Coal gasification

Coal gasification refers to the conversion of a solid fuel into synthesis gas (chiefly carbon monoxide and dihydrogen) through high-temperature partial oxidation. This process belongs to the family of partial oxidation technologies, which also encompasses the gasification of heavy fuel oils and petroleum residues. Within the fertiliser sector, it offers an alternative route to steam methane reforming for ammonia manufacture, particularly in regions endowed with sizeable coal reserves yet limited gas resources.

Technological principles

Two gasifier designs have been commercialised for ammonia production: moving-bed gasification and entrained-flow gasification^[1]. In the latter arrangement, coal is fed either as dry dust (via lock hoppers and rotary feeders) or as a concentrated coal-water slurry (pumped by piston pumps)^[1]. Operating pressures range from 30 to 80 bar^[1]. The non-catalytic reaction proceeds at temperatures around 1,500 °C, with steam injection to moderate the heat^[1]. The raw gas exhibits low methane content (0.4 %), moderate carbon dioxide levels, and a CO/H2 ratio exceeding unity^[1].

The entrained-flow gasifier essentially embodies the concept of heavy hydrocarbon partial oxidation, carried out within an empty pressurised chamber^[1]. Variations chiefly concern the fuel injection method^[1]. Hot-gas cooling and solids removal may be accomplished by different devices depending on the process, notably through water quenching or in a waste-heat boiler, with varying degrees of integration^[1]. Mineral ash is collected as slag at the gasifier base^[1].

Integration into ammonia production

The raw syngas must undergo several purification stages before supplying the ammonia synthesis loop^[2]. An air separation unit provides the oxygen required for partial oxidation and the nitrogen needed for the synthesis reaction as well as for final liquid-nitrogen wash^[2][3]. This liquid-nitrogen scrubbing removes residual impurities and adjusts the hydrogen/nitrogen ratio of the syngas^[3]. The gas then passes through the standard steps of CO shift conversion, CO2 removal, and compression before being routed to the ammonia synthesis reactor^[4].

Sulphur compounds present in the feedstock are converted to hydrogen sulphide during gasification^[3]. In the heavy-oil route, heavy metals (nickel, vanadium) introduced with the fuel appear suspended as oxides and partly as salts in the aqueous soot-washing circuit; a purge of these waters is required to prevent their build-up, and the resulting paste rich in vanadium and nickel is retrieved and marketed to the metallurgical sector^[1].

Energy and environmental balance

Compared with natural-gas reforming, the coal route shows markedly higher consumption and costs. According to benchmark estimates for modern technologies under European economic conditions, relative energy consumption stands at 1.7 (natural-gas basis = 1.0), capital costs at 2.4, and production costs at 1.7^[5]. Ammonia derived from coal carries a carbon footprint nearly double that from natural gas: based on Fertilizers Europe (2016) data for 2014 production, urea manufacture from coal-based ammonia generates 6.53 kg CO2 equivalent per kg of urea nitrogen, versus 3.50 kg for a European facility running on gas^[6]. For ammonium nitrate, the gap widens further: 11.06 kg CO2 equivalent per kg of nitric nitrogen in China (coal), against 3.42 kg in Europe according to the same figures^[6].

Outlook and market position

Despite its lower energy efficiency, coal gasification remains strategically important in certain economies with abundant coal reserves. It helps set the "price floor" in the nitrogen fertiliser market when demand is weak, this threshold being determined by the producing region with the highest feedstock energy costs (natural gas or coal)^[7]. Over a very long-term horizon (50 to 200 years), global coal reserves could grant it an increasing role should gas resources become depleted^[3][5]. However, the emergence of green and blue ammonia and regulatory pressure on greenhouse-gas emissions, notably through the EU ETS and CBAM, are calling into question the competitiveness of this carbon-intensive pathway in Europe.