Chapter 11

Fuel Processing Subsystem Design

Lecture NotesDr. Sammia Shahid

Why is Fuel Cell Technology Important?

Why is Fuel Cell Technology Important?

Since conversion of the fuel to energy takes place via an electrochemical process, not combustion

It is a clean, quiet and highly efficient process- two to three times more efficient than fuel burning.

Fuel Cell

Fuel Cell Stack

Importance of Hydrogen

Fuel Cells require highly purified hydrogen as a fuel

Researchers are developing a wide range of technologies to produce hydrogen economically from a variety of resources in environmentally friendly ways

Importance of Hydrogen

Hydrogen is a secondary energy resource, meaning it must be made from another fuel

Hydrogen can be produced from a wide variety of energy resources including:

Fossil fuels, such as natural gas and coal Nuclear energy Renewable resources, such as solar, water, wind and biomass

Hydrogen Production

The biggest challenge regarding hydrogen production is the cost

Reducing the cost of hydrogen production so as to compete in the transportation sector with conventional fuels on a per-mile basis is a significant hurdle to Fuel Cell’s success in the commercial marketplace

Hydrogen Production

There are three general categories of Hydrogen production

Thermal ProcessesElectrolyte ProcessesPhotolytic Processes

HYDROGEN GENERATION

PROCESSES Steam reforming of Natural Gas/Naphtha Partial oxidation of hydrocarbons Thermal cracking of Natural Gas Coal/Bio mass Gasification Electrolysis – Electricity from renewable sources like

solar, wind, hydel etc.

HYDROGEN PRODUCTION

World wide production From Natural gas (mostly steam reforming) - 48% Oil (mostly consumed in refineries) – 30% Coal – 18% Electrolysis –4%

Nearly all H2 production is based on fossil fuels at present.

H2 OPTIONS FOR PAKISTAN

Hydrocarbon Liquid Fuels Natural GasSolar / Wind power for electrolysisCoalBio-massOther options like Chlor-Alkali Units & Co-generation

electricity from Bagasse at sugar mills

STORAGE OPTIONS

Storage as gas under pressure (250 – 350 bar)

Cryogenic storage as liquid hydrogen (Temp. –253 0 C)Storage as metallic hydridesCarbon adsorption and glass microsphere storage techniques (under development)

Fuel Processing

Reformer

A vessel within which fuel and other gaseous recycle stream(s) are reacted with water vapor and heat, usually in the presence of a catalyst, to produce hydrogen-rich gas for use within the fuel cell. There are a number of different sources of hydrogen and methods for reforming.

Steam reformationFuel reacts with water over a catalystRequires heat input

Partial oxidationFuel reacts with air (w/o catalyst)Typically produces heat

Auto-thermalCombination of above (fuel, water, air)No net heat required or produced

Other – e.g., CyclicOxidation states of metalsAlternatively pass fuel and oxidant over parallel bedsCalcium oxide, calcium carbonate for carbon

absorption

Fuel Processing

Fuel Processor

Fuel Processing Subsystem

Fuel processing subsystem will probably consists of at least three primary reactor processes:

1.Fuel Reforming (labeled no. 3)

2.Water gas shift reaction (labeled no. 4)

3.Carbon monoxide clean-up (labeled no. 5)

Fuel Processing Subsystem

The overall goal of fuel reforming is to convert a hydrocarbon (HC) fuel into a hydrogen-rich gas. The primary conversion may be accomplished with or without a catalyst via one of five major types of fuel-reforming processes:

1.Steam reforming (SR)

2.Partial oxidation (POX) reforming

3.Auto thermal reforming (AR)

4.Gasification

5.Anaerobic digestion (AD)

Fuel Reforming Overview

Steam Reforming

Steam reforming is endothermic reaction:

CxHy + x H2O x CO + (x + 1/2y) H2

The SR typically has a H2 yield of 76% on a dry basis.Catalytic system: CuO/ZnO, CuO/SiO2, CuO/ZnO/SiO2

For Methanol: 250-2600C

Ethanol : >3000C

Advantages : Maximum Hydrogen generation

Disadvantages: Indirect Heat transfer

Partial Oxidation Reforming

POX is an exothermic reaction that combines a HC fuel with some oxygen to partially oxidize the fuel into a mixture of CO and H2. e.g. the complete combustion of propane is:

C3H8 + xO2 yCO2 + zH2O

Advantages: Any type of hydrocarbon Direct Heat transfer

Disadvantages:

Low H2 production

Dilution of gas with N2

Soot formation

Auto thermal Reforming

Combination of Steam Reforming and partial oxidation and water gas shift reaction in a single process.

CH4 +H2O CO+3H2 H=+206.16 kJ/mol

CH4+1/2O2 CO+2H2 H= -36 MJ/kmol

Reactions are balanced in such a way net energy

requirement is Zero (∆H = 0)

C + 1/2O2 CO2 C + H2O CO2 + H2

C + CO2 2COCO + H2O CO2 + H2

CO + 3H2 CH4 +H2O

Gasification of solid fuels

Stationary fuel cell systems may also utilize fuel gases produced from solid fuels through a process known as gasification. The process of gasification typically reacts a solid fuel containing carbon (such as coal) at high temperature (700 1400°C) under pressure with O2 and H2O to produce H2, CO2, CO and other gases.

Catalytic decomposition

CH3OH 2 H2 + CO

• Mostly suitable for Alcohols

• Soot formation and carbon deposition for Hydrocarbons

Desulphurisation

Gas phase Desulphurisation

ZnO + H2S ZnS (s) + H2O (g)

2 to 3 kg ZnO sufficient for one year Automobile operation

Liquid Fuel Desulphurisation

Gasoline 30-40 ppm 1-2ppm Sulphur

For high sulphur Fuels hydro treatment followed by gas phase

Desulphurisation

Adsorption Chemical reaction

High temperature & Low temperature Shift reaction

CO + H2O CO2 + H2

HT Shift: Iron and Chromium Oxide Catalyst.

Temperature 350-4500C

LT Shift: Copper and Zinc Oxide Catalyst.

Temperature 200-2500C

Carbon Monoxide Clean-up

Chemical• Preferential oxidation

CO + ½ O2 CO2

H2 + ½ O2 H2O Pt, Pd, Rh, Ru are catalysts (~ 1200C)

• Methanation CO + 3 H2 CH4 + H2O Ru, Rh are catalysts. Temperature 100-2200C Physical

• Pressure Swing Adsorption (PSA)• Membranes-Metal or polymeric • Solvent Absorption

Pressure Swing Adsorption

Pressure Swing Adsorption (PSA) processes rely on the fact that under pressure gases tend to be attracted to solid surfaces, or adsorbed. The higher the pressure, the more gas is adsorbed; when the pressure is reduced, the gas is released, or desorbed. PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly. If a gas mixture such as air, for example, is passed under pressure through a vessel containing an adsorbent bed that attracts nitrogen more strongly than it does oxygen, part or all of the nitrogen will stay in the bed, and the gas coming out of the vessel will be enriched in oxygen. When the bed reaches the end of its capacity to adsorb nitrogen, it can be regenerated by reducing the pressure, thereby releasing the adsorbed nitrogen. It is then ready for another cycle of producing oxygen enriched air.

Pressure Swing Adsorption

Pressure swing adsorption is known to be one of the most economic and widespread commercial processes for hydrogen purification.

It can produce 99.99% pure hydrogen.

A PSA unit operates with atleast two adsorption beds.

Each adsorption process is batch process.

Primary Reformer

CO + H2O = CO2 + H2

- - - - - - - - - - - - - - - - - H2

FuelAirH2O

H2,CO,N2H2, CO2, N2

To Fuel Cell

Fuel Processor using Membrane Reactor

Novel Reformer Technologies

Solvent enhanced reforming

Calcium Oxide along with steam reforming catalyst is added. Composition 90% H2, 10% CH4, 0.5% CO2 and <50ppm CO

Downstream processing load is reduced. Ion transport membrane reforming

Oxygen on one side of the membrane (1-5 psig) Methane & steam on the other side of the membrane (100-

500 psig)

Plasma Reformers

HT plasma (3000-100000C) is generated by electric arc in

plasmatron

Heat Exchangers

A device used to transfer heat from a fluid (liquid or gas) to another fluid where the two fluids are physically separated. Heat exchangers are needed to cool fuel cells and maintain a consistent operating temperature. They are used primarily by the lower-temperature fuel cells (AFC, PEMFC and PAFC).The typical cooling mechanism is water. There are a number of types of heat exchangers:

Gas - Gas Gas - Liquid Condensers Humidifiers

Some of the development areas to date are: Size and weight reduction (transportation markets) Heat transfer and size (portable power markets)

Power Conversion and Electronics

Fuel cells produce direct current or DC power. Today, most of our buildings and appliances require alternating current or AC power to operate.

Consequently, a conversion device is necessary as part of the fuel cell system.

There are in fact many "appliances" that can or do use DC power, such as computers and lighting.

Today, the issue with this technology is to reduce the efficiency losses, increase the reliability, and the cost.

General Power Electronics Capabilities

Convert direct current (DC) to alternating current (AC) when required. Control current and/or voltage Feedback to control system Surge and short-circuit protection Connect to and manage energy storage Connect to and/or control loads

Dedicated load Motor controller, motors

Synchronize Electrical grid Other generators

Balance of PlantBalance of plant refers to supporting and/or auxiliary components based on the power source or site-specific requirements and integrated into a comprehensive power system package.

FiltersFilters remove solid material from a medium, such as a gas (air) or fluid. In the case of air, filters can be installed as part of a heating/cooling system through which air flows for the purpose of removing particulates before or after the air enters the mechanical components. In the case of a liquid or gas, it could be a pre-step to reformation (e.g. sulfur removal).

Auxiliary Components

Seals and Gaskets

A gasket or seal is used to prevent the leakage of fluids and to maintain the pressure in an enclosure. Specifically, a gasket is used between two static surfaces to provide a seal. Sealing is an important issue for all applications. Some of the developmental areas today include: High temperature seals

SOFC and MCFC applicationsFuel processing components

Good performance versus: Load swingsThermal gradientsRapid transients (pressure, temperature)Multiple cycles

ValvesA device used in piping to control the fuel supply to any section of a system of piping or to fuel utilization equipment.

Automatic. A device consisting essentially of a valve and operator that controls the fuel supply to the burner during normal operation of the equipment. The operator may be actuated by application of fuel pressure on a flexible diaphragm, by electrical means, by mechanical means or by other means.

Diaphragm Type. A device consisting essentially of an automatic valve actuated by means of the application of fuel pressure upon a flexible diaphragm.

Electric Type. A device actuated by electrical energy for controlling the fuel supply. These consist of the following:Modulating. A valve designed so the valve opening is controlled within narrow limits throughout the entire range from the "full open to the "closed" position.Motor. An electric control valve that is automatically closed by a spring or other mechanical means in the event the electric circuit is broken.Solenoid. A valve that is opened or closed by the action of an electrically excited coiled wire magnet upon a bar of steel attached to the valve disc.Step (Manual). A valve having a rotating plug usually with three positions and different rates of fuel flow for each. The plug is actuated by a solenoid or motor-driven rack and pinion and a cam arrangement. This apparatus and a combination push-button switch determine the position assumed by the plug.

Valves

Burner. A manually or mechanically operated valve which permits control of the flow of fuel.

Combustion (input) Control. An automatic control valve for regulating fuel input.

Latching Type. A manual gas valve which requires at least two separate actions or movements to turn the valve on, as for example, pushing in on the valve handle to unlatch the valve before the valve handle can be rotated to turn on the fuel.

Lubricated Plug. A valve of the plug-and-barrel type that has bearing surfaces designed to be re-lubricated without disassembly of the valve.

Main Burner, Individual. A valve that controls the fuel supply to an individual main burner.

Valves

Valves

Semi-Automatic. A valve that is opened manually and closed automatically, or vice versa.Shutoff, Manual. A manually operated valve in a fuel line for the purpose of completely turning on or shutting off the fuel supply to fuel utilization equipment.Shutoff, Manual Main. A manually operated valve in the fuel line for the purpose of completely turning on or shutting off the fuel supply to fuel utilization equipment, except to a pilot provided with independent shutoff valves.Shutoff, Safety. A valve that is automatically closed by the safety control system or by an emergency device. Such valve may be of the automatic or manually opened type.

TYPES OF FUEL CELLS

Temp.°C Application• Alkaline (AFC) 70-90 Space• Phosphoric Acid 150-210 Commercially available

(PAFC) • Solid Polymer 70-90 Automotive application (PEMFC)• Moltan Carbonate 550-650 Power generation

(MCFC)• Solid Oxide 1000-1100 Power generation

(SOFC)• Direct Methanol 70-90 Under development

(DMFC)