Mitigation of greenhouse gas emissions from power generation through cofiring coal and raw glycerol: a theoretical feasibility study

Abstract

A theoretical technical feasibility investigation aiming to decrease the rate of greenhouse emissions per unit of generated power from fossil fuel plants is presented. To that end, the FGSIG/GT (fuel glycerol integrated gasifier/gas turbine) concept is applied. High-ash coal and raw glycerol—that last deriving from renewable fuel (biodiesel) production—are mixed to form a slurry, which is pumped into the gasifier. The gas stream emerging from that reactor is cleaned by cyclones and filters to decrease the concentration of suspended solids in the stream as well as its granulometry. Before combustion, the fuel gas temperature is decreased to values below the dew points of alkaline compounds in it, thus significantly reducing their concentrations in the gas stream, therefore allowing its injection into commercial gas turbines. Energy recovering is accomplished by two Rankine cycles. The exergetic efficiency is chosen as an objective function to optimize the gasification process. The results reveal a diverging aspect of the RG/solid-fuel ratio influence on the gasification efficiency when comparing with the obtained in previous works where biomasses were fed to the FGSIG/GT process. Due to that highly pressurized process, the whole power generation is optimized resulting in first law efficiencies around 45%, which is noticeable having in mind the relatively low heating values of the presently considered fuels. Additionally, it is important to stress that the objective of this investigation is not just to decrease the overall CO₂ emitted per unit of power output from a station consuming fossil fuel and shift, at least part of that, to one derived from a renewable source.

Appendices

Appendix 1

Important tables mentioned in the text are presented below (Tables 5, 6, 7).

Table 5 Main results for each rate of RG injection into the gasifier

Table 6 Most important parameters related to gasification operations for various DSC

Table 7 Produced gas composition (molar percentage) for each case

Appendix 2

To provide a more complete view, the following figures illustrate a few aspects of the gasifier operation under those conditions. Those have been obtained by the application of CeSFaMB^© software.

Figures 9 and 10 picture the temperature profiles in the bed and freeboard, respectively.

Fig. 9

Temperature profiles inside the gasifier bed

Fig. 10

Temperature profiles inside the gasifier freeboard

As expected at bubbling fluidized bed operations, the temperatures are uniform and relatively low, which allows for easier control as well many advantages when compared with other techniques [18, 35].

Figure 11 shows that complete cracking and coking of tar generated during the fuel pyrolysis near its feeding position (Z = 0.5 m) could be achieved. Thus, the produced gas would be free of Tar. Such prevents operational problems and maintenance costs to clean the produced gas.

Fig. 11

Molar fractions of Tar, H₂S, and NH₃ throughout the gasifier

Figure 12 allows observing the fast development and destruction of Tar near the fuel feeding position, which is far below the bed top. Therefore, well-designed equipment would allow sufficient bed height to complete those reactions before gas with Tar passes to the freeboard region; otherwise, the risk of jeopardizing the gas cleaning system increases. Additionally, the fuel feeding position is above the oxidation region, thus allowing the volatiles—rich in hydrogen, methane, and carbon monoxide—to survive and improve the quality of the produced gas.

Fig. 12

Rates of Tar decomposition in the emulsion phase

Figure 13 shows other important heterogeneous reactions occurring in the emulsion. It becomes clear how fast fuel oxidation is when compared with gas–solid reactions. That also explains the surge of temperatures near the distributor (Z = 0), as shown in Fig. 9, as well as the fast decline of oxygen near that position (Fig. 14).

Fig. 13

Main gas–solid reaction rates

Fig. 14

Concentration of CO₂, CO, and O₂ throughout the equipment

The concentration profiles of other important gas species throughout the gasifier interior are presented in Fig. 15.

Fig. 15

Concentration of H₂O, H₂, and CH₄ throughout the equipment

Drying and evaporation of water entering with the slurry is paramount for hydrogen production (see reaction R.3 in Appendix 3). The rates of those reactions are also pictured in Fig. 9, while Fig. 16 illustrates the main homogeneous ones.

Fig. 16

Main gas–gas reaction rates

That figure demonstrates the key role played by the shift reaction (R.41 in Appendix 3) throughout the bed, which continues in the freeboard.

Appendix 3

A list of main reactions and processes considered by the simulator (CeSFaMB^©) is presented below:

$${\text{CH}}_{{a_{32} }} {\text{O}}_{{a_{33} }} {\text{N}}_{{a_{34} }} {\text{S}}_{{a_{35} }} + \left( {\frac{{a_{32} }}{4} - \frac{{a_{33} }}{2} + \frac{{a_{34} }}{2} + a_{35} } \right){\text{O}}_{2} \to {\text{CO}}_{2} + \frac{{a_{32} }}{2}{\text{H}}_{2} {\text{O}} + a_{34} {\text{NO}} + a_{35} {\text{SO}}_{2}$$

(R.1)

$${\text{CH}}_{{a_{32} }} {\text{O}}_{{a_{33} }} {\text{N}}_{{a_{34} }} {\text{S}}_{{a_{35} }} + (1 - a_{33} ){\text{H}}_{2} {\text{O}} \leftrightarrow \left( {1 + \frac{{a_{32} }}{2} - a_{33} - a_{35} } \right){\text{H}}_{2} + {\text{CO}} + \frac{{a_{34} }}{2}{\text{N}}_{2} + a_{35} {\text{H}}_{2} {\text{S}}$$

(R.3)

$${\text{CH}}_{{a_{531} }} {\text{O}}_{{a_{551} }} {\text{N}}_{{a_{546} }} {\text{S}}_{{a_{563} }} + {\text{CO}}_{2} \leftrightarrow 2{\text{CO}} + a_{551} {\text{H}}_{2} {\text{O}} + \left( {\frac{{a_{531} }}{2} - a_{551} - \frac{{3a_{546} }}{2} - a_{563} } \right){\text{H}}_{2} + a_{34} {\text{NH}}_{3} + a_{35} {\text{H}}_{2} {\text{S}}$$

(R.4)

$${\text{CH}}_{{a_{531} }} {\text{O}}_{{a_{551} }} {\text{N}}_{{a_{546} }} {\text{S}}_{{a_{563} }} + \left( {2 - \frac{{a_{531} }}{2} + a_{551} + \frac{3}{2}a_{546} + a_{563} } \right){\text{H}}_{2} \leftrightarrow {\text{CH}}_{4} + a_{551} {\text{H}}_{2} {\text{O}} + a_{34} {\text{NH}}_{3} + a_{35} {\text{H}}_{2} {\text{S}}$$

(R.5)

$${\text{CH}}_{{a_{32} }} {\text{O}}_{{a_{33} }} {\text{N}}_{{a_{34} }} {\text{S}}_{{a_{35} }} + (2 - a_{33} ){\text{NO}} \leftrightarrow \left( {\frac{{a_{32} }}{2} - a_{35} } \right){\text{H}}_{2} + {\text{CO}}_{2} + \left( {1 + \frac{{a_{34} }}{2} - \frac{{a_{33} }}{2}} \right){\text{N}}_{2} + a_{35} {\text{H}}_{2} {\text{S}}$$

(R.6)

$$\text{Fuel}_\text{daf} \to \text{Volatile} + \, \text{Char}_{1}$$

(R.7)

$${\text{Volatile }} \to {\text{Gases}} + {\text{Tar}}$$

(R.8)

$${\text{Wet}}\;{\text{Carbonaceous}}\;{\text{Solid }} \rightleftarrows {\text{ Dry}}\;{\text{Carbonaceous}}\;{\text{Solid}} + {\text{H}}_{{2}} {\text{O}}$$

(R.10)

$${\text{CH}}_{{a_{32} }} {\text{O}}_{{a_{33} }} {\text{N}}_{{a_{34} }} {\text{S}}_{{a_{35} }} + (2 - a_{33} ){\text{NO}} \leftrightarrow \left( {\frac{{a_{32} }}{2} - a_{35} } \right){\text{H}}_{2} + {\text{CO}}_{2} + \left( {1 + \frac{{a_{34} }}{2} - \frac{{a_{33} }}{2}} \right){\text{N}}_{2} + a_{35} {\text{H}}_{2} {\text{S}}$$

(R.12)

$${\text{CH}}_{{a_{531} }} {\text{O}}_{{a_{551} }} {\text{N}}_{{a_{546} }} {\text{S}}_{{a_{563} }} + (1 - a_{551} ){\text{N}}_{2} {\text{O}} \leftrightarrow \left( {\frac{{a_{531} }}{2} - a_{563} } \right){\text{H}}_{2} + {\text{CO}} + \left( {1 + \frac{{a_{34} }}{2} - \frac{{a_{33} }}{2}} \right){\text{N}}_{2} + a_{35} {\text{H}}_{2} {\text{S}}$$

(R.13)

$${\text{Tar}} \to {\text{ Char}}_{{2}}$$

(R.14)

$${\text{CO }} + {\text{ H}}_{{2}} {\text{O }} \rightleftarrows {\text{ CO}}_{{2}} + {\text{ H}}_{{2}}$$

(R.41)

$${\text{2CO}} + {\text{O}}_{{2}} \rightleftarrows {\text{2CO}}_{{2}}$$

(R.42)

$${\text{2H}}_{{2}} + {\text{O}}_{{2}} \rightleftarrows {\text{2H}}_{{2}} {\text{O}}$$

(R.43)

$${\text{CH}}_{{4}} + {\text{2O}}_{{2}} \rightleftarrows {\text{CO}}_{{2}} + {\text{2 H}}_{{2}} {\text{O}}$$

(R.44)

$${\text{2C}}_{{2}} {\text{H}}_{{6}} + {\text{7O}}_{{2}} \rightleftarrows {\text{4CO}}_{{2}} + {\text{6H}}_{{2}} {\text{O}}$$

(R.45)

$${\text{4NH}}_{{3}} + {\text{5O}}_{{2}} \rightleftarrows {\text{4NO}} + {\text{6H}}_{{2}} {\text{O}}$$

(R.46)

$${\text{2H}}_{{2}} {\text{S}} + {\text{3O}}_{{2}} \rightleftarrows {\text{2SO}}_{{2}} + {\text{2H}}_{{2}} {\text{O}}$$

(R.47)

$${\text{N}}_{{2}} + {\text{O}}_{{2}} \rightleftarrows {\text{2NO}}$$

(R.48)

$${\text{Tar}} + {\text{O}}_{{2}} \rightleftarrows {\text{Combustion}}\;{\text{Gases}}$$

(R.49)

$${\text{Tar}} \to {\text{Gases}}$$

(R.50)

$${\text{Tar}} + {\text{H}}_{{2}} \to {\text{CH}}_{{4}} + {\text{Other}}\;{\text{Light}}\;{\text{Gases}}$$

(R.51)

$${\text{C}}_{{2}} {\text{H}}_{{4}} + {\text{3O}}_{{2}} \rightleftarrows {\text{2CO}}_{{2}} + {\text{2H}}_{{2}} {\text{O}}$$

(R.52)

$${\text{2C}}_{{3}} {\text{H}}_{{6}} + {\text{9O}}_{{2}} \rightleftarrows {\text{6CO}}_{{2}} + {\text{6H}}_{{2}} {\text{O}}$$

(R.53)

$${\text{C}}_{{3}} {\text{H}}_{{8}} + {\text{5O}}_{{2}} \rightleftarrows {\text{3CO}}_{{2}} + {\text{4H}}_{{2}} {\text{O}}$$

(R.54)

$${\text{2C}}_{{6}} {\text{H}}_{{6}} + {\text{15O}}_{{2}} \rightleftarrows {\text{12CO}}_{{2}} + {\text{6H}}_{{2}} {\text{O}}$$

(R.55)

$${\text{4HCN}} + {\text{3O}}_{{2}} \rightleftarrows {\text{4CO}} + {\text{2N}}_{{2}} {\text{O}} + {\text{2H}}_{{2}}$$

(R.56)

$${\text{CH}}_{{4}} + {\text{H}}_{{2}} {\text{O }} \rightleftarrows {\text{CO}} + {\text{3H}}_{{2}}$$

(R.71)

$${\text{C}}_{{2}} {\text{H}}_{{4}} + {\text{2H}}_{{2}} {\text{O}} \rightleftarrows {\text{2CO}} + {\text{4H}}_{{2}}$$

(R.72)

$${\text{C}}_{{2}} {\text{H}}_{{6}} + {\text{2H}}_{{2}} {\text{O}} \rightleftarrows {\text{2CO}} + {\text{5H}}_{{2}}$$

(R.73)

$${\text{C}}_{{3}} {\text{H}}_{{6}} + {\text{3H}}_{{2}} {\text{O}} \rightleftarrows {\text{3CO}} + {\text{6H}}_{{2}} {\text{C}}_{{3}} {\text{H}}_{{6}} + {\text{3H}}_{{2}} {\text{O }} \rightleftarrows {\text{3CO}} + {\text{6H}}_{{2}}$$

(R.74)

$${\text{C}}_{{3}} {\text{H}}_{{8}} + {\text{3H}}_{{2}} {\text{O}} \rightleftarrows {\text{3CO}} + {\text{7H}}_{{2}}$$

(R.75)

$${\text{C}}_{{6}} {\text{H}}_{{6}} + {\text{6H}}_{{2}} {\text{O}} \rightleftarrows {\text{6CO}} + {\text{9H}}_{{2}}$$

(R-76)

$${\text{CH}}_{{4}} + {\text{CO}}_{{2}} \rightleftarrows {\text{2CO}} + {\text{2H}}_{{2}}$$

(R-77)

$${\text{2NO}}_{{2}} + {\text{2NH}}_{{3}} \rightleftarrows {\text{4NO}} + {\text{3H}}_{{2}}$$

(R.81)

$${\text{2NO}} + {\text{2NH}}_{{3}} \rightleftarrows {\text{2N}}_{{2}} {\text{O}} + {\text{3H}}_{{2}}$$

(R.82)

$${\text{3N}}_{{2}} {\text{O}} + {\text{2NH}}_{{3}} \rightleftarrows {\text{4N}}_{{2}} + {\text{3H}}_{{2}} {\text{O}}$$

(R.83)

$${\text{NO}}_{{2}} + {\text{2H}}_{{2}} \rightleftarrows {\text{NO}} + {\text{H}}_{{2}} {\text{O}}$$

(R.84)

$${\text{2NO}} + {\text{2H}}_{{2}} \rightleftarrows {\text{N}}_{{2}} {\text{O}} + {\text{H}}_{{2}} {\text{O}}$$

(R.85)

$${\text{N}}_{{2}} {\text{O}} + {\text{H}}_{{2}} \rightleftarrows {\text{N}}_{{2}} + {\text{H}}_{{2}} {\text{O}}$$

(R.86)

$${\text{NO}}_{{2}} + {\text{CO}} \rightleftarrows {\text{NO}} + {\text{CO}}_{{2}}$$

(R.87)

$${\text{2NO}} + {\text{CO}} \rightleftarrows {\text{N}}_{{2}} {\text{O}} + {\text{CO}}_{{2}}$$

(R.88)

$${\text{N}}_{{2}} {\text{O}} + {\text{CO}} \rightleftarrows {\text{N}}_{{2}} + {\text{CO}}_{{2}}$$

(R.89)

$${\text{NH}}_{{3}} + {\text{CO}} \rightleftarrows {\text{HCN}} + {\text{H}}_{{2}} {\text{O}}$$

(R.90)

$${\text{2N}}_{{2}} + {\text{O}}_{{2}} \rightleftarrows {\text{2N}}_{{2}} {\text{O}}$$

(R.91)

$${\text{2N}}_{{2}} {\text{O}} + {\text{O}}_{{2}} \rightleftarrows {\text{4NO}}$$

(R.92)

$${\text{2NO}} + {\text{O}}_{{2}} \rightleftarrows {\text{2NO}}_{{2}}$$

(R.93)