producción de hidrógeno a partir de energías renovables_a.monzón

1CATALYSIS FOR ENERGY: NEW CHALLENGES FOR A SUSTAINABLE ENERGETIC DEVELOPMENT. 18th to 20th August, Palacio de la Magdalena Santander

Hydrogen Production from Renewable Resources

Prof Antonio MonzónProf. Antonio MonzónDepartment of Chemical & Environmental Engineering

Institute of Nanoscience of Aragón (INA). University of Zaragozae‐mail: [email protected]

http://www.unizar.es/creg/



Outline Introduction to Renewable

Hydrogen ProductionHydrogen Production Electrolysis Biomass to Hydrogen Biomass to Hydrogen Fermentation Thermolysis Photolysis Conclusions



Renewable EnergyRenewable Energy

"Energy obtained from the continuous or repetitive currents of energy recurring in therepetitive currents of energy recurring in the

natural environment”

“Energy flows which are replenished at the same rate as they are used”



Renewable EnergyRenewable Energy"Energy obtained from the

ti titicontinuous or repetitive currents of energy recurring in the natural environment”in the natural environment

“Energy flows which are replenished at the samereplenished at the same rate as they are used”



What is the Focus?What is the Focus?

Environmental focus – Global Warming

Replace fossil fuels burned for electric power generation with renewable resources that have near zero net CO2with renewable resources that have near zero net CO2

Energy Security Focus

Replace imported oil with alternative fuels



World Energy Matrix: Present ScenarioMTonnes oil equivalent

Coal

NGNG

Oil

World primary energy consumption grew by 1.4% in 2008, below 10‐year average (the weakest year since 2001).Oil remains the world’s dominant fuel though it has steadily lost market share to coal and NG in recent yearsOil remains the world s dominant fuel, though it has steadily lost market share to coal and NG in recent years. Oil’s share of the world total has fallen from 38.7% to 34.8% over past decade.Oil consumption and nuclear power generation declined last year, while NG and coal consumption, as well as hydroelectricity generation, increased.


BP Statistical Review of World Energy 2009


Future Evolution of Fossil Fuels

• Oil Production will peak in this decade (Multicyclic Hubbertmethod) becoming critically low in 40‐50 years’ time.

• Natural Gas It will run out in 60 years’ time.

C l R ill t i 130 ’ ti• Coal Reserves will run out in 130 years’ time.

• NG and Coal will peak in ca. 40 years

* Note that they are key raw materials for the chemical industry and consumer products



Hubbert Curve: Peak Oil ProductionHubbert Curve: Peak Oil ProductionThe Hubbert peaktheory: “For any given geographical area, from an individual oil‐producing

www.almc.army.mil

individual oil producing region to the planet as a whole, the rate of petroleum production tends to follow a bell‐shaped curve” yQ(t)=Qmax/(1+a.exp(‐b.t))

tmax=ln(a)/b

World oil resources are on track to critically deplete within 40 years.

Oil has a host of useful industrial applications and to irreversibly burn oil jeopardizes the future.

* The vertical scale is in arbitrary relative units, but to get an idea of scale, world production averaged at about 80 million barrels per day in 2008.


D. Abbott, Proc. of the IEEE (2010)


What are the Options?

Non‐renewable/Non‐fossil Máximum Power

( ) *


Nuclear energy (fission) 2 TW*

Nuclear energy (fusion) Unpredictable* In 2008: 0 9 TW In 2008: 0.9 TW

Renewable Máximum Power

Biofuels: ethanol & biodiesel Food conflict

Biomass Unknown

River hydroelectric 7 TW

Ocean thermal 100 TW

Wind 72 TW

Geothermal 44 TW Geothermal 44 TW

Desert solar 7650 TW1TW=1012 Watt



Scale of the Problemf

The current population on the planet consumes 15 TW.

The total amount of solar energy utilized by all plant life is 90 TW where, 65 TW is from land plants and 25 TW is from algae.

Th l t E th fl t 30% f th i id t 166 PW l The planet Earth reflects 30% of the incident 166 PW solar power back into space and therefore the total power our planet absorbs is 116 PW.

The total power output of our Sun is enormous at 3.6x1026 W, and for our galaxy it is 5x1034 W.

The message here is that humankind’s energy consumption of 15 TW is absolutely tiny when compared to the typical power levels in our cosmos. Thus for our future energy needs we need to look to our stars, f f gy ,with our nearest one being the Sun.



Orders of Magnitude for Power (Watts)


Orders of Magnitude for Power (Watts)

1 Quadrillon BTU (1 Quad): the amount of energy in 45 million tons of coal or 1 trillion cubic feet of natural gas1 Quadrillon BTU (1 Quad): the amount of energy in 45 million tons of coal, or 1 trillion cubic feet of natural gas, or 170 million barrels of crude oil. In 1988, total world energy consumption was about 1 quad every 26 hours.1 Quad: 1015 BTU= 1.055x1018 J = 1.055 Exajoules




Conversion Factors for Current Hydrogen UnitsConversion Factors for Current Hydrogen Units

Weight Gas Liquid

pounds(lb)

kilograms(kg)

cubic feet(scf)

cu meters(Nm3)

gallons(gal)

liters(l)(lb) (kg) (scf) (Nm3) (gal) (l)

1 pound 1.0 0.4536 192.0 5.047 1.6928 6.408

1 kilogram 2.205 1.0 423.3 11.126 3.377 14.128

1 scf gas 0.00521 0.00236 1.0 0.02628 0.00882 0.03339

1 Nm3 gas 0.19815 0.08988 38.04 1.0 0.3355 1.2699

1 gallon liquid 0.5906 0.2679 113.4 2.961 1.0 3.785

R. Guerrero Lemus and J.M. Martínez Duart;d ( )

1 liter liquid 0.15604 0.07078 29.99 0.7881 0.2642 1.0

Scf (standard cubic foot) gas measured at 1 atmosphere and 70°F. Nm3 (normal cubic meter) gas measured at 1 atmosphere and 0°C.


Int. J. Hydrogen Energy, 35 (2010) 3929‐3936Liquid measured at 1 atmosphere and boiling temperature

Power Available From Renewable Sources


The current population on the planet consumes 15 terawatt (TW)




A Different View!

Usually it is accepted that "To make an energy fix, we need an energy mix“ The solution to the global energy supply problem is to diversify with a mix of power sources such as oil solar

ff

the global energy supply problem is to diversify with a mix of power sources, such as oil, solar, wind, biomass etc Energy policy of many governments around the globe.

From a world‐scale, long‐term perspective, the pertinent question to ask is: "Is there a single , g p p , p q gtechnology that can supply the world’s 15 TW power consumption in a clean sustainable way?”

A solar hydrogen economy has a sustainable and vastly higher total power output potential than all other sources combined focus on a dominant solution rather than a solution based on diversification of energy sources.

A S l H d E h ld b th fi l l f t li I th t iti A Solar Hydrogen Economy, should be the final goal of current energy policy. In the transitionperiod towards such a sustainable energy cycle, the current approach promoting a mix of energy sources is required.

However, having a dominant end vision can help to better analyze the most viable energy policy mix for the transition period.




Critical Importance of Energy Conservation

Supply of the world’s energy has long‐term viability if are put into place policies that i i

Critical Importance of Energy Conservation

create incentives to save energy.

See the following simple example Suppose in the near future we have 1 billion (109) domestic dwellings in the world.g

Basic assumptions: (i) dwellings have an average sized with a surface area of 500 m2, (ii) walls 0.1 m thick, (iii) each house is a cube with no doors or windows, (iv) thermal conductivity of brick material: 1 W/m K and (v) average T between inside and outsideconductivity of brick material: 1 W/m.K, and (v) average T between inside and outside (either heating or cooling): 5 ºC The power consumed will then be 25 kW /house25 TW total !!.

If it is used wall insulation, so that we drop the thermal conductivity to 0.1 W/m.K, we immediately save 22.5 kW/house 22.5 TW total saving!!, which is well over the current world power consumption!.




Primary Energy Overview 1949‐2008 (USA)Primary Energy Overview 1949 2008 (USA)

1 Quad BTU: 1015 BTU= 1.055x1018 J = 1.055 Exajoules


US Energy Information Administration / Annual Energy Review 2008


Critical Importance of Energy ConservationCritical Importance of Energy Conservation

It is absolutely vital to responsibly conserve as well as to responsibly generate energy on large scales.

“Think Globally, Act Locally”



Hydrogen as a Fuel


y g

Advantages

GHG reduction

Available all over the world

Reduction of local contamination (key factor in big cities)

Can be used in ICE or in FC: the latter, moreCan be used in ICE or in FC: the latter, more efficient, non‐contaminant, less noisy

Disadvantages

Low energy density (volume basis)

Requires either high pressure or liquefaction

High cost of production/purification (pollution)



Hydrogen Production AlternativesHydrogen Production Alternatives

lNatural Gas + Reforming

Hydro/Wind/Waste +Waste +

Electrolysis Natural Gas + Reforming & CO Removal Biomass +CO2 Removal Biomass +

Pyrolysis, Gasification, ReformingReforming

IntermediatesSolar +

Electrolysis

Today Future


y


Renewable Pathways for Hydrogen Production


Benefits of Hydrogen from Renewable Resources


Benefits of Hydrogen from Renewable Resources

Reduce dependence on imported oil and enhance energy security through domestic production

Reduce emissions of greenhousef ggases and criteria pollutants that affect air quality

A clean and secure energy future includes hydrogen as an energy carrier.



Lif C l A t f H d F l P d ti PLife Cycle Assessment of Hydrogen Fuel Production Processes

Eco‐Points of Electricity Generation Processes

El t i it G ti E P i tElectricity Generation System

Eco‐Points (LCA)

Lignite 1735gOil 1398

Carbon 1356Natural Gas 267Nuclear 672Eolic 65Eolic 65

Hydraulics (small) 5Solar Photovoltaic 461

* Eco‐point: A measure of the overall environmental impact of a particular product or process. More Eco‐points indicate higher environmental impact.


R. Guerrero Lemus and J.M. Martínez Duart; Int. J. Hydrogen Energy (2010)


CO2 Emissions for Different H2 Production TechnologiesCO2 Emissions for Different H2 Production Technologies

PV: The worst environmental performance due to the manufacturing process of PV d l ith hi h i t l i t Th ll ffi i f PV i lmodules, with a high environmental impact. The overall efficiency of PV is very low.


Guerrero Lemus & Martínez Duart; Int. J. Hydrogen Energy (2010) ; Koroneos et al., Int. J. Hydrogen Energy (2004)


Electrolysis of Watery f

The decomposition of H2O into O2 and H2 gas due to an electric current being passed through the water.

R i i d di l i (DC) f Requires no moving parts and a direct electric current (DC) one of the simplest ways to produce H2.

Is reliable and clean and when H O vapor is removed from the Is reliable and clean and, when H2O vapor is removed from the product, capable of producing ultra‐pure (> 99,999%) H2.

Electrolytic production of H2 with carbon‐free electricity sources is the y p f 2 f yonly way to produce large quantities of H2 without emitting CO2.

Typical commercial electrolyzer system efficiencies are 56‐73%, which corresponds to 70‐53 kWh/kg.



Electrolysis of Watery fThere are two basic types of low temperature electrolyzers:

(i) alkaline and (ii) polymer electrolyte membrane (PEM)

Alkaline electrolyzer

Well‐established technology.

Uses an aqueous solution of 25‐30 wt% KOH l l h d h iKOH as electrolyte, that conduct the ions between the electrodes.

Commercial alkaline electrolyzers work ywith current densities in the range of 100‐300 mA/cm2

Anode :

Cathode:


Cathode:



Proton exchange membrane (PEM)

A t th t i t lid t A system that incorporates a solid proton‐conducting membrane that is not electrically conductive.

h b h d l The membrane has dual purpose: (i) as a gas separation device, and(ii) an ion (proton) conductor.

High‐purity deionized (DI) water is required in PEM‐based electrolysis, but avoids the hazards surrounding KOH.

DI water is introduced at the anode of the cells, and a potential is applied across the cell to dissociate the water.

Anode :

Cathode:

The H+ are pulled through the membrane under an electric field and rejoin with e‐ being supplied by the power source at the cathode, to form H gas


Cathode:form H2 gas.



Proton exchange membrane (PEM) (cont.)

PEM electrolyzers are operated at higher current densities (> 1600 mA/cm2) 1 order of magnitude higher than alkalines.

Stack efficiency decreases as current density i b hi h d i iincreases, but high current density is necessary to increase H2 production to offset the higher capital costs of PEM cells.

PEM advantage over alkaline electrolyzers the capacity to maintain a significant P across the anode and cathode, avoiding the risk of high‐pressure O2. Anode :

Cathode:


Cathode:



Coupling renewable energy systems with H2‐generating electrolyzers has the potential to provide low‐cost, environmentally friendly electricity and H2.

Using available wind and solar energy offers a large potential for H2 production ff g p f 2 pvia electrolysis.

To meet the DOE cost goals of 2‐3 USD/kg (1.5‐2.3 €/kg); H2 production via electrolysis needs installations where electrolyzer capital costs are low, less than 800 USD/kW (606 €/kW); and to electricity that costs less than 0.055 USD/kWh (0 042 €/kWh)(0.042 €/kWh).




Alkaline and PEM electrolyzers are commercially l bl f ll l b d l h havailable from small laboratory models to high‐

production systems (higher than 2MW).

Active research areas in electrolysis include:y(i) Study of high pressure operation, with

reduced compression in H2 systems, and (ii) Operation at elevated temperatures to

i ffi i iimprove efficiencies.(iii) Integration of electrolyzers with renewable

energy systems

Two pathways can use wind energy as an electricity source: (1) Distributed: wind energy can be put onto the electric power system and then transferred

to the H2 generation point via the grid; or (2) Central i d l t i it b d t d H d id l t i it t th i d it(2) Central: wind electricity can be used to coproduce H2 and grid electricity at the wind site.


Central vs. distributed H2 production via electrolysis


Central vs. distributed H2 production via electrolysis

Central Distributed

Both systems, Central and Distributed, have the potential to cost 2‐3 USD/kg H2; including production delivery and dispensingproduction, delivery, and dispensing.

Central H2 production makes fiscal sense if the wind/H2 system can be optimized in such a way that cost reductions of the combined system compensate delivery costs.


Both systems will require increases in electrolyzer efficiencies and decreases the capital costs.



Research approaches:

Exploring synergies from coproduction of electricity and H2 to address the intermittent nature of wind power To build a ready source of electricity for times when the wind does not blow or the demand for electricity is hightimes when the wind does not blow or the demand for electricity is high.

Comparing electrolyzer technologies (alkaline and PEMs) to measure their efficiencies and abilities to be brought on and off line quickly as well as ACefficiencies and abilities to be brought on‐ and off‐line quickly as well as AC–DC and DC–DC converters to directly couple the wind turbine to the electrolyzer to achieve efficiency gains.

There are opportunities for reducing the capital and operating costs of electrolyzers; however, electricity prices are key to H2 cost via electrolysis.



Biomass to Hydrogen

Biomass has long been considered a leading near term source of renewable H2.

The resources of lignocellulosic biomass in the USA amount to between 0.4 and 1.3 billion dry tons/yr.

Th ti ti t t th tThe more conservative estimate represents the near‐term economic potential for the annual production of 40 Mtons of H2, enough to fuel more than 150 million fuel cell vehicles.

Production of H2 by gasification is the most promising economic route for the conversion of syn‐gas to transportation fuels 1.38 USD/kg H2 at the plant gate for large‐scale gasification.

Centralized and distributed production scenarios based on bi ibl F d t k l i ti d th k t illbiomass are possible. Feedstock logistics and the market will determine the best technology options and scale of operation.



Biomass to Hydrogen

There are 3 major thermochemical technologies:

(i) gasification, (ii) pyrolysis/reforming, and (iii) high‐pressure water/steam treatment.

Major technical barriers of thermochemicalMajor technical barriers of thermochemical conversion are:

(i) Effect of variable feedstock composition on d t idownstream processing,

(ii) Efficient and durable catalysts for gas conditioning,

(iii) Efficient heat integration(iii) Efficient heat integration.



Biomass to Hydrogen

Heat integration is very important for small‐scaleHeat integration is very important for small scale systems for distributed H2 production. Costs for distribution & storage costs remain high and work against centralized production plants.

Pyrolysis vapor and bio‐oil reforming are a better fit for distributed production because of potentially lower costs at smaller scale.costs at smaller scale.

Thermochemical conversion of biomass has the potential to provide a important part of future t t ti f ltransportation fuels.

Before the advent of the fossil fuel based economy in the 19th century, biomass was the major source of y j fenergy. However, the energy efficiency in the conversion of biomass was very low.



Biomass to Hydrogen

Technologies to produce H2 from various sources can be classified in 3 categories: g p 2 f f g

(i) net positive emission of CO and CO2,(ii) CO2 free emissions, and (iii) CO2 neutral emissions.(iii) CO2 neutral emissions.

Examples of type (i): steam reforming (SR), partial oxidation (PO) and autothermal reforming (ATR) of HCs such as NG, and the WGS reaction of gaseous products of coal gasificationgasification.

NG gas and coal require a lot of energy for mining and processing, adding to the CO2emissions during H2 production, thus increasing the environmental impact of hydrogen g 2 p , g p f y gproduction from these sources.

Examples of type (ii): CH4 decomposition or CH4 aromatisation.



Biomass to Hydrogen

H d i b i ll f i dl l if h d H iH2 production can be environmentally friendly only if the resource used to extract H2 is carbon neutral.

CO neutral hydrogen production can be achieved by the conversion of biomass:CO2 neutral hydrogen production can be achieved by the conversion of biomass:

(i) Gasification, (ii) Pyrolysis of bio‐oils, (iii) SR of biomass derived higher alkanes and alcohols and(iii) SR of biomass derived higher alkanes and alcohols, and(iv) Aqueous phase reforming (APR) of oxygenated hydrocarbons.

Biomass derived H2 can be classified as carbon neutral because the CO2 released during H d ti i d b f th bi ti ( l ti th CO d dH2 production is consumed by further biomass generation (neglecting the CO2 produced from the fossil fuel energy required for operating the hydrogen production unit).



Possible Fuel & Energy Conversion VectorsPossible Fuel & Energy Conversion Vectors

“Any fuel or form of energy can be converted into any other fuel or form of energy in a modern industrial economy, with appropriate conversion efficiencies”


J. Barton and R. Gammon, J. of Power Sources (2010)


Energy contents of different fuels

Fuel Energy content (MJ/kg)

Energy contents of different fuels

Fuel Energy content (MJ/kg)Hydrogen 120.0Liquefied natural gas 54.4Propane 49.6Aviation gasoline 46.8Automotive gasoline 46 4Automotive gasoline 46.4Automotive diesel 45.6Ethanol 29.6thanol 9.6Methanol 19.7Coke 27.0Wood (dry) 16.2Bagasse 9.6



Typical Biomass Sources & Compositionyp pTypes of lignocellulosic biomass (plant biomass):1. Forestry wastes: logging wastes, sawmill wood waste & trees’ & shrubs’ residues.2 Agricultural residues: animal and crop wastes (e g corn stover)2. Agricultural residues: animal and crop wastes (e.g. corn stover).3. Energy crops: corn, sugarcane, grasses and aquatic plants like water hyacinth.

Biomass type Marine Freshwater Herbaceous Woody Woody WoodyName Giant brown

kelp (algae)Water hyacinth

Bermuda grass

Poplar Sycamore Pine

Component (dry wt. %)Celluloses 4 8 16 2 31 7 41 3 44 7 40 4Celluloses 4.8 16.2 31.7 41.3 44.7 40.4Hemicelluloses 55.5 40.2 32.9 29.4 24.9Lignins 6.1 4.1 25.6 25.5 34.5Mannitol 18.7Algin 14 2Algin 14.2Crude protein 15.9 12.3 12.3 2.1 1.7 0.7Ash 45.8 22.4 5.0 1.0 0.8 0.5TOTAL 99.4 112.5 93.3 102.9 102.1 101.0

Typical composition: 75‐90 wt.% of sugar polymers, 10–25 wt.% lignin (large organic aromatics).

Minor components: triglycerides, alkaloids, pigments, resins, sterols, terpenes, terpenoids, waxes.


Huber and Dumesic, Catal. Today (2006)

H d & Li id F l P d ti f Bi S


Hydrogen & Liquid Fuels Production from Biomass Sources


Tanksale et al.; Renewable and Sustainable Energy Reviews (2010)

Biomass Gasification



Biomass gasification is similar to coal gasification, except that the biomass ifi ti i d t d t h l t tgasification is conducted at a much lower temperature.

This is because biomass contains many more functionalities than coal and is th f titherefore very reactive.

Gasification is achieved at T > 700 ºC in the presence of O2/air and/or steam.

Tar free gasification requires much higher temperatures.

S (CO CO ) i d d h i d f h ifi iSyn‐gas (CO2, CO, H2) is produced when oxygen is used for the gasification as opposed to a producer gas (CO2, CO, H2, CH4, N2), in which case air is used for gasification.




A combination of pyrolysis, partial oxidation and/or steam reforming of gaseous alkanes and char takes place during gasification


alkanes and char takes place during gasification.

Biomass Gasification Process


f


Biomass Gasification Reactors


f

Fluidized‐bed gasifier

Downdraft gasifier

Updraft gasifier


Navarro et al., Chemical Reviews (2007).

Typical Gasification Reactions


Typical Gasification Reactions(*) (kJ/gmol)at 27 oCReaction Type Reactor Equation Enthalpy (*)

C H O (1 x)CO + (y/2) H + C 180Pyrolysis

CxHyOz (1‐x)CO + (y/2) H2 + C

CxHyOz (1‐x)CO + ((y‐4)/2) H2 + CH4

180

300CxHyOz + (1/2)O2 xCO + (y/2) H2 ‐71

Partial Oxidation CxHyOz + O2 (1‐x)CO + CO2 + (y/2) H2

CxHyOz + 2O2 (x/2)CO + (x/2)CO2 + (y/2) H2

‐213

‐778

C H O + H2O xCO + (y/2) H2310

Steam Reforming

CxHyOz + H2O xCO + (y/2) H2

CxHyOz + nH2O aCO + (x‐a)CO2 + (y/2) H2

CxHyOz + (2x‐z)H2O xCO2 + (2n+y/2‐z) H2

230

64W G Shif CO H O CO H 41Water Gas Shift CO + H2O CO2 + H2 ‐41Methanation CO + 3H2 CH4 + H2O ‐206

The presence of O2 or air promotes partial oxidation over pyrolysis reactions.

Fast pyrolysis reactions produce bio‐oils, tar and charcoal (and some few gas).

WGS reaction is conducted in a separate catalytic reactor (CuO–ZnO or Fe catalyst depending T).




f

Catalytic cracking, gasifier type, design, heating rate, temperature and residence time can be optimi ed to ma imi e the efficienc of gasificationresidence time can be optimized to maximize the efficiency of gasification with minimum tar formation.

Tars are aromatic hydrocarbons produced from the condensation of Tars are aromatic hydrocarbons produced from the condensation of organic matter in the gasifier, or further downstream, at low temperatures.

Thermal cracking of the tar is possible at T > 1000 ºC but using catalysts Thermal cracking of the tar is possible at T > 1000 ºC, but using catalysts like dolomite or olivine can be obtained 100% tar removal at lower T.

Ni Pt Pd Ru and alkaline metal oxides supported on dolomite or CeO /SiO Ni, Pt, Pd, Ru and alkaline metal oxides supported on dolomite or CeO2/SiO2catalyze the gasification to reduce tar formation and improve the product gas purity and conversion efficiency.




f

Rh/CeO2/SiO2 is the most effective catalyst to reduce tar formation, but Ni2 2based catalysts are also highly active for tar destruction.

Ni catalyze the steam reforming of tars and WGS reactions to produce H2.2

Alkali metal salts decrease the tar yield, but increase char and ash contents not suitable for commercial use.

The inorganic contents (Na, K, Ca and other alkali) are converted to ash, collected at the bottom of the gasifier or carried away as fly ash.

Deposition of ash: May cause sintering, fouling, agglomeration and slagging.



Feedstocks and H Production during Biomass Gasification

Feedstock Reactor Type Catalyst Temperature (K) H2 (% vol)Sawdust Fluidized Bed Unknown 1073 57 4

Feedstocks and H2 Production during Biomass Gasification

Sawdust Fluidized Bed Unknown 1073 57.4Fluidized Bed Ni 1103 62.1

Sawdust Fluidized Bed K2CO3 1237 11.3CaO 1281 13 3CaO 1281 13.3

Pine Sawdust Fluidized Bed Unknown 973‐1073 26‐42Bagasse 973‐1073 29‐38Eucaliptus Gobulus 973‐1073 35‐37Eucaliptus Gobulus 973 1073 35 37Pinus Radiata 973‐1073 27‐35Sewage Sludge Downdraft Unknown 10‐11Almond Shell Fluidized Bed La‐Ni‐Fe 1073 62.8Almond Shell Fluidized Bed La Ni Fe 1073 62.8

Perovskite 1173 63.7Switchgrass Moving Bed Cu‐Zn‐Al 27.1

Using a fluidized bed gasifier along with suitable catalysts, it is possible to attain a 60% of H2.

Such high conversion efficiency makes biomass gasification an attractive H2 alternative.


Navarro et al., Chemical Reviews, 2007

Biomass Gasification in Supercritical Water


f pBiomass gasification to syn‐gas is limited to 35% of water‐content.

If % water > 35% Gasification under supercritical water conditions (SCWG) i e above If % water > 35% Gasification under supercritical water conditions (SCWG) i.e. above water critical point (374.3 ºC and 221.2 bar).

At T > 600 ºC, hydrothermolysis of the biomass products will lead to gases (H2, CO, CO2 andAt T > 600 C, hydrothermolysis of the biomass products will lead to gases (H2, CO, CO2 and CH4) with 100% conversion.

Advantage over conventional O2/steam gasification: No formation of tar and char. 2

SCWG can be classified into 2 operating conditions based on their temperature:

(i) Low temperature SCWG is operated at 350 < T < 600 ºC and(i) Low temperature SCWG is operated at 350 < T < 600 ºC, and (ii) High temperature SCWG is conducted at T > 600 ºC.

100% gasification is achieved only at T > 600 ºC LT SWCG needs a catalyst.

Catalysts: Ru or Ni supported on TiO2, ZrO2, or Carbon, which are stable under severe oxidizing and corrosive conditions of the SCWG.


Reaction mechanism in HyPr RING gasification process


Reaction mechanism in HyPr‐RING gasification process

Gasification & WGS reactions are combined in one reactorGasification & WGS reactions are combined in one reactor with simultaneous absorption of CO2 to increase the H2yield, while maintaining T = 650 ºC.

The reactions take place in a two‐step process: The reactions take place in a two‐step process:

1st step: H2O reacts with HCs to produce H2 and CO2(endo). H2 can then be used to generate power.

2nd step: CO2 is absorbed by Ca(OH)2, which is in turn produced at high pressure by hydration of CaO.

CaCO3 is separated and regenerated to produce CaO and release pure CO2 for sequestration.

With the high water content of the biomass, the gasification can proceed with little or no dditi l t li d f th tiadditional water supplied for the reaction


Biomass Gasification Carbo‐V Process


The aim of the Carbo‐V process is to produce tar free syn‐gas for H2 or liquid fuel production.

Takes place in 2 stages: 1st step: Biomass is converted to tar containing gas and charcoal in a pyrolyzer at 500 ºC. The tar containing gas is combusted in a HT gasifier by co‐feeding OThe tar containing gas is combusted in a HT gasifier by co feeding O2. 2nd step: Charcoal from the 1st reactor is gasified to syn‐gas at 1500 ºC in an entrained flow gasifier. The remaining ash is converted to slag which can be used as construction material.

Advantages of Carbo V process: Increases efficiency (>80%) via (1) chemical quenching by Advantages of Carbo‐V process: Increases efficiency (>80%) via‐(1) chemical quenching by blowing charcoal into the hot gas, and (2) decreasing the heat losses in the combustion.

The LT pyrolysis reactor increases the feedstock flexibility as the solid feed is converted to


combustible gas and charcoal.http://www.choren.com/en/biomass_to_energy/carbo‐

v_technology/

Biomass Gasification BIOLIQ Process


Biomass Gasification BIOLIQ Process

In the BIOLIQ process, lignocellulosic biomass such as straw and other non‐woody biomass is first liquefied by fast pyrolysis at many local small plants.

The resulting bio‐oil and char slurry is transported to a central facility where large pressurized entrained flow gasifiers combusts the slurry to produce tar free syn‐gas.

The gasifier is operated at 26 bar and at 1200 < T < 1600 ºC ( above the ash melting point). Burners at the top of the gasifiers are fed by a nozzle that atomizes the incoming slurry pneumatically by pure O2 (at stoichiometric ratio of 0.3–0.7) at the gasifier pressure. The gasification takes place in a downward flame reaction in ca. 1 s.

The resulting syn‐gas and molten ash exits the bottom of the gasifier into a quench zone where it is cooled by injecting water.

The process attains high gasification efficiency (70% at 1200 ºC) and veryThe process attains high gasification efficiency (70% at 1200 C) and very high carbon conversion (>99%).


http://www.bioliq.com/

Hydrogen production from syn‐gas


Hydrogen production from syn gas

Relatively pure H2 can be obtained from syn‐gas produced from the biomass gasification by steam reforming followed by a WGS reactorgasification by steam reforming followed by a WGS reactor.

WGS reaction: CO and H2O react on a catalyst to form CO2 and H2.

Reversible reaction steam is added in excess to shift the equilibrium towards H2.

WGS reaction can be carried out at 2 temperature ranges: (1) HT reaction with Fe and/or Cr oxide catalysts 350 < T< 500 ºC and(1) HT reaction, with Fe and/or Cr oxide catalysts, 350 < T< 500 ºC, and (2) LT reaction, Cu‐Zn oxide catalysts, 200 < T 250 ºC.

LT WGS reactions have also been carried out on transition metal catalysts and Au supported on Al O CeO and CeO ZrOsupported on Al2O3, CeO2 and CeO2‐ZrO2.

HT shift catalysts based on oxides of Fe and Cr are used in the industry.


Hydrogen production from syn gas from biomass gasification


Hydrogen production from syn‐gas from biomass gasification

The gasified biomass stream is filtered in a heated particulate filter and purified to remove tars in a guard bed dolomite reactor at 600 ºC.p f g

Syn‐gas may contain light HCs and tar is converted to H2 and CO by SR using a supported Ni catalyst at a temperature range of 750‐850 ºC.

CO from the SR reactor is converted feeding the gas to a HT and LT WGS reactor to increase H2 yield.

SR & WGS catalysts usually suffer deactivation in a long run due to sintering and coke deposition.

Stage of H2 purification (e.g. CO‐PROX or Catalytic Membrane Reactor) to decrease CO content to sub‐ppm


level, because CO is a poison for the anode catalyst used in the PEM fuel cells.

Membrane Reactors for Hydrogen Separation


Membrane Reactors for Hydrogen Separation

Membrane reactors improve WGS reaction performance with the in situ separation of products.

It is possible to overcome thermodynamic constraints and increase the CO conversion significantly It is possible to overcome thermodynamic constraints and increase the CO conversion significantly.

If steam re‐forming syn‐gas is used as the feed gas, the H2 concentration may reach 99.6%.

Using Pd, or other inorganic H2‐selective membranes, CO conversion close to 100% are attained.Using Pd, or other inorganic H2 selective membranes, CO conversion close to 100% are attained.

Pd membranes have a high cost and show instability in the presence of HCs or steam.

Silica membranes are very attractive for H2 production by WGS reactions.

H

2HCO 2H

CO


2H

Biomass Pyrolysis


Pyrolysis of biomass: Thermal decomposition of y y f p fcellulosic matter in the absence of air or oxygen.

Slow heating process, 300 < T < 900 ºC charcoal formation Not attractive for producing H2.

Fast pyrolysis: fast heating rates, HT (400‐3000 ºC) and short residence times (ca 1s)and short residence times (ca. 1s).

Fast pyrolysis:Fast pyrolysis:

(1) Produces solid, liquid and gases depending upon the feed and temperature

(2) Requires elevated heating rates and fine particle size to attain high heat transfer ratesInitiates the primary pyrolysis reactions at HT, releasing volatiles and the remaining char.


Fast Pyrolysis Reactor


Fast Pyrolysis Reactor

Dried and finely ground biomass is fed into the pyrolysis reactor char, organic vapors, and gases py y , g p , g concentrations depend upon severity of pyrolysis.

Chars can be used for heating purposes.

Organic vapors condensed to produce bio‐oils.

H2 yield can be increased from the gases coming out of the condenser

The bio‐oils are separated into water‐soluble and water‐insoluble componentswater insoluble components.

Insoluble organics to produce chemicals

Soluble organics to SR to produce H2.

Alternative All the bio‐oil can be treated in an ATR or SR to produce H2.


Biomass Pyrolysis


Physical characteristics of bio‐oil vary depending upon the feed type and the severity of pyrolysis.

Bio‐oils contain very high oxygen content (30‐40%) and a lower heating value (16‐19 MJ/kg) compared to heavy fuel oil (0.1% and 40 MJ/kg, respectively).

Most of the oxygen is present as water (15‐30%) and is also responsible for decreasing the heating value of the bio‐oils.

h b l h ll bl d h l High oxygen content bio‐oils chemically unstable unwanted chemical reactions with increasing time and temperature increase in viscosity and the cloud point temperature.

Other problems: poor volatility, high viscosity, coking, cold flow and corrosiveness.

Refining of bio‐oils is essential to make it suitable as a liquid fuel. However, distillation of bio oils can lead to resid al astes of p to 50% b eight (1% for hea f el oil)bio‐oils can lead to residual wastes of up to 50% by weight (1% for heavy fuel oil).

An alternative to refining bio‐oils is to increase the severity of pyrolysis conditions in order to produce more gases (CO, H2, CO2 and CH4).


2 2 4

H d P d ti f F t P l i d Bi il


Hydrogen Production from Fast Pyrolysis and Bio‐oils

Gaseous products can be obtained from fast pyrolysis of biomass by increasing the pyrolysis temperature.

Depending upon the feed, the H2 yield increase from 35 %vol to 47 %vol of the total gas yield The H yield is still too low for it to be commercially attractivegas yield. The H2 yield is still too low for it to be commercially attractive.

Methods to increase the H2 yield: (1) Catalytic pyrolysis of bio‐oils (e.g. Waterloo Fast Pyrolysis Process‐WFPP)(1) Catalytic pyrolysis of bio oils (e.g. Waterloo Fast Pyrolysis Process WFPP)

Very fast heating rates and short residence time of the biomass (ca.1 s). (2) Steam reforming of bio‐oils.

SR reaction competes with the gas phase thermal decomposition coke SR reaction competes with the gas phase thermal decomposition coke formation can plug the reactor and deactivate the catalysts Very high steam/carbon ratio (>7) to avoid coke deposition Increase of energy demand to produce the excess of steam.


Hydrogen Production from


Hydrogen Production from Autothermal reforming of Bio‐oils

Autothermal reforming (ATR) is an attractive alternative to SR of bio‐oils.

ATR is a combination of SR and partial oxidation (PO) of the HCs to produce CO, CO2 and H2. 2 2

The overall reaction for ATR can be written as:

Advantages of ATR over SR:

(i) The heat generated from the exothermic PO can compensate the endothermic SR.(ii) The water produced as a by‐product can be utilized during SR which reduces the

external water and energy required for producing excess steam.

The oxygen concentration in the feed can be adjusted to match the heat required for SR only, or for preheating and SR.or for preheating and SR.


Hydrogen Production by Catalytic Decomposition of Bio oils


Hydrogen Production by Catalytic Decomposition of Bio‐oils

Hydrogen can be produced from bio‐oils using the sequential catalytic decomposition (CD).

Bio‐oil sequential cracking is a two step process in which CD of feed is alternated with a catalyst regeneration step.

This process is similar to catalytic decomposition of methane (CDM):This process is similar to catalytic decomposition of methane (CDM):

During the cracking step the hydrocarbon feed decomposes on a metal catalyst surface to produce H2 and solid carbon on the metal site which accumulates as coke.

During the regeneration step the coke deposited on the metal site is combusted or gasified to produce CO2 and this restores the catalytic activity in the process. 2

If two or more reactors are placed in parallel, H2 can be produced continuously by cyclically switching the hydrocarbon and O2 feed between the reactors.

CD has an added advantage in that the H2 and CO2 are produced in different steps, thereby, saving the energy required to purify H2.

Platinum group metals supported on Al2O3, ZrO2, and CeO2‐ZrO2 have been used.


Hydrogen Production by Aqueous Phase Reforming


Catalytic aqueous phase reforming (APR) convert biomass‐derived oxygenated HCs with C:O ratio of 1:1, into H2, CO, CO2 and gaseous alkanes using supported metal catalysts.


Feed (Oxygenated HCs): Methanol, ethylene glycol, glycerol, glucose and sorbitol. APR is carried out at 200 < T < 250 ºC and 10 < P < 50 bar to maintain the liquid phase.

Advantages: (i) Moderate T and P, which favors the WGS reaction in the same reactor, (ii) Low CO level in the gas stream (100‐1000 ppm) FC application, (iii) Lower energy requirement: Oxygenated HCs feed and water are in the liquid phase and(iii) Lower energy requirement: Oxygenated HCs feed and water are in the liquid phase, and (iv) Feedstock is non‐hazardous which makes its storage relatively easier.

The overall reaction for APR is similar to the SR reaction:

Thermodynamically is favored the production of H2 and CO2, but in these conditions the products can further react to form alkanes and water:products can further react to form alkanes and water:

Pt was is the best catalyst for APR, but the cost is prohibitive for large‐scale applications.


Huber and Dumesic, Catal. Today (2006)


Reaction pathways for production of H from conversion ofReaction pathways for production of H2 from conversion of ethylene glycol with water

Pathway I is desired C‐C cleavage to form adsorbed CO. Pathway II represents undesired C‐O cleavage followed by hydrogenation to produce ethanol, leading to formation of methane and ethane Pathway III is the desired WGS reaction Pathway IV represents undesired methanation


ethane. Pathway III is the desired WGS reaction. Pathway IV represents undesired methanationand FT reactions to produce alkanes.



One of the pathways, similar to the SR is to produce adsorbed CO on the metal site by the C–C bond cleavage. It is followed by the WGS reaction to produce H2 and CO2.


The combined reaction is:

FT reaction to form alkanes and water presents a serious selectivity problem because it consumes hydrogen.

A ll l i d h h h l f C O b d f l h l A parallel reaction proceeds through the cleavage of C–O bonds to form alcohols.

Other pathways involve dehydration/hydrogenation and dehydrogenation/rearrangement of the oxygenated HCs to form alcohols and acids which reacts with water to give alkanes H and COoxygenated HCs to form alcohols and acids, which reacts with water to give alkanes, H2 and CO2.

These reaction pathways present a parallel selectivity challenge which should be overcome to maximize the hydrogen yield. y g y

The optimum catalytic pathway for the production of H2 and CO2 by APR involves cleavage of C–C bonds as well as C–H and/or O–H bonds to form adsorbed species on the catalyst surface.




A good catalyst for production of H2 by APR must be highly selective for C–C bond cleavage and promote removal of adsorbed CO species by the WGS reaction

y g y q f g

and promote removal of adsorbed CO species by the WGS reaction.

Catalyst must not catalyze C–O bond cleavage and hydrogenation of CO and CO2.

APR activity (on SiO2 support) decreased as follows: Pt = Ni > Ru > Rh = Pd > Ir.

Pt was is the best catalyst for APR, but the cost is prohibitive for large‐scale applications. y f p f g pp

Acidic supports, Al2O3, are more alkane‐selective and basic/neutral supports, Carbon are more H2‐selective.

pH of the solution should be neutral for higher H2‐selectivity.

H2‐selectivity decreases with increasing number of carbon atoms in the feed molecule as follows: methanol>ethylene glycol>glycerglycerol>sorbitol > glucose.


f d d i f i


Summary of Hydrogen Production from Biomass

(a) Energy ratio = heating value of product H2/heating value of biomass feed. The energy ratio does not include the fossil fuel energy required(a) Energy ratio heating value of product H2/heating value of biomass feed. The energy ratio does not include the fossil fuel energy required for biomass production.

(b) The energy ratio value is calculated assuming 100% supercritical water gasification of glucose, where glucose is produced from the enzymatic hydrolysis (yield 75‐95%) of crystalline + amorphous cellulose (80% of dry biomass feed).



S mmar of H drogen Prod ction from Biomass


Summary of Hydrogen Production from Biomass

Biomass is an important renewable resource for producing H2. (e.g. more than 50 p f p g 2 ( gmillion tons of H2 could be produced annually in the USA in the near term from available biomass resources.

Two main technology pathways are being explored:

(i) Gasification to syn‐gas followed by gas conditioning and WGS(ii) Pyrolysis to bio‐oil followed by catalytic reforming.(ii) Pyrolysis to bio oil followed by catalytic reforming.

The gasification approach has the potential to produce H2 for less than 1.5 USD/kg at a scale of 2000 tons per day.

The bio‐oil approach is a potential low‐cost process for distributed reforming.



Hydrogen Production by Fermentation



Direct fermentation of carbohydrate feedstocks by microorganisms is a potential technology for producing renewable hydrogen if several technical barriers are overcome.

H2 yields have been low and it is uncertain whether this technology can be developed to provide high yields of H and become economically competitive with other pathwaysprovide high yields of H2 and become economically competitive with other pathways.

Many anaerobic microorganisms carry out the dark fermentation reaction during which the metabolism of sugars, amino acids, and fatty acids results in the production of H2, CO2the metabolism of sugars, amino acids, and fatty acids results in the production of H2, CO2and other reduced end products.

Hydrogen production is catalyzed via the hydrogenase enzyme according to the following equation:

d h l ll d b l b f

2122 HeH

Hydrogenase is present in phylogenetically diverse microbes exploring microbes for their H2 production potential is an active research subject.

CATALYSIS FOR ENERGY: NEW CHALLENGES FOR A SUSTAINABLE ENERGETIC DEVELOPMENT. 18th to 20th August, Palacio de la Magdalena Santander

73


Pathways of H2 Production during Glucose Fermentation

Glucose fermentation pathways, via glycolysis, to H2 production C6H12O6

f p y , g y y , 2 poccurs in 2 types of microbes:

(1) Enteric bacteria, e.g. Escherichia coli, and(2) Strict anaerobes, e.g. clostridial species,

The H2 molar yield depends the pathways that microbes usemicrobes use:

(i) H2 molar yield is 2 in enteric bacteria (type 1).

(ii) Yield is 4 in clostridial microbes (type 2).


74


Pathways of H2 Production during Glucose Fermentation

Clostridial

Escherichia coliEscherichia coli


75



Advantages of fermentation:

y g y

(1) Simple reactor design and operation (darkness);

(2) Fermentative microbes are readily available in sewage sludge, garden soils, and anaerobic compost;

(3) Diverse waste materials can be used as feed; and

(4) High rates of H2 production unsurpassed by other biological processes.(4) High rates of H2 production unsurpassed by other biological processes.

Fermentation thus has significant potential provided that several technical b i bbarriers can be overcome:

(i) High cost of glucose feedstock and

(ii) Low H2 molar yield( ) 2 y


76


Potential of H2 Production by Fermentation

Glucose is the ideal substrate, yet it is too costly at present.

f 2 y

, y y p

Agricultural residues and food wastes are rich in carbohydrates could serve as feedstock.

Lignocellulosic biomass is a sustainable feedstock for H2 production 70% of biomass is hemicellulose (xylose) and cellulose (glucose polymer) the bulk of which is fermentable if monomeric sugars can be readily released.

Challenge of using biomass: Crystallinity and Heterogeneity prevents its direct utilization by most microbes.

Even after chemical pretreatment, the cellulose still has to be further hydrolyzed via a suite of cellulase enzymes to produce the more fermentable glucose.

Main goal of research programs To lower the cost of biomass‐derived sugar for the bioethanolrefinery.


77



The more challenging barrier of fermentation is its low H2 molar yield.

f 2 y

Considering the energy content in 1 mol of glucose (678.2 kcal), H2 molar yield of 2‐4 thus recovers only 17‐33% of the chemical energy in glucose:

0

M t i b d i t f t d t ( ti f i b t i d l ti

molkJGHCOAcetateOHGlucose 4.182;4222 0222

molkJGHCOButyrateGlucose 1.257;22 022

Most microbes produce an mixture of waste products (acetic, formic, butyric, and lactic acids, and alcohols) that provide multiple pathways to consume NADH and regenerate NAD+ at the expense of hydrogen.

This metabolic diversification lowers the hydrogen molar yield.


78



The decrease in pH (< 4.5) of the medium, from acid accumulation, metabolic shift of

f 2 y

p ( ) f , f , f fthe microbe acids are re‐assimilated toward solvent production lowering H2 yield. Controlling medium pH is necessary to improving H2 yield.

To compensate for the low H2 yield, the cost of feedstock has to be decreased significantly for fermentation to be cost competitive.

Techno‐economic analysis (NREL): If glucose price is 0.11 USD/kg, and assuming a H2molar yield of 4, a minimum H2 selling price of 2.47 USD/kg could be achieved.

Thi i i H lli i i b d l f d k 75% f h ll iThis minimum H2 selling price is based on only feedstock cost, 75% of the overall cost, is near to the target of the H2 US‐DOE cost goal of 2‐3 USD/kg.

This encouraging study guides new research approaches to overcoming the twoThis encouraging study guides new research approaches to overcoming the two barriers.


79


Status of H2 Production by Fermentation

Maximum H2 molar yield is 4, but experimental results are close to 2‐3 (using either pure culture

f 2 y

or mixed microbial and glucose, sucrose, molasses, starch, and food wastes as the substrates.

This range of molar yield is 50‐75% efficient biologically, although only reaching 17‐25% efficienc if based on the energ content in gl coseefficiency if based on the energy content in glucose.

To improve feasibility of fermentation, less expensive and more abundant alternative feedstock has to be exploredhas to be explored.

Research results indicate that converting sugars, food waste, and hemicellulose to hydrogen is a feasible process if suitable microbes are used.f p f

This demonstrates that lignocellulosic biomass is a feasible substrate for H2 production.

Improvement of molar yield could be reached via genetic modification of microbes More research is needed to overcome the barriers.


80


Hydrogen Production by Fermentationy g y

Research approaches

To identify microbes that can utilize hemicelluloses and cellulose directly This eliminates the use of expensive cellulase enzymes and simplifies biomass pretreatment.

The mining of various cellulolytic bacteria thereby offers a promising solution to converting cellulose to hydrogen in a one‐step consolidated process.

Genomics and molecular biology are effective tools to redirect metabolic pathways toward maximal H2 production, especially when genetic engineering is done in cellulolytic microbes.

A better understanding of the underlying biochemical metabolism is needed to target pathways that yield the greatest improvements in H2 molar yield.


81


Integrated Scheme of Dark Fermentation and PhotofermentationIntegrated Scheme of Dark Fermentation and Photofermentation

An integrated approach Photofermentation of waste organic acids of dark fermentation toAn integrated approach Photofermentation of waste organic acids of dark fermentation to generate additional H2, catalyzed by the nitrogenase enzyme of the photosynthetic bacteria.

Substances present in waste acids (acetate, formic, lactic, and butyric acids) are converted into additional H according to:additional H2 according to:

Theoretically, 1 mol acetate could yield 4 mol H2, while butyrate would yield 10 mol H2.

22)222( xCOHxyOz)H-(2x OHC 2zyx

Theoretically, 1 mol acetate could yield 4 mol H2, while butyrate would yield 10 mol H2.

The total sum of the 2 processes (n1+n2) could approach to 12 mol H2; the equivalent of the energy content in glucose.


The photobioreactor must be optimized to ensure the success of the more complex integrated process.


Hydrogen Production by Solar‐Driven Thermochemical Reactions

Water is a virtually limitless source of Hydrogen, but it is very stable molecule large energy input to decompose in H2 an O2.

y g y

large energy input to decompose in H2 an O2.

A carbon neutral and strictly renewable process Solar Energy.

Solar energy can be supplied as:Solar energy can be supplied as:(i) Heat (Thermochemical), (ii) Light (photochemical), or (iii) Electricity (electrolysis).

Solar energy collected as heat is the most efficient path No inefficiencies of the photochemical processes or of the conversion to electricity followed by electrolysis.

Water thermally decompose, at significant extent, at T > 2500 K. Need of separate H2and O2 to avoid an explosive mixture

Temperature can be reduced by carrying the reaction out high temperature cycles.p y y g g p y

Temperature requirements (> 800 K) of thermochemical water splitting dictate that concentrating solar systems be used to collect solar energy and convert it to heat.


84


Thermochemical routes for Production of Solar Hydrogenf f y g

Concentrated Solar Energy

H OH O H OFossil Fuels

(NG Oil C l)H2OH2O H2O(NG, Oil, Coal)

SolarThermolysis

SolarThermochemical

Cycles

SolarReforming

SolarCracking

SolarGasification

CO2/CSequestration

Solar Hydrogen

Sequestration


85

A. Steinfeld, Solar Energy (2005)


Hydrogen Production by Water‐Splitting Thermochemical Cyclesy g y p g y

Water‐splitting thermochemical cycles bypass the H2/O2 separation problem and operate at T<1200 K.

These cycles required multiple steps inefficiencies associated with heat transfer and product separation at each stepat each step.

Optical systems for large scale solar concentration attain solar 5000 suns* conversion of solar toattain solar 5000 suns conversion of solar to thermal reservoirs at 2000 K, needed for 2‐step thermochemical cycles using metal oxide redoxreactions:

‘‘Rotating‐cavity’’ solar reactor for thermal dissociation of ZnO@ 2300 K

222

22

2

21

),(

),(2

OHOH

exosolarnonyHOMOyHxM

endosolarOyxMOM

yx

yx

222 21 OHOH


86A. Steinfeld, Solar Energy (2005)*1 sun=1 kW/m2



Water is the only input. H2 and O2 are the outputs. All other chemicals are recycled.

Selection of temperature ranges thermodynamics, cost, chemicals’ availability ,Selection of temperature ranges thermodynamics, cost, chemicals availability , environmental, safety & health factors.



Hydrogen Production by Water‐Splitting Thermochemical Cycles

Types of solar concentrating systems:

(i) One axis tracking parabolic trough,

y g y p g y

( ) O e a s t ac g pa abo c t oug ,

(ii) towers with a field of 2 axis‐tracking heliostats, and

(iii) dish systems.

Effi i iEfficiencies:

(i) Parabolic troughs: ca. 100 suns, 800K, 60% efficiency;

(ii) Tower configurations: ca. 1000 suns, 1000K, 75% efficiency.

(iii) Dishes: 10000 suns, 2200 K, 85% efficiency.

Overall energy efficiency: Combination of solar & chemical cycle efficiencies.

The minimum efficiency desired is that for conventional electrolysis. Many chemical cycles have the potential to be 40–50% efficient.

The current status of concept development is directed to stand‐alone operations.

There may be potential for producing electricity with waste heat.


88




89


Hydrogen Production by Water‐Splitting Thermochemical Cycles

Barriers to achieving potential

y g y p g y

g p

To adjust the chemical processes to the daily and annual solar cycles

Storing energy, as heat for off‐sun operation, can extend the time for H2 production and allow some process operations to occur at night.

Research approaches

Fundamental work on engineering solar concentrators, solar receivers, reaction kinetics gas separation and materials of construction is requiredkinetics, gas separation, and materials of construction is required.

The demands on performance in all these areas increase with the temperature requirements.requirements.


90


Photolysis – Photoelectrochemistry

The thermodynamic potential for splitting water at 25 ºC is 1.23V.

Adding overvoltage losses and some energy to drive the reaction at a reasonable rate, a voltage of 1.6‐1.8V is needed commercial l t l t t 1 7 1 9Velectrolyzers operate at 1.7–1.9V.

The energy of 1.9 eV corresponds to a =650 nm, red light, the lowest energy portion of the visible spectra Nearly the entirelowest energy portion of the visible spectra Nearly the entire visible spectrum has enough energy to split water into H2 and O2.

The key: To find the right combination of a light harvesting system and a catalyst that can efficientlyThe key: To find the right combination of a light harvesting system and a catalyst that can efficiently collect the energy and direct it toward the water‐splitting reaction.

Direct Photoelectrochemical (PEC) splitting of water: one‐step process for producing H2 with solar i di tiirradiation.




Semiconductor electrode Counter‐electrode

Photoelectrochemical mechanism

Semiconductor electrode Counter electrode

Conduction band Light is absorbed in the semiconductor and water issemiconductor and water is split at the semiconductor surface.

Conduction band

22 )(222 HOHeOH 22 422 OHhOH

Major criteria of viability:

(i) The light harvesting system must generate sufficient voltage to decompose water(ii) The system must be stable in an aqueous environment.




Advantages of direct conversion PEC systems:

(i) Eliminate most of the costs of the electrolyzer, and

(ii) increase the overall efficiency.

Basic requirements for PEC water‐splitting semiconducting material:

(1) Stable in an aqueous electrolyte.

(2) R b d (E ) 1 7 V E 2 2 V t hi th ti f(2) Range band gap (Eg): 1.7 eV < Eg < 2.2 eV to achieve the energetic for electrolysis, allowing maximum absorption of solar spectrum.

(3) To have a high quantum yield (>80%) across its absorption band to reach the efficiency necessary for a viable deviceefficiency necessary for a viable device.

(4) To span the H2 and O2 half‐reactions redox potentials with its conduction and valence band edges.

(5) To have a pathway to low cost high volume synthesis(5) To have a pathway to low‐cost, high volume synthesis.




PEC production of H2 is based on solar illumination Very large energy resource.

% l ld d b id f h i A 10% solar‐to‐H2 PEC system would need about 1 MHa to provide H2 for the entire U.S. fleet (310 M people, 236 M vehicles). ~ 2 MHa in EU (500 M people, 380 M vehicles).

Very high resource potential (sunlight) for PEC Economical production distribution Very high resource potential (sunlight) for PEC Economical production, distribution, and storage of hydrogen are necessary.

Fujishima and Honda in 1972 showed that H2 generation via splitting of water withFujishima and Honda in 1972 showed that H2 generation via splitting of water with visible light was possible at a semiconductor electrode 38 years later, a visible light‐driven water‐splitting system that is efficient and stable still remains an elusive goal.

The needed material properties for a working PEC device requires a unique combination of physical, chemical, structural, and economic properties that no known material satisfies.

Th h f ifi t i l i f d th th i d h t i ti fThe research for a specific material is focused on the synthesis and characterization of materials, primarily mixed metal oxides Combinatorial techniques.


Fujishima and Honda, Nature (1972)

CONCLUSIONS


1) Hydrogen can be produced from the renewable energy resources water and biomass by

CONCLUSIONS

1) Hydrogen can be produced from the renewable energy resources, water and biomass, by a variety of processes (e.g. photolysis, electrolysis, thermochemical, and biochemical).

2) Electrolysis of water is the simplest technology for producing hydrogen. The electrolytic2) Electrolysis of water is the simplest technology for producing hydrogen. The electrolytic production of hydrogen is currently the only way to produce large quantities of hydrogen without emitting the traditional byproducts associated with fossil fuels.

3) Biomass‐to hydrogen processes (gasification, pyrolysis, and fermentation) are less well‐developed technologies. These processes offer the possibility of producing hydrogen from waste materials (cellulosic biomass and sewage).

4) Hydrogen production may be the most promising economic route for the conversion of syngas to transportation fuels. Solar energy can be used to produce hydrogen in the form of heat (thermochemical), light (photochemical), or electricity (electrolysis). f ( ), g (p ), y ( y )



CONCLUSIONS

5) Solar energy collected as heat may be the most efficient solar path to hydrogen from water

CONCLUSIONS

since it does not have the inefficiencies associated with photochemical transformations or the conversion of solar energy to electricity followed by electrolysis.

6) Photoelectrochemical water splitting and photobiology are also options for producing6) Photoelectrochemical water splitting and photobiology are also options for producing hydrogen with solar energy.

7) Hydrogenase‐containing organisms (cyanobacteria and green algae) can extract7) Hydrogenase containing organisms (cyanobacteria and green algae) can extract reductants from water and achieve very high light conversion efficiencies, and they photoproduce hydrogen without the input or output of carbon‐based molecules. These technologies are in the development stage.

8) The contribution that each technology will have to future energy economies will depend of factors such as: (i) Energy‐conversion efficiency, (ii) Greenhouse gas emissions, and (iii) Overall cost through the complete chain from primary energy source to delivered hydrogenOverall cost through the complete chain from primary energy source to delivered hydrogen at pressure.


producción de hidrógeno a partir de energías renovables_a.monzón

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