Exergy Analysis of Waste Incineration Plant: Flue Gas Recirculation and Process Optimization ^†

Abstract

Simulations of two incineration processes, with and without flue gas recirculation, have been carried out performing an exergy analysis to investigate the most critical equipment unit in terms of second-law efficiency. Flue gas from the economizer outlet is employed to partially replace secondary combustion air to reduce, at the same time, incinerator temperature and oxygen concentration. Conversely, in the proposed configuration, the recirculated flue gas flow rate is used to control incinerator temperature, while the air flow rate is used to control the oxygen content of the fumes, leaving the incinerator as close to 6% as possible—i.e., the minimum allowed for existing plants to ensure completion of the combustion reactions and according to environmental regulations—and determines the corresponding minimum flue gas flow rate. The flue gas recirculation guarantees a larger level of energy recovery (up to +3%) and, at the same time, lower investment costs for the lower flow rate of fumes actually emitted if compared to the plant configuration without flue gas recirculation. Various operating parameters were varied (incinerator’s effluent gas temperature, air flowrate and flue gas recirculation flowrate) to investigate their influence on process exergy efficiency. Exergy analysis allowed the individuation of the equipment units characterized by larger exergy destruction and demonstrated that the flue gas recirculation led to an overall process exergy efficiency increase of about 3%.

1. Introduction

The quick population growth and high raw materials consumption are leading to a substantial increase in the municipal solid waste (MSW) produced worldwide, which has a remarkable negative impact on life quality [1]. MSW incineration, among other possible waste treatment processes, represents a well-known technology that can also allow waste to be transformed into mechanical power, according to the waste to energy (WtE) approach [2]. WtE can be considered an important choice for waste management: indeed, in 2015, about 2200 waste incineration plants were operative worldwide, with a capacity of 280 Mt/d [3]. It has been estimated that a reduction of 10–15% of GHG (GreenHouse Gas) emissions can be reached by improving MSW management [4]. Despite the notable advantages of MSW incineration, some disadvantages have still reduced the overall efficiency and limited the environmental benefits of this process: by-product production (bottom and fly ashes, on which intensive research on their reuse is currently ongoing [5]), combustion instabilities, toxic gaseous pollutants and heavy metals emissions [6]. In this framework, exergy analysis may be a fundamental tool to individuate thermodynamic irreversibility and waste streams based on a second principle analysis, overcoming the drawbacks of the first principle analysis of existing and new power plants.

The present work reports the exergy analysis of a previous simulated incineration plant, integrated with steam and work production cycle but without considering the flue gas treatment system [7].

2. Materials and Methods

The process was built in a PRO/II simulation environment (see Figure 1).

The MSW (101) enters the incinerator with air compressed with a blower (C-101) up to 1.04 bar (103), then the flue gas (104) is sent to the boiler, represented by three heat exchangers (H-101 the vaporizer, H-102 the superheater and H-103 the economizer), and the produced steam is then sent to the turbines for the work generation by steam expansion. Then, the obtained condensed water is recirculated (by the pump P-102). The exhaust gas (108) is sent to the treatment units. The temperature of the flue gas and the oxygen %mol (kept > 6% on wet basis, according to current legislation [7]) were controlled by manipulating air inlet flowrate and flue gas recirculation. The equipment is indicated as follows: C-Compressor, P-Pump, S-Splitter, M-Mixer, H-Heat Exchanger, T-Turbine and VR-Regulator valve.

The exhaust gas treatment section aimed to reduce HCl and SO₂ concentration to the EC regulation (Directive 2010/75/EU) and their modelling was limited to the pressure drop and power of the pump estimation for energy and exergy analysis. The temperature of stream 108, exhaust gas exiting from the boiler, was set equal to 200 °C based on the actual trend for energy efficiency increase. The thermal input was 22 MW (8.28 t/h of MSW), whereas the heat loss from the incinerator as radiative heat loss was 0.93 MW. More details are reported in Liuzzo et al., 2007 [7]. The MSW characteristics (proximate and ultimate analysis) are reported in

Table 1. MSW characteristics (from [7]).

MSW Analysis	U.M.	Value
Moisture	%wt	25.2
Ash	%wt	24.4
Fixed carbon and volatile matter	%wt	50.4
C	%wt	50.8
H	%wt	6.8
O	%wt	40.3
N	%wt	1
Cl	%wt	0.8
S	%wt	0.3
LHV *	kJ/kg	9570

* LHV is the lower heating value.

.

The same overall process was simulated adopting flue gas recirculation (FGR) (see Figure 2).

The simulations were carried out at two different temperatures of the combustion chamber—i.e., the temperature of exiting flue gas (950 °C and 1100 °C) and the operative conditions—and assumptions related to the other plant units are reported in

Table 2. Equipment characteristic parameters (from [7]).

Plant Unit Parameter	U.M.	Value
Turbine adiabatic efficiency	%	84
Losses at turbine discharge	bar	0.056
Pumps and blowers efficiency	%	70
ΔP boiler water side	bar	3.04
ΔP boiler fumes side	bar	0.05
Discharge head of P-102	bar	47.62
ΔP VR-101	bar	2.03
ΔP VR-103	bar	2.03
Steam pressure at condenser	bar	0.078
Superheated steam temperature	°C	360

(ΔP [bar] represents pressure drop term).

As reported in

Table 3. Simulation results (from [7]).

Parameter	U.M.	Temperature 1		Temperature 2
Temperature	°C	950		1100
Air	kg/h	64,675	40,567	53,073	40,563
FGR	kg/h	-	26,761	-	13,376
Flue gas (to treatment)	kg/h	72,356	48,163	60,538	48,159
Net overall efficiency	%	21.7	23.5	22.8	23.7

, in this case, various benefits were achieved, such as larger net overall efficiency (defined as the multiplication of boiler efficiency and the efficiency of thermal energy conversion into electric energy), lower inlet air flowrate and flue gas flowrate to the stack after its treatment (i.e., lower investment and operative costs of treatment units).

For each piece of equipment and for the overall process, the physical and chemical exergy fluxes (in/out streams) were calculated using the following equations [8]:

$$E{x}_{in/out}^{ph}=M_{in/out}\left[\left(H-H_0\right)-T_0\left(S-S_0\right)\right]$$

(1)

$$E{x}_{in/out}^{ch}=M_{in/out}\left({\displaystyle \sum }_i^n{x}_{i}e{x}_i^{ch}+R{T}_0{\displaystyle \sum }_i^n{x}_i\ln \left(x_i\right)\right)$$

(2)

where Ex_in^ph [kJ/h or kW] is the inlet/outlet physical exergy, M [kmol/h] is the molar flowrate of the considered stream, H [kJ/kmol] is the molar enthalpy of the stream at its P and T, H₀ [kJ/kmol] is the molar enthalpy of the stream at P = 1 atm and T = T₀ = 25 °C—i.e., the pressure and temperature of the dead state, S [kJ/K·kmol] is the entropy of the stream at its P and T and S₀ [kJ/K·kmol] is the entropy of the stream at dead state conditions [8], Ex_in^ch [kJ/h or kW] is the inlet/outlet chemical exergy, n is the number of chemical species, x_i is the mole fraction of i species, ex_i^ch (kJ/kmol) is the standard chemical exergy of i species at P = 1 atm and T₀ = 25 °C taken from [9,10] or calculated with the procedure reported by Gharagheizi and co-workers [9], R [kJ/K·kmol] is the gas constant.

In the calculations, the kinetic and potential exergy are usually neglected since their order of magnitude is lower with respect to those of chemical and physical exergy contributions; therefore, the exergy efficiency and destroyed exergy were calculated as:

$$\eta =\frac{E{x}_{prod}^{tot}}{E{x}_{feed}^{tot}}=1-\frac{E{x}_d^{tot}}{E{x}_{feed}^{tot}}$$

(3)

$$E{x}_d^{tot}=E{x}_{feed}^{tot}-E{x}_{prod}^{tot}=T_0{S}_{gen}$$

(4)

where Ex^tot_prod and Ex^tot_feed are the total exergy produced from the system and fed to the system, respectively, and S_gen [kJ/K·kmol] is the entropy generated.

3. Results

3.1. Exergy Analysis Results without FGR

Table 4. Equipment exergy analysis results at 950 °C.

Unit	Exergy Produced (kW)	Exergy Feed (kW)	Irreversibility (kW)	Waste (kW)	η
C-101	Ex,103–Ex,102	Ex,C-101	22.93	0	0.706
	55.07	78
P-101	Ex,119–Ex,118	Ex,P-101	0.47	0	0.842
	2.53	3.00
P-102	Ex,122–Ex,121	Ex,P-102	10.89	0	0.768
	39.56	46
T-101	Ex,T-101	Ex,112–Ex,113	580.89	0	0.84
	−3058	3639
T-102	Ex,T-102	Ex,114–Ex,115	184.22	0	0.837
	−963	1147
T-103	Ex,T-103	Ex,116–Ex,117	374.56	0	0.818
	−1681	2056
H-101	Ex,110–Ex,109	Ex,104–Ex,106	3055.56	0	0.620
	4988.89	8044
H-102	Ex,111–Ex,110	Ex,106–Ex,107	211.11	0	0.857
	1261	1472.22
H-103	Ex,109–Ex,123	Ex,107–Ex,108	22.22	0	0.989
	1972.22	1994.44
H-104	Ex,120–Ex,119	Ex,125–Ex,126	13.57	0	0.961
	338.85	352.42
H-105	Ex,q,H-105	Ex,126+Ex,117-Ex,118	699.17	0	0.508
	723.12	1422.29
C.Chamber	Ex,104	Ex,101+Ex,103	5692.18	Ex,Ash	0.701
	13,359.15	19,051.43		0.09

,

Table 5. Cycle exergy analysis results at 950 °C.

Exergy Produced	Exergy Feed	Irreversibility (kW)	Waste	η
(kW)	(kW)		(kW)
Ex,T-101+Ex,T-102+Ex,T-103+Ex,q,H-105	Ex,101+Ex,102+Ex,P-101+Ex,P-102+Ex,C-101	10,851	Ex,108+Ex,Ash	0.31
5702.00	18,401		1848

,

Table 6. Equipment exergy analysis results at 1100°C.

Unit	Exergy Produced (kW)	Exergy Feed (kW)	Irreversibility (kW)	Waste (kW)	η
C-101	Ex,103–Ex,102	Ex,C-101	18.79	0	0.706
	45.21	64
P-101	Ex,119–Ex,118	Ex,P-101	0.42	0	0.860
	2.58	3.00
P-102	Ex,122–Ex,121	Ex,P-102	11.89	0	0.752
	39.56	46
T-101	Ex,T-101	Ex,112-Ex,113	593.67	0	0.84
	−3123	3717
T-102	Ex,T-102	Ex,114-Ex,115	186.44	0	0.837
	−983	1169
T-103	Ex,T-103	Ex,116–Ex,117	383.00	0	0.818
	−1717	2100
H-101	Ex,110–Ex,109	Ex,104–Ex,106	2988.89	0	0.630
	5088.89	8078
H-102	Ex,111–Ex,110	Ex,106–Ex,107	313.89	0	0.806
	1300	1613.89
H-103	Ex,109–Ex,123	Ex,107–Ex,108	175.00	0	0.918
	1972.22	2147.22
H-104	Ex,120–Ex,119	Ex,125–Ex,126	14.75	0	0.959
	345.71	360.45
H-105	Ex,q,H-105	Ex,126+Ex,117-Ex,118	713.22	0	0.509
	738.50	1451.72
C.Chamber	Ex,104	Ex,101+Ex,103	5291.96	Ex,Ash	0.722
	13,734.58	19,026.63		0.09

and

Table 7. Cycle exergy analysis results at 1100 °C.

Exergy Produced	Exergy Feed	Irreversibility (kW)	Waste	η
(kW)	(kW)		(kW)
Ex,T-101+Ex,T-102+Ex,T-103+Ex,q,H-105	Ex,101+Ex,102+Ex,P-101+Ex,P-102+Ex,C-101	10,702	Ex,108+Ex,Ash	0.32
5823.00	18,373		1848

summarize the results obtained at 950 °C and 1100 °C.

3.2. Exergy Analysis Results with FGR

Table 8. Equipment exergy analysis results at 950 °C.

Unit	Exergy Produced (kW)	Exergy Feed (kW)	Irreversibility (kW)	Waste (kW)	η
C-101	Ex,103–Ex,102	Ex,C-101	14.48	0	0.705
	34.52	49
C-102	Ex,129–Ex,128	Ex,C-102	26.56	0	0.818
	119.44	146.00
P-101	Ex,119–Ex,118	Ex,P-101	0.31	0	0.896
	2.69	3.00
P-102	Ex,122–Ex,121	Ex,P-102	5.56	0	0.889
	39.56	46
T-101	Ex,T-101	Ex,112–Ex,113	605.33	0	0.843
	−3253	3858
T-102	Ex,T-102	Ex,114–Ex,115	4342.67	0	0.837
	−1024	5367
T-103	Ex,T-103	Ex,116–Ex,117	397.11	0	0.818
	−1789	2186
H-101	Ex,110–Ex,109	Ex,104–Ex,106	3095.39	0	0.637
	5443.49	8539
H-102	Ex,111–Ex,110	Ex,106–Ex,107	411.11	0	0.746
	1206	1616.67
H-103	Ex,109–Ex,123	Ex,107–Ex,108	68.61	0	0.968
	2102.78	2171.39
H-104	Ex,120–Ex,119	Ex,125–Ex,126	17.10	0	0.955
	359.42	376.51
H-105	Ex,q,H-105	Ex,126+Ex,117-Ex,118	744.09	0	0.508
	769.28	1513.37
C.Chamber	Ex,104	Ex,101+Ex,103+Ex,129	4894.10	Ex,Ash	0.757
	15,279.16	20,173.35		0.09

,

Table 9. Cycle exergy analysis results at 950°C.

Exergy Produced	Exergy Feed	Irreversibility (kW)	Waste	η
(kW)	(kW)		(kW)
Ex,T-101+Ex,T-102+Ex,T-103+Ex,q,H-105	Ex,101+Ex,102+Ex,P-101+Ex,P-102+Ex,C-101+Ex,C-102	10,478	Ex,127+Ex,Ash	0.340
6266.00	18,444		1900

,

Table 10. Equipment exergy analysis results at 1100°C.

Unit	Exergy Produced (kW)	Exergy Feed (kW)	Irreversibility (kW)	Waste (kW)	η
C-101	Ex,103–Ex,102	Ex,C-101	14.89	0	0.696
	34.11	49
C-102	Ex,129–Ex,128	Ex,C-102	7.55	0	0.801
	30.45	38.00
P-101	Ex,119–Ex,118	Ex,P-101	0.31	0	0.896
	2.69	3.00
P-102	Ex,122–Ex,121	Ex,P-102	5.56	0	0.889
	39.56	46
T-101	Ex,T-101	Ex,112-Ex,113	605.33	0	0.843
	−3253	3858
T-102	Ex,T-102	Ex,114–Ex,115	4342.67	0	0.837
	−1024	5367
T-103	Ex,T-103	Ex,116–Ex,117	397.11	0	0.818
	−1789	2186
H-101	Ex,110–Ex,109	Ex,104–Ex,106	3639.84	0	0.599
	5443.49	9083
H-102	Ex,111–Ex,110	Ex,106–Ex,107	516.67	0	0.700
	1206	1722.22
H-103	Ex,109–Ex,123	Ex,107–Ex,108	155.56	0	0.931
	2102.78	2258.33
H-104	Ex,120–Ex,119	Ex,125–Ex,126	17.10	0	0.955
	359.42	376.51
H-105	Ex,q,H-105	Ex,126+Ex,117-Ex,118	744.09	0	0.508
	769.28	1513.37
C.Chamber	Ex,104	Ex,101+Ex,103+Ex,129	4094.10	Ex,Ash	0.788
	15,209.08	19,303.27		0.09

and

Table 11. Cycle exergy analysis results at 1100°C.

Exergy Produced	Exergy Feed	Irreversibility (kW)	Waste	η
(kW)	(kW)		(kW)
Ex,T-101+Ex,T-102+Ex,T-103+Ex,q,H-105	Ex,101+Ex,102+Ex,P-101+Ex,P-102+Ex,C-101+Ex,C-102	10,398	Ex,127+Ex,Ash	0.344
6311.00	18,335		1872

summarize the results obtained at 950 °C and 1100 °C.

4. Discussion

Besides H-105, the exergy efficiency of machinery and heat exchangers was always higher than 60%, and the loss of exergy was due only to the irreversibility of the process (compression/heating, etc.). The H-105 was characterized by lower exergy efficiency because the heat transfer occurred with two mixed fluids entering the unit, streams 117 and 126, where stream 117 was characterized by a pressure of 0.081 bar and a low physical exergy with respect to that of streams 126 and outlet stream 118. Furthermore, the chemical exergy of stream 117 was one order of magnitude higher than that of stream 118. Due to the high chemical exergy of MSW in comparison with that of flue gas produced, the exergy efficiency of the combustion chamber was lower than 80%, even when FGR was adopted. FGR also led to exergy efficiency improvement, besides the well-known environmental and economic benefits. The FGR allowed an increase η of 3% at 950 °C and about 2.5% at 1100 °C. The increase in temperature in the combustion chamber also led to exergy efficiency improvement, because of the larger power produced by the three turbines with respect to the exergy feed flow, and, therefore, lower irreversibility and waste. The FGR allowed a reduction in exergy loss as waste, since an aliquot of FGR will be recirculated and will not be sent to pollution treatment units. The second configuration required an additional compressor (C-102), but the FGR adoption permitted a reduction in the power consumption of the first compressor unit because of the lower oxidant inlet flowrate and lower exergy feed flow.