Thermodynamic and economic analysis of acombined CHP system fuelled with rice husk

Chang T. Chang 1, Michela Costa 2*, Maurizio La Villetta 2, Adriano Macaluso 3, Laura Vanoli 3, Daniele Piazzullo 4

1) Department of Environmental Engineering, National Ilan University of Taiwan, Taiwan 2) Istituto Motori – CNR, Viale Marconi, 8, 80125 Naples, Italy

3) University of Naples “Parhenope”, Department of Engineering, Centro Direzionale, Isola C4, 80143 Naples, Italy 4) University of Rome “Tor Vergata”, Department of Industrial Engineering, Via del Politecnico 1, 00122 Rome, Italy

*m.costa@im.cnr.it

BIWAES 2017Biennal International Workshop Advances in Energy Studies

Naples, September 25-28 2017

IntroductionCombined Heat and Power (CHP) plants fuelled with biomass is an emerging technology on the market, with promising prospects for the near future, such as new perspectives for residue biomass utilization in district heating&cooling and/or in industrial-commercial activities.

Stateoftheartonbioenergy systems

Aim ofpresent workThermodynamiccharacteristics

Gasifiers+ICEs ORCs

Specificbiomassconsumption[kg/kWhe]

1.2-1.7 2.5-3.5

Electricefficiency%[-] ~25 ~12

Thermalefficiency%[-] ~25 ~70

Heattemperatureavailable[°C]

80-500 30-80

Operationtime[h/y] 7000 8000

SpecificCost[€/kWe] 3000-5000 5000-7000

A possible application is the coupling between a biomass gasifier with an Internal Combustion Engine (ICE).Waste heat recovery (WHR) through Organic Rankine Cycle (ORC) can be considered to increase the overall efficiency. Aimof present work• The gasifier is fed with rice husk • The ICE exhaust gases are exploited to preheat air for the rice husk drying process. • The ICE cooling water powers an ORC.• The ORC produce power to cover the internal demand of electricity.

A wide range of biomasses and technologies can be used for heat and/or power production.

SystemLayout

System layout developed in Thermoflex environment:• Dried biomass and air enter the gasifier;• Syngas temperature is decreased (350°C) to warm up the air for the drying section (HE1); recover of heat

from the exhausts exiting the ICE (HE2);• ICE cooling circuit as hot source in the evaporator for the ORC(R245fa as working fluid).

SystemLayout

Fixed objectives:• ---- 450 m3/min at 120°C, supposed to dry ---- 2,5 kg/s of rice (initial moisture content of 26%). • ---- Constant flow rate of 0.5 kg/s of rice husk biomass in the gasifier (15% of moisture content). • Gasifier equivalence ratio = 0.3

db %[w/w] daf %[w/w]

VM(VolatileMatter)

54,40 68,51

FC(FixedCarbon)

25,00 31,49

A(Ashes) 20,60 -C 31,44 39,60H 4,76 6,00O 42,64 53,70N 0,56 0,70

LHV [MJ/kg] 12,36 15,57

HHV[MJ/kg] 13,40 16,80

RiceComposition

ORCCharacteristics

• Cooling water temperature variation fixed from 82°Cto 92°C.

• R245fa chosen as working fluid.• Cooling water at the condenser varies between 20°C

and 30°C.• Pressure levels: 8 bar for the evaporation and 2.5 for

condensation.

Property Value

Chemical Name Pentafluoropropane

Chemical NotationCF3CH2CHF2 (1,1,1,3,3-pentafluoropropane)

Critical Temperature (°C) 154.01

Critical Pressure (bar) 36.51

Liquid Density (kg/m³) 0°C 1338.54

ANSIASHRAE Standard 36-1992SafetyGroup Classification B1

RelativeMolarMass (kg/mol) 134.05

Vapour Density (kg/m³) 25°C 8.55

GWP 1030

ODP Non-ozone depleting

REACH Approved

ATEL/ODL (kg/m³) 0.19

Practical Limit(kg/m³) 0.19

Low Flammability Level Non flammable

AIT(°C) ND

Item Specification

ASHRAE# R-245fa

Class Molecule

Refrigerant Type SingleComponent Fluid

Typical Lubricant POE

Replaces CFC-11

NotesEquipment redesign, ORCheattransfer fluid

Economic analysis

tot

tot Operation

JSPB=R -C

Specificcostofgasifier+ICE cGas+ICE 3500€/kW Specificcostofnaturalgascnat.gas 0.400€/Sm3

SpecificcostofORCcORC 4000€/kW LHVnaturalgas 34.5MJ/Sm3

Incentivepriceofelectricity(DM06/2016)csell 0.14€/kWh Specificdisposalcostofashcdisp.ash 100€/ton

Purchasepriceofelectricitycpurch 0.06€/kWh Yearlyhoursofoperation 2080h

Specificdisposalcostofricehuskcdisp.husk 200€/ton ConventionalcombustionchamberefficiencyηCC

97.0%

The profitability of the system is assessed by estimating the Simple PayBack, expressed by the ratio between the totalinvestment cost Jtot and the sum of operating costs and economic savings.

tot O&M disp,ahs.

disp,ahs. ash disp.ash

O&M tot

C =C +CC =m ×cC =0.05×J

⎧⎪⎨⎪⎩

tot Dry Gas.+ICE ORC HE

Gas.+ICE nom.ICE Gas+ICE

ORC nom.ORC ORC

J = J +J +J +JJ =P c

J =P c

⎧⎪

⋅⎨⎪

⋅⎩

&

&

tot el,sell el,av.purch th,av disp.av.

el,sell el,net sell

el,av.purch selfcons puch

disp,av. husk disp.husk

R =R +R +R +RR =E ×c

R =E ×cR =m c

⎧⎪⎪⎨⎪⎪ ⋅⎩

Referring technology :traditional biomass butch dryer

Gasifier ModelValidationReliability of the gasifier model assessed considering different initial biomasses. Syngas composition (equivalence ratio = 0.3) compared to experimental measurements and 0D thermo-chemical equilibrium model* results.

* Costa M, La Villetta M , M assarotti N , Optimaltuning of a thermo-chemical equilibr iummodel for downdraft biomass gasif iers,Chem Eng Trans, 2014, 37:343-348.

Results

ICE ORCPower output: 1150 kWel Power output: 76,3 kWelElectrical efficiency: 27.96%. Thermal efficiency: 6.5%

Dryer air final temperature:124 °C

%[v/v] %[w/w]

CO 22,58 26,84

CO2 13,76 25,71

H2 25,58 2,2

CH4 0,0006 0,0004

N2 38,07 45,26

Syngas species composition (daf basis):

InvestmentcostsJDry JGas.+ICE JORC JHE JTot SPB

494k€ 4.03k€ 305k€ 88.7k€ 4.91M€

5.32YearlyrevenuesRel,sell Rel,av.purch Rth,av Rdisp,av RTot

347k€ 4.37k€ 90.6k€ 734k€ 1.13M€

OperationalcostsCO&M Cash,disp CTot246k€ 7.49k€ 253k€

Economic analysis

The SPB calculated amount to 5.32 years, which canbe considered competitive.

Geometricalfeature- Standardaxialflow–

(GasifierE.R.=0.3) HE1 HE2

UA 1.60kW/K 0.7kW/K

Heattransferarea 189m2 88.7m2

Corelength 84.1cm 210cm

Corewidth 114cm 106cm

Coredepth (stackheight) 63.3cm 60.3cm

No.oflayerperexch. 63 60

Parametric Study

If the equivalence ratio of thegasifier increases, CH4, H2 and COreduce, leading to a reduction of therelated syngas LHV and to anincrease of the gasificationtemperature (approachingstoichiometric conditions).

The reduction of the ICE electricalefficiency is accompanied by anincrease of the thermal energycontent of the exhaust gases.

The ORC system, working at thesame efficiency, produces lesselectrical power as the gasifierequivalence ratio increases, due to areduction in the flow rate of theworking fluid.

The air that is supposed to enter thedryer increases from 83.5 °C to 162.4°C thanks to the heat transfer thatoccurs in the two heat exchangers: theincrease in the syngas temperaturehas a stronger effect with respect tothe slight reduction that occurs in theexhaust gases energy.

Conclusions

The CHP system fuelled with syngas by the rice husk gasification coupled with apuddy dryer is analyzed from a thermodynamicand economic point of view:• the system is perfectly capable to produce the required desiccant current for

puddy drying process, in terms of both temperature and mass flow rate of hotair.

• a parametric study of the power output of both the ICE and the ORC isperformed as a function of the increasing equivalence ratio of the gasifier,showing a reduction of the electric efficiency of both the systems, and anincrease of the thermal efficiency of the ICE.

• due to the high capacity of the dryer, which guarantees a high syngasproduction and consequently a good net production of electricity, the system ischaracterizesby a very good value of SPB, equals to 5.32 years.