Air emissions from fertiliser manufacturing: Which pollutant is generally absent or negligible in direct process emissions from a typical nitrogen fertiliser (ammonia/urea) plant?

Introduction / Context:
Different fertiliser plants (nitrogenous vs. phosphatic) have distinct emission profiles. Identifying typical pollutants informs stack monitoring, scrubber selection, and compliance planning.

Given Data / Assumptions:

• Plant considered is a nitrogen fertiliser complex (ammonia/urea) rather than a phosphatic acid/superphosphate unit.
• Focus is on process emissions rather than auxiliary boilers.

Concept / Approach:
Ammonia/urea plants feature synthesis loops and urea prilling/granulation. Fugitive and stack emissions often include unreacted NH3 (ammonia slip) and NOx from fired reformers. Phosphatic fertiliser and sulphuric acid units would emit SOx, but in a pure nitrogen fertiliser facility significant SO2/SO3 is not characteristic unless sulphur-based utilities are present. Carbon monoxide (CO) is typically minimal in properly operated reformers and is largely converted to CO2 via shift before synthesis; residual CO in process vents is negligible compared to other pollutants.

Step-by-Step Solution:

Verification / Alternative check:
Plant permits typically list NH3 and NOx as primary controlled pollutants for urea units; CO limits are often applicable only to utility boilers rather than process.

Why Other Options Are Wrong:

Common Pitfalls:
Conflating boiler stack pollutants with process vents; always distinguish plant type and emission point.

Final Answer:
CO