Over the past decades, selective catalytic reduction (SCR) using aqueous urea sprays as ammonia precursor has become the prevalent technique for NO${}_{\mathrm{x}}$ emission control in mobile applications. Preparation of ammonia from urea water sprays still represents a challenge in aftertreatment engineering as complex interactions of multi-phase physics and chemical reactions have to be handled. Increasingly stringent emission legislations and the ongoing development of fuel-efficient engines and close-coupled aftertreatment systems raise high demands to SCR systems. Due to highly transient conditions and short mixing lengths, incomplete spray evaporation can result in liquid/wall contact and formation of solid urea deposits lowering ammonia selectivity and homogeneity. This article reviews the ongoing development of SCR systems with focus on the efficient evaporation and decomposition of the injected spray for a homogeneous ammonia distribution in front of the SCR catalyst. Critical aspects of spray evaporation and impingement, liquid film and deposit formation are pointed out and potentials for system optimization are discussed.

在过去的几十年里，使用尿素水溶液喷雾作为氨前体的选择性催化还原 (SCR) 已成为 NO 的流行技术${}_{\mathrm{X}}$移动应用中的排放控制。由于必须处理多相物理和化学反应的复杂相互作用，因此从尿素水喷雾中制备氨仍然是后处理工程中的一个挑战。日益严格的排放法规以及节油发动机和紧密耦合后处理系统的持续发展对 SCR 系统提出了更高的要求。由于高度瞬变的条件和短的混合长度，不完全的喷雾蒸发会导致液体/壁接触并形成固体尿素沉积物，从而降低氨的选择性和均匀性。本文回顾了 SCR 系统的持续发展，重点是喷射喷雾的高效蒸发和分解，以在 SCR 催化剂前实现均匀的氨分布。