Water (H₂O) within coal seams is capable of competing the occurrence space with methane (CH₄), thereby, weakening its adsorption, diffusion, and flow in coal reservoirs. H₂O coexisted in coals will reduce CH₄ adsorption capacity and diffusion coefficient by up to 62.37% and 80%, respectively. Based on existing literature and data, this paper mainly reviews the occurrence of H₂O within various rank coals and bridges the current research gaps. Results show that H₂O content in coals varies between 0.5% and 25%, showing an L-shaped decreasing trend with coal rank. This trend is associated with the variation of oxygen-containing functional groups, pore specific surface area and pore volume, and clay minerals of coals. The modified Brunauer-Emmet-Teller (BET) model is applicable to describe the adsorption equilibrium of H₂O on coals with multiple correlation coefficient (R ²) above 0.9997. The separation of water vapor adsorption and desorption hysteresis loop on coals into low relative pressure (P/P _S) below 0.3 and high relative pressure above 0.3 can effectively distinguish the influence of coal matrix swelling effect, ink-bottle shaped pore effect and capillary condensation on H₂O occurrence. Considering that traditional methods cannot well determine H₂O in the closed pores, SANS and SAXS are predictive to estimate the occurrence space of H₂O within coals. Moreover, the low-temperature plasma ashing can remove the organic matter from coals and maintain mineral structure, which is expected to determine the content distribution and spatial distribution characteristics of H₂O in inorganic minerals and organic matters of coals. Finally, the accurate measurement of CH₄ adsorption and desorption on moist coals via volumetric method should be focused on. Specifically, the terahertz waves with humidity monitor sensitivity being 62 ppm are recommended to detect H₂O content in bulk phase for CH₄ adsorption and desorption system. Thus, incorporating terahertz waves into volumetric method is helpful to generate reliable data regarding CH₄ adsorption and desorption on moist coals.

煤层中的水（H ₂ O）能够与甲烷（CH ₄）竞争发生空间，从而削弱其在煤层中的吸附，扩散和流动。煤中共存的H ₂ O分别使CH _4的吸附能力和扩散系数分别降低62.37％和80％。在现有文献和数据的基础上，本文主要综述了各种等级煤中H ₂ O的存在，并弥补了目前的研究空白。结果表明，H ₂煤中O含量在0.5％到25％之间变化，随着煤级的升高呈L形下降趋势。这种趋势与含氧官能团，孔比表面积和孔体积以及煤的粘土矿物的变化有关。修正的Brunauer-Emmet-Teller（BET）模型可用于描述H ₂ O在具有0.9997以上的多重相关系数（R ²）的煤上的吸附平衡。将煤中水蒸气的吸附和解吸滞回回路分离为相对低压（P / P _S低于0.3和0.3的较高相对压力可以有效地区分煤基质膨胀效应，瓶形孔隙效应和毛细管冷凝对H ₂ O发生的影响。考虑到传统方法无法很好地确定封闭孔中的H ₂ O，SANS和SAXS可预测煤中H ₂ O的发生空间。此外，低温等离子灰化可以去除煤中的有机物并保持矿物结构，这有望决定煤无机矿物和有机物中H ₂ O的含量分布和空间分布特征。最后，精确测量CH ₄应重点研究通过体积法对湿煤的吸附和解吸。具体而言，建议使用湿度监控器灵敏度为62 ppm的太赫兹波来检测CH ₄吸附和解吸系统本体相中的H ₂ O含量。因此，将太赫兹波并入体积法有助于生成有关CH ₄在湿煤上吸附和解吸的可靠数据。