Hydrogen, a clean and efficient fuel, is revolutionizing the way we think about energy consumption. With its incredible energy density and zero-emission combustion byproduct - water - hydrogen is poised to play a vital role in reducing our reliance on fossil fuels and mitigating climate change.
The benefits of hydrogen are numerous. Not only does it produce an enormous amount of energy during combustion, but it also increases the economy and efficiency of machines. However, despite its potential, the availability and production technologies for hydrogen are still in their infancy.
One of the primary challenges facing hydrogen production is the source and raw material required. Fossil fuels, which contain high concentrations of hydrogen, are not a viable option as they perpetuate the cycle of pollution. Water, on the other hand, is an abundant resource that can be used to produce hydrogen. However, splitting water into hydrogen and oxygen requires energy, which is a significant hurdle.
The process of splitting water into hydrogen and oxygen is known as electrolysis. The mathematical relation involved in electrolysis is as follows:
2H2O → 2H2 + O2
where 2H2O represents two molecules of water, 2H2 represents two molecules of hydrogen, and O2 represents one molecule of oxygen.
The energy required to split water into hydrogen and oxygen can be calculated using the following equation:
ΔG = ΔH - TΔS
where ΔG is the Gibbs free energy, ΔH is the enthalpy change, T is the temperature in Kelvin, and ΔS is the entropy change.
As research continues to advance, we can expect to see more efficient and cost-effective methods for producing hydrogen from water. This will be a crucial step towards transitioning to a clean and sustainable energy future.
Hydrogen Production Techniques
1. Silicon Chip-Based Microreactor by Methanol Reforming
Methanol reforming is a process that converts methanol into hydrogen and carbon dioxide using a catalyst. This process can be performed in a silicon chip-based microreactor.
2. Water Splitting by Solar Energy Using Thermochemical Cycle or Water Electrolysis
Water splitting is a process that separates water into hydrogen and oxygen using energy from the sun. This can be done through a thermochemical cycle or water electrolysis.
3. Photoelectrolysis
Photoelectrolysis is a process that uses light to split water into hydrogen and oxygen. This process uses a photoelectrode to convert light energy into chemical energy.
4. Reforming of Natural Gas
Natural gas reforming is a process that converts natural gas into hydrogen and carbon dioxide using a catalyst.
5. Gasification of Coal and Biomass
Gasification is a process that converts coal or biomass into a gas mixture containing hydrogen, carbon monoxide, and carbon dioxide.
6. Photo-Biological Method with High-Temperature Decompositions
The photo-biological method uses microorganisms to produce hydrogen through photosynthesis. High-temperature decompositions can also be used to produce hydrogen from biomass.
7. Hydrocarbon Reforming
Hydrocarbon reforming is a process that converts hydrocarbons into hydrogen and carbon dioxide using a catalyst.
8. Ammonia Cracking
Ammonia cracking is a process that converts ammonia into hydrogen and nitrogen using a catalyst.
9. Pyrolysis
Pyrolysis is a process that converts biomass into a gas mixture containing hydrogen, carbon monoxide, and carbon dioxide through thermal decomposition.
10. Aqueous Reforming
Aqueous reforming is a process that converts biomass or organic compounds into hydrogen and carbon dioxide using a catalyst in an aqueous solution.
11. High-Temperature Electrolysis
High-temperature electrolysis is a process that splits water into hydrogen and oxygen using electricity at high temperatures.
12. Photo-Electrolysis (Photolysis)
Photo-electrolysis, also known as photolysis, is a process that uses light to split water into hydrogen and oxygen.
13. Photo-Biological Production (Biophotolysis)
Photo-biological production, also known as biophotolysis, is a process that uses microorganisms to produce hydrogen through photosynthesis.
All above technologies depend on the raw material used for the production of hydrogen, the mostly known feedstocks and process dependents are Algae, Gas, Oil, Wood, Coal, biomass and Power.
Efficiency metrics of the 13 hydrogen production techniques:
 |
| Hydrogen production techniques |
| Technique | Efficiency Range (%) | Energy Efficiency (%) | Cost-Effectiveness ($/kg H2) | Environmental Impact (g CO2/kg H2) |
|---|---|---|---|---|
| Silicon Chip-Based Microreactor by Methanol Reforming | 70-90 | 80 | 2-3 | 100-200 |
| Water Splitting by Solar Energy Using Thermochemical Cycle | 10-20 | 15 | 5-6 | 50-100 |
| Photoelectrolysis | 10-20 | 12 | 6-7 | 40-80 |
| Reforming of Natural Gas | 70-90 | 85 | 1-2 | 150-300 |
| Gasification of Coal and Biomass | 40-60 | 50 | 3-4 | 200-400 |
| Photo-Biological Method with High-Temperature Decompositions | 10-20 | 12 | 6-7 | 40-80 |
| Hydrocarbon Reforming | 70-90 | 80 | 2-3 | 100-200 |
| Ammonia Cracking | 50-70 | 60 | 2-3 | 80-150 |
| Pyrolysis | 40-60 | 50 | 3-4 | 200-400 |
| Aqueous Reforming | 50-70 | 60 | 2-3 | 80-150 |
| High-Temperature Electrolysis | 70-90 | 80 | 2-3 | 100-200 |
| Photo-Electrolysis (Photolysis) | 10-20 | 12 | 6-7 | 40-80 |
| Photo-Biological Production (Biophotolysis) | 10-20 | 12 | 6-7 | 40-80 |
a) Hydrogen from NG
We have three chemical production processes
1. Steam reforming
CH4 + H2O + heat → CO + 3H2 ( Primary reforming reaction)
CO + H2O → CO2 + H2 + heat ( water-gas shift reaction)
CO + H2O → CO2 + H2 + heat ( water-gas shift reaction)
Mathematical Model:
Assumptions:
- The reactions are carried out in a plug-flow reactor.
- The system is at steady state.
- The reactions are irreversible.
- The heat of reaction is negligible.
Model Equations:
Mass balance for CH4:
- ∂(FCH4)/∂z = -r1 * A
Mass balance for H2O:
- ∂(FH2O)/∂z = -r1 * A - r2 * A
Mass balance for H2:
- ∂(FH2)/∂z = 3 * r1 * A + r2 * A
Mass balance for CO:
- ∂(FCO)/∂z = r1 * A - r2 * A
Mass balance for CO2:
- ∂(FCO2)/∂z = r2 * A
Reaction Rates:
Steam reforming reaction rate:
- r1 = k1 * (FCH4 / Ftot) * (FH2O / Ftot)
Water-gas shift reaction rate:
- r2 = k2 * (FCO / Ftot) * (FH2O / Ftot)
Boundary Conditions:
Inlet conditions:
- FCH4(0) = FCH4_in
- FH2O(0) = FH2O_in
- FH2(0) = 0
- FCO(0) = 0
- FCO2(0) = 0
Outlet conditions:
- ∂(FCH4)/∂z(L) = 0
- ∂(FH2O)/∂z(L) = 0
- ∂(FH2)/∂z(L) = 0
- ∂(FCO)/∂z(L) = 0
- ∂(FCO2)/∂z(L) = 0
Parameters:
- k1: steam reforming reaction rate constant
- k2: water-gas shift reaction rate constant
- A: cross-sectional area of the reactor
- L: length of the reactor
- Ftot: total molar flow rate
- FCH4_in: inlet molar flow rate of CH4
- FH2O_in: inlet molar flow rate of H2O
This model can be solved using numerical methods, such as the method of lines or finite difference methods, to obtain the profiles of the species concentrations along the reactor length.
Here, we will use the finite difference method to discretize the model equations and solve them using MATLAB.
% Define the parameters
k1 = 0.1; % steam reforming reaction rate constant
k2 = 0.05; % water-gas shift reaction rate constant
A = 1; % cross-sectional area of the reactor
L = 10; % length of the reactor
Ftot = 1; % total molar flow rate
FCH4_in = 0.5; % inlet molar flow rate of CH4
FH2O_in = 0.5; % inlet molar flow rate of H2O
% Define the grid size
N = 100;
dz = L / (N - 1);
z = linspace(0, L, N);
% Initialize the arrays to store the solution
FCH4 = zeros(N, 1);
FH2O = zeros(N, 1);
FH2 = zeros(N, 1);
FCO = zeros(N, 1);
FCO2 = zeros(N, 1);
% Set the inlet conditions
FCH4(1) = FCH4_in;
FH2O(1) = FH2O_in;
FH2(1) = 0;
FCO(1) = 0;
FCO2(1) = 0;
% Solve the model equations using finite difference method
for i = 2:N
FCH4(i) = FCH4(i-1) - k1 * (FCH4(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz;
FH2O(i) = FH2O(i-1) - k1 * (FCH4(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz - k2 * (FCO(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz;
FH2(i) = FH2(i-1) + 3 * k1 * (FCH4(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz + k2 * (FCO(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz;
FCO(i) = FCO(i-1) + k1 * (FCH4(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz - k2 * (FCO(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz;
FCO2(i) = FCO2(i-1) + k2 * (FCO(i-1) / Ftot) * (FH2O(i-1) / Ftot) * dz;
end
% Plot the results
figure;
plot(z, FCH4, 'b-', z, FH2O, 'r-', z, FH2, 'g-', z, FCO, 'c-', z, FCO2, 'm-');
xlabel('Reactor Length (m)');
ylabel('Molar Flow Rate (mol/s)');
legend('CH4', 'H2O', 'H2', 'CO', 'CO2');
This code solves the model equations using the finite difference method and plots the molar flow rates of the species along the reactor length. The results show the conversion of CH4 and H2O to H2, CO, and CO2 along the reactor length.
 |
| Conversion of Natural gas to Hydrogen along the Plug Reactor |
CH4 + 1 / 2O2 → CO + 2H2 + heat
3. Auto thermal reforming the above two methods are used simultaneously.
b) Hydrogen from Coal
By coal gasification in a fixed bed or fluidized bed gasifier hydrogen can be produced
C(s) + H2O + heat → CO + H2
c) Hydrogen production from water by splitting
H2O + electricity → H2 + 1 / 2O
d) Alkaline electrolysis cell
 |
Alkaline electrolysis |
Cathode: 4 H+ + 4e–↔2H2
Anode: 4OH– ↔ O2 + 2H2O + 4e–
Sum: 2H2O ↔ O2 + 2H2
e) Polymer electrolyte membrane (PEM) electrolysis
Anode: H2O→ 1 / 2O2 + 2 H+ + 2e–
Cathode: 2H+ + 2e–→ H2
Photosynthesis: 2H2O → 4H+ + 4e– + O2
Hydrogen Production: 4H+ + 4e–→ 2H2
f) High-temperature decomposition
Thermo-chemical water splitting
(850 °C): H2SO4 ↔ SO2 + H2O + 1 / 2
(120 °C): I2 + SO2 + 2H2O → H2SO4 +
(450 °C): 2HI → I2 + H2
SUM: H2O → H2 + 1 / 2O2
Hydrogen storage in solid form:
- Using microporous metal-organic materials
- Carbon nanotubes and graphite nanofibers
- Platinum and palladium nonporous films
- Using boron nitride nanotubes
Hydrogen storage in liquid form:
- Composite tanks
- Glass microspheres
- Cryogenic liquid hydrogen (LH2)
- NaBH4 solutions: NaBH4 (l) + 2H2O (l) → 4H2 (g) + NaBO2 (s) (ideal reaction)
- Rechargeable organic liquids