CONVENTIONAL AND ADVANCED HYDROGEN PRODUCTION METHODS
5.1.2. Conversion of Coal
The most abundant fossil fuel on earth is coal. Its conversion to liquid and gaseous hydrocarbons has been commercially employed worldwide. In the first half of this century, it was the principal method of hydrogen production in the coke furnace process. The gas production from coal besides the anaerobic combustion process is realized by means of a gasification medium which reacts with the coal at temperatures > 800 °C. All organic constituents of the coal will be converted with long enough residence times. The gasification medium used is either steam ("steam-coal gasification") or hydrogen ("hydro-gasification").
If air or oxygen is injected into the gasifier, part of the coal is burnt directly leading to an autothermal reaction.
5.1.2.1. Steam-Coal Gasification
During steam-coal gasification, two consecutive chemical processes take place [30, 50]. The first process, the pyrolysis reaction
CHxOy <—» (1-y) C + y CO + x/2 H2
CHxOy <—> (l-y-x/8) C + y CO + x/4 H2 + x/8 CH4
is a very rapid expelling of all volatile constituents of the coal. In the subsequent, much slower step, steam of 600 - 1000 °C is needed to convert the residual organic solids in the heterogeneous water gas reaction:
C + H2O *—» CO + H2 - 163 kJ/mol
with further increase of the H2 yield in the shift reaction. The thermodynamic equilibrium composition of the product gas mixture is a function of temperature and pressure. As can be drawn from Fig. 5-6, the production of hydrogen and carbon monoxide is optimal at high temperatures and low pressures. The heat of the hot gas is recuperated for use in the production of high-pressure steam; the heat withdrawal must be quick to avoid reverse chemical reactions.
Despite its small hydrogen contents, coal gasification with oxygen and steam is used to generate about 18 % of the world hydrogen production. Also the production of methane or methanol or gasoline from the coal conversion step by modifying downstream treatment is conventional chemical engineering. Pulverized coal is rapidly partially oxidized by oxygen and steam in a fluidized bed at about atmospheric pressure. 30 - 40 % of the coal are transformed into CO2 to supply splitting energy of water. The reaction rate strongly increases with temperature. Temperatures up to 2000 °C and pressures up to 3 MPa are typically chosen.
Pertinent criteria for applicability and economy of steam-coal gasification are the characteristics of the coal to be gasified. The (geologically) older the coal, the smaller is its reactivity and the higher is the temperature required [30]. Main disadvantages of coal gasification are the handling of solid material streams, which is generally more difficult
100 01 MPa 100- 4MPa
0
600 700 800 900 1000 °C 1200 600 700 800 900 1000 °C 1200 Temperature ——>• Temperature ——*
Fig. 5-6: Equilibrium composition of the product gas after steam-coal gasification as a function of temperature and (left) 0.1 MPa, (right) 4 MPa, from [30]
and more expensive than gaseous or liquid streams, and the large amounts of CC>2, SC>2, and ash produced during the process.
Different types of autothermal coal gasification processes used on a large scale are existent as shown in Fig. 5-7, Lurgi, Winkler, Koppers-Totzek, and their modifications, respectively. Their characteristic features are summarized in Table 5-1.
The TEXACO gasification process operates at high pressures (« 5.5 MPa) and high steam contents. The synthesis gas typically contains 34 % H2 and 48 % CO. Hydrogen of > 97 % purity is yielded at a pressure of 4 MPa to reduce energy consumption during H2 pressurization [55].
At present, coal gasification is primarily used for ammonia synthesis in the fertilizer industry and for synthesis gas production to be employed in the hydrocarbon and methanol synthesis with large-scale production facilities, in particular, in the developing countries.
Hydrogen production is of minor importance.
The option of integrating the steam-coal gasification process in a combined heat and power facility with a gas turbine step preceding the water/steam process, is currently considered the cleanest and most efficient (34 —»• 38 %) coal-fueled technique so far, and the combined-cycle portion could be switched to different fuels, e.g., natural gas or oil.
The Integrated Gasification Combined Cycle (IGCC) technology with its intermediate stage of synthesis gas allows the removal of most carbon components before combustion.
The separated COi stream is of high purity and therefore suited for disposal. Adopting improvements in the turbine system, the overall efficiency is anticipated to be raised to 52
% [64]. Respective facilities have been realized so far in Germany and the United States
Table 5-1: Characteristic features of the steam-coal gasification processes after Lurgi,
Fig. 5-7: Gas generator types for steam-coal gasification according to Lurgi (left), Winkler (middle), and Koppers-Totzek (right), from [66]
[30]. Some 4000 MW(e) of these IGCC power systems are planned for installation in the USA, Europe, and Asia [54]. The world's largest IGCC plant with 300 MW(e) is almost completed in Spain with an expected efficiency of 45 %.
A 300 MW IGCC power plant with electricity and methanol cogeneration has been proposed where 88 % of the carbon introduced by the coal can be extracted as CC>2 with an acceptable amount of energy and investment cost which is partially utilized in combination with H2 from an external carbon-free source for methanol production. The overall balance for a 354 MW power plant (234 MW for the gas turbine and 120 MW for the steam generator) is an input of 2300 t of coal, 780 t of hydrogen, and 5500 t of CC>2 (as intermediate product) per day to achieve a daily output of 3800 t of methanol and a total net power output of 310 MW [44].
At a very early R&D level is the use of coal gasification integrated in a high-temperature fuel cell as a stationary power system (see section 7.2.2.3.).
5.1.2.2. Hydro-Gasification
In the hydro-gasification process, coal is converted to synthetic natural gas in a fluidized bed at temperatures of around 800 °C in an exothermal reaction:
C + 2 H2 -» CH4 + 86 kJ/mol
Parallel reactions are the steam reforming and the water-gas shift reactions to produce synthesis gas. The feed gas hydrogen could be taken either from a partial conversion of methane from the product gas in a steam reformer or from a conversion of the residual char with oxygen and steam in a high temperature Winkler process. A high gasification degree can be reached already with relatively short residence times (9 - 80 min) [50]. In order to obtain a high conversion rate of coal, the CHU fraction in the product gas of the reforming step (which will then be the feed gas for the hydro-gasification) should be no higher than 5 %, which requires a low-temperature separation step. Both reactions are carried out at system pressures of 4 MPa. The amount of 1.5 Nm3 of hydrogen is required to produce 1 Nm3 of synthetic natural gas [32].
After cleaning and separation, the methane is either utilized as a final product or fed into the reformer while the product hydrogen is available after the reforming process in the synthesis gas, subtracted by the portion that is required for the gasification step.
The advantage of hydro-gasification compared with steam-coal gasification is its 200 K lower pre-heating temperature which reduces potential corrosive attack. A major drawback, however, is the relatively large amount of residual coke of up to 40 % [13]. Standardized reformer equipment for power industries is available.
5.1.2.3. Steam-Iron Process
The steam-iron process is another old hydrogen generation process. Although based on coal, it is actually a cycle process where hydrogen is generated from the decomposition of steam by reacting with iron oxide. The cycle, however, is not completely closed, since
coal is consumed and CO2 is emitted. The synthesis gas produced in the coal gasification step with steam is then reacted with iron oxide to generate reduced forms of iron oxides:
Fe3O4 + H2 -» 3 FeO + H2O Fe3O4 + CO -» 3 FeO + CO2
FeO + H2 -* Fe + H2O FeO + CO -» Fe + CO2
In the following step, the reduced species are re-oxidized with water to form the original oxides plus a hydrogen enriched gas:
Fe + H2O -» FeO + H2
3 FeO + H2O -+ Fe3O4 + H2 815 - 870 °C Reduction and oxidation step in two separate reactors allow for a continuous hydrogen production. A major drawback is the small conversion rate of 60 % of the synthesis gas in the reduction step. The effluent from the steam-iron reactor contains 37 % H2 plus 61 % steam and 96 % H2, respectively, if the steam is condensed. The remaining heating value plus sensible heat at 825 °C, however, can be used to cogenerate electricity. With a plant capacity of 110,000 Nm3/h of H2, the byproduct electric power is 158 MW [55].
5.1.2.4. Gasification in a Molten-Iron Process
Temperatures of around 1400 °C in the molten-iron process, MIP, are capable of cracking almost any chemical compound, in particular that of coal. Product gases are CO and H2. Other constituents are remaining as slag in the iron bath. Process heat is provided by the exothermal partial oxidation of C to CO. The MIP process is still in the development stage.
5.1.2.5. Coal Cracking Process
The HYDROCARB coal cracking is an advanced process where natural gas is synthesized from the hydrogenation of the carbon containing raw material. Coal is routed to a thermal cracker where it is decomposed to carbon black as a clean fuel and hydrogen as a byproduct fuel. HYDROCARB represents the possibility of a CO2-free fossil-based hydrogen economy. The reaction in the hydropyrolyzer operating on bituminous coal is:
CHo.gOo.o8So.oi6No.oi5 + -» C + 0.32 H2 + 0.08 H2O + 0.008 N2 + ash + CaCO3 ash + 0.016 CaS
The HYDROCARB process is expected to become highly competitive with all other coal-based processes [55].
5.1.2.6. Byproduct from Coke Production
Coke furnace gas was formerly the major source for hydrogen until replaced by more efficient petroleum-based processes and by coal gasification. However, it is still an important and convenient method to obtain hydrogen on places where economically attractive.
CHo.gOo.2 -» 0.8 C + 0.2 CO + 0.4 H2
Modem coke furnaces produce about 350 m3 of coke furnace gas out of 1 ton of coal.
Half of it is used to run the furnace at 750 - 850 °C and atmospheric pressure, the other half is processed for hydrogen extraction by means of either low temperature condensation or by pressure swing adsorption.
From the worldwide coke production from hard coal, an estimated (theoretical) 60 billion Nm3 of hydrogen could be recovered [14].