2.            STATE OF THE ART TECHNOLOGY

Hydrogen

Hydrogen can be produced from any primary energy sources, but not all production sources and methods are equally attractive from an economic and environmental point of view.

Generally, it is preferred to produce hydrogen from hydrocarbons, as the energy consumption is less than when producing hydrogen by generating electricity and subsequent electrolysis. Hydrogen produced from coal, natural gas or oil, however, involves emissions of CO_2. Therefore it will only be attractive from an environmental point of view, if a permanent method can be found for storing the CO₂ produced.

As shown below, there are different production processes that can be used to generate hydrogen.

Hydrogen production technologies

Hydrogen is a realistic energy carrier for the future energy system. From an environmental point of view the introduction of hydrogen technology will be attractive. When hydrogen is used together with renewable energy in the electricity or the transport sector CO₂ emission from the energy system is considerably lower.

Hydrogen can be produced by a number of processes:

• Electrolysis: using electricity from renewable resources such as wind power, hydropower and solar photovoltaic cells to separate water into Hydrogen and Oxygen.
• The direct splitting of water using light. This method produces no carbon dioxide.
• Biomass conversion: using microorganisms to convert sewage into Hydrogen. This method does produce carbon dioxide, but it is all recycled through the growth of more biomass.
• Reformation: converting natural gas into hydrogen.

Electrolysis

Electrolysis uses electric power to decompose water into hydrogen and oxygen. Hydrogen is formed at the cathode and oxygen at the anode in an electrolytic cell when at least 1.6 Volts are applied between the electrodes. The electrolyte is an aqueous solution.

There are three main kinds of water electrolysis: alkaline water electrolysis, solid polymeric electrolysis (SPE) and high temperature electrolysis (700 – 1000 °C). The principle of this latter type of electrolysis is that the total energy amount, or the major part of it, comes from light. In principle it can be considered as a hydrogen producing photovoltaic cell.

At present, the most common kind of electrolysis is alkaline electrolysis, usually in small or medium-sized plants (0.5 – 5 MW, that is 100 – 1000 Nm³/h). The amount of electricity needed for alkaline electrolysis is about 4 kWh/Nm³, including the energy loss, the energy for pumps etc. The electrolysis is carried out at temperatures of 70 – 100 °C.

Coal gasification

Oxygen and hydrogen are the only products of water electrolysis. From an environmental point of view, this process does not involve any emissions; however, emissions may occur from the production of the electricity used for electrolysis.

Coal gasification

Coal gasification is a process in which hydrogen is produced by the addition of oxygen at high temperatures and under increased pressure. Coal is gasified at high temperatures by adding steam and oxygen to produce a synthetic gas consisting of a mixture of CO and H₂, which must be cleaned before the next process. When producing pure hydrogen, CO is converted in the exothermal catalytic process:

                                   CO + H₂O ® CO₂ + H₂

This process takes place at lower temperatures. The remaining traces of CO are removed by a catalytic methane reaction. Pure hydrogen is finally obtained by cryogenic or other gas separation methods.

Coal gasification is a new technology in connection with electricity and CHP production. In the coal gasification plant, coal is gasified and the gas is burned in a gas turbine, which together with a degas boiler constitutes a combined cycle plant. The plant is very expensive, and it is based on the utilization of coal. Therefore it is suitable as base load plant, but might also be used as intermediate load plant.

A coal gasification plant is characterized by the ability to obtain high desulphurisation degrees of about 96 – 100 %. If a coal gasification plant is used in combination with a fuel cell, there will be no NO_x emissions. Burning of coal either directly or by gasification will always result in discharge of CO₂.

Production of hydrogen can take place in a CHP plant already established as a base load or medium load plant using coal gasification. The plant can function as an electricity and heat production unit during base load periods, whereas when electricity is not required, hydrogen can be produced in the coal gasification plant instead of electricity (for instance during periods when plenty of electricity is being generated from renewables). The renewable energy can in this way produce electricity for direct use, rather than converting it into hydrogen to be stored for later use.

Biomass gasification

Biomass gasification takes place in a gasification process similar to the steam reforming of natural gas. The process can be divided into three main stages:

1)      Generation of syngas (the reformer part),

2)      Addition of water to the gas (the shift-process) and

3)      Gas purification (the PSA-system).

Biomass contains a surplus of water and therefore less steam is needed in biomass gasification than for reforming natural gas.

From an environmental point of view the big advantage of biomass over coal gasification is that this process will - in connection with hydrogen - be environmentally more attractive.