Quality issue in urea production:\nWhich compound is the well-known undesirable by-product formed when urea is overheated or held too long, especially during prilling or granulation?

Introduction / Context:
Urea (46% N) is the most widely used nitrogen fertilizer. During manufacturing, especially in high-temperature hold-ups like melt conveyance, prilling, or granulation, secondary reactions can occur. Biuret formation is a central quality concern because elevated biuret levels can be phytotoxic, affecting seed germination and foliar applications.

Given Data / Assumptions:

• We consider thermal side reactions of urea.
• The by-product in question is “undesirable” for agronomic quality.
• Plant stages include synthesis (carbamate), decomposition to urea, then finishing (prills/granules).

Concept / Approach:
Biuret forms by condensation of two urea molecules with release of ammonia, yielding H2N–CO–NH–CO–NH2. Its concentration in the product rises with higher melt temperature and longer residence time. Fertilizer-grade specifications therefore limit biuret (often below about 1.0–1.5% for general use, tighter for certain crops) and emphasize careful temperature control and residence-time minimization in finishing equipment.

Step-by-Step Solution:
Recognize thermal pathway: urea → biuret + NH3 under overheating/long hold.Identify agronomic impact: biuret inhibits growth at elevated levels.Thus, among the listed choices, “Biuret” is the characteristic undesirable by-product.

Verification / Alternative check:
Process and agronomy references consistently flag biuret, not carbonate or carbamate, as the product-quality issue during finishing. Ammonium carbamate and carbon dioxide are intermediates/by-products of synthesis, not the main quality-limiting impurity in finished urea.

Why Other Options Are Wrong:
Ammonium carbonate/carbamate are upstream species; CO2 is a normal synthesis by-product; cyanuric acid is associated with different condensation/polymerization chemistries and is not the primary fertilizer impurity of concern.

Common Pitfalls:
Assuming any nitrogenous by-product is equally harmful—biuret is the key specification driver in urea fertilizers.

Final Answer:
Biuret