Fuels and Fuel Processing - Värmeöverföring

Fuel Cell Technology

Fuels and Fuel Processing

Docent Jinliang YuanNovember, 2008

Department of Energy Sciences Lund Institute of Technology (LTH), Sweden


SOFC-Gas Turbine Hybrid Power Plant


Fuel Cell System

Source: Presentation from Monash University, Australia, 2001


Automotive PEM-System Operated On Gasoline

Adapted from Volkswagen presentation, 2002Adapted from Volkswagen presentation, 2002

AA

CCAir

Water

Fuel

+ H

2S Mixingchamber ATR HTS H2S-trap LTS PrOx/Selox

Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C

Water (For Cooling)St

eam

and

air

Exhaust

Bypass at high COBypass at high CO

Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP

~ 0.2 kg/kW ~ 0.2 kg/kW

~ 0.1 kg/kW(including H2S-trap)

~ 0.3 kg/kW ~ 0.2 kg/kW


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C


eam

and

air

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C


eam

and

air

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP

~ 0.2 kg/kW ~ 0.2 kg/kW


~ 0.3 kg/kW ~ 0.2 kg/kW


Pathways to Hydrogen

Source: Presentation from Monash University, Australia, 2001

Industrial coproducts


Hydrogen Production

•48% of H2 is produced from natural gas, 30% from petroleum (mostly consumed in refineries), 18% from coal, and the remaining 4% by water electrolysis;

•H2 costs around $5.60 /GJ when produced from natural gas, $10.30 /GJ from coal, and $20.10 /GJ from electrolysis of water

•About half of the H2 produced is consumed in the refineries, 40% is used in ammonia synthesis, and the rest in the manufacture of methanol and in other processes


Fuel Specification Required by Different Types of Fuel Cells

a In reality CO reacts with H2 O to produce H2 and CO2 by water gas shift reaction and CH4 reacts with H2 O to form H2 and CO faster than reacting as a fuel at the electrode. b

A fuel in the internal reforming MCFC and SOFC. c CO2 is a poison for AFC.


Fuel Processing Trains


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C

Water (For Cooling)

Stea

m a

nd a

ir

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP

~ 0.2 kg/kW ~ 0.2 kg/kW


~ 0.3 kg/kW ~ 0.2 kg/kW


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C

Water (For Cooling)

Stea

m a

nd a

ir

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C

Water (For Cooling)

Stea

m a

nd a

ir

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP

~ 0.2 kg/kW ~ 0.2 kg/kW


~ 0.3 kg/kW ~ 0.2 kg/kW


Feedstocks for H2 Production

•Natural gas, gasoline, diesel, methanol, ethanol, ammonia, and biomass are common feedstocks (renewable or non-renewable) will be discussed.

Methane is usually the major constituent of natural gas (NG), ranging between 75–96 vol.%.

Other hydrocarbons (ethane, propane etc.), nitrogen, hydrogen sulfide, water, and carbon dioxide presented may pose particular processing difficulties.

NG is the most abundant and cleanest primary fuel, and the cheapest source of industrial H2 production by steam reforming.


••Liquid Petroleum Gas (LPG),Liquid Petroleum Gas (LPG), main component propane, having good energy density at low storage pressure, and available worldwide, no pollution.

•Liquid Hydrocarbon Fuels: Gasoline and Diesel, the most popular fuels available, high energy density of gasoline and diesel (diesel LHV is 42.5 MJ/kg while methanol LHV is only 19.9 MJ/kg, reforming temperatures higher than 700oC.

••Alcohols:Alcohols: Methanol and Ethanol are convenient for storage, especially for portable applications:

Methanol is produced via reaction of CO and H2, reformed at a relatively low temperature (250oC), high H2-to-carbon ratio, no C-C bond (minimizing the risk of soot formation), high H2 content in the product stream (up to 75%), and no sulfur in the fuel.

Ethanol produced by fermentation of starch or sugar or lower-cost vegetation such as crop wastes with competitive price with petroleum fuels, catalyst reforming reaction is possible but ethanol-air mixtures are flammable over a wide composition range.


•Ammonia generates H2 from a single-step decomposition, and is synthesized by catalytically reacting H2 with nitrogen under high pressure, can be kept in liquid form with acceptable energy density (18.6 MJ/kg, LHV). It does not contain carbon atoms, coking is not a concern. Trace ammonia on the anode and membranes limited in PEMFC applications.•Biomass is renewable resource of organic matters, such as forest and mill residues, agricultural crops and wastes, wood and wood wastes, animal wastes, livestock operation residues, aquatic plants, fastgrowing trees and plants, and municipal and industrial wastes. Biogas, mainly methane and carbon dioxide by hydrogasification, anaerobic digestion, or pyrolysis.

Methanol can be synthesized from the syngas obtained from biomass/biogas. Ethanol can be produced by biomass fermentation, and then used as a

H2 source. The production of H2 from sugar by catalytic reactions has been

demonstrated.


Bio-Energy

•

Bio-energy

(or biomass energy) is a renewable energy produced from organic matter

and achieved

by a conversion of the complex carbohydrates in organic matter to energy.

•

Organic matter may either be used directly as a fuel

or processed into liquids and gases.

• Wood

is the largest source of bio-energy and has been used to provide heat for thousands of years.

• Cultivation of energy crops, such as fastgrowing trees and grasses, which are so- called bio-energy feed-stocks.


Bio-Fuel

• Gaseous, liquid or solid fuels produced from raw biological material or bio-mass through gasification, decomposing or fermentation.

•

Heat can be used to chemically convert bio-mass into a fuel, which can be burned like petroleum to generate electricity. Bio-mass can also be burned directly to produce steam for electricity production or manufacturing processes.

• Above processes are cheap but wasteful uses of the resource, and result in deforestation and detriment to human health.


Two-stage processes for bio-mass gasification with typical temperatures


Typical bio-fuel gas composition

Bio- fuels

Landfill gas (LFG)

Anaerobic digestion gas (ADG)

BG (downdraft)

Composi -tion

57% CH4 , 42% CO2 ,

0.5% N2 , 0.2% H2 , 0.2% O2

50-70% CH4 , 30-40% CO2 , 1-10% N2 , 0-

1% H2

40-50% CH4 , 22-

27% CO, 10-15% H2 , 10-15% CO2

Traces

Alkanes, aromatics,

clorocarbons, sulphur

compounds, higher

hydrocarbons

O2 , sulphur compounds

(up to 3000 ppm

H2 S),

halogens, organics

Heavy hydrocarbon, particulates


Power output from a SOFC running on bio-gas at 850 oC

•

Steady increase in power output from the cell over the range of 15-45% methane content in the bio-gas, due to an effective consumption of the methane and carbon dioxide.

• With high methane contents (over 45%)

and low

levels

CO2, catalytic methane decomposition

happens


•In general, the higher the operating temperature of the fuel cell, the more tolerant to the impurities in the feed.

•SOFC and MCFC can utilize carbon monoxide via the water gas shift reaction, while a very small concentration of carbon monoxide can poison the PEM fuel cell.

•PAFC is insensitive to CO2 , AFC is very sensitive to the acid gas that results in carbonate formation in the alkaline electrolyte.

•MCFC require the presence of CO2 in the cathode to compensate for the carbonate transfer to the anode.

•PAFC is found to be the most tolerant toward sulfur presence in the feed.

Fuels VS. Fuel Cells


•Desulfurization: Hydrodesulfurization (HDS), Sulfur Removal by Adsorption

Fuel Processing

•Fuel Reforming: Steam Reforming, Partial Oxidation, Autothermal Reforming

•Water-Gas Shift Reaction

•Carbon Monoxide Removal: Preferential Oxidation, Methanation, Membrane Separation

•HD-5 Propane Processing for Direct Carbonate Fuel Cell Applications


•Hydrocarbon fuels contain significant amounts of sulfur containing compounds, e.g., sulfur levels in diesel and gasoline are around 500 and 300 ppmw, respectively.

Desulfurization

•Sulfur is added to NG for safety purposes

•Presence of sulfur compounds at certain concentrations has a big effect on the performance of catalysts in fuel processor and the fuel cell stacks.•In contrast to CO poisoning, adding Ru to Pt to the anode catalysts has no effect on increasing MEA tolerance toward H2 S poisoning.•Hydrocarbon fuels fed to PEMFC need to be desulfurized to concentrations less than 0.1 ppm.

Details skipped here…


General Reforming Process

ReformerFuel

WaterAir

Products(Reformate or Syngas)

H2 , CO2 , CO, CH4, N2

Cn Hm Op

Cn Hm Op + xO2 + (2n-2x-p)H2 O = nCO2 + (2n-2x-p+m/2)H2 Cn Hm Op + xO2 + (2n-2x-p)H2 O = nCO2 + (2n-2x-p+m/2)H2

–ΔHr = nΔHCO2 – (2n-2x-p)ΔHH2O – ΔHfuel –ΔHr = nΔHCO2 – (2n-2x-p)ΔHH2O – ΔHfuel

General Reforming Reaction (Developed By Argonne National Laboratory):

Enthapy of Reaction:

Definition: The process of converting liquid or gaseous hydrocarbon fuels into a gas consisting of mainly

hydrogen and carbonmonooxide (syngas).

(Chemical Reactor)


Reforming IssuesReforming Issues• Hydrocarbon into hydrogen by reforming

(negligible NOx formation) • Steam reforming- hydrocarbon+CO+steam

reformed into CO2 +H2 ; • Partial oxidative reforming (POX)-partial

combustion of hydrocarbon; • Autothermal reforming -heat release of

combustion to balance steam reformation process (more responsive to unsteady load than SR, and higher hydrogen conversion efficiency than POX reformation)

• Reformation temperature is important for fuel choice - methanol@200-300 oC attractive for PEMFCs, gasoline/diesel @800 oC for SOFC).


Integration Methods in Reformers

Options Integration Methods

In situ heat generation via a coupled reaction

Chronologically segregated heat generation, regenerative

heat exchangeSpatially segregated heat

generation, recuperative heat exchange


Integration Methods in Reformers

Options Integration Methods

In situ

Reactants for both reactions are mixed; All the reactions run in parallel;Direct heat transfer appears within the reactingmixture.

Chronolo- gical

segregation

The fixed bed heated during the exothermic cycle;Stored heat consumed in the endothermicprocess, and the fixed bed is cooled.

Spatial segregation

Catalytic reactor is comprised of closely spacedand catalytically coated thin metal plates;Heat transfer resistances between reaction sitesare effectively decreased.


Chemical Reactions

CH4 + H2 O → CO + 3H2 , ∆h= 226 kJ/mol CO + H2 O → CO2 + H2 , ∆h= -41

kJ/mol

CH4 + 2H2 O → CO2 + 4H2 , ∆h= 165

kJ/mol

Heat balance is crucial for local reactions and temperature gradient/thermal stress.

CH4 + 2O2 → CO2 + 4H2 O, ∆h= -800.8

kJ/mol


Reforming Reactions and Mechanisms

The asterisk refers to active sites

Equilibrium steam reforming product composition using a steam-to-carbon

ratio of 2 and at atmospheric

pressure


Pre-reforming and Others

•To convert heavier hydrocarbons to a mixture of smaller molecules, such as CH4 , H2 and carbon oxides at low temperatures.

•This methane-rich gas mixture can then be used directlyas feedstock to internally produce H2 in some fuel cells, called a direct carbonate fuel cells (SOFCs and MCFCs).

• Possible steam reforming of propane, alcohols (methanol and ethanol), etc. Steam reforming of fossil fuel mainly produces H2 and carbon monoxide, steam reforming of alcohols mainly produces H2 and carbon dioxide. At room temperature, steam reforming of methanol requires about four times less heat than natural gas.


Partial Oxidation•Partial oxidation (POX), an exothermic reaction, occurs when the feed reacts directly with air or pure oxygen.

•POX can reform liquid hydrocarbons such as gasoline, diesel fuel and heavy fuel oil even without the use of a catalyst.

•Non-catalytic partial combustion of the hydrocarbon feed with oxygen in the presence of steam in a combustion chamber occurs at flame temperatures between 1300 and 1500oC, and water is not reactant - a simpler system but less H2 .


•Low H2 partial pressure is also due to the introduction of nitrogen into the stream, when air is used as the oxygen source in partial oxidation.

•Low H2 partial pressure in the anode causes lower efficiencies.

•POX does not scale down well, it is not suitable for fuel cell mobile applications, which prefer compact fuel processors.

POX Limitations & CPO

•Catalytic Partial Oxidation (CPO), using catalysts and flameless, can reduce the operating temperature by hundreds of degrees enabling the use of inexpensive materials of construction and reducing the CO concentration in the reformate.


•COP catalysts may contain either noble metals (Pt, Pd, and Rh) or non-noble transition metals (Ni, Cu, Fe, and Co) supported on zirconia, ceria, or alumina and they can be active at temperatures as low as 450–850°C depending on the feedstock.

•COP experiences some major operating problems such as over-heating or hot spots due to the exothermic reactions.

•Coking may be a critical issue.

COP


Autothermal Reforming•Combination of POX and steam reforming.

•The fuel is mixed with steam and sub-stoichiometric amounts of oxygen or air, where the ratios of oxygen to carbon (O:C) and steam to carbon (S:C) are properly adjusted so that the partial combustion supplies the necessary heat for endothermic steam reforming.

•The autothermal reformer consists of two zones; the thermal zone and the reforming zone.

•The reactions can either be run in a single reactor or in separated reactors that are in good thermal contact.


Water-Gas Shift Reaction (WGSR)•CO concentration in the reformate varies with the reforming type and feedstock used.

•CO is poisonous in PEMFC and PAFC. The Pt preferentially adsorbs CO, and blocks access of H2 to the catalytic sites and results in significant decreased performance.

•In WGSR, CO reacts with excess steam to form CO2 and H2 to increase H2 yield, and decrease poisonous CO.

•Simultaneous WGSR and steam reforming reaction in the reformers, but should be run at lower temperatures. Separate ones, such as HTS and LTS…


•At low temperature (315–400°C), the equilibrium of HTS is shifted toward CO conversion to CO2 , and 80 to 95% conversion is obtained.

•The reaction mixture’s temperature increases due to the exothermic nature of the reaction leading to enhancement in the reaction rate, however, adversely affecting the equilibrium.

•To obtain a satisfactory CO conversion, the reaction mixture is cooled again via an inter-stage cooler and supply to a second stage (LTS) at temperature 200–250oC.

HTS & LTS


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C

Water (For Cooling)

Stea

m a

nd a

ir

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP

~ 0.2 kg/kW ~ 0.2 kg/kW


~ 0.3 kg/kW ~ 0.2 kg/kW


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C

Water (For Cooling)

Stea

m a

nd a

ir

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP


AA

CCAir

Water

Fuel

+ H


Cat.-Burner

PEM-FC

Condenser

Air

Air

400°C 800°C 400°C 350°C 200°C 150°C

80°C

650°C

Water (For Cooling)

Stea

m a

nd a

ir

Exhaust


Fuel

incl

udin

g su

lphu

r

DeS Preheater

Evaporator

Superheater

GT

CP

~ 0.2 kg/kW ~ 0.2 kg/kW


~ 0.3 kg/kW ~ 0.2 kg/kW

Two methane Two methane WGSRsWGSRs bring the CO bring the CO concentration down to around 0.5% (ok for concentration down to around 0.5% (ok for

PAFCsPAFCs but not for but not for PEMFCsPEMFCs). Methanol ). Methanol reformate may contain as little as 0.1%, still reformate may contain as little as 0.1%, still

high for high for PEMFCsPEMFCs. . Extra CO remover??Extra CO remover??


Hydrogen Fuel Storage and Delivery

•Roadblock to mass commercialization of fuel cell applications;

•Several ways in near future: compressed-, liquefied-hydrogen, metal hydride or hydrocarbon (released through a reformation process);•High-pressure compressed storage-upto 34 MPa tanks, but difficult market acceptance due to safety and low density (31g/L at 34MPa);

•Liquid hydrogen-high density (71g/L) but evaporation /health concern;

•Metal hydrides (most used) releasing at high temperature, but too heavy/cost /refueling time/sensitive to impurities.


LoadingLoading (while submarine in harbour)

Cooling (by harbour cooling device)

H2

UnloadingUnloading (while submarine on mission)

Heating (by waste heat from FC)

H2• Hydrogen storage in metal hydrides

• Loading/unloading of Hydrogen


•Partial oxidation reformers are more suitable for short startup applications than steam reformers because of low weight, rapid starting, compact, and dynamically responsive.

•Steam reformers have higher energy efficiency due to the H2 generation from water molecules, which makes them more attractive for long startup applications.

Final Remarks•Development of coke-resistant catalysts makes it possible to process heavy hydrocarbon oil. Most steam-reforming catalysts are also good catalysts for POX. But most commercial reforming catalysts are (Ni- or Cu-based catalysts) for stationary processes and not for onboard application (compact and stable at higher space velocities).

•The activity of catalysts depends on the particle surface area.

•Commercial Ni-based reforming catalysts are usually active at temperatures above 450°C.

Fuels and Fuel Processing - Värmeöverföring

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