design considerations, modeling and assessment for the thermochemical production of diesel fuel from...

8/11/2019 DESIGN CONSIDERATIONS, MODELING AND ASSESSMENT FOR THE THERMOCHEMICAL PRODUCTION OF DIESEL FU

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utilities consumption, and GHG emissions. The results of

this technical assessment shall help in the earlyelimination of one or more of the routes and will be the

base of a subsequent study aiming to estimate the cost of1 barrel of diesel using the different routes in Egypt.

2 DESIGN CONSIDERATIONS

2.1 Gasifier technologyFor the large scale commercial production required

for this work, the entrained-flow gasification technologyis known to be the most suitable [23]. It is alsocharacterized by high carbon conversion and minimal tar

production [20], and hence it is selected to be the gasifiertechnology in this work. According to [24], nearly 75%

of the gasification plants that generate electricity fromcoal throughout the world use two entrained flow

gasification technologies; namely, Shell and Texaco. In

this work, Shell entrained flow gasification technologyhas been selected because of its successful experiencewith co-gasification in Buggenum plant [11], and itshigher thermal efficiency compared to Texaco [25].

2.2 Feeding rate

To have common basis for comparing the differentperformance parameters, the same feeding rate andcapacities (in MWth) were assumed for the three proposed

routes. As the maximum capacity of biomass gasifiers is400 MWth [3,12], the feeding rate was adjusted to bearound this value or one of its multiples. Other guides for

choosing feeding rate as well were the commercial Shell

units currently operating [26] in addition to the provenreserves of Maghara coal mine. The balance of these

factors has finally led to select the feeding rate as 115 t/husing 2*400 MWth units. The impact of changing the

feeding rate on the syngas compositions at constant

thermal power output was studied as a part of this work.The coal-biomass blend mass ratio in the CBtL route

was taken as 70-30 as this is nearly the blend ratio

commercially applied in Buggenum plant [27].

2.3 Flow rates of gasifying agents

Oxygen, air, steam or mixtures of them are the most

widely used gasifying agents. While the air gasification ischeaper than oxygen and steam, the correspondingheating value of the produced syngas is much lower (4-7MJ/Nm3 vs. 10-14 MJ/Nm3). When steam is used, the

rate of the endothermic water gas and methane steamreforming reactions increase leading to a decrease in thesyngas temperature and an increase in the hydrogencomposition of the syngas [7]. Entrained flow gasifiersgenerally use oxygen and steam as gasifying agents. The

ratios of oxygen and steam to the solid feed (coal,

biomass, or their mixture) are important designparameters which affect the resulting syngas temperatureand composition, and accordingly the downstream

processes will be much affected. The oxygen to feed ratiois generally represented by the symbol (lambda) whichis defined as follows [28]:

(d.a.f)inputfuelofUnit

tsrequiremenOtricStoichiome

(d.a.f)supplyFuel

SupplyOExternal

2

2

(1)

The value of lambda for biomass gasification is

generally between 0.2 and 0.5 [28, 29], where its valuefor entrained flow gasifiers specifically lies between 0.35

and 0.5 [29, 30]. For coal entrained flow gasification,lambda lies between 0.4 and 0.9 [25]. On the other hand,

the steam to biomass ratio for biomass gasification is inthe range of 0.5-1.5 [5, 28], while the steam to coal ratio

falls between 0.01 and 0.35 [25, 31].

In this work, a parametric study was performed forthe lambda value and steam to feed ratio in the threeproposed routes where their effects on the syngas

composition and outlet temperature were investigated.The results of this parametric study were crucial inselecting the operational lambda and steam to feed ratio

in each of the three proposed routes.

2.4 Energy integrationPinch technology analysis (PTA) was employed in

this work to perform energy integration in each of the

proposed routes. PTA is a tool which aims to minimizethe energy consumption by estimating the heating andcooling targets through thermodynamic calculations [32,33]. PTA first starts by categorizing the process streamsinto groups of hot and cold streams, and then temperature

interval diagram and composite curves are accordingly

created. The pinch point is defined as the point of closestapproach between the hot and cold composite curves [7].By avoiding the inclusion of heat exchangers passing

through the pinch temperature in the heat exchangernetwork (HEN) schemes, the process heating and coolingtargets will be minimized. In large scale processes as

those encountered in this work, the minimization of

energy consumption is a crucial factor in the productioncost and hence the feasibility of the proposed routes.

3 PROCESS DESCRIPTION

3.1 Pre-processingThe proximate and ultimate analysis of Miscanthus

energy crop (M*G) and Maghara coal (MC) is presentedin Table 1. As shown, the moisture content in both feedsis low, and accordingly no drying step is included for

both. Accordingly, the feed preparation section is just

feed storage and size reduction. After being stored in thesilos, coal and biomass are separately conveyed togrinding facilities to reduce the average particle size to 1mm or even lower. Such low particle size is required in

the entrained flow gasification technology to acquireextremely fast heating rates.

Table I: Proximate and ultimate analysis of the feedmaterials used in this work

Feed Type Miscanthus Maghara coal

Proximate analysis in wt% (as received)Fixed carbon 9.5 38.4Volatile matter 78.8 49.3

Ash 2.7 6.8Moisture 10 5.5

Ultimate analysis in wt% (dry basis)C 49.5 70.7

H 6.1 6.0N 0.1 1.2

S 0.006 1.7

O 41.2 13.1

22nd European Biomass Conference and Exhibition, 23-26 June 2014, Hamburg, Germany

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bed reactor, or slurry bed reactor [5].

The general form of FT reactions can be summarizedas (Eq. 2 and Eq. 3) [18]:

OHnHCHnCOn nn 2222)12( (2)

OHnOHHCHnCOn nn 2122 )1(2 (3)

The idea of FT reactions can be expressed according toan Anderson-Schulze-Flory (ASF) distribution (Eq. 4). The

mole fraction of hydrocarbon molecules Pncontaining ncarbon atoms depends on which is the probability that ahydrocarbon chain combines with another instead of

terminating [5].

)1(1nnP (4)

3.5 HydroprocessingAs shown in Fig. 1, the product from FT reactor is

then cooled to 40oC and sent to a 3-phase separator wheregas is separated from a hydrocarbon phase and water

phase. The hydrocarbon phase is separated in a

distillation column supplied with steam and equippedwith side stripper into a top product rich in LPG andnaphtha, middle product composed of crude diesel, and aheavy product rich in wax. The latter is pumped to 50 barand heated to 345oC before being directed to a

hydrocracker. The other feed to the hydrocracker ishydrogen which is separated using pressure swingadsorption (PSA) unit from the gas stream produced fromFT reactor. The heavy wax is cracked to form about 5%light gases, 15% naphtha, and 80% diesel [12] with about

80% overall conversion [18].

The hydrocracker effluent is cooled and directed intoa distillation column similar to the first one where thebottom product is recycled to the hydrocracker as well.

The top product from both columns is directed to a 3-phase separator where the liquid water and light gases areseparated from the naphtha rich stream. The latter stream

is directed to a third column to separate residual light

compounds followed by a fourth column to separate LPGfrom naphtha. To increase the recovery of the LPG andnaphtha products, the gas stream produced from FTreactor is fractionated in a fifth column where an

ammonia refrigeration cycle is employed. The bottom

product of this column is rich in LPG and naphtha and isdirected to the fourth column, while the top product issplitted to a part routed to the PSA and the other to the

combined heat and power (CHP) section.

3.6 Power generation

To utilize the chemical energy stored in theunconverted syngas, it is routed to CHP section which iscomposed of gas turbine, heat recovery steam generator(HRSG), and steam turbine. The gas turbine consists ofair compressor, combustor, and expander. Huge amount

of air, much higher than the theoretical amount neededfor combustion, is compressed in order to cool the bladesand vanes. Then the compressed air is mixed with theunconverted syngas in the combustor forming hot flue

gases which are then directed to the multi-stagedexpander to generate power.

The hot flue gases after the gas turbine contain muchamount of energy that should be utilized. Thus, the fluegases are routed to HRSG which is composed of gas-gas

heat exchangers heating liquid water into steam [34]. Partof this steam will be used as gasifying agent for theentrained flow gasifier, while the majority is sent to themulti-stage steam turbine to generate excess power.

4 SIMULATION MODEL

Aspen Plus process simulation software is used to

model and simulate the three proposed routes in thiswork. Aspen Plus is the most widely used software in the

technical and economic studies of coal/biomassgasification for the generation of chemicals/power [3, 18,

34, 36, 37]

4.1 Components and thermodynamic packageTwo component types are used in the Aspen Plus

model of each of the three routes; namely, Conventionaland Non-Conventional. The first type is applicable for allthe gaseous and liquid components, while the other is

used for the solid feed. As the products of FT reactor aremainly naphtha, diesel, and wax, the components to be

entered to the model should represent all these fractions.This work has followed the methodology used by Sudiro

and Bertucco [18] in selecting the different components.

Sudiro and Bertucco [18] have chosen all the linear andsaturated hydrocarbons from C5H12to C30H62, C32H66and C36H74 which are already available in the library ofthe simulator. The same authors [18] have also definednew components to the simulator to have better

representation for the wax fraction. These components

are namely C37H76, C38H78, C39H80, C40H82, C45H92,C50H102, C55H112and C60H122. The olefins produced fromFT reactions are represented in this work by ethene,

propene and butane, while the alcohols are represented bycomponents by ethanol, n-propanol, i-propanol, n-butanoland i-butanol [38]. On the other hand, the solid feed of

each route is modeled as Non-Conventional (NC)

component whose composition is defined through theproximate and ultimate analysis. The latter are entered in

the Aspen model as Component Attribute.The correct choice of the thermodynamic package is

crucial for equilibrium calculations and hence the

compositions of the different streams in the flow sheet.For the current work, Peng-Robinson equation of statewith Boston-Mathias modifications (PR-BM) was

selected as the thermodynamic package [18]. For the NCcomponents, HCOALGEN and DCOALIGT models areused for the enthalpy and density calculations

respectively [39].

4.2 Modeling syngas production and processingThe entrained flow gasifier modeling can be

classified into modeling the gasification reaction, and

modeling the heat recovery section. The gasificationreaction is modeled using 2 reactors in series; RStoicrepresenting the solid feed pyrolysis, and RGibbsrepresenting the char gasification. In RStoic, the reactionmodeled is the decomposition of the solid feed into O2,

N2, H2, H2O, C, S and ash. The stoichiometry of every

product is calculated separately based on the proximateand ultimate analysis using a user-specified FortranCalculator. For the high temperatures encountered in the

entrained flow gasifiers, thermodynamic equilibriumconditions can be assumed [3, 5, 37], and accordinglyRGibbsis used to model the char gasification. The slag is

then separated from the produced syngas using a simple

Separator (Sep), while the syngas will be transferred tothe heat recovery section.The radiant cooling is modeledusing aHeat Exchanger (HeatX)where a water stream atroom temperature is converted to a superheated stream

whose flow rate is adjusted to cool the syngas to 815oC[34]. The water quench section is then modeled as a


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Mixerwhich mixes the cooled syngas and quench water

followed by a Flash separator which separates thesyngas and the resulting wastewater stream.

The cooled syngas is then splitted using a (FSplit)block so that part is directed to the WGS reactor and the

other bypasses it. The WGS reactor was modeled asREquil where the WGS reaction is specified [37]. The

WGS product is then mixed with the rest of the syngas

before being cooled to 40oC using a utility heatexchanger (Heater) and another wastewater stream isseparated using a Flash separator. The last syngas

processing step before the FT section is the acid gasremoval which is modeled using a simple Separator(Sep) [18].

4.3 Modeling FT and hydroprocessing

The syngas is heated to 240oC, the temperature of FTreactor, using a utility heat exchanger (Heater). The

syngas is then directed to FT reactor which is modeled as

RStoic [3] where a total of 48 reactions have beendefined. The product stream is then cooled to 40oC usinga utility heat exchanger (Heater) before being separatedin aFlash separatorinto gas product, liquid product, anda wastewater stream. The liquid product is throttled using

a Valveand then fed together with a steam stream to a

PetroFrac column (PetroFrac1) equipped with a sidestripper. The locations of the different feed and productstreams are given together with the molar flow rate of the

distillate and side product streams. An optimization block(O-1) was used to maximize the recovery of C11 in thedistillate stream by varying the flow rates of the main

steam and side product streams. Three constraints were

added to the optimization block to adjust the carbonnumbers of the top, side and bottom product streams. The

side product stream was thus adjusted to be in the rangeof C10 and C20, and thus it is considered a crude diesel

oil product. The top product ofPetroFrac1 is thencooled

to 40oC using a utility heat exchanger (Heater) beforebeing separated in a flash separator into light gases,liquid product, and a wastewater stream. The liquid

product is then pumped and distilled in a RadFraccolumn (RadFrac1) separating any amount of light gases.The pressure inRadFrac1 was adjusted to separate gases

lighter than C3 using cooling water in the condenser. The

bottom product of RadFrac1 is then throttled using aValveand thendistilled in aRadFraccolumn (RadFrac2)into an LPG top product and crude naphtha bottomproduct. An optimization block (O-2) was used to

maximize the recovery of C4 inRadFrac2by varying thefeed pressure and reflux ratio and adding a constraintthatmakes the minimum temperature after the condenser40oC so that it can be attained using normal coolingwater.

The bottom product of PetroFrac1 which contains a

lot of waxes is pumped and heated using a utility heatexchanger (Heater) before being fed to the hydrocracker.The latter is modeled as RYield reactor [18] which is

often used in case of knowing the product distributionwithout knowing the exact reaction mechanism. Thedifferent component yields were calculated in a separate

Excel spreadsheet to achieve the required product

distribution. The product of RYield reactor is throttled,cooled and separated using a Flash separator into aliquid and gas streams. The liquid stream is directed toPetroFrac column (PetroFrac2) equipped with a side

stripper while the gas stream is cooled further andseparated using a Flash separator into a gas stream rich

in LPG and a liquid stream also directed to PetroFrac2.

Using the same methodology of PetroFrac1, anoptimization block(O-3) was created to adjust the carbon

numbers of the top, side and bottom product streams. Thebottom product rich in waxes is then pumped and

recycled to the hydrocracker to crack it further to lighterproducts. The product of the hydrocracker was taken as a

Tear Stream in order to facilitate the solution of the

recycle loop. The side product of PetroFrac2 togetherwith that of PetroFrac1 are fed into a Decanter toseparate the accompanying water and acquire the crude

diesel fraction product. The top product of PetroFrac2istreated the same as the top product of PetroFrac1whereit is separated from water and then pumped to RadFrac1

followed byRadFrac2.The gas stream produced from the Flash separator

following the FT reactor is fed to a distillation column(RadFrac3) to recover as much as possible of LPG and

naphtha. An optimization block (O-4) varying the reflux

ratio and the distillate flow rate is created to maintain thecondensers temperature above -32oC so that an ammoniarefrigeration cycle can be used. The bottom product isthen directed to RadFrac2 to achieve the final LPG andcrude naphtha products. The top product of RadFrac3is

splitted using a (FSplit) block so that a small part is

directed to the PSA unit for separating hydrogen requiredfor the hydrocracking, while the other part bypasses itand is directed to the CHP section. The PSA is modeled

using a simple Separator, where theseparated Hydrogenis compressed using (Compr) to the hydrocrackerspressure.

4.4 Modeling power generationAir is compressed on 3 stages using (Compr) where a

(FSplit) block follows each stage so that some air coolsthe gas turbine blades. Compression ratio of 2.5 is used in

each stage. The compressed air is then mixed with the

unconverted syngas and directed to a combustor modeledas RStoic block where combustion reactions for all thecombustible components in the syngas are added. The

exhaust gases from the combustor are expanded on 3stages using (Compr) at an expansion ratio of 2.5. Theexhaust gases from each stage are mixed with the air

splitted after each compressor stage. After the last

expander stage, the exhaust gases are cooled on 4 stagesusing 4 heat exchangers (HeatX). The cooling stream ineach HeatX is water or steam produced from the HRSGsection. After the first 3HeatXblocks, the water becomes

superheated steam which is expanded in a high pressuresteam turbine section using (Compr) block. The resultingsteam is then further heated in the 4thHeatX block andsplitted using a (FSplit) block so that part is used as afeed steam to the gasifier, and the other is directed to 2

consecutive steam expander stages producing work. The

discharge steam is then mixed with make-up water andseparated using a Flash separator into steam and liquidwater which is pumped to exchange heat with the exhaust

flue gases from the gas turbine section.

4.5 Energy Integration

Aspen Energy Analyzersoftware was used to conduct

the energy integration of the three studied routes in thiswork. Aspen Energy Analyzer was used to extract thedata of the heat exchangers from the modeled flow sheet,and it accordingly determines the pinch temperature(s)

and plots the composite curves. The temperature of thecooling water utility was fixed at 25oC while that of the


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refrigerant was selected to be -40oC. The program then

generates alternative HEN schemes satisfying all theheating and cooling requirements.

5 PROCESS PERFORMANCE ANALYSIS

This section highlights the main outputs of the

simulation model of each of the three routes. The resultswill be presented in terms of process performanceparameters. Nine performance parameters were chosen in

this work; namely, the flow rates of the oxygen and steamgasifying agents, yields of the liquid fuel products, yieldsof CO2 (the main byproduct), yields of wastewater,

cooling water requirements, net electricity production,overall thermal efficiency, and GHG emissions.

5.1 Parametric study results of the feed rates and ratios

In operation, sometimes it happens that the feeding

rate may be varied according to the feed supply and themarket demand. For sensible process equipment like theentrained flow gasifier, the feeding rate variation shouldbe done carefully as this will affect the syngastemperature and composition. In order to minimize big

variations in downstream process equipment, the syngas

outlet temperature and thermal calorific value (mainlyrepresented in the H2/CO ratio) should be held constant.This was studied on Aspen Plus for the three proposed

routes, and similar behaviors were observed. Fig. 2ashows the results for the CBtL route. It appears that incase of decreasing the feed rate, the corresponding O2

and steam flow rates should be increased. The increase in

the flow of the gasifying agents leads to the increase inthe CO2 fraction of the syngas, and hence the load on the

acid gas removal unit will increase.For the three proposed routes in the is work, it was

found that for constant solid feed rate (115 t/h in this

work), and by holding the syngas calorific value at 800MWth, the increase in the lambda value leads toremarkable increase in the syngas outlet temperature as

shown in Fig. 2b for the CBtL route. About 9% increasein the lambda value results in approximately 190oCincrease in the syngas outlet temperature, and vice versa.

This may be attributed to the increased carbon oxidation

leading to higher exothermicity. On the other hand, theincrease in the lambda value leads to a small decrease inthe H2/CO ratio. This decrease may be attributed to be thenet effect of the increased rates of carbon oxidation,

reverse water gas shift reaction and forward water gasreaction according to Le Chatelier principle. On the otherhand, the effect of steam to solid feed ratio was studied atthe same feed rate and adding the syngas calorific valueas a new variable. As shown in Fig. 2c for the CBtL

route, as the steam to solid feed ratio increases, the

syngas outlet temperature decreases and the calorificvalue of the syngas increases. This may be attributed tothe increase in the rate of the endothermic water gas

reaction.The contradictory effects observed for the oxygen

and steam on the syngas temperature and H2/CO ratio

indicates the necessity of the careful choice of their

operational flow rates. The criteria for selecting thevalues of lambda and steam to solid feed ratios for eachof the three proposed routes were to have the outletsyngas temperature 1315oC as mentioned in the process

description, have the thermal power of syngas 800 MW thas mentioned in section 2.2, and to have the selected

ratios within the ranges mentioned in section 2.3. Fig. 3

presents the values that satisfy these criteria for the threeproposed routes. The significantly greater oxygen flow

rate for the CtL route may be attributed to the loweroxygen content of MC compared to M*G.

0.5

0.6

0.7

0.8

0.9

800

1000

1200

1400

1600

1800

0.45 0.50 0.55 0.60 0.65

H2

/COratioinsyn

gas

SyngastemperatureinoC

Lambda

Temperature

H2/CO

780

790

800

810

820

830

840

1150

1200

1250

1300

1350

1400

0.1 0.2 0.3 0.4

SyngascalorificvalueinMW

th

Syngaste

mperatureinoC

Steam/feed ratio

Temperature

Calorific value

(a)

(b)

(c)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0

0.2

0.4

0.6

0.8

90 100 110 120

CO2

fractioninsyngas

Gasifyingagentratio

Feed flow rate in t/h

Lambda

Steam/Feed

CO2 in syngas

Figure 2: Parametric study results of feed rates andratios. a) effect of feed rate, b) effect of lambda, c) effect

of steam

0 0.2 0.4 0.6 0.8

BtL

CtL

CBtL

Fraction

Steam to

Feed

Lambda

Figure 3: Selected gasifying agents ratios of the three

routes

5.2 Syngas flow and compositionDue to the differences in the proximate and ultimate

analysis of the feed solids in the three routes, the

occurrence of some variations in the syngas flow and


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composition is predicted. Table II presents the syngas

flow and composition in different processing stages andbefore being introduced to the FT reactor. Compared to

the BtL route, the syngas in the CtL route wascharacterized by high fraction of CO and low fraction of

CO2. These two facts may be attributed to the low oxygento carbon ratio of the parent coal. The amount of water

vapor in the syngas generated in the BtL route was much

higher than the CtL route mainly because of the highercontents of moisture and volatile matter in M*G. Aspredicted, the syngas produced from the CBtL route has

intermediate behavior between both CtL and BtL routes.The high CO content in the syngas produced from theCtL route has resulted in much lower H2/CO ratio

compared to the BtL route, and hence the fraction ofsyngas directed to the WGS reactor is higher. This should

lead to higher WGS reactor volume for the CtL route, afactor that should be taken in account in the economic

assessment. In the WGS reactor, CO reacts with steam

forming CO2 and H2. As the fraction of syngas directed tothe WGS reactor in the CtL route is higher, much amountof CO2 was formed, and thus the total amount of CO2produced from both the gasifier and WGS in the threeroutes were comparable. As shown in Table II as well,

the compositions of the different syngas components after

the WGS are within the same range, and they are nearlyidentical after CO2removal.

The factor which is different in the three routes is the

flow rate of the syngas after the different processingstages. After the WGS and water removal, the flow rateof the syngas in the BtL route becomes 32% and 26%

lower than those of the CtL and CBtL respectively

despite being higher than both routes directly after thegasifier. The reason behind that is the higher amount of

water vapor in the syngas in the BtL route, and whichhave been condensed in the water removal section. The

net syngas flow rate directed to the FT reactor after the

syngas processing is of great importance as this willaffect the yields of the different products after thehydroprocessing steps in addition to the amount of

electric power generated. The CtL has net syngas flowrate 10% and 28% higher than the CBtL and BtL routesrespectively, and this is considered a significant point of

strength.

5.3 Yields of products, byproducts and waste streamsIn addition to the diesel, naphtha and LPG products,

the process generates some saleable byproducts like slag

and CO2and it also generates big amounts of wastewaterstreams. Table III presents the yields of the products,byproducts and waste streams for BtL, CtL and CBtLroutes using both absolute and specific units. Aspredicted from the previous section, the CtL has the

maximum total yields of the different products, and the

BtL has the lowest yields. The CBtL route proved to bevery competitive to the CtL route in terms of the mainproducts yields. The diesel and naphtha yields in the

CBtL route are only 3% and 2% respectively lower thanthe CtL, while they are 29% and 36% lower in the BtLroute. It is also clear that the LPG yields in all the routes

are low in accordance with [18, 20]. The calculation of

the main products yields using specific units (kg/t feed) isuseful in defining and calculating a process performance

Table II: Syngas composition and flow at different

processing stages

Feed Type BtL CtL CBtL

Directly after the gasifier

Total flow in t/h 226.1 190.9 200.9CO in mol.% 24.8 60.2 48.2CO2in mol.% 10.8 0.5 4.6H2in mol.% 28.2 38.4 36.8H2O in mol.% 36.2 0.9 10.3

Directly after the WGS and water removalTotal flow in t/h 163.5 240.9 220.3

CO in mol.% 24.8 25.9 25.8CO2in mol.% 25.3 22.0 22.3

H2in mol.% 49.7 51.9 51.7H2O in mol.% 0.3 0.3 0.3Directly after the CO2removal

Total flow in t/h 81.5 113.2 102

CO in mol.% 33.1 33.1 33.1

CO2in mol.% 0.3 0.3 0.3H2in mol.% 66.2 66.3 66.3H2O in mol.% 0.4 0.3 0.3

parameter namedLiquid fuel recovery (LFR)as shown inequation 5. The LFR was found to be 16.6, 23.9, and

23.2% for BtL, CtL and CBtL respectively confirming

the high competitiveness of the CBtL route.

100*amountfeedSolid

yieldNaphthayieldoilDieselLFR (5)

Table III shows that CO2is the main byproduct fromthe three routes, with the CtL having the highest valueand the BtL the lowest probably because of the difference

in the fixed carbon content of MC and M*G. It is

interesting to note that the CO2yields are about 4.5 timesthe yields of the main products indicating that the CO2utilization will significantly affect the plant economics.

In addition to the possibility of carbon capture andstorage, which is a costly technique mainly targeting themitigation of CO2emissions to the global warming, CO2

can be a source of revenue because of its potentialapplication in oil wells, chemical industries including

urea, methanol and carbonated soft drinks, and fireextinguishers. Not only shall these applications generatea source of revenue to the plant, but also this should helpreduce CO2 emissions in a way similar to the zero-

discharge concept. The plant site selection shouldconsider the marketing of CO2 gas among the otherfactors. Another important byproduct from the threeroutes is the slag produced from the gasifier. As shown in

Table III, the slag yield in the CtL route is the highestwhile that of the BtL is the lowest simply because of the

higher ash content in MC compared to M*G. Accordingto [40], the slag can be used as a construction material,

and hence it can also generate additional revenues for theplant.

Huge amount of wastewater is generated in the three

routes especially in the BtL route as shown in Table III.

The main wastewater streams are that separated after theWGS reactor in addition to that separated after FTreactor. The wastewater daily amount will affect both thecapital cost of the corresponding wastewater treatment

plant in addition to the operating cost of the related

chemicals and electricity. Any idea to minimize theamount of generated wastewater through techniques likerecycling and segregation shall have a significant effect

on the process economics. One idea is to recycle part of


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Table III: Yields of products, byproducts and wastes

(Values in t/h, and the values in brackets in kg/t feed)


Main products

Diesel oil 14 (120) 19.2 (167) 18.6 (162)Naphtha 5 (46) 8 (72) 8 (71)

LPG 1 (9) 3 (25) 2 (19)

ByproductsSlag 2 (22) 8 (69) 7 (56)CO2 94 (818) 126 (1093) 117 (1017)

Waste streamsWastewater 199 (1734) 83 (722) 113 (986)

the wastewater either as it is or after partial treatment to

be used in quenching the product gases in the entrainedflow gasifier.

5.4 Energy integration analysisThe hot and cold composite curves and the grand

composite curves of the three routes were found to havesimilar trends, with just some changes in the values ofheat duties and temperature. Fig. 4a and 4b showrespectively the cold and hot composite curves and the

grand composite curves of the CBtL route. These twofigures show that the process doesnt need externalheating utilities, while much external cooling utilities arerequired. These findings are in accordance with [18]. Fig.

4a has also shown that the heat integration conductedthrough the pinch analysis has saved much energy in the

so-called heat recovery region. As shown in Table IV,the cooling duties in the three routes are comparable with

the CtL being the maximum and the BtL the minimum.The corresponding cooling water requirements are

massive in the range of 20,000 t/h. Of course, theeconomics of any process shall be bad in case of using an

open cycle for the cooling water. Accordingly, closedcycle cooling towers will be installed in the process so asto minimize the cooling water requirements. Due to thewater lost by evaporation and blow down in the cooling

towers, make-up water in the range of 3-5% is required[41], and hence the cooling water requirements will be

much lowered as shown in Table IV. Taking the makeupcooling water requirements as another process

performance indicator, the CBtL clearly outperformsCtL. However, still the makeup water requirements arehuge, and accordingly the transformation to air coolersmay be a viable alternative in case of lacking the access

to cheap source of water.

Table IV:Heating and cooling duties in the three routes


Heating duty (MJ/h) 0 0 0

Cooling duty (MJ/h) 8.07E+05 8.69E+05 8.1E+05

Total cooling water

(m3/h)

19,046 20,612 18,999

Makeup coolingwater (m3/h)

757 819 755

Makeup coolingwater (m3/t feed)

6.6 7.1 6.6

5.5 Net energy analysisThe net electricity generation and overall thermal

efficiency are significantly important performanceindicators that may overbalance a route over another.

Most of the energy consumption and generation values

were generated by the simulation models of the three

routes. The specific power consumption of oxygenproduction was taken as 300 kWh/ton O2and that of the

CO2removal was taken as 1 MJ/kg CO2 [5]. The overallprocess energy efficiency for the three routes was

calculated using the following equation [5]:

100*

,,

.

,,

.

infu elinfu el

netou tfu elou tfu eloverall

LHVm

yElectricitLHVm (6)

Fig. 5 compares the main power consumption and

generation sources in the three routes. It is clear that boththe power consumption and generation values are thehighest for the CtL route and lowest for the BtL. Fig. 5also shows that the O2 production is the highest power

consumption source in the plant, and that the gas turbineis the biggest power generator. Table V presents both the

overall thermal efficiency and the net electricitygeneration (export power) in the three routes. As shown,

the export power in the CtL route is higher than the otherroutes. On the other hand, the overall thermal efficiency

of the BtL route has the highest value while that of theCtL route is the lowest in accordance with [8]. Table Valso shows that the overall thermal efficiency values of

all the routes were significantly improvedby adding the net electricity generation in consideration.The CBtL route has intermediate net electricity

generation and overall thermal efficiency values.

Table V:Electricity consumption and generation sourcesof the three routes


Overall thermal efficiency (%)Excluding net

electricity

47.6 37.8 42.4

Including netelectricity

52.1 44.1 48.1

Net electricity

generation (MW)

22.6 58.0 45.2

5.6 GHG emission analysis

The degree of GHG emissions is one of the importantperformance indicators from the sustainability point ofview. The main sources of GHG emissions in the threeroutes are the process CO2 produced after gasification

and WGS reactor, the amounts which are separated in the

CO2 removal section, in addition to the CO2 produced

from combusting the syngas in the CHP section. Table VIshows that the total CO2produced from the BtL route isthe lowest followed by the CBtL and CtL. Knowing that

the biomass is carbon neutral as it just emits the CO2absorbed during its growth, the GHG emissions of theBtL route is considered to be zero and hence it is themost environment-friendly route from the climate change

point of view. Using the same concept, only 70% of theCO2 emissions of the CBtL route were considered to be

GHG emissions. Thus, the CBtL route outperforms theCtL in this performance indicator as well.

Table VI:GHG emissions in the three routes

Feed Type BtL CtL CBtLProcess CO2 (t/h) 94.0 125.7 116.9

CHP CO2 (t/h) 28.4 47.9 41.3

Total GHG (t/t feed) 0 1.5 1.0


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(a)

(b)

BtL CtL CBtL

Steam Turbines 3.8 24.5 17.2

Gas Turbines 91.7 157.0 134.1

CO2 Removal -3.3 -4.5 -4.1

O2 Compression -6.5 -9.4 -8.3

O2 Production -12.8 -21.9 -19.2

CHP Air Compression -49.2 -84.1 -71.7

-150.0

-100.0

-50.0

0.0

50.0

100.0

150.0

200.0

ElectricityCons

umption/GenerationinMW

Figure 4:Composite curves of the CBtL route. a) hot and cold composite, b) grand composite

Figure 5:Sources of electricity consumption and generation in the three routes


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6 SUMMARY AND CONCLUSIONS

The technical performance of BtL, CtL and CBtL

routes using Miscanthus energy crop and MagharaEgyptian coal as feedstocks was evaluated by building

process models in Aspen Plus and defining processperformance indicators to be used in comparison. The

performance indicators chosen in this work are the flow

rates of the gasifying agents (steam and oxygen), yieldsof the liquid fuel products, yields of CO2 (the mainbyproduct), yields of wastewater, cooling water

requirements, net electricity production, overall thermalefficiency, and GHG emissions.

The CtL route was found to have the highest oxygen

consumption and the lowest steam consumption and viceversa with the BtL route, while the CBtL route had

intermediate consumption figures. After the WGS andCO2 removal, the compositions of the different syngas

components are nearly identical for the three routes. On

the other hand, the factor which is different in the threeroutes is the flow rate of the syngas which is maximumfor the CtL route and minimum for the BtL route.Defining the Liquid fuel recovery (LFR) percentage asthe amount of liquid fuels produced per ton of solid feed,

it was found that the LFR percentages for BtL, CtL and

CBtL were 16.6, 23.9, and 23.2 respectively. Thisindicates that the CBtL route is competitive to the CtLroute in this performance indicator. CO2 is the main

byproduct from the three routes, with the CtL having thehighest value and the BtL the lowest. It was observed thatthe CO2yields are about 4.5 times the yields of the main

products indicating that the CO2 utilization will

significantly affect the plant economics for the threeroutes. On the other hand, huge amount of wastewater is

generated in the three routes especially in the BtL route,while the CtL route is the lowest. Accordingly,

minimizing the amount of generated wastewater through

techniques like recycling and segregation shall have asignificant effect on the process economics of each route.The energy integration performed through the PTA

realized that the process in each route doesnt needexternal heating utilities, while much external coolingutilities are required. The cooling duties in the three

routes are comparable with the CtL being the maximum.

Taking the makeup cooling water requirements asanother process performance indicator, the CBtL clearlyoutperforms CtL with 6.6 vs. 7.1 m3/t feed. However, stillthe makeup water requirements are huge, and accordingly

the transformation to air coolers may be a viablealternative in case of lacking the access to cheap sourceof water. The net electricity generation in the CtL route is257% and 128% higher than BtL and CBtL routesrespectively. On the other hand, it has the lowest overall

thermal efficiency with 44% compared to 52% and 48%

for BtL and CBtL respectively. The GHG emissions ofBtL are zero, while the CBtL outperforms CtL again with1.0 versus 1.5 t GHG/t feed.

Thus, as expected, the BtL route will be excluded as acurrently possible route for the production of diesel fuelin Egypt. In addition to not being yet commercialized,

BtL suffers from bad performance indicators such as low

liquid fuel yields, low export power, and hugewastewater amounts. On the other hand, and based on theperformance indicators chosen in thus study, CtL andCBtL seems to be promising routes for the production of

diesel fuel in Egypt. CBtL (70-30 Maghara Coal-Miscanthus blend) was found to be very competitive to

CtL. In addition to having nearly comparable liquid fuel

yields, CBtL route surpasses the CtL in four out of thenine performance indicators chosen in this study; namely

less O2 consumption, higher overall thermal efficiency,less cooling water requirements, and less GHG

emissions.In addition to this technical perspective, the

utilization of the Egyptian Maghara coal in either CtL or

CBtL will solve a national strategic problem. Magharacoal mine historically faces marketing problems becauseof its non-coking nature inhibiting its use in iron and steel

industry, and the potential environmental problems whichmay take place if used in conventional power productionplants because of its high sulfur content. Accordingly, the

results of this study encourages performing economicassessment of both the CtL and CBtL routes in Egypt,

and this will be the subject of a subsequent paper.

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