 260°C and 4.69 MPa (500°F and 680 psi). Determine the benefit of 5 cm thick mineral wool molded pipe insulation for the following conditions:

Tambient=20°CKpipe=45W⋅m/m2° CKinsulation=0.06W⋅m/m2°Chcair=9Wm2°Chcsteam =14,200Wm2°C

Solution.

Compute the four resistances (refer to Chapter 10, equation 10.5). Note that heat flow through a hollow cylinder is given by:

(9.3)Q=2πLKΔT/ln(r2/r1)Watts,

where L = length of pipe, m

K = thermal conductivity of cylinder, W·m/m2°C

ΔT = temperature difference between inner and outer walls, °C

r2 = outer radius, m

r1 = inner radius, m.

The resistances are found as follows (per unit length of 1 m):

Rinnersurface=1(hcsteam)(A)=1(14,200)(π)(0.25)=m°C/W8.97×10−5Rpipe=1n(r2/r1)2πKpipe=1n(12.98/12.5)(2π)(45)=13.33×10−5Rins=1n (r3/r2)2πKins=1n (17.98/12.98)(2π)(0.06)=0.865Rair=1(hcair)(A) =1(9)(π)(0.369)=0.096Rtotal=0.96.

Since the same quantity of heat flows through each thermal resistance,

Ts −TambientRtotal=Tins−TambietRair260−200.96=T ins−200.096=44°C(111°F).

And, since

Q=ΔTR=260−200.962=50W/m,

we can compute the annual heat loss for 30 m and 8,760 hours/year as:

E=(250)(30) (8,760)(3,600)=236GJ/yr.

This is seen to be about 5 percent of the bare line loss shown in Figure 9.8.

Hot Water and Water Pumping.

The heating and transport of water and other fluids requires utilization of energy to raise the water temperature, to make up heat losses from pipelines, to pressurize fluids, and to overcome the resistance to fluid flow of pipelines.

The heat input necessary to raise the temperature of a fluid is given by

(9.4)Ein=mCp(Tf−Ti),

where

Ein = heat input, joules

m = mass, kg

Cp = specific heat at constant pressure, joules/kg°C

Tf = final temperature, °C

Ti = initial temperature, °C.

The power required to move an incompressible fluid through an external piping system is given by

Pumpingpow er=(massflow)(workdoneperunitmass).

This may be rewritten as:

(9.5)pp=m˙(ΔPρi)watts ,

where

pp = pumping power, watts

m˙ = mass flow rate, kg/sec

ΔP = system pressure drop, N/m2

ρi = fluid density at pump inlet, kg/m3.

Or in more convenient form, the power expended in pumping a fluid is related to the volume pumped:

(9.6)ppV˙ΔPwatts,

where now

V˙=volumepummped,m3 /sec

The pressure drop depends on the system (pipes, channels, orifices, bends, etc.) and must be determined for each case. In the case of round pipes, it is given by the following equation:

(9.7)ΔP=fρ¯v2L2DeNm2,

where

f = friction factor (dimensionless)

ρ¯ = average density, kg/m3

v = velocity, m/sec

L = length of pipe, m

De = pipe diameter, m.

The losses through bends, enlargements and contractions, and valves and fittings are similarly proportional to velocity squared.

For incompressible fluids, the volume flow rate V˙ is related to the velocity v by:

V˙= A⋅vm3/sec

solving,

v2=V˙2 A2.

Substituting these results in the equation for pumping power we obtain:

(9.8)pp=V˙3fρ¯L2A2Dewatts.

This shows that once the pipe size is fixed, the power required for pumping increases as the cube of the volumetric flow rate. Conversely, for a fixed flow rate, the pumping power decreases in proportion to the fifth power of the diameter.

For a wide range of Reynolds numbers f = 0.022 for clean, commercial steel pipe. This holds true in either American/British or SI units, since f is a dimensionless ratio having units of length/length. If American/British units are used, a conversion factor (g = 32.2 ft/sec2) must be inserted in the denominator of the pumping power equation.

Pressure drops for complete piping systems require determination of losses through all components. This can be done approximately using data such as figure 9.9, which converts flow resistances of typical fittings to equivalent lengths of pipe.

The above equations can also be used to estimate pressure drops for compressible fluid flow, when pressure drops and temperature changes do not cause large density changes (say less than 5 to 10 percent). Otherwise more rigorous calculations should be made.

The relative importance of these quantities can be illustrated in a sample calculation,

Example.

Problem. Find the power required to heat, pressurize, and pump 3.1 × 10–3 m3/sec of water through a 2-inch piping system. The pressure drop of pipe fittings and valves is equivalent to a pipe twice as long as the piping run.

Calculations

To heat the water:

Q=m˙cp(Tf−Ti)=(3.1)(4,180)(60−20)=518kW.

To raise the pressure:

pp1=V˙(Pf−Pi)=(3.1×10−3)(600−100)×103=1.55kW.

To overcome pressure drops:

ΔP=f ρ¯v2L/2De=(0.022)( 1,000)(2.25)(1,000+2,000) (2)(0.051)=1.46×106N /m2pp2=(3.1×10−3)(1.46×106)=4.53kW.

Total pumping power:

ppt=pp1+pp2=6.08kW.

Assume the pump is 81 percent efficient; then the input power required is ~ 7.5 kW (10 hp), which is less than 2 percent of the total input power. In this example, the major energy management target clearly should be reduction of heat losses or heat recovery.

Losses in hot water systems can be reduced by the following steps:

Reduce thermostat settings

Cover open tanks

Insulate tanks and pipes

A major source of loss in hot water systems is the standby losses which occur when tanks are continuously maintained at elevated temperatures.

Heat recovery is another useful technique. Waste process heat (for example, refrigeration compressor cooling water) can often be reclaimed to heat or preheat water, as shown in the following example:

Example.

An industrial building has an electric domestic water heating tank with a 760 1 (200 gallon) capacity (Figure 9.10(A)). The average demand for hot water is about 700 1/hr at 50°C. Measurements indicated that the water heater uses approximately 27.5 kW during the summer and 40.7 kW during the winter. Water input temperature ranges from 4 to 18°C.

In the same building there is a reciprocating chiller unit rated at 40 tons (140 kW) — 140 kW is equivalent heat value of 40 tons — with a compressor motor rated at 37 kW (50 hp). The heat available from the condensing unit is equal to the heat removed from the building (40 tons or 140 kW) plus the heat of the compressor, pumps, and other equipment. If the chiller operates at maximum load, this can be seen to be equal to or greater than 140 + 37 = 177 kW (Figure 9.10(B)).

Since the average power required for water heating is 34 kW, even if the chiller operates at half load, it rejects three times as much heat as is required for water heating.

The Freon-22 refrigeration cycle has the following temperatures:

The cycle is sketched in figure 9.11. Since hot water is required at 50°C, and since the refrigerant discharge temperature is 71°C, heat recovery from the refrigerant in the superheat region is adequate to provide the required temperatures.

Several approaches can be considered for heat recovery:

Install a heat exchanger in the hot water tank and flow hot refrigerant through it before entering the condenser.

Use a second tank to preheat hot water with a heat exchanger heated by refrigerant.

Use a second tank, a heat exchanger, and a recirculating pump.

This third approach was selected as the best means of optimizing heat recovery with a variable hot water load. The heat recovery system with manual controls is sketched in figure 9.12. Note that an alternative approach with automatic controls could also be developed.

Costs for this system were estimated as follows:

The economic savings were estimated to be

(32−1kW)(8,760hrs/year)(0.06$/kWh)=17,345$/yr

(In the above estimate, 32 kW was the average load year round, 24 hours per day.) This gives a simple payback of 1.4 years. Note that allowance has been made for approximately 1 kW of power used by the heat recovery system. (See Chapter 12 for a more detailed economic analysis of this example.)

Pumping system losses can be reduced by lowering system pressures, reducing friction losses, and stopping leaks. Major energy management opportunities can be summarized as:

Reduce system pressure

Reduce friction losses (increase pipe size, eliminate pressure reducing valves)

Stop leaks

Use storage tanks or accumulators so pumps can be shut down part time or operated off-peak

Recycle or reuse water

It is a common practice to provide water at the pressure required to meet the highest pressure load. An alternate approach which will sometimes save energy is to provide water at the pressure needed by most of the load, and provide booster pumps for the high pressure loads.

Direct- and Indirect-Fired Furnaces and Ovens.

It is estimated that nearly 50 percent of all the energy used in the United States is for process or space heating (see reference 1, Chapter 10, p. 342).

Direct-fired furnaces and ovens rely on heating directly by the products of combustion (fuel-fired) or by electric heating elements. Indirect-fired furnaces involve some type of heat exchanger for transferring heat from the heat source to the process.

There are three basic approaches for managing the efficient use of process heat. These are:

Reduce heat losses

Use more efficient equipment and processes

Recover heat

Reduce Losses.

Reduction of heat losses can be accomplished by insulation or improved designs. Generally, the economically optimum amount of insulation for furnaces, ovens, and pipes depends on the temperature range and fuel costs, so no simple rule of thumb can be given. Each case will usually have to be analyzed on its own merits.

One useful approach is to consider an analysis on a unit area basis. Consider an oven or tank which loses 3.5 kW/m2 from its uninsulated walls under normal operating conditions. Analysis indicates that this could be reduced to 0.7 kW/m2 if 2.5 cm of rockwool insulation is added. Energy costs are $5.00/GJ (~4.73$/MBtu) and annual operation is 2,000 hours per year. A one-year payback is required. The analysis indicates:

Unit Energy Loss Without Insulation:

(3.5kW/m2)(2,000hr/ yr)(0.6×106J/kWh)=25,2GJ/m2⋅yr.

Unit Energy Loss with Insulation:

(3.7kW/m2)(2,000hr/yr)(3.6×10 6J/kWh)=5,04GJ/m2⋅yr.

Energy Savings:

25.2−5.4=20.16GJ/m2⋅yr.

Cost Savings:

(20.16 GJ/m2⋅yr)(500$/GJ)=100.80$/yr .

Therefore, if the installed cost of the insulation is equal to or less than 100$/m2, a one-year payback will be obtained.

Other forms of heat losses which should be evaluated include:

Heat absorbed by the work or product and lost as the work cools

Heat absorbed by auxiliary equipment (conveyors, trays, etc.)

Heat lost up stacks or outdoors

More Efficient Equipment.

More efficient equipment designs generally require improved heat transfer capability, although sometimes a different process (e.g., microwave heating) will lead to savings. There are so many potential possibilities that it is impossible to enumerate them here. Suffice it to say that the general approach should be to examine carefully the specific needs of the process, and then attempt to provide heat only when and where its use is essential.

Examples of this approach would include using immersion heaters for tanks rather than underfiring, or using induction heating rather than furnaces. Both of these approaches concentrate the heat in the product, and reduce the potential for external heat losses. Another example is jet impingement heating, common in the European metals industry. This allows the heat to directly impinge on the object being heated, thereby penetrating the surface film barrier and increasing heat transfer efficiency.(4)

Process heat savings also result from converting batch-type processes to continuous operation. This conserves fuel by eliminating or reducing heat-up and cool-down periods. Often, productivity is increased as well. For example, one company replaced a group of batch type kilns with a gas-fired walking beam kiln, and found the new kiln operated with 20 percent less fuel, and had 2.5 times the product throughput. Labor savings also resulted, since only two personnel (rather than ten) were required to operate the new unit.(4)

Excess air is provided to certain types of ovens for diluting the exhaust air. For example, in solvent drying ovens, air is introduced to create an air-gas mixture which is below the lower explosive limit (LEL). Many ovens use excessive amounts of dilution air, thereby wasting heat and fuel.

Typical industry practice is to operate in the range up to 25 percent LEL concentrations. With automatic controls, operation up to 50 percent of LEL limits is possible. Many ovens in use today operate below 25 percent LEL and may be operating as low as 5 percent LEL. This is equivalent to about four times the excess air actually needed, or about twice the energy use actually required.(4)

Heat Recovery.

Heat recovery is an important tool for the energy manager. The potential list of applications for this technique is too long to include here, but representative examples will illustrate the possibilities.

Recover heat from building exhaust air using run-around systems

Recover heat from milk in dairy operations using heat pumps

Recover heat from air compressor cooling water and preheat hot water with it

Recover heat from steam condensate and use it to preheat feedwater

Recover heat from chillers and use for hot water or space heating

The variables which determine the feasibility of heat recovery include the value of the heat, when it is available, the cost of the installation, and the uses to which the heat can be put. Typical uses for recovered heat include hot water, process heat, space heating, drying, and preheating fuels or materials. Sometimes heat recovery leads to other economies, such as reducing fan or pump operation, or permitting cooling tower operation to be reduced. Several examples of heat recovery are given in this chapter.

Heat can often be recovered from hot process streams, exhausts, or stacks. Often the heat exchange equipment is a critical item, depending on the temperature range and the corrosiveness of the hot effluent streams. The selection of the optimum heat recovery system size involves a balancing of the costs of the equipment compared to the benefit of recovering still more heat. This is discussed more fully in reference 1, and illustrated with several case studies.

2.4 Complete and incomplete combustion

During complete combustion, the reactant burns in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, nitrogen will yield nitrogen dioxide, and sulfur will yield sulfur dioxide.

In most industrial applications and in fires, air is the source of oxygen (O2). Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted to nitrogen oxides (NOx).

CH4+O2+N2 →CO2+2H2O+N2+CO +NOx+heat

Incomplete combustion occurs when there isn't enough oxygen to allow the hydrocarbon to react completely with the oxygen to produce carbon dioxide and water, and also when the combustion is quenched by a heat sink such as a solid surface or flame trap.

Complete or incomplete combustion (an indicator of the combustion efficiency) is a calculation of how well the equipment is burning a specific hydrocarbon, shown in percent. Complete combustion efficiency would extract all the energy available in the hydrocarbon. However 100% combustion efficiency is not realistically achievable. Common combustion processes produce efficiencies from 10% to 95%. Combustion efficiency calculations assume complete hydrocarbon combustion and are based on three factors: (i) the chemistry of the hydrocarbon, (ii) the net temperature of the stack gases, and (iii) the percentage of oxygen or CO2 by volume after combustion.

Combustion efficiency relates to the part of the reactants that combine chemically. Combustion efficiency increases with increasing temperature of the reactants; increasing time that the reactants are in contact; increasing vapor pressures, surface areas, and stored chemical energy. One way of increasing the temperature of the reactants and their vapor pressures is to preheat them by circulating them around the combustion chamber and throat before being injected into the combustion chamber. The specific heat of combustion is a chemical property that refers to the amount of energy that can theoretically be extracted from a hydrocarbon at 100% combustion efficiency. The heating value is a more realistic term and does not include the condensation of the water vapor produced. It is thus more easily applied to real combustion processes.

Air preheating is one method used in steel works, for instance, to increase combustion efficiency. This uses the heat in the flue gases to heat one of a pair of chambers and the inlet air passes through the other one. The use of the chambers is switched as soon as one chamber has reached temperature, so the air passes through the heated chamber. This is one of the simplest and best methods of increasing combustion efficiency in this kind of process; such preheaters are standard equipment these days for larger systems.

In this same context, hydrocarbon efficiency is a form of thermal efficiency of a process that converts chemical potential energy contained in a carrier hydrocarbon into kinetic energy. Overall hydrocarbon efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous energy profile.

5 Complete and incomplete combustion

In complete combustion, the reactant burns in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, nitrogen will yield nitrogen dioxide, and sulfur will yield sulfur dioxide.

In most industrial applications and in fires, air is the source of oxygen (O2). Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted to nitrogen oxides (NOx):

CH4+2O2+N2→CO2+2H2O+N2+CO+NOx+heat

Incomplete combustion occurs when there isn’t enough oxygen to allow the fuel to react completely with the oxygen to produce carbon dioxide and water, and also when the combustion is quenched by a heat sink such as a solid surface or flame trap.

Complete or incomplete combustion (an indicator of the combustion efficiency) is a calculation of how well the equipment is burning a specific fuel, shown in percent. Complete combustion efficiency would extract all the energy available in the fuel. However, 100% combustion efficiency is not realistically achievable. Common combustion processes produce efficiencies from 10 to 95%. Combustion efficiency calculations assume complete fuel combustion and are based on three factors: (1) the chemistry of the fuel; (2) the net temperature of the stack gases; and (3) the percentage of oxygen or CO2 by volume after combustion.

Combustion efficiency relates to the part of the reactants that combine chemically. Combustion efficiency increases with increasing temperature of the reactants, increasing time that the reactants are in contact, increasing vapor pressures, increasing surface areas, and increasing stored chemical energy. One way of increasing the temperature of the reactants and their vapor pressures is to preheat them by circulating them around the combustion chamber and throat before being injected into the combustion chamber. The specific heat of combustion is a chemical property that refers to the amount of energy that can theoretically be extracted from a fuel at 100% combustion efficiency. The heating value is a more realistic term and does not include the condensation of the water vapor produced. It is thus more easily applied to real combustion processes.

Air preheating is one method used in steel works, for instance, to increase combustion efficiency. This uses the heat in the flue gases to heat one of a pair of chambers and the inlet air passes through the other one. The use of the chambers is switched as soon as one chamber has reached temperature, so the air passes through the heated chamber. This is one of the simplest and best methods of increasing combustion efficiency in this kind of process; such preheaters are standard equipment these days for larger systems.

In this same context, fuel efficiency is a form of thermal efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous energy profile.

1 INTRODUCTION

The complete combustion of methane by Pt and Pd catalysts has been studied in relation to pollution control of emissions from natural gas vehicles (NGV) [1], as well as for the oxidation of methane in turbines for power generation [2]. Supported Pt catalysts are often prepared from Cl-containing precursors such as H2PtCl6, and it has been reported [3–5] that Cl poisons the oxidation activity. The state of the active catalyst’s surface and the effect of Cl poisoning on the activity, however, have not been elucidated.

Liebske et al. [6] were among the first to propose a model of the various phases that could be present as a function of pretreatment conditions in a Pt/Al2O3 catalyst prepared from Cl precursors. These phases, however, were not correlated with the catalyst’s activity. Based on temperature-programmed reduction (TPR) of Pt catalysts oxidized at different temperatures, Hwang and Yeh [7] concluded that four types of oxide species could be formed. These authors proposed that at room temperature there is a surface oxide, PtO0.7. At 100 °C PtO is formed, while at 300 °C Pt further oxidized to PtO2. Oxidation at 600 °C leads to metallic Pt and platinum aluminate. In addition, preparation of the catalyst from a PtCl4 precursor results in a PtOxCly complex, which can be removed by reduction in H2 at 400 °C [3]. In a similar study, Burch and Loader [8] concluded that the oxidation activity of Pt catalysts was optimal for a partially oxidized and reduced surface.

Farrauto et al. [3] reported that the presence of Cl on the catalyst reduced the methane oxidation activity of Pd/Al2O3, and that removal of Cl increased the catalyst’s activity. Similarly, Marceau et al. [4] found that elimination of Cl from Pt/Al2O3 catalysts at 450 °C led to higher activity. Roth et al. [5] also confirmed that removal of Cl from a Pd(Cl)/Al2O3 catalyst led to the same activity as Cl-free Pd catalysts and suggested that the active sites are PdO that slowly deactivates to form a less active Pd(OH)2.

The objective of this work is to determine the structure of the active Pt species for methane oxidation and to characterize the state of the surface on Cl-free and Cl-containing Pt/alumina catalysts. EXAFS spectroscopy probes the local structure around a selected element – important for a study of highly dispersed cartalysts for which long range order does not exist and x-ray diffraction may not be an appropriate technique.

Balancing Coal, Oxygen and Water Feeds

The complete combustion reactions, R-3.5, R-3.6, and R-3.7, are very exothermic release a great deal of heat, but the product gasses have no further combustion value. The steam and CO2 gasification reactions (R-3.9 and R-3.10), are endothermic; which means that the product gasses have a greater heating value than the reactants. To maximize the heating value of the syngas, we want to drive the endothermic gas-forming reactions as much as possible. These reactions do not occur spontaneously, so we rely on exothermic reactions with oxygen to raise the mixture temperature to the desired gasification temperature and to provide heat for the endothermic gas-forming reactions.

The endothermic steam gasification, R-3.9, and CO2 gasification, R-3.10, reactions can be driven by an external source of heat rather than by feeding oxygen to the gasifier. Sources of heat that have been considered include solar heat, nuclear heat, and external char or gas combustion. With an external heat source, heat transfer is complicated by selecting conductive heat transfer materials that will withstand high gasification temperatures and corrosive atmospheres.

The typical feeds to a gasifier are coal, oxygen, and water. Oxygen may be fed as a nearly pure oxygen stream from an air separation unit, or as air. Water enters the gasifier as coal moisture, coal slurry water, or as steam. The combined oxygen and water feeds should be sufficient to completely gasify the feed. If the oxygen feed is excessive, then the reaction becomes more like combustion than gasification, and low heating value gasses are produced. The temperature is controlled by varying the oxygen/water balance. The reactions with oxygen are all exothermic, so oxygen tends to increase the gasifier temperature. The steam gasification reaction, R-3.9, provides additional gas formation. Steam gasification is endothermic, so it tends to reduce the gasifier temperature. To optimize gasifier operation one must find the correct (O2 + H2O)/coal ratio and the correct O2/H2O ratio.

Carbon dioxide can be used in place of steam, but the CO2 gasification reaction, R-3.10, is slower than the steam gasification reaction, R-3.9. Most syngas applications also favor the higher H2/CO ratio produced by the steam gasification reaction.

The oxygen/water/coal ratios depend on the gasifier configuration, the operating conditions, and the choice of coal. For example, the BGL gasifier, a moving bed gasifier; coal enters at the top, and slag is removed from the bottom. Oxygen and steam enters near the bottom; and gas flows up through the bed of coal and out near the top of the gasifier. In a recent study,5 a bituminous coal, Illinois No. 6, was gasified. Hot gasses from the bottom of the gasifier rise through the incoming coal, preheating the coal, driving off volatiles, and drying coal. Consequently, the exiting syngas has a relatively low temperature, 537 oC. This energy efficient counter current design also results in a relatively low 0.54 oxygen/dry coal mass ratio. The water/dry coal mass ratio was 0.40. Steam was 81% of the water feed, and the balance was moisture in the feed.

In another study,6 the same coal, was gasified using a Shell gasifier. This is a high temperature, entrained flow design. Coal and oxygen enter near the top. Syngas and slag are removed near the bottom. Syngas exits the gasifier at 1,427 oC. The higher operation temperature results in a relatively high 0.83 oxygen/dry coal mass ratio. The water/dry coal ratio was 0.16. Steam was 67% of the water feed, and the balance was moisture in the feed.

Since O2 and H2O are both sources of oxygen for gasification; one can compare molar oxygen, O, not O2, to carbon ratios for both gasifiers. This ratio is 0.9669 for the BGL gasifier and 1.015 for the Shell gasifier. Despite the large difference in operating conditions, the oxygen feed requirement is nearly the same. The difference is in the source of the oxygen. The O2/H2O mass ratio is 1.336 for the BGL gasifier and 5.145 for the Shell gasifier. Due to the higher operating temperature for the Shell gasifier; more of the oxygen must come from O2, rather than H2O.

In some gasifiers, the water feed is greatly in excess of the stoichiometric quantity required to gasify the feed. In this case, the oxygen feed must also be increased to raise the excess water to the gasification temperature. This is especially true if water is fed as liquid water, rather than steam, because heat is required to boil the water.

12.3 Applications of glycerol in the fuel sector

The complete combustion of glycerol (Fig. 12.2) generates 4195 kcal per kg, but there are many difficulties associated with this process. Incomplete burning may generate acrolein, which is highly toxic to humans. The salts present in the crude glycerol from biodiesel production may deteriorate the equipment, leading to corrosion and other problems. All these drawbacks make the direct combustion of glycerol economically less attractive than the chemical or biochemical transformation.

Glycerol can also be used in the production of ethanol, an important biofuel used worldwide. Historically, ethanol has been produced mainly from sugars and carbohydrates via microbial fermentation. Speers and co-workers (2012) developed a microbial co-culture for the conversion of glycerol into ethanol and electricity in bioelectrochemical systems. Ethanol can be used as a feedstock for the transesterification of vegetable oil for biodiesel production and the electricity can be used to partially offset the energy needs of the biodiesel plant. The platform includes a glycerol-fermenting bacterium, Clostridium cellobioparum, which produces ethanol and other fermentative byproducts (lactate, acetate, formate, and H2), and Geobacter sulfurreducens, which can convert the fermentative byproducts into electricity. Both organisms were adaptively evolved for tolerance to industrially relevant glycerol concentrations and co-cultivation of these strains stimulated microbial growth and resulted in ethanol and complete conversion of fermentative byproducts into electricity.

The yeast Saccharomyces cerevisiae utilizes the general glycolytic pathway for the majority of its energy production. The carbohydrates are converted to pyruvic acid, which is decarboxylated to acetaldehyde, and then to ethanol. Starting from glycerol, the pathway involves formation of di-hydroxy-acetone (DHA) and pyruvic acid (Fig. 12.3). To further increase ethanol production and evaluate fermentative performance, the genes involved in the conversion of pyruvate to ethanol were overexpressed (Yu et al., 2012). These genes included pyruvate decarboxylase, which is involved in the decarboxylation of pyruvate and thus controls the first step in the production of ethanol from pyruvate, and alcohol dehydrogenase, which is the enzyme involved in the ethanol production pathway from acetaldehyde.

There have been intensive efforts to describe methods for the efficient conversion of glycerol to ethanol via metabolic pathway engineering of E. coli, to minimize byproducts. The engineered E. coli strain produced 21 g/L of ethanol from 60 g/L of pure glycerol, with a volumetric productivity of 0.216 g/L/h under anaerobic conditions (Yazdania and Gonzalez, 2008).

Glycerol gasification to synthesis gas, a mixture of CO and H2, has also been studied (Soares et al., 2006). The reaction is endothermic by 83 kcal/mol, but can be carried out at temperatures around 350°C over Pt and Pd catalysts. Synthesis gas is used in many industrial processes, like methanol production and Fischer–Tropsch synthesis of hydrocarbons.

Another approach to use glycerol from biodiesel production in the fuel sector is the development of glycerol ethers, acetals/ketals and esters with potential use as fuel additives. The glycerol molecule has about 50% of its mass in terms of oxygen atoms, which makes it a good platform for the production of oxygenated additives.

The acid-catalyzed reaction of glycerol with isobutene affords tert-butyl-glyceryl ethers (Klepacova et al., 2005), which are considered as an octane booster for gasoline (Wessendorf, 1995). Ethyl glyceryl ethers (Fig. 12.4) can be produced through the acid-catalyzed reaction between glycerol and ethanol (Pariente et al., 2009), being a completely renewable molecule. These ethers are potential additives for biodiesel, improving the cold flow properties (Pinto, 2009). For instance, addition of 0.5 vol% of glyceryl ethyl ethers in the soybean and palm biodiesel led to a reduction of up to 5°C in the pour point, indicating that these ethers can be used in blends with biodiesel.

Glycerol acetals and ketals are another class of derivatives with potential use as fuel additives. They are produced through the acid-catalyzed reaction of glycerol with aldehydes and ketones, respectively. The reaction of acetone with glycerol produces one ketal, known as solketal, whereas reaction with formaldehyde solution affords two acetal isomers (da Silva et al., 2009) (Fig. 12.5). Solketal is a potential additive for gasoline (Mota et al., 2010). Within 5 vol% addition, it improved the octane number and significantly reduced gum formation, without affecting other important properties of the gasoline, such as the vapor pressure. Although acetone is produced today from petrochemical feedstock, it can be produced from sugars, through fermentation procedures (Jones and Woods, 1986), making solketal a completely renewable oxygenated compound with potential to be used in the fuel sector.

Acetals produced in the reaction between glycerol and n-alkylaldehydes have found application as additives for biodiesel, improving the cold flow properties (Silva et al., 2010b). The best results were found with the acetals of glycerol and butyraldehyde. As the aldehyde chain increases, the effect in the pour point is less relevant. In addition, the glycerol conversion decreases with the increase in the hydrocarbon chain. Glycerol acetals can also be used as antioxidants. The acid-catalyzed reaction of glycerol with aromatic aldehydes, such as benzaldehyde, anisaldehyde and furfural, affords acetals with a benzylic C–H bond (Fig. 12.6). These molecules showed antioxidant properties in the diphenylpicrylhydrazyl (DPPH) test, which is a known procedure to estimate the antioxidant activity of a compound (Molyneux, 2004). An antioxidant is a hydrogen donor, forming a delocalized, more stable, free radical. The benzylic C–H bonds of the aromatic glycerol acetals can afford highly delocalized radicals, explaining their antioxidant properties (Fig. 12.7). However, tests of the aromatic glycerol acetals with soybean biodiesel did not lead to significant improvement in the oxidation resistance, measured according to the EN 14112 method, but the concomitant use of a commercial antioxidant, such as butyl-hydroxy-toluene (BHT) and the acetals gives much better results (Table 12.2), indicating a synergistic effect (Soares, 2011).

Table 12.2. Oxidation stability of soybean biodiesel with furfural/glycerol acetals, according to EN 14112 standards

The acetins or glycerol acetates are useful compounds. Triacetin or glycerol triacetate is important in the tobacco industry and, more recently, has been tested as a fuel additive, especially for biodiesel, improving the viscosity and the pour point (Melero et al., 2007). The most traditional method of preparation of the acetins is the direct esterification of glycerol with acetic acid in the presence of an acidic catalyst (Gonçalves et al., 2008), yielding a mixture of the acetins. To increase the selectivity to triacetin, a large excess of acetic acid should be used. Another approach is to use acetic anhydride (Fig. 12.8) (Liao et al., 2009). Use of zeolite beta or K-10 montmorillonite as catalysts for the acetylation of glycerol with acetic anhydride leads to 100% selectivity to triacetin within 20 minutes of reaction time (Silva et al., 2010a).

Esterification of the free hydroxyl group of glycerol ketals and acetals has also been reported in the literature (Garcia et al., 2008). The reaction of solketal with acetic anhydride in the presence of triethylamine produces solketal acetate in 90% yield (Fig. 12.9). This product may be used to improve the viscosity of biodiesel, without affecting the flash point. It may also reduce the formation of particulates in diesel.

16.2.4.4 Carbon monoxide emission

During the complete combustion of hydrocarbon fuel, the end product of the reaction must be carbon dioxide (CO2) and water (H2O). Hence, the presence of higher concentration of carbon monoxide (CO) in the exhaust gives an indication of the incompleteness of combustion and represents a loss of chemical energy [74]. Apart from engine and fuel type, the fuel-air equivalence ratio is also an influencing factor regulating CO emissions [9]. Increase in load/speed, combustion temperature, and air mass flow decrease CO emissions. In general, higher engine speed increases excess air factor (λ).

Many researchers [26,75–77] reported that the CO emission was reduced with biodiesel and its blends. Singh et al. [70] have reported 32% and 27% reduction in CO for jatropha and microalgal BDs, respectively. Buyukkaya et al. [78] reported that with the application of trout oil biodiesel, CO emission reduces by 9% and 17.2%, for low and high load, respectively. The reason cited is the presence of 10.37%–12.25% of inbuilt oxygen in biodiesel which burns the fuel more completely. The presence of lesser carbon in biodiesel also reduces CO emission [35]. The higher cetane number of biodiesel reduces ignition delay [79]. Further, advanced injection and combustion were reported for biodiesel [80,81]. Both the above factors also justified the CO reduction with the oxygenated fuel.

Acronyms

AC

Alternating Current

Adiabatic Isochoric Complete Combustion (AICC)

Boiling Water Reactor

Canada Deuterium Uranium

Core-Concrete Interaction

Core Damage State

Containment Fltered Venting System

Calandria Tube

Direct Containment Heating

Defense-in-depth

Emergency Mitigating Equipment

Fueling Machine Duct (FMD)

In-Vessel Retention

Loss of Coolant Accident

Light-Water Reactors

Molten Core Concrete Interaction

Nuclear Energy Agency

Nuclear Power Plant

Organization for Economic Co-operation and Development

Passive Autocatalytic Recombiner

Pressurized Heavy Water reactor

Phenomena Identification and Ranking Table

Pressure Tube

Pressurized Water Reactor

High-Powered Channel-Type Reactor (Reaktor Bolshoy Moshchnosti Kanalnyy)

Severe Accident Guides

Severe Accident Management

Severe Accident Management Guideline

Station Blackout

Spent Fuel Pools

Steam Generator Tube Rupture

Structures, Systems, and Components

Working Group on Analysis and Management of Accidents

6.9.2 Combustion and Oxidation

In a complete combustion reaction, a compound reacts with an oxidizing element, such as oxygen, and the products are compounds of each element in the fuel with the oxidizing element. The oxidation with oxygen is the most commonly occurring phenomena in nature. It is because of the abundance of oxygen as well as the ability of oxygen to react at all temperatures. In terms of generating energy, most notably heat generation, is through oxidation of hydrogen. Even though, in nature it is rarely the case, the oxidation of hydrogen produces the most intense heat in presence of a flame (2000 °C). This is the principle used in rocket engines. The second most intense heat is with carbon (1000 °C). This is the principle used in all forms of fossil fuel burning. Unlike hydrogen and oxygen, this reaction is natural and takes place at all temperatures, albeit as a strong function of temperature. The low-temperature oxidation is continuous and follows Arrhenius equation, which is an exponential relationship with temperature. However, oxidation of elemental carbon (e.g., graphite and diamond) are both rare because of rarity of those elements, compared to compound form of carbon. For instance, diamond and graphite both burn at 800 °C in presence of oxygen but in absence of oxygen they melt at very high temperature (3600 °C for graphite and 3800 °C for diamond). The next most heat generating combustion is with methane. This reaction is written as follows

(6.3)C H4(g)+2O2(g)→CO2(g)+2H2O(g)+Σ

The standard enthalpy of reaction for methane combustion at 298.15 °K and 1 atm is −802 kJ/mol. The symbol Σ signifies the time function that stores information regarding intangibles (Islam et al., 2010a), such as the history of methane (organic or otherwise), history of oxygen (organic or mechanical, as well as the collection of all elements that are present in nonmeasurable quantities. The usefulness of Σ is in its ability to track the history in order to chart the future pathway in terms of harm and beneficial quality. For instance, if the oxygen supply is restricted, the following reaction will take place, in stead of Eqn (6.2).

(6.4)2C(s)+O2(g) →2CO(g)+Σ

This reaction is typical of industry-standard producer gas that is produced by injecting oxygen through hot coke. The resulting gas is a mixture of carbon monoxide (25%), carbon dioxide (4%), nitrogen (70%), and traces of hydrogen (H2), methane (CH4), and oxygen (O2). In addition to this information, Σ will also contain information regarding any other trace elements that can be present due to use of catalyst, heating mechanism, existence of flames, etc. In essence, Σ is the tracker of intangibles.

Any combustion reaction is known to be accelerated dramatically in presence of a flame. A flame is a mixture of reacting gases and solids emitting visible, infrared, and sometimes ultraviolet light, the frequency spectrum of which depends on the chemical composition of the burning material and intermediate reaction products. A standard and beneficial flame is fire, arising from burning wood. This process of heat and light generation is entirely sustainable (Chhetri and Islam, 2008) and produces no harmful or by-product, therefore, it is waste-free (Khan and Islam, 2012). The fundamental characteristic of this wood flame is that combustion is incomplete, thereby generating incandescent solid particles, called soot. It comes with red-orange glow of fire. This light has continuous spectrum, similar to sunlight spectrum. Even though it is rarely talked about, the orange glow of wood fire is also similar to the glow of sun. See Picture 6.8.