硼氫化鈉水解產氫系統之設計與控制

Abstract

世界能源正面臨一個新的轉捩點，在能源消費結構中，開始從石油為主要能源逐步向多元能源結構過度，氫能源便是替代能源中極受囑目之一。使用氫能源的一大問題的氫氣的儲存，其中化學儲氫法的硼氫化鈉(NaBH4)具有潛力達到美國能源局(DOE)所訂定的2015年儲氫系統能源密度標準(5.5wt%) [28]。 此篇研究係建立硼氫化鈉(NaBH4)連續式產氫系統之數學模式，以描述該系統的行為，在等溫、絕熱、部份熱移除等不同的操作狀態下，探討反應器不同的熱移除量與系統儲氫量之關聯。在連續式產氫系統中，偏硼酸鈉(NaBO2)的濃度會受到高反應熱釋出所產生的大量水蒸氣之影響，而較高的反應器出口溫度可以有效提升NaBO2飽和溶解度，因此產氫系統傾向操作在部份熱移除的情況下，會有較好的能量密度表現。接著，為了解決大量氣體(包括水蒸氣和氫氣)充滿塞流式反應器，造成NaBH4溶液無法有效與觸媒表面活性位置接觸之問題，本研究同時提出一個新的反應器設計架構，即在塞流式反應器中，增設一層針孔膜，藉此分離出液相與氣相通道，使NaBH4水解反應更易進行，並可省去裝設氣液分離器的重量，進一步增加系統的能量密度。 此外，本研究針對控制目標，即加快氫氣產能需求改變時之動態響應，並避免NaBO2析出的問題，提出了二個控制架構如下，一、以進料當作產能調節變數；二、以氫氣出口流量做為產能調節變數。在此二種控制架構下，當氫氣需求量改變時，皆無NaBO2析出的問題，並且氫氣流量皆獲得合理的動態響應。 為了使產氫系統能即時、快速地供應燃料電池使用，本研究建立之冷進料的啟動策略係脈衝連結一階式的進料形態，在絕熱環境下操作，並且結合前次啟動後儲存在反應器中的氫氣，使硼氫化鈉(NaBH4)產氫系統達到DOE的目標啟動時間(即5~15秒) [28]。

NaBH4 hydrolysis for on-board hydrogen generation has received much attention recently due to its higher theoretical energy capacity and zero emissions. In this work, three different operating modes (adiabatic, isothermal, Partially-insulated) of a continuous hydrogen generation system using the NaBH4 hydrolysis reaction are explored. Partially-insulated operation is recommended for this system since it has a higher outlet temperature and lower temperature distribution in reactor, which both mitigate the NaBO2 precipitation problem so that a larger energy density can be achieved. A novel reactor design is proposed to overcome the effects of gas generation and multiphase flow from the NaBH4 hydrolysis reaction. With a pinhole membrane set in the middle of reactor tube to provide gas and liquid channels respectively, the multi-phase flow problem is reduced and energy capacity increased since a separator is not needed for gas-liquid separation. Next, two control structures are developed and both give reasonable dynamic results. The on-supply structure gives fast response while the on-demand structure provides a simple control loop to adjust hydrogen generation directly. After control policies are designed, a cold start-up strategy is developed using a pulse plus step function feed input and reserved hydrogen from previous reaction to give rapid and sufficient hydrogen gas to supply a PEM fuel cell.