The payback period for the WtE-ASU plant is evaluated taking into account the CAPEX index of Equation (4) and the annual incremental income of Equation.
Energetic, economic and environmental analysis of integrated waste to energy- cryogenic air separation unit plant Alessio Tafonea, Fabio Dal Magroa, Alessandro Romagnolib a
Energy Research Institute @ NTU, 1 Cleantech loop, 637141, Singapore School of Mechanical and Aerospace Engineering, 50 Nanyang Avenue, 639798, Singapore
b
Abstract: The main benefit of using oxygen enrichment in an incineration system is represented by the reduction of the sensible heat since the quantity of nitrogen in the flue gas is decreased. At the same time, due to the electricity required by the Air Separation Unit, a penalty in energy efficiency which may put at stake the economic investment, occurs. Therefore, in order to overcome that criticality, the possibility to exploit the byproducts of the cryogenic air separation (mainly liquid nitrogen) by means of a novel technology - the Dearman Engine - has been considered. Under the assumption that all the output products from the Air Separation Unit can be used and monetized, the proposed research investigates the feasibility of the combined system (Waste-to-Energy plant, Air Separation Unit and Dearman Engine) in terms of different technical, economic and emissions performance indices such as power output, economic savings and incremental income, and pollutant emissions savings. The results show that, under opportune conditions, the penalty in energy efficiency coming from the integration between the Waste-to-Energy plant and the Air Separation Unit can be compensated, both economically and environmentally, by means of the by-products valorisation.
Keywords: Waste-to-Energy plant (WtE), Air Separation Unit (ASU), Dearman Engine (DE), Liquid nitrogen, Liquid air, Oxygen Enriched Combustion (OCE),
1. Introduction Oxygen enriched combustion (OCE) is a technique introduced in industrial production processes to enhance combustion process using an oxidant which contains higher molar concentration of oxygen than that present in the air [1].The use of this technique has grown significantly in the last decades due to the main general advantages such as: -
-
-
-
Increase thermal efficiency. The flue gas heat losses are reduced because the flue gas mass decreases as the oxygen molar concentration in the oxidant gas increases: instead of heating up inert nitrogen, more steam is produced in the coupled Rankine cycle. Lower emissions. Oxygen enriched combustion systems can achieve lower levels of pollutants (e.g. nitrogen oxide) and products derived from incomplete combustion (e.g. carbon monoxide, aromatic polycyclic hydrocarbons and chlorinated organic compounds). Improve temperature stability and heat transfer. Increasing the oxygen content allows more stable combustion and higher combustion temperatures that can lead to better heat transfer within the combustive agent the load. Increase productivity. By means of oxygen enrichment of oxidant gas, the throughput of the plant can be increased for the same fuel input because of higher flame temperature, increased heat transfer to the load and reduced flue gas.
While in steel, iron, glass and cement industry the OEC has been accepted as a current practice [1], in the incineration process such a technique is not so extensively used. Despite the stringent emissions level enforced on Waste-to-Energy (WtE) plants and the global trend for further
minimizing pollutant emissions [2], the economic penalties associated with the production of oxygen required to enrich the combustion in WtE plants overcome the operative and environmental benefits [3]. In fact, although the OEC leads to higher thermal efficiencies (which in turn correspond with higher electricity production), the electricity required by the Air Separation Unit (ASU) to produce oxygen, is more than the extra energy produced by the WtE plant, thus resulting in an overall reduction in power supply capacity [4],[5]. A possible way to enhance the economic feasibility of an integrated plant composed by a WtE plant and an ASU is offered by the opportunity to use the byproducts coming from the ASU (mainly nitrogen streams in gaseous or liquid form). A promising technical solution is represented by a new engine concept, the Dearman Engine [6], which uses liquid air (or liquid nitrogen, LN2 ) as main energy vector. The introduction of liquid air vehicles could produce substantial economic and environmental benefits to the integrated plant WtE-ASU since it allows monetizing all the products from the ASU. Indeed the DE could be used in a number of configurations [6]: as the ‘prime mover’ or principal engine of a zero emissions vehicle; combined with an internal combustion engine to form a ‘heat hybrid’ engine that converts waste heat from the ICE; or as a ‘power and cooling’ refrigeration unit. In the present work two configurations for two different commercial sectors have been analysed: a cold and power refrigeration unit (DE-TRU) for the transport of frozen goods and a waste heat recovery and air conditioning unit employed in the public transport (DE-bus). The analysis has been carried out to assess the technical, economic and environmental feasibility of the two selected configurations (DE-TRU & DE-bus) coupled with the integrated plant (WtE-ASU). In Section 2 the baseline cases, the integrated systems1 and the main key performance indices will be introduced and detailed. In Section 3, the results from the energy, economic and environmental analysis will be shown and finally the main conclusions will be drawn.
2. Methodology and approach 2.1. The baseline cases – Waste to energy plant and diesel engines In order to compare and assess the possible benefits introduced with the integrated systems, two different baseline cases have been considered depending on the technology and the commercial sector that will make use of the liquid nitrogen. A model of WtE plant has been developed by using Aspen Hysys [7]. The main inputs are summarized in Table 1: the air is assumed to be composed by a fixed molar composition of oxygen and nitrogen, (21% and 79 % respectively), neglecting the presence of other components (Argon, CO2, etc.) in minor concentrations. It has been assumed that the WtE plant consumes 25% of the electricity produced thus selling the remaining portion (75%). Table 1 Assumption for WtE plant. Parameters Air inlet composition (% molar) Waste mass flow (kg/s) Yearly hours of operation (h/year) Plant auxiliary consumption (%) Net Power (MWe) Thermal efficiency (-)
Values 21% O2, 79 % N2 11.6 8000 25 22.62 0.256
Two configurations of diesel engines have been considered in the study: an auxiliary diesel engine (~19 kW) to power a TRU for frozen good transport and a EURO VI (~200 kW) diesel engine for 1
In the paper the following notation will be used: ‘integrated plant’ for the WtE-ASU and ‘integrated system’ for the WtE-ASU-DE
City-Buses. The air conditioning system of the City-Buses (water cooled condenser chiller with COP ~ 2 [8]) consumes 25% of the total energy produced by the engine [9]; these assumptions have been considered throughout the analysis of the DE-Bus. While the 200 kW engine is fuelled with road diesel (or white diesel), the auxiliary engine for TRUs runs on red diesel which is a cheaper but more sooty combustible. It is worth remarking that from an environmental point of view, TRUs belong to non-road engine types which emit more air pollution (NOx and PM) than a modern Euro VI diesel engine since TRUs emissions are effectively unregulated compared to road diesel engine [8].
2.2. Description of the integrated system: WtE plant - ASU - Dearman engine In order to increase the oxygen enrichment of the air fed into the WtE plant, a cryogenic ASU has been considered. Depending on the enrichment required by the WtE plant and on the required purity of the ASU products, two different output streams have been considered: a gaseous oxygen and a liquid nitrogen streams. Taking into account the specific consumption of an average cryogenic ASU designed to provide a certain quantity of gaseous oxygen per day [10], the value of the ASU specific consumption considered for our specific case has been set to 0.549 kWhe/kgLN2. The technical assumptions regarding the ASU are summarized in Table 2. Table 2 Assumption for cryogenic Air Separation Unit. Parameters Values Air inlet composition (% molar) 21% O2, 79 % N2 Specific consumption (kWhe/kgLN2) 0.549 Output stream 1 ( Gaseous O2 concentration) 99.5 % Output stream 2 ( Liquid N2 concentration) 99.999% Once the gaseous oxygen and liquid nitrogen are produced, the oxygen stream is supplied to the WtE plant and mixed with the main air flow whereas the liquid nitrogen is used to fuel the DE. The DE is a new engine concept driven by the vaporisation and expansion of liquid air or LN2 to produce high pressure gas that can generate clean cold and power [11]. In fact, besides the mechanical work produced by means of the pistons, as the liquid air or LN2 expand into a gas, they also give off large amounts of valuable cold, which can be used to provide ‘free’ refrigeration or air conditioning.
Electrical auxiliaries
Evaporator
Mechanical power
Evaporator
Mechanical power
Generator
LN2 Tank
DE Cryogenic Pump
LN2 Tank
DE Cryogenic Pump
HTF Mechanical power
Cooling load
Cooling load
Main diesel engine
Mechanical power
Chiller
(a) Figure 1 a) DE-TRU and b) DE-Bus configurations
HTF
(b)
The DE cycle requires the use of a heat transfer fluid inside the cylinder of the engine as a source of heat to augment the efficiency of the expansion of the liquid air or LN 2 by resembling a nearly
isothermal expansion. Ambient or low grade waste heat is used as an energy source with the liquid air or LN2 in order to enhance the system efficiency. In Figure 4 it is given a description of the proposed two configurations under study. The DE-TRU configuration provides refrigeration for the frozen goods by two means: the latent heat of vaporisation of the LN2 extracted from the refrigerated compartment (corresponding to approximately 0.101 kWh/kgLN2 of cooling energy) and the mechanical work produced by the DE which is partially used to drive a vapour refrigeration cycle to provide approximately 0.080 kWh/kgLN2 of cooling energy. The DE-Bus configuration instead, provides refrigeration only from the vaporization of the LN2 employing the mechanical work produced by the piston to partially power the main diesel engine.
2.3. Key performance indices and assumptions In order to carry out a comparative analysis among the baseline and the integrated systems, the following performance parameters have been considered: ▪ Net electric power output [MWe] from the integrated WtE-ASU plant:
PWtE ASU PWtE PASU ,
(1)
▪ Annual incremental income (MUSD) between the WtE plant and the integrated WtE-ASU plant:
income income el .energy income gatefee income LN 2 ,
(2)
where the three main economic components of the integrated WtE-ASU plant that contribute positively (+) or negatively (-) to the annual incremental income over the baseline case are represented by:
income el .energy ( EWtE ASU EWtE ) * ET : the annual incremental income due to electric energy (E) sold to the grid (-)2 at the current electric tariff (ET); o income gatefee : the annual incremental income due to gate fee paid by local authority per ton of waste3 (+). o incomeLN 2 : the annual incremental income due to the tons of liquid nitrogen sold to the commercial company (+); o
▪ Annual economic savings (MUSD) for the company that operates the refrigerated trucks/bus fleet, computed as the difference between the yearly operative costs before and after the introduction of the DE:
Savings econ, yearly C Diesel * kgavoidedDiesel C LN 2 * kg LN 2 ,
(3)
where Cdiesel (USD/kgdiesel) and CLN2(USD/kgLN2) represent the cost of diesel and liquid nitrogen respectively. ▪ Capital cost of the cryogenic ASU (MUSD):
2
The negative impact is due to the ASU which consumes some of the electric energy produced by the WtE plant. Gate fee is the payment that the landfills or WTE plants earn per ton of waste received from the local government. The source of this part of subsidy mainly comes from government and waste disposal fee charged to local residents [14]. 3
CAPEX ASU CPASU * PASU ,
(4)
where CPASU and PASU are the cost per unit power (MUSD/MW) and the electric power required by the ASU respectively. ▪ Incremental Capital cost of the DEs fleet (MUSD):
CAPEX DE CPDE * N DE ,
(5)
where ΔCPDE and NDE are the incremental costs per each refrigerated truck/bus (USD/unit) and the number of DE units required. The payback period for the WtE-ASU plant is evaluated taking into account the CAPEX index of Equation (4) and the annual incremental income of Equation (2) while the payback period for the DEs fleet is computed considering the incremental CAPEX of equation (5) and the annual economic savings of Equation (3). ▪ Annual total emissions savings computed as the difference between the annual emissions of the baseline and the integrated systems for each specific emission species:
Savings i , yearly Emissions i ,baseline Emissions e,int egrated ,
(6)
The following assumptions have been made throughout the whole analysis: ▪ No additional fuel is introduced in the combustion chamber of the WtE plant under OEC operations; ▪ The economic analysis neglects the social costs linked with CO2, NOx and PM emissions [6]; ▪ The CO2 emissions (ton/year) related with the WtE plant have been considered to be constant for each level of oxygen enrichment4; ▪ The electric energy required by the ASU must not exceed the net electric energy produced by the WtE plant; ▪ The red diesel price is assumed to be half price of the respective road diesel price [6].
3. Results and discussion 3.1. Energy analysis In order to analyze the effect of oxygen enrichment over the WtE plant performance, a standard WtE plant has been simulated in Aspen Hysys. The WtE plant model consists of a conversion reactor (i.e. a combustion chamber) coupled with a classic dual pressure Rankine cycle designed for a steam pressure and temperature of 35 bar and 370 °C respectively, and a condensing pressure of 0.1 bar. From the energetic perspective, the results showed that the integrated plant (WtE + ASU) leads to an overall penalty in terms of net electric power output. In Figure 2 the net electric power output has been computed for four different oxygen concentration: 21%, 23%, 25% and 27%. The first and the last oxygen concentrations represent the extreme scenarios of the analysis: the baseline case, in which there is no oxygen enrichment in the combustion process (basically this corresponds to the case in which there is no ASU) and the zero net electric power in which all the net electric energy produced by the WtE is completely consumed to operate the ASU.
4
Since no additional fuel is added in the combustion chamber for waste processing, no fuel savings occurs and therefore any significant reduction of CO2 does not takes place in our analysis [1].
Net Power [MWe]
25 P_WtE-ASU
20 15 10 5 0 0.19
0.21
0.23
0.25 xO2[-]
0.27
0.29
Figure 2 Net electric power production of WtE-ASU as a function of oxygen molar concentration
Waste mass flow [tonn/day]
Another relevant effect linked with oxygen enrichment is represented by the increase of the throughput of the WtE plant. In fact, as already mentioned, the higher temperatures associated with an oxygen enriched combustion enhances the heat transfer to the load (i.e. the waste being incinerated) thus increasing the material processing rates through the combustion chamber. Since the simulated model is not able to capture the increase of mass flow with the oxygen enrichment, the increased capacity has been estimated taking as reference the work carried out by Melo et al.[5] in which it was estimated that the waste mass flow rate can be increased up to 60% for an oxygen concentration of 30%. Assuming a linear dependence between waste mass flow and oxygen enrichment, the increase of the waste mass flow has been computed by means of a linear interpolation and the results are given in Figure 5. 1400 1200 1000 800 600 400 200 0 0.21
0.23 xO2 [-]
0.25
0.27
Figure 3 Waste mass flow as a function of oxygen molar concentration. The byproduct of the ASU (i.e. the LN2) is used to feed a fleet of refrigerated trucks or buses for frozen goods or civil transport application respectively. The transportation means and the parameters used in the energy analysis are summarized in Table 1. Table 3. Assumptions for the energy analysis Vehicle Fuel Cooling specific consumption [6][8][9] City Bus Road diesel 2.27 kWhc/kgdiesel 40 ft trailer with Red diesel 2.17 kWhc/kgdiesel TRU DE-Bus Road diesel/LN2 0.101 kWhc/kgLN2 40 ft trailer with DE- Red diesel/LN2 0.182 kWhc/kg LN2 TRU
Mechanical work specific consumption [6][8] 3.02 kWhm/kgdiesel
LN2 (ton/day) [6][8] -
1.09 kWhm/kgdiesel
-
0.08 kWhm/kg LN2
0.185
0.02 kWhm/kg LN2
0.275
25
x 1000
x 1000
Assuming an utilization factor of 100% for the liquid nitrogen produced by the ASU, by means of the above mentioned assumption, the number of units powered by the DEs and the tons of diesel annually saved for each respective application have been computed and the results are reported in Figure 4 for three different oxygen molar concentrations of the oxidant. Due to the higher cooling energy released per kg of LN2 by the DE-TRU as compared with the DEBus, the former configuration allows saving approximately 40% more diesel than the latter. It is worth noting that at 0.25 oxygen molar concentration, the number of DE-Bus units would fit well with the average bus fleet operated in Singapore [12], a location where due to the particular climate condition, the cooling energy consumption for a public bus is almost steady throughout the whole year.
20
40 35 30 25
15
20 10
15 10
5
5 0 0.23
0.25
0.27
xO2 [-] DE-Bus Number units Diesel saved (ton)
0 0.23
0.25 0.27 xO2 [-] DE-TRU Number units Diesel saved (ton)
Figure 4 Comparison of Dearman engine applied to City-Bus and 40 ft refrigerated trailer in term of number of units and tons of diesel saved for different oxygen molar concentration.
3.2. Economic analysis In order to carry out the economic feasibility of the integrated systems over the baseline case the following assumptions have been made (refer to Table 4). Table 4. Nominal assumptions for the economic analysis. Parameter Value Electric tariff- ET (USD/kWhe) [13] 0.102 Gate fee (USD/ton) [14] 20 LN2 price (USD /kg) [6] 0.07 CAPEXASU (M USD /MWe) [15] 0.35 Red Diesel price (USD /kg) [1] 0.7959 Road Diesel price (USD /kg) [16] 1.5918 ΔCAPEXDE- Bus (USD /unit) [6] 7677 ΔCAPEXDE-TRU (USD /unit) [8] 5117 Figure 5 allows to better visualize the effect of the oxygen molar concentration over the total annual incremental income disaggregated into its various economic components. As we move along the xaxis, for a contextual decrease of the income from the electricity sold to the grid (due to the higher electric energy consumed by the ASU), the integrated plant increases its annual incremental income due to the increased processing capacity (i.e. increase of the income from the gate fee) and the sale of the liquid nitrogen.
Annual WtE-ASU Δincome [MUSD]
30 20 10 0
-10 -20 0.23 Energy Sale
0.25 xO2 [-] Gate fee LN2 sale
0.27 Total
0.16
0.16
0.14
0.14
0.12
0.12
0.10
0.10
0.08
0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
ET [USD/kWh] (continuous lines)
LN2 price [USD/kg] (dashed lines)
Figure 5 WtE-ASU annual incremental income components as a function of oxygen molar concentration.
0.00 -10
-5
0 5 10 Annual ASU Δincome [MUSD]
15
20
Gate fee 5 $
Gate fee 20 $
Gate fee 40 $
Gate fee 5 $
Gate fee 20 $
Gate fee 40 $
Figure 6 WtE-ASU annual savings for xO2 =0.25 as a function of: a)LN2 price for a defined ET ( 0.102 USD /kWhe) and for different gate fees; b)ET for a defined LN2 (0.07 USD /kg) and for different gate fees. In order to assess the influence of the ET (USD /kWh) and the price of liquid nitrogen (USD/kgLN2) over the economic feasibility of the integrated plant, a sensitivity analysis has been carried out for both these parameters as illustrated in Figure 6. From the figure is apparent that the economic investment is more convenient if the electric energy tariff decreases from its nominal value (0.102 USD /kWh) to lower values. In fact, since the annual incremental income depends on three factors5 - refer to Equation (2) - as the electric tariff decreases, the annual incremental income due to the electricity sold to the grid increases. The break-even point is achieved in the range of USD 0.13÷0.15 USD/kWh and 0.04÷0.055 USD/kgLN2 (i.e. threshold values for the integrated plant economic feasibility) for the ET and liquid nitrogen price, respectively.
5
The gate fee and amount of LN2 sold have been fixed in this case.
10
25
5
20
0
15
-5
10
-10
5
-15
Pay back Period [year] (continuos lines)
Annual WtE-ASU Δincome [MUSD] (dashed lines)
Another interesting factor that affects the economic feasibility of the integrated WtE-ASU plant system is represented by the so called liquid nitrogen utilization factor, defined as the ratio between the tons of liquid nitrogen required by a potential fleet of Dearman engines over the total produced by the ASU. Figure 7 highlights that the integrated plant produces positive annual incremental income only for an utilization factor higher than approximately 70%; however, in order to obtain an economically advantageous investment the utilization factor should be at least higher than 75% thus allowing to achieve a pay-back period below 10 years.
0 0%
20%
40% 60% 80% LN2 utilization factor [%]
100%
xO2 = 0.23
xO2 = 0.25
xO2 = 0.27
xO2 = 0.23
xO2 = 0.25
xO2 = 0.27
Figure 7 WtE-ASU incremental annual savings and pay-back period as a function of LN2 utilization factor for different oxygen molar concentrations for a defined gate fee of 20 USD/ton. Finally, under the hypothesis of 100% LN2 utilization factor, the economic feasibility of the combined investment (ASU + DE) has been investigated. In Figure 8 and Figure 9 the annual economic savings, incremental incomes and the pay-back period of the investment is compared for each configuration (either DE-TRU or DE-Bus) in order to evaluate the threshold values of LN2 and diesel price that make the investment attractive. The LN2 price has two opposite effects as shown by the trend of the dashed lines in Figure 8; indeed, it may guarantee positive annual incremental income for the integrated plant but at the same time it may limit the opportunity to consider the DE. Figure 6 shows that the minimum price of LN2 which guarantees positive incremental income for the integrated plant is around 0.05 USD /kgLN2 (if a gate fee of 20 USD/ton is considered, as indicated in Table 2). Figure 8 shows that an optimum LN2 price range which allows pay-back periods below 5 years for both the DE configurations and the integrated plant, falls between 0.0550.075 USD /kgLN2. Nevertheless, taking into account the longer technical life of ASU compared to Dearman engine system, it is advisable to operate with LN2 price closer to the lower end of the price range (~ 0.055 USD /kgLN2) in order to always guarantee an economic interest toward the DE investment. In fact without the economic feasibility of DE technology, the WtE-ASU is deprived of its commercial counterpart to which sell the LN2 byproduct. In addition, due to lower price of red diesel (half price of road diesel), the DE-TRU configuration is the most sensitive to the increase of LN2 price variation: at 0.08 USD /kgLN2 the annual savings approaches the zero value while for DE-Bus configuration the pay-back period is still acceptable. Finally, the combined effect of the diesel and the liquid nitrogen prices on the DE annual economic savings and pay-back period is shown in Figure 9, where three different prices for road diesel [18] (e.g. Singapore, USA and EU) are marked with red and blue dots. For the assumed nominal liquid nitrogen cost (0.07 USD/kg), although, as pointed out earlier, the DE-Bus configuration may benefit from the Singapore climate condition, the current road diesel price (1.08 USD /kg) leads to longer (>20 years) pay-back period as compared with the European situation. The same scenario applies to the USA context where the lower price of both red and road diesel does not allow to achieve an economic advantage from the DE investment. On the contrary, the European market seems to be the
20
45 40 35 30 25 20 15 10 5 0 0.14
15 10 5 0 -5 -10 0.00
0.02
0.04
0.06 0.08 0.10 LN2 price [USD/kg]
0.12
WtE-ASU
DE-TRU
DE-Bus
WtE-ASU
DE-TRU
DE-Bus
Pay-back period [year] (continuous lines)
Annual ΔIncome/savings [MUSD] (dashed lines)
one most favourable for the DE penetration due to the highest red and road diesel price that leads to attractive pay-back period inferior to 5 years. Nevertheless, considering the steep gradient of payback period curves, especially remarkable for DE-Bus, even a 20% decrease of road diesel cost, may lead to significantly higher annual economic savings with pay-back period for Singapore context inferior to 10 years. The same effect has been observed if the cost of liquid nitrogen is decreased to the above mentioned value of 0.055 USD/kg: in Singapore the investment for both DE configurations comes again to produce positive annual savings, especially remarkable for DE-bus where a pay-back period of 4 years is achieved.
Singapore
15
20
10 15 5 10 0
USA
Singapore 5 USA 0 0.25
EU
EU
-5 -10
0.75
1.25
1.75
50
25
45
Singapore
20
40 35
15
30 25
10
20 5
15 USA
10 5
Singapore
USA
0 0.25
0 EU
EU
-5
0.75
1.25
1.75
Diesel Price [USD/kg]
Diesel Price [USD/kg]
DE-Bus
DE-TRU
DE-Bus
DE-TRU
DE-Bus
DE-TRU
DE-Bus
DE-TRU
Annual Savings [MUSD] (continuous lines)
20
Pay-Back Period [year] (dashed lines)
25
Annual Savings [MUSD] (continuous lines)
Pay-Back Period [year] (dashed lines)
Figure 8 WtE-ASU, DE-TRU and DE-Bus incremental annual savings and pay-back period as a function of LN2 price for xO2 =0.25 for a defined gate fee of 20 USD/ton and diesel price of 1.5918 USD/kg.
Figure 9. DE-TRU and DE-Bus pay-back period as a function of Diesel price for xO2 =0.25 and for LN2 price of a) 0.07 USD /kgLN2 (left) b) 0.06 USD /kgLN2(right).
3.3. Environmental analysis In integrated plants, despite the penalty efficiency introduced, oxygen enriched combustion could lead to several environmental benefits. Firstly, mainly due to the increase of incineration capacity
Annual Emission savings
and removal of pollutants caused by a more complete reactive combustion in presence of high oxygen concentration and the decrease of the flow rate of the flue gas that in turn leads to lower specific emissions. Besides the pollutant reduction related with oxygen enriched combustion, a substantial environmental benefit is achieved with the avoided diesel consumption substituted by the alternative energy vector, the liquid nitrogen. Although the possible presence of hazardous pollutants such as polychlorinated biphenyls (PCBs) and principal organic hazardous constituents (POHCs) in the emitted flue gas of the waste incinerator, the present analysis will focus only on three pollutant species, CO2, NOx and PM. The emissions factor (g/kgwaste) for the different oxygen enrichments are computed by interpolating the results obtained by Verdone et al. [17] for the extreme cases of 0 and 100 % oxygen concentrations; the assumptions on the Diesel engines emissions are based on the works carried out by Strahan [6],[8]. The results related to the emissions reduction of the pollutants, on annual basis, compared to the baseline cases are shown in Figure 10 for both the DE configurations. 200 150 100 50 0 DE-Bus 0.23
DE-Bus 0.25
DE-Bus 0.27
DE-TRU 0.23
Nox WtE-ASU [ton/year]
PM WtE-ASU [ton/year]
NOx DE [ton/year]
PM DE [ton/year]
DE-TRU 0.25
DE-TRU 0.27
CO2 DE [kton/year]
Figure 10 Integrated systems annual emissions reduction for different xO2 and DE configurations allocated for the subsystem analysed. The introduction of DE is the main driver of the integrated plant emissions reduction. Although both DE configurations allows a substantial limitation of the emission, in this context the emissions reduction associated with the DE-TRU is predominant due to the less severe regulations for nonroad mobile machinery as compared with the normative involving road diesel engines. In addition to this, it is worthwhile noting that in the case of the DE-TRU, the CO2 emissions reduction (ranging from 9 to 23 ktonCO2/year) is due to both reduction of diesel consumption and leaks of HFC refrigerant gases employed for TRU cooling unit which account approximately for a 80 % and 20 % of the total greenhouse emissions of the vehicle, respectively.
4. Conclusions In this paper, an energetic, economic and environmental analysis of WtE-ASU-DE integrated system was studied. Baseline and integrated systems have been described in details and the assumptions adopted have been highlighted as well as the main key performance indices. The energy analysis confirms that the proposed WtE-ASU solution leads to a penalty in thermal efficiency due to the ASU power consumption; on the other hand, if the by-product of ASU is entirely sold to a commercial company, it has been illustrated the possibility to daily save a substantial quantity of diesel for both DE configurations (TRU and bus) as high as 34 kton/year. From the economical perspective, the disadvantage introduced with the ASU may be compensated principally by the new income in LN2 selling. In the EU scenario, the global investment seems to be attractive, with pay-back period inferior to 5 years for the both integrated plant and DE configurations, if two conditions are simultaneously fulfilled: the LN2 utilization factor is higher
than 75 % and the ASU could guarantee a LN2 price not superior to 0.075 USD /kgLN2 and 0.085 USD /kgLN2 for the DE-TRU and DE-Bus fleet, respectively. In addition, it is worth pointing out that the less the electric energy produced by WtE is remunerated or subsidized, the more the economic investment in an WtE-ASU tends to be interesting. Analysing the diesel price influence on the DE economic feasibility in other scenarios (e.g. Singapore and USA), it turns out that the sensible lower diesel prices lead to negative annual economic savings. Nevertheless, the sensitivity analysis has shown that even a small variation of liquid nitrogen and/or diesel price may substantially altered the economic feasibility leading to attractive economic scenarios. Finally, substantial environmental benefits are achieved by means of ASU and DE implementation especially notable for the DE-TRU configuration where the lack of an effective regulation for TRU allows to save as high as 140 ton/year of NOx and 17 ton/year of PM.
Nomenclature E P xO2
Energy, MWh Power, MW Oxygen molar concentration, molO2/moltot
Abbreviations DE Dearman Engine ET Electric Tariff ($/kWhe) EU European Union HTF Heat Transfer Fluid ICE Internal Combustion engine LN2 Liquid Nitrogen OEC Oxygen Enhanced Combustion TRU Transport Refrigeration Unit WtE Waste to Energy Subscripts and superscripts c cooling m mechanical
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