Marine Methanol

Production methods

The by far dominating production method of methanol synthesis is through the synthesis gas process first developed during the 1920s. A gas mixture of hydrogen and carbon monoxide (usually also carbon dioxide), known as synthesis gas (syngas) is the basis for almost all methanol production today [1].

+ Methanol synthesises with synthesis gas

The production of methanol usually consists of three basic steps independent of feedstock material: synthesis gas preparation, methanol synthesis and methanol purification.

In order to properly understand the challenges of different processes to produce the synthesis gas, we first need to understand the process from synthesis gas to methanol, the methanol synthesis. In essence the process consists of the three following equations:

CO + 2H2 ⇌ CH3OH ΔH298K= -21.7 kcal/mol (1) CO2 + 3H2 ⇌ CH3OH + H2O ΔH298K=-11.9 kcal/mol (2) Reverse WGS CO2 + H2 ⇌ CO+H2O ΔH298K= 9.8 kcal/mol (3)

All three equations are reversible and thus the process conditions regarding temperature, pressure and synthesis gas mixture are important to control. It can also be noted that equation (1) and (2) are exothermic, i.e. the processes produce heat and require cooling. Some heat is normally recovered and used for other parts of the synthesis.

While it was originally believed that the main process that produce methanol was the reaction between carbon monoxide and hydrogen (equation 1) it is now understood that carbon dioxide is just as important in the synthesis process. CO2 even used to be scrubbed from the reactant mixture but a scrubber failure at Imperial Chemical Industries (ICI) with a resulting increase of methanol production showed that CO2 was active and important in the reaction. Subsequent studies has shown that it is mainly the CO2 that is converted to methanol while CO act as an reducing agent for oxygen at the surface of the catalyst [4].

Equation (3) describes the reverse water gas shift reaction that produces carbon monoxide from carbon dioxide and hydrogen. The carbon monoxide then reacts with hydrogen to produce methanol (equation 2). Equation 2 is actually merely the sum of (1) and (3) [2].

To synthesise methanol, not only is a specific H2/CO ratio of 2 in the synthesised gas needed but also a (H2-CO2)/(CO+CO2) ratio, called stoichiometric number, equal to or slightly above 2 [3].

+ Biomass

In contrast to ethanol, methanol can rather easily be produced from virtually all biomass such as wood, algae, agricultural waste and municipal waste through gasification. Production from biomass does however offer challenges that need to be addressed especially regarding the cost of production. Due to the composition of biomass the production plants inquire high capital investment costs and has lower energy conversion efficiency compared to natural gas and coal [7].

The conventional method to produce methanol from biomass is through gasification of the feedstock material. An attractive alternative is enzymatic conversion, although most research in this area is currently focused on ethanol production. Another alternative mainly for sea growing plants like macro- and microalgae as well as water hyacinth and cattail etc. is anaerobic digestion that produces methane that could be used in the same way as natural gas. As with enzymatic conversion the methods is still in need of further research and development before large scale commercial implementation is possible[2].

The gasification process of biomass is similar to the synthesis gas process from coal. For gasification of biomass the feedstock is first dried and pulverized. The moist content should generally be no higher than 15-20 wt%. The first step in a two-step gasification process is called pyrolysis, or destructive distillation. The dried biomass is heated to 400-600 °C in an oxygen deficient environment to prevent complete combustion. Carbon monoxide, carbon dioxide, hydrogen, methane as well as water and volatile tars are released. The remaining biomass (≈10–25 wt%),called charcoal. is further reacted with oxygen at high temperature (1300-1500 °C) to produce mainly carbon monoxide.

The synthesis gas produced from the pyrolysis and charcoal conversion is purified before the methanol synthesis. Compared to coal biomass consists of much less sulphur but the tar content offers operational challenges as it condense easily in pipes, filters and boilers. This can to an extent be controlled by choosing the right operational pattern and technique according to the composition of the available biomass. A one step partial oxidation process is an attractive alternative but the technical challenges have so far prevented large scale operation.

Production from biomass is possible at small scale but as with natural gas and coal large scale production is preferred due to the high system costs. The logistical challenges for a biomass plant are however great due to the lower energy content in biomass compared to natural gas and coal implying a large demand of feedstock material. The quantities needed to feed a 2500 MTPD plant is estimated to 1.5 million ton biomass per year [2] with put large strains on collection, transportation and storage of biomass and might be one of the largest hurdles towards the construction on mega sized plants [7].

An alternative that has been proposed to solve or ease the transportation demands is to first convert the biomass to bio-crude through fast pyrolysis. Dried and atomized biomass is quickly heated to about 400-600 °C in atmospheric pressure and then quenched to avoid cracking [2]. The result is a black liquid called bio crude that can be transported more easily.

+ Black liquor from pulp

Black liquor from the pulp industry has been identified as an interesting feedstock for renewable energy. Black liquor is formed as pulpwood is mixed with chemicals (white liquor) to produce pulp as a pre stage to paper production. Black liquor can be gasified and used for methanol synthesis. The chemicals are recovered and reused.

Black liquor is available in large quantities worldwide and offers a feasible way to produce methanol. An industrial scale demonstration plant at the Smurfit Kappa paper mill in Piteå, Sweden has been operational since 2010 producing DME.

+ Synthesis gas production from natural gas

Before natural gas is processed to synthesis gas impurities needs to be removed. The most important are sulphur compounds (such as H2S) because of the poisonous effects these have on catalysts downstream[5]. Other impurities such as carbon dioxide, nitrogen a

+ Steam reforming

The traditionally dominating method is through steam reforming where methane gas and steam is mixed at high temperature and pressure and with the help of catalysts form carbon monoxide and hydrogen (Equation 4). The gas mixture is typically led through pipes coated with catalysts in a tube in shell heat exchanger in order to provide the necessary heat (≈850 °C) for the reaction to take place.

Steam reforming 2CH4 + 2H2O ⇌ 2CO + 6H2 ΔH298K= 49.1 kcal/mol (4) Water gas shift CO + H2O ⇌ CO2 + H2 ΔH298K= -9.8 kcal/mol (5)

Carbon dioxide is typically added to the gas mixture before the methanol synthesis but can also be present in the natural gas used as feedstock.

Remembering that the H2/CO ratio should be 2 for methanol synthesis we observe that steam reforming produces an excess of hydrogen which needs to be subtracted and is usually burnt to provide heat for the reformation to take place but can also be used for other purposes.

The synthesis gas production is strongly endothermic and requires a lot of thermal energy. The methanol synthesis produce some heat that can be recovered but heat is normally also provided by burning a portion of the natural gas. The synthesis gas production and subsequent compression stands for a large amount of the investment cost in a methanol production plant and most of the energy need to power the process and represents as much as 60 % of the capital cost [1][6].

One step reforming through steam reforming used to be the dominating process but is today mainly considered for smaller plants up to 2500 MTPD where CO2 is available at low cost or contained in the natural gas [1].

+ Partial oxidation

Another basic rout for synthesis gas production is partial oxidation. Originally developed by Shell in the 1950s [3].

CH4 + 0.5O2 ⇌ CO+2H2 ΔH_298K= -8.6 kcal/mol (6) CO + 0.5O2 ⇌ CO2 ΔH_298K= -67.6 kcal/mol (7) H2 + 0.5O2 ⇌ H2O ΔH_298K= -57.7 kcal/mol (8)

The partial oxidation process for methane is slightly exothermic. The process occurs in the gas phase via radical reaction within the flame of the burner. A small excess of oxygen is needed to favour some oxidation to carbon dioxide and water in order to bring up the temperature to the desired 1000-1200 °C [3], unfortunately that produces carbon dioxide and water that lowers the stoichiometric number to about 1.6 which is below the preferred 2 but an improvement over steam reforming.

A large cost for a partial oxidation plant is the air separator needed to produce oxygen. While it is possible to use air most modern plants use pure oxygen to avoid the need of separating mainly nitrogen from the synthesis gas after the oxidation step.

+ Two-step reforming

A combination of steam reforming and partial oxidation offers a mean to improve the overall process and to better control the composition of the produced synthesis gas. The right system configuration depends on the composition of the natural gas used as feedstock. Two step reforming also requires that the steam reforming operates with high methane slip, usually 35-45 %, [1] to provide a high enough methane content for the partial oxidation step.

The technique is fairly new and was first used in a 2400 MTPD commercial plant in Norway 2007 and a 5000 MTPD plant with similar technology in Saudi Arabia 2008 [1].

+ Dry reforming

By reacting methane and carbon dioxide synthesis gas is produced in a process called dry reforming as no steam is used.

Dry reforming CH4 + CO 2 ⇌ 2CO + 2H2 ΔH298K= 59.1 kcal/mol (9)

The reaction is more endothermic than steam reforming and produces a gas with significant hydrogen deficit for methanol synthesis. While this is a disadvantage for methanol synthesis the gas mixture has the right composition for other applications.

+ Synthesis gas from coal

The process to convert coal to synthesis gas is a combination of partial oxidation and steam treatment called gasification. [2].

C + 0.5*O2 ⇌ CO ΔH298K= -29.4 kcal/mol (10) C + H2O ⇌ CO + H2 ΔH298K=31.3 kcal/mol (11) CO + H2O ⇌ CO2 + H2 ΔH298K= -9.8 kcal/mol (12) CO2 + C ⇌ 2CO ΔH298K=40.8 kcal/mol (13)

The design and processing conditions vary greatly depending on the composition of the coal used as feedstock. The synthesis gas produced have a deficit of hydrogen and must be subjected to the water gas shift reaction (equation 12) in improve the H2/CO ratio. The synthesis gas produced from coal is usually in higher need of purification than that produced from natural gas, especially sulphur compounds must be removed before the methanol synthesis to protect the sensitive catalysts from poisoning.