余热回收技术

余热回收技术

废热是与离开过程并进入大气的空气、废气和/或过程产品的废物流相关的能量。它是在各种过程中产生的能量，没有投入任何实际使用，而是损失或浪费到大气中。它是在足够高的温度下从过程中排出的能量，以便以经济的方式将部分能量回收用于有用目的

在废热的定义中，这意味着携带热量的废物流最终会与大气或地下水混合，并且这些流中包含的能量无法作为有用的能量。环境对废弃能源的吸收通常被称为热污染。

余热回收可以通过不同的余热回收（WHR）技术进行，以提供有价值的能源并降低整体能源消耗。有几种 WHR 技术可用，可用于捕获和回收废热。

工业过程中使用的大量能源作为热量以废气、气流和离开过程的液体/固体的形式被浪费掉。回收所有废热在技术上和经济上都是不可行的。更多地使用 WHR 技术也有助于减少温室气体 (GHG) 排放。

WHR 技术包括捕获过程中的废热，并将气体、液体或固体作为额外的能源传输回系统。能源可用于产生额外的热量或产生电能和机械能。可以在任何温度下排出废热。通常余热温度越高，余热质量越高，WHR工艺的优化就越容易。因此，重要的是要从一个过程中发现最大潜力的最大可回收热量，并确保 WHR 系统实现最大效率

废热的来源通常包括通过产品、设备和过程的传导、对流和辐射传递的热量损失以及燃烧过程释放的热量。热损失可分为（i）高温热、（ii）中温热和（iii）低温热。每种余热都可采用WHR技术，以实现WHR的最佳效率。

高温 WHR 包括回收高于 400 摄氏度的余热，中温余热范围为 100 摄氏度至 400 摄氏度，低温余热范围为低于 100 摄氏度的温度。一般大多数高温区余热来自直接燃烧过程，中温区余热来自燃烧装置排气，低温区余热来自工艺装置的零部件、产品和设备。

根据废热的类型和来源，为了证明可以使用哪种废热回收系统的合理性，有必要检查可从工艺中回收的热量的数量和等级。在废热的量化中使用了三个重要参数。这些参数是 (i) 数量、(ii) 质量和 (iii) 时间可用性。

可用废热的数量或数量可以使用等式 Q =V x d x Cp x (T1-T2) 计算。式中，Q为热容，V为物质携带热量的流量，d为物质的密度，Cp为物质的比热，(T1-T2)为物质之间的温差系统出口的最终最高温度（T2）和入口的初始温度（T1）。可用废热的量也可以用废物流的焓流表示，并由等式 H =m x h 给出，其中 H 是废物流的总焓率，m 是废物流的质量流量，并且h 是废物流的比焓。

质量可以粗略地用废物流的温度来表示。温度越高，可用于回收的废热就越多。来自较低温度源的 WHR，例如来自机器和冷凝器的冷却水，通常比较困难，通常需要使用热泵将温度升高到合适的温度以进行回收。

时间可用性是衡量废热在需要时的可用性。将余热的可用性与最终负荷相匹配是 WHR 有效性的一个重要考虑因素。因此，余热的有用性不仅取决于可用的数量，还取决于其质量是否符合潜在负荷的要求以及是否在需要时可用（时间可用性）。

具有成本效益的 WHR 和再利用涉及确定具有足够质量、数量和时间可用性的废热源，以及可以再利用回收的废热的热负荷。在低温到中温范围内有几种工艺可以重复利用废热。这些过程用于不同的行业。例如，某些蒸馏操作是开环热泵系统的理想选择，该系统对“塔顶”蒸馏蒸汽进行机械再压缩，然后在再沸器中冷凝，使蒸馏塔中的“底部”产品蒸发。这些应用通常涉及较小的温差，并且通常比使用燃料燃烧加热再沸器和冷却塔排出馏分中的热量更具成本效益。

评估 WHR 的可行性需要对废热源以及热量将要传递到的流进行表征。需要确定的重要废热流参数包括 (i) 热量，(ii) 热温度/质量，(iii) 成分，(iv) 最低允许温度，以及 (v) 运行时间表、可用性和其他物流.这些参数允许分析流的质量和数量，还可以洞察可能的材料/设计限制。例如，在 WHR 中传热介质的腐蚀是一个相当重要的问题，即使在流的质量和数量都可以接受的情况下也是如此。

WHR 选项和技术

WHR 的方法包括 (i) 在气体和/或液体之间传递热量，(ii) 将热量传递给进入熔炉的负载，(iii) 产生机械和/或电力，或 (iv) 使用废热用于加热或冷却设施的热泵。 WHR 技术的术语经常因行业而异。下面介绍主要的 WHR 技术。

热交换器

热交换器通常用于将热量从燃烧废气传递到进入炉膛的燃烧空气。由于进入炉膛的预热燃烧空气温度较高，因此燃料提供的能量较少。空气预热常用的技术有换热器、炉膛回热器、燃烧器回热器、回转回热器和被动式空气预热器。

恢复器 – 换热器在中高温应用中回收废气的废热。换热器可以基于辐射、对流或两者的组合。

一个简单的辐射换热器由两个同心长度的管道系统组成。热废气通过内部管道，热传递主要辐射到墙壁和外壳中的冷空气。预热的壳空气然后行进到炉燃烧器。对流式或管式换热器（热交换器）使热气体通过较大外壳中直径相对较小的管子。进入的燃烧空气进入外壳并在管道周围挡板，从废气中吸收热量。另一种选择是组合辐射/对流换热器。该系统包括一个辐射部分和一个对流部分，以最大限度地提高传热效率。

换热器由金属或陶瓷材料制成。金属换热器用于温度低于 1100 摄氏度的应用，而较高温度下的热回收更适合陶瓷管换热器。它们可以在高达 1550 摄氏度的热侧温度和 1000 摄氏度左右的冷侧温度下运行。

再生器 – 蓄热器有两种类型，即 (i) 熔炉蓄热器和 (ii) 旋转蓄热器或热轮。在炉式蓄热器的情况下，蓄热式炉由两个砖格子工作室组成，冷热气流交替通过。当燃烧废气通过一个燃烧室时，砖块会从燃烧气体中吸收热量，从而使温度升高。然后调整空气流量，使进入的燃烧空气通过热格子工作，将热量传递给进入炉子的燃烧空气。使用了两个燃烧室，一个燃烧室从废气中吸收热量，另一个燃烧室将热量传递给燃烧空气。气流的方向在一段时间后改变。再生器最常用于焦炉，历史上也用于平炉，较早用于炼钢。再生器还用于预热提供给炼铁中使用的高炉的热风。然而，高炉中的蓄热器不是热回收应用，而只是将气体燃烧释放的热量传递给热风空气的方法。再生器系统特别适用于排气脏污的高温应用。炉式蓄热室的一个主要缺点是尺寸大和投资成本高。  

在旋转蓄热器的情况下，它们的操作类似于固定蓄热器，因为通过在多孔介质中储存热量以及通过在蓄热器中交替流动冷热气体来促进热传递。旋转蓄热器有时也称为空气预热器和热轮。他们使用一个旋转的多孔盘放置在两个平行的管道上，一个包含热废气，另一个包含冷气体。由高热容材料组成的圆盘在两根管道之间旋转，将热量从热气管传递到冷气管。由于高温产生的热应力，热轮通常仅限于低温和中温应用。两根管道之间的较大温差会导致膨胀差和大变形，从而损害管轮气封的完整性。在某些情况下，陶瓷轮可用于更高温度的应用。热轮的另一个挑战是防止两种气流之间的交叉污染，因为污染物可以在热轮的多孔材料中传播。

热轮的一个优点是它可以设计为从清洁气流中回收水分和热量。当使用吸湿材料进行设计时，水分可以从一根管道转移到另一根管道。这使得热轮在空调应用中特别有用，其中进入的湿热空气将热量和水分传递给冷的排出空气。除了主要应用于空间供暖和空调系统外，热轮还有限地用于中温应用。

被动式空气预热器 – 被动式空气预热器是用于中低温用途的气对气热回收设备，用于防止两股气流之间的交叉污染。无源预热器可以有两种类型，即（i）板式和（ii）热管式。

板式换热器由多个平行板组成，这些板为冷热气流创建单独的通道。热流和冷流在板之间交替，并允许大量区域进行热传递。与热轮相比，这些系统不太容易受到污染，但它们通常体积更大、成本更高，并且更容易出现结垢问题。

热管式换热器由几根末端密封的管子组成。每个管道都包含一个毛细芯结构，该结构有助于工作流体在管道的热端和冷端之间移动。在这种热交换器中，热气体通过热管的一端，导致管内的工作流体蒸发。沿管道的压力梯度导致热蒸汽移动到管道的另一端，在那里蒸汽冷凝并将热量传递给冷气体。然后冷凝水通过毛细作用循环回到管道的热端。

蓄热式/蓄热式燃烧器 – 这些燃烧器包含再生或回热系统。与独立的蓄热式炉或换热器相比，它们在设计和构造上更简单、更紧凑。与使用环境空气运行的燃烧器相比，这些系统提供了更高的能源效率。自恢复燃烧器将热交换表面作为燃烧器主体设计的一部分，以便从离开的烟道气中捕获能量，然后通过主体返回。自蓄热式燃烧器将废气通过燃烧器主体进入耐火介质箱，并以类似于蓄热式炉的方式成对运行。通常，蓄热式燃烧器系统的热交换面积较小，而蓄热式燃烧器系统的质量比独立单元低。因此，它们的能源回收率较低，但成本较低且易于改造，使其成为能源回收的有吸引力的选择。

翅片管换热器/省煤器 – 翅片管换热器用于从低温到中温废气中回收热量以加热液体。翅片管由带有附加翅片的圆管组成，可最大限度地提高表面积和传热率。液体流过管子并从流过管子的热气体中接收热量。翅片管换热器，锅炉废气用于给水预热，通常被称为锅炉“省煤器”。

余热锅炉 – 余热锅炉（WHB）是一种水管锅炉，利用中高温废气产生蒸汽。 WHB 有多种容量可供选择，允许从 1500 立方米/小时到 150 万立方米/小时的气体摄入量。如果废热不足以产生所需水平的蒸汽，通常会添加辅助燃烧器或加力燃烧器以获得更高的蒸汽输出。蒸汽可用于工艺加热或发电。产生过热蒸汽一般需要在系统中增加一个外部过热器。

负载预热

负载预热是指使用离开系统的废热来预热进入系统的负载的任何努力。最常见的例子是锅炉给水预热，其中省煤器将热燃烧废气中的热量传递给进入锅炉的水。其他应用利用燃烧废气和进入炉内的固体材料之间的直接热传递。

虽然锅炉给水预热是一种标准做法，但在直接燃烧系统中熔化之前对材料进行负载预热并没有广泛使用。这是由于多种原因造成的，包括难以控制产品质量、与环境排放相关的问题，以及建造先进的熔炉装载/热回收系统的复杂性和成本增加。然而，通过负载预热进行热回收近年来受到了越来越多的关注。不同负载预热炉的可用技术和障碍因炉子类型和相关负载而有很大差异。

低温能量回收选项和技术

虽然经济性通常限制了低温 WHR 的可行性，但在一些应用中，低品位废热已被成本有效地回收以供使用。大量余热可在40℃~200℃范围内利用，其回收利用存在固有挑战，需要对低温余热进行单独深入研究。

大部分工业余热处于低温范围。例如，锅炉等燃烧系统经常使用回收技术，在 150 摄氏度至 180 摄氏度左右排放废气。此外，工业冷却水和冷却空气中也存在大量废热。例如，仅空气压缩机的冷却就占了每年大量的废热。日本一家综合钢铁厂（ISP）成功安装了一座容量为 3.5 兆瓦的发电厂，其冷却水温度仅为 98 摄氏度。

在燃烧废气的情况下，如果气体中包含的水蒸气被冷却到较低的温度，则可以回收大量的热量。通常采用大约 120 摄氏度至 150 摄氏度的最低温度限制，以防止废气中的水在热交换器表面冷凝和沉积腐蚀性物质。然而，进一步冷却烟道气可以通过回收汽化潜热来显着提高热回收率。潜热包括废气中所含能量的很大一部分。在将废气冷却到冷凝点以下的同时，可以最大限度地减少化学侵蚀的技术可以通过回收蒸发潜热来显着提高能源效率。图 1 显示了不同烟囱出口温度下的能量回收情况。如果气体从 150 摄氏度冷却到 60 摄氏度，那么效率会提高 3%。将气体进一步冷却至 38 摄氏度会捕获一部分潜热，并可以将效率提高 11%。

图 1 不同烟囱出口温度下的能量回收  

低温热回收面临三个挑战。这些挑战如下所示。

• 换热器表面有腐蚀。随着废气中所含的水蒸气冷却，其中的一部分会凝结并在热交换器表面沉积腐蚀性固体和液体。热交换器的设计应能承受这些腐蚀性沉积物的暴露。这通常需要使用先进的材料，或者经常更换热交换器的组件，这通常是不经济的。
• 传热需要较大的热交换表面。传热率是热交换材料的热导率、两个流体流之间的温差以及热交换器的表面积的函数。由于低温废热涉及两个流体流之间的较小温度梯度，因此需要较大的表面积来进行热传递。这限制了热交换器的经济性。
• 对低温热量有要求。只有当工厂需要低温热量时，在低温范围内回收热量才有意义。潜在的最终用途是低温工艺加热。其他选项包括使用热泵将热量“升级”到更高的温度，以服务于需要更高温度的负载。此外，低温发电技术正在慢慢兴起。

低温热交换技术

低温热交换技术可以将气体冷却到露点温度以下以回收低温废热。技术选择包括深度省煤器、间接接触冷凝回收、直接接触冷凝回收和最近开发的运输膜冷凝器。由于成本高以及设施缺乏回收热的最终用途，这些技术的商业化受到限制。当设施缺乏废热的最终用途时，将使用其他回收手段，包括热泵和低温发电。这些技术也经常受到经济限制。

深度节能器 – 深度省煤器旨在将废气冷却至 70 摄氏度左右，并能承受其表面沉积的酸性冷凝水。节能器的设计可以有不同的选择。它可以在省煤器的冷端安装一个“一次性”部分。冷端的管道会随着时间的推移而退化，需要经常更换。更换频率取决于烟气成分和结构材料。其中一种选择是用不锈钢管设计省煤器。与建筑中通常使用的低碳钢相比，不锈钢可以更好地承受酸性气体。在另一种设计中，大部分热交换器使用 C 钢，但在出现酸性沉积物的冷端使用不锈钢管。使用玻璃管换热器（主要用于空气预热器等气-气应用）或铁氟龙等先进材料可以是其他替代方案。

间接接触冷凝回收 – 间接接触冷凝回收装置将气体冷却到 40 摄氏度左右。在此范围内，气体中的水蒸气几乎完全冷凝。间接接触式换热器由壳管式换热器组成。它们可以采用不锈钢、玻璃、铁氟龙或其他先进材料进行设计。

直接接触冷凝回收 – 直接接触冷凝回收涉及工艺流和冷却液的直接混合。由于这种类型的回收不涉及传递热量的分隔壁，因此它避免了间接接触回收装置所需的大传热表面的一些挑战。在这种类型的回收中，当烟气进入热交换器时，它们被装置顶部引入的冷水冷却。加热的水流通过交换器的底部离开并为外部系统提供热量。直接接触冷凝的一个挑战是水会被烟气中的物质污染。

运输膜冷凝器 – 传输膜冷凝器 (TMC) 是一种正在开发的技术，用于从废气流中的水蒸气中捕获水（连同水的潜热）。通过采用毛细管冷凝从温度高于露点的烟气中提取水，然后再循环到锅炉给水中。与直接接触热回收一样，TMC 直接从烟气中提取热水。然而，由于 TMC 通过膜传输来回收水，因此回收的水不会像直接接触回收那样受到污染。该技术已被证明可用于燃烧天然气的锅炉中的清洁废气流。然而，TMC 需要进一步开发先进材料，才能广泛应用于更脏的废物流。

热泵或低温余热改造 – 上述热交换技术涉及从高温到低温最终用途的能量“下坡”流动。当废热温度低于给定热负荷所需的温度时，这会限制热回收的机会。例如，废热可以以 35 摄氏度左右的热水形式提供，而需要 85 摄氏度左右的热水。在这种情况下，热泵可以提供机会将热量“升级”到所需的最终使用温度。热泵使用外部能量输入来驱动一个循环，该循环从低温源吸收能量并在较高温度下将其排出。根据设计，热泵可以提供两种功能。它们既可以将废热升级到更高的温度，也可以将废热作为能量输入来驱动吸收式冷却系统。热泵最适用于流程工业中的低温产品流。

在某些情况下，根据所需的温差以及燃料和电力的相对成本，升级热量可能是经济的。如果设施的热负荷温度略高于废热源，则热泵有时可以比通过燃烧额外燃料获得的热量更有效地提供热量。性能系数 (COP) 是热泵性能的量度，由热输出和功输入确定，并由等式 COP =Q/W 给出，其中 Q 是热泵的有用热输出，W 是功输入。

确定热泵可行性的一个重要考虑因素是废热温度和所需的温度提升。使用的循环类型和选择的工作流体类型影响热泵可以接收或排出热量的温度，以及确定可实现的最大温度提升。热泵的效率随着所需温度的升高而降低。

闭合压缩循环 – 在封闭压缩循环中，热泵用于降低冷却水的温度，同时利用提取的热量来提高工厂其他地方使用的工艺水的温度。热泵由蒸发器、压缩机、冷凝器和膨胀阀组成。在蒸发器中，能量从废热源传递到制冷剂。然后制冷剂进入压缩机，温度升高。然后过热的制冷剂进入冷凝器并将热量传递给散热器。最后，制冷剂在返回蒸发器之前在膨胀阀中被节流。

开式循环蒸汽再压缩 – 开式循环蒸汽再压缩使用压缩来增加废蒸汽的压力（从而提高温度）。机械蒸汽再压缩使用机械压缩机，而热蒸汽再压缩使用蒸汽喷射器，因此是热驱动而不是机械驱动

吸收式热泵 – 吸收式热泵与封闭式压缩循环非常相似，只是压缩机被更复杂的热驱动吸收机制所取代。根据工厂需要，系统可以通过多种方式进行配置。在一种类型中，热泵可以使用较低和较高温度的热量输入来排出中等水平的热量（例如升级低温热量）。在另一种类型中，热泵可以使用中等温度输入来排出一个较低温度流和一个较高温度流中的热量。第二种应用可用于空调和/或制冷。

发电

从废热发电通常涉及使用来自锅炉的废热来产生机械能，然后驱动发电机。这些动力循环得到了很好的发展。然而，正在开发可以直接从热量发电的新技术，例如热电和压电发电。在考虑 WHR 发电技术时，需要牢记的一个重要因素是不同温度下发电的热力学限制。发电效率很大程度上取决于废热源的温度。一般来说，余热发电仅限于中高温余热源。然而，交替动力循环的进步可以增加在低温下发电的可行性。虽然在这些温度下的最大效率较低，但这些方案在从废热中回收大量能量方面仍然是经济的。

下面介绍利用机械能发电的三种方法。

蒸汽朗肯循环 – 最常用的废热发电系统是利用余热产生蒸汽，然后驱动蒸汽轮机。传统的蒸汽朗肯循环是从温度高于 340 摄氏度的废气流中回收废热的最有效选择。在较低的废热温度下，蒸汽循环的成本效益较低，因为低压蒸汽需要更大的设备。此外，低温余热不能提供足够的能量使蒸汽过热，这是防止蒸汽冷凝和涡轮叶片腐蚀的必要条件。因此，低温热回收应用更适合有机朗肯循环或卡利纳循环，它们使用的流体与蒸汽相比沸点温度较低。

有机朗肯循环 – 有机朗肯循环 (ORC) 的运行类似于蒸汽朗肯循环，但使用有机工作流体而不是蒸汽。替代品包括硅油、丙烷、卤代烷烃（例如氟利昂）、异戊烷、异丁烷、对二甲苯和甲苯，它们的沸点比水低，蒸气压高。这使得 ORC 能够在显着降低的废热温度下运行。最合适的温度范围取决于所使用的流体，因为流体的热力学特性会影响不同温度下的循环效率。与水蒸气相比，流体具有更高的分子量，从而能够实现紧凑的设计、更高的质量流量和更高的涡轮效率。然而，由于 ORC 在较低温度下起作用，因此整体效率较低，并且取决于冷凝器和蒸发器的温度。虽然效率低于高温蒸汽发电厂，但重要的是要记住低温循环本质上比高温循环效率低。效率限制可以根据卡诺效率来表示，卡诺效率是在两个温度之间运行的热机的最大可能效率。卡诺发动机在 150 摄氏度的热源下运行并在 25 摄氏度的温度下拒绝它的效率仅为 30% 左右。 In this light, a low efficiency in the range of 10 % to 20 % in case of ORC is a substantial percentage of theoretical efficiency, especially in comparison to other low temperature alternatives, such as piezoelectric generation, which are only 1 % efficient.

Although the economics of ORC, heat recovery need to be carefully analyzed for any given application, it is a useful alternative in those industries which do not have in-house use for additional process heat or no neighbouring plants which can make economic use of the heat.

Kalina cycle – The Kalina cycle is a variation of the Rankine cycle, using a mixture of ammonia and water as the working fluid. A key difference between single fluid cycles and cycles which use binary fluids is the temperature profile during boiling and condensation. For single fluid cycles, the temperature remains constant during boiling. As heat is transferred to the working medium (water), the water temperature slowly increases to boiling temperature, at which point the temperature remains constant until all the water has evaporated. In contrast, a binary mixture of water and ammonia (each of which has a different boiling point) increases its temperature during evaporation. This allows better thermal matching with the waste heat source and with the cooling medium in the condenser. Consequently, these systems achieve considerable greater energy efficiency. The cycle was invented in the 1980s.

Direct electrical conversion technologies

Whereas traditional power cycles involve using heat to create mechanical energy and ultimately electrical energy, new technologies are being developed which can generate electricity directly from heat. These include thermoelectric, thermionic, and piezoelectric technologies. However, these technologies are in development stage. A few have undergone some prototype testing in applications such as heat recovery in automotive vehicles.

Thermoelectric generation – Thermoelectric (TE) materials are semiconductor solids which allow direct generation of electricity when subject to a temperature differential. This technology is based on a phenomenon known as the Seebeck effect which states that when two different semiconductor materials are subject to a heat source and heat sink, a voltage is created between the two semi-conductors. Conversely, TE materials can also be used for cooling or heating by applying electricity to dissimilar semiconductors. Thermoelectric technology has existed for a long time (the thermoelectric effect was first discovered in 1821), but has seen limited use due to low efficiencies and high cost. Most TE generation systems in use have efficiencies in the range of 2 % to 5 %. These have mainly been used to power instruments on spacecraft or in very remote locations. However, recent advances in the nano-technology have enabled advanced TE materials which can achieve conversion efficiencies 15 % or higher.

In a recent study, it has been concluded that advanced TE packages are appropriate in medium to high temperature, high flow rate exhaust streams where facilities have little use for recovered waste heat. However, more development work is needed in this area. Low cost, high volume production methods for TE materials need to be developed in order to achieve this goal. Also, maintaining a high temperature differential across thin TE devices present a significant engineering challenge. Obtaining high heat transfer rates require advances in heat transfer materials and heat exchange systems with high heat transfer coefficients.

Piezo-electric power generation – Piezo-electric power generation (PEPG) is an option for converting low temperature waste heat in the range of 100 deg C to 150 deg C to electrical energy. Piezo-electric technology converts mechanical energy in the form of ambient vibrations to electrical energy. A piezo-electric thin film membrane can take advantage of oscillatory gas expansion to create a voltage output. However, there are several technical challenges associated with PEPG technologies. These include (i) low efficiency (only around 1 % efficient), (ii) difficulties remain in obtaining high enough oscillatory frequencies (current devices operate at around 100 Hz, and frequencies needed are close to 1,000 Hz), (iii) high internal impedance, (iv) complex oscillatory fluid dynamics within the liquid/vapour chamber, (v) need for long term reliability and durability, and (vi) high costs.

While the conversion efficiency of PEPG technology is currently very low (1 %), there can be prospects to use PEPG cascading, in which case efficiencies can reach about 10 %. Other key issues are the costs of manufacturing piezoelectric devices, as well as the design of heat exchangers to facilitate sufficient heat transfer rates across a relatively low temperature difference.

Thermionic generation – Thermionic devices operate similar to thermo-electric devices. However, whereas thermoelectric devices operate according to the Seebeck effect, thermionic devices operate via thermionic emission. In these systems, a temperature difference drives the flow of electrons through a vacuum from a metal to a metal oxide surface. One key disadvantage of this technology is that it is limited to applications with high plying electricity to dissimilar semiconductors. Thermo-electric technology has existed for temperatures above 1,000 deg C. However, some development has enabled their use at around 100 deg C to 300 deg C range.

Thermo photo voltaic generator – Thermo photo voltaic generators can be used to convert radiant energy into electricity. This technology involves a heat source, an emitter, a radiation filter, and a photo voltaic (PV) cell (like those used in solar panels). As the emitter is heated, it emits electro-magnetic radiation. The PV cell converts this radiation to electrical energy. The filter is used to pass radiation at wave-lengths which match the PV cell, while reflecting remaining energy back to the emitter. This technology can potentially enable new methods for WHR. A small number of prototype systems have been built for small burner applications and in a helicopter gas turbine.

WHR and iron and steel industry

The iron and steel industry employs several high temperature furnaces for coke, sinter, hot metal, and steel production and accounts for high energy consumption. While recovery from clean gaseous streams in the industry is common, heavily contaminated exhaust gases from coke oven, blast furnace (BF), basic oxygen furnace (BOF), and electric arc furnace (EAF) continues to present a challenge for economic WHR. Heat recovery techniques from these dirty gaseous streams are available, yet implementation has been limited due to high capital investment costs.

The steel industry has made the biggest progress in reducing its energy intensity. Such progress has been achieved by continuous casting and optimization of BF operation, and also through steel recycling and replacement of fossil fuels with recycled by-product gases (coke oven gas, blast furnace gas, and converter gas). In-situ waste heat recovery has been implemented wherever possible, for example, by recirculating hot flue gases inside the furnace where they were created to lower external energy demand, or by using hot flue gases to preheat combustion air or fuel gas. Such energy efficiency improvements still leave residual waste heat recovery opportunities, e.g. to produce steam for other parts of the process or to produce electricity.

WHR in case of steel plants is described below.

Coke production

Production of coke is an essential burden material for BF operation. Coke is produced in coke ovens, where coal is heated in an oxygen limited atmosphere. There are two methods for producing coke namely (i) the byproduct process, and (ii) the non-recovery process. In the byproduct process, chemical byproducts (crude tar, ammonia, and light oils) in the coke oven gas are recovered, while the remaining coke oven gas (COG) is cleaned and recycled within the steel plant. In the non-recovery process, the entire COG is burned in the process. The most common type of process is still the byproduct process and this is discussed below.

Byproduct cokemaking process has two areas of sensible heat loss namely (i) COG which is cooled in the gas cleaning process, and (ii) waste gas leaving the coke oven. The coke making process employs several coke oven chambers separated by heating flues. Recycled COG, and sometimes other gases such as BF gas, are used as the fuel source in the heating flue and supply heat to the oven chamber where coal carbonization takes place. As coal is carbonized in the oven chamber, gas and moisture (accounting for around 8 % to 11 % of charged coal) are driven off and leave through the pipes. The COG has a high heat content ranging from around 4000 kcal/cum to 4400 kcal/cum and hence it can be recycled for use as a fuel after undergoing a cleaning process.

The temperature of the crude COG at the oven outlet ranges from 650 deg C to 1000 deg C.   At this point, the COG gas is a source of sensible heat. However, the heat is universally wasted due to the high amount of tar and other materials which can cause build up on heat exchanger surfaces. Upon leaving the oven, the COG is cooled by ammonia liquor spray followed by primary coolers. Different technologies are then used for removing tar, sulphur compounds, ammonia, and light oils. After cleaning, the COG is used as a fuel throughout the steel plant. In this arrangement, only the chemical energy of the COG is recovered when recycled, while the sensible heat is wasted.

While most of the steel plants do not employ heat recovery from COG, a limited level of heat recovery from COG is possible, as shown by the success of this practice in Japan. Coke oven facilities in Japan have successfully applied heat recovery through use of a low pressure heat transfer medium. In general, the minimum allowable temperature for the COG in the heat exchanger is around 450 deg C. At lower temperatures, tar condenses and leads to soot formation on the heat exchanger surface.   Cooling to 450 deg C enables only about one third of the sensible heat to be recovered. However, it is unlikely that ISPs in other countries are going to pursue new technologies for heat recovery from crude coke oven gas. This is since ISPs are facing cost barriers with heat recovery from dirty exhaust streams. Also, the byproduct coke making process can become irrelevant in future years. It is likely that the ISPs are going to move away from the byproduct process to the non-recovery process due to environmental considerations. In the non-recovery process, the COG gas is burned within the process, and a WHB used to recover the sensible heat in the off gases.

Another source of sensible heat loss in coke ovens is the waste gases from the combustion of recycled fuel gases. The recycled fuel gases are used in the heating flue, which is adjacent to the oven chamber. Combustion of the fuel gases generates hot exhaust gases which leave the oven flue and pass through a regenerator to transfer heat to incoming combustion air and/or fuel. Waste gases leave the regenerator at temperatures averaging around 200 deg C. In some plants, the heat content of the waste gases are further recovered by use of a heat pipe or for preheating coal charge and reducing its moisture content. In this case, the temperature of the exhaust gases drops to around 60 deg C.

Production of sinter

Sintering plant consists of two major sections, sintering section and sinter cooling section. Heat recovery from both parts has been developed namely (i) from sintering section exhaust gas, and (ii) from cooling section cooling gas. There is large temperature difference depending on the position of the section. Average gas temperature in both sections is in the level of 100 deg C to 150 deg C, too low for effective heat recovery. Heat recovery is to be limited to high gas temperature zone, the final part of sintering section and primary part of cooling section, where gas temperatures of 300 deg C or higher are available. Although heat recovery zone is limited, the gas volume of sintering process is large enough for practical heat recovery.

The waste gas energy recovery system consists of hood, dust catcher, heat recovery boiler, circulation fan and de-aerator. Sintering machine exhaust gas is corrosive containing some dusts. Heat recovery is generally limited to high gas temperature zone as aggregated average temperature is low for heat recovery. At the same time, due to its corrosiveness, the gas temperature after heat recovery is to be kept above acid due point of the gas. Cooling gas is basically atmosphere air containing some dust. In case of sinter cooler, it is same as sintering machine heat recovery. Due to gas temperature distribution along with the cooler, heat recovery is limited to high gas temperature zone.

Sintering machine exhaust gas heat recovery can be categorized to circulation type and non-circulation type. In circulation type, gas after heat recovery are circulated to sintering machine as cooling gas replacement, whereas in non-circulation type, the gas after heat recovery is lead to gas treatment facility directly. Circulation type is adopted to improve heat recover efficiency.

In case of cooler heat recovery, the cooler gas is air. The cooler heat recovery system can be categorized as circulation type and non-circulation type. In case of non-circulation type, after heat recovery from hot gas zone, cooling gas is released to the atmosphere. In case of circulation type, after heat recovery from hot gas zone, cooling gas is led to cooler and reused for sinter cooling. Cooler gas temperature rises through recirculation and consequently results to higher heat recovery. On the other hand, cooling gas temperature rises up to the level of 180 deg C, cooling capability can decrease. Sinter temperature at outlet of the cooler is higher around 30 deg C in circulation type. Temperature difference is small enough and does not affect sinter plant operation. Recovered energy increases by 50 % in circulation type compared to non-circulation type. Fan power consumption is larger in case of circulation type. However, recovered power is far larger.

Hot metal production in BF

BF is one of the main units in ISPs. It converts iron ore into hot metal. Raw materials are charged from the top, including iron containing materials (lump iron ore, sinter, or pellets), additives (flux), and coke, while hot air and supplemental fuels are injected through tuyeres at the bottom of the furnace. The burden moves down through the BF and meets a rising current of hot gases. The hot air entering the BF is provided by several auxiliary hot blast stoves. In the hot blast stove, mixed gas consisting of BF gas (BFG) and COG are combusted. The heat from the combustion exhausts is transferred to a checker work regenerator. When the regenerator reaches an appropriate temperature, the flow of air is reversed and cold air is forced through the regenerator, which transfers heat to the cold air. The heated air is then injected into the furnace. The system operates according to the same principles as a regenerator used for heat recovery. However in this case, the regenerator is not a ‘waste heat’ recovery unit, but rather the mechanism for transferring heat from the stove to the hot blast. Sources of off gas waste heat in BF include both the exhaust gases from the hot blast stove and the BFG leaving the BF.

There is sensible heat loss from BFG. New BFs are designed for efficient heat transfer, resulting into hot gases at the BF top in the low temperature range. The BFG is recovered for use as a fuel in blast air heating, rolling mill reheating furnaces, coke oven heating, power production, and steam generation. Since BFG has low calorific value, it is often mixed with COG or converter gas. BFG is required to be cleaned before it can be used as a fuel, and the sensible heat contained in the gas is rarely recovered. In some cases, BF operates at a sufficiently high pressure (2.5 atm or higher) to economically use a top pressure recovery turbine (TRT) for recovering of the ‘pressure energy’ of the BFG. The gas is to be cleaned before entering the TRT, which is generally accomplished via wet cleaning, with the result that sensible heat of the off gas is lost. An alternative to wet cleaning technology is dry cleaning, in which the temperature of the gas entering the TRT can be raised to around 120 deg C. Dry type TRT technology is already working in several places. However, it is more expensive.

Another opportunity for WHR is from the combustion exhaust gases leaving hot blast stoves. The gases are at temperatures of around 250 deg C. The blast stove exhaust gas is relatively clean and is more compatible with heat recovery devices, making heat recovery from blast stoves a more common practice. The heat can be used to preheat combustion air and/or fuel gas. Heat exchangers used include rotary regenerators, fixed plate heat exchangers, and circulating thermal medium systems.

Production of liquid steel in BOF

BOF uses oxygen to oxidize impurities in the hot metal. Operation is semi-continuous:hot metal and scrap are charged to the furnace, oxygen is injected, fluxes are added to control erosion, and then the metal is sampled and tapped. The temperature required to melt the metal is supplied by the exothermic oxidation reaction and hence, no external heat source is needed.

The off gases from the BOF are at a high temperature. It has a high concentration of CO (carbon monoxide). Like COG and BFG, BOF gases offer opportunities for recovery of chemical energy and sensible heat. Challenges to WHR include high capital costs and the substantial maintenance problems resulting from hot dirty gases. Contaminants include iron oxides, heavy metals, SOx, NOx, and fluorides.

Various commercial methods for WHR are available. The two main methods for heat recovery are ‘open combustion’ and ‘suppressed combustion’. In open combustion systems, air is introduced to the BOF gas duct to combust the CO. The heat generated is recovered with a waste heat boiler. In the ‘suppressed combustion’ method, a skirt is added to the converter mouth to reduce air infiltration and combustion of the CO. The gas is then cleaned, collected, and used as a fuel. It is also possible to recover both the gas and the sensible heat via a combined boiler/suppressed combustion gas recovery system.  

Liquid steel production by EAF

The steel industry has experienced significant growth in manufacture from recycled scrap via electric smelting. EAF and induction furnace are the two types of furnaces used to melt ferrous scrap for electric smelting. Out of these two, EAF is the prominent furnace. The furnace is refractory lined and typically covered by a retractable roof, through which C electrodes are lowered. Charge materials are lowered through the roof. Fluxes and alloying agents are also added to help control the quality of the material. The electrodes are then lowered to about an inch above the metal, and the current provides heat for melting the scrap. During furnace operation, several gases and particulate emissions are released, including CO, SOx, NOx, metal oxides, volatile organic compounds (VOCs), and other pollutants. Off gas temperatures at peak loads can equal anywhere from 1,350 deg C to 1,950 deg C. Exhaust gases are responsible for losses of around 20 % of the power input. Half of these losses are due to the chemical energy in the gases, while the other half is sensible heat. Additionally, around 8 % to 10 % of energy input is also lost to EAF cooling water ‘jacket’.

The most common method for heat recovery is scrap preheating, which has been widely used. The use of off gases to preheat scrap can save from 5 % to 10 % of total EAF energy consumption. Initial designs for scrap preheat required piping off gases to the charging bucket. Some of the challenges with these systems include the need to transport preheated scrap containing semi-burned non scrap materials (e.g., plastics), as well the evaporation of volatiles which create odour and environmental control problems. Alternatives to the bucket preheating system include the Consteel process, the Fuchs shaft furnace, and the Twin shell furnace. These processes have been installed at various places.

The Consteel process involves continuous charging of scrap and uses a scrap conveyer, a feeding system, and a preheater. The preheater is a refractory lined tunnel where off gases flow opposite the flow of scrap charge. Air is introduced into the preheater to burn the CO and CO2 and thus both the chemical and sensible heat in the off gas is used. An afterburner is sometimes installed to burn remaining CO and other compounds.The Fuchs shaft furnace involves a shaft immediately above the arc furnace roof. The charge is loaded via baskets in three stages. The baskets are refractory lined and designed with a seal which prevents the escape of fumes. Scrap heating is further assisted by auxiliary oxy-fuel burners. Additionally, afterburners are installed to completely combust all the CO. One additional benefit of the system is that charge acts as a dust filter, capturing around 40 % of the dust and returning it to the furnace, thus enabling slight increases in yield.

The benefits and drawbacks of scrap preheating systems depend on the specific operation. In some cases, it enables reduced electricity consumption and increased productivity. In other cases, scrap preheating systems are difficult to maintain. As EAFs become increasingly efficient and tap to tap times are reduced, scrap handling can reduce productivity and possibly create burdensome maintenance demands. In one case, the energy savings enabled by scrap preheating are reduced by about one half when tap to tap times are reduced by a third.

Power plant boilers

Boilers in ISPs normally use BFG and COG as fuel. The exhaust gas temperature for the boilers varies with the boiler’s age and the controls used. Temperatures can be fairly high (340 deg C to 450 deg C), with O2 content varying from 3 %–7.5 %. The waste heat is in the form of clean, contamination-free gases and does not require further conditioning.  The areas of waste heat and recovery from boilers and steam systems include (i) use of exhaust gases to preheat BFG and COG, (ii) use of low-temperature power generation if economically justifiable, (iii) preheating service water or river water for use in the plant, if possible and required, and low-pressure steam can be condensed and reused for the boiler water system instead of venting.

Reheating furnace

Reheating furnace is a key equipment of the hot rolling mills. Its function is to continuously heat billets, slabs or blooms of different sizes and grades upto 1,250 deg C. Most of the new reheat furnaces are ‘walking beams furnaces’ (WBF). On the WBF, the heating is done over and under the products which are handled from charging side to discharging side by means of insulated and cooled beams (skids). A key performance criterion for reheating furnaces is heating homogeneity. 20 % to 30 % of the energy input is typically wasted divided between several thermal losses namely (i) the temperature of the exhaust gas between the combustion air recuperator and the stack is at 250 deg C to 300 deg C with natural gas fuel and higher with lower calorific value fuel, (ii) the product handling systems inside the furnace with skids and post cooling system, and (iii) wall and doors losses, hardly recoverable.

Water is used to constantly cool the skid system which is in contact with a very hot atmosphere in the furnace. This water loop typically enters at 40 deg C and is heated by 15 deg C before being directed to a dedicated cooling system.

At several places, WHR is carried out on the skid cooling system by producing steam when it is needed in the plant for other purposes. On its own, this installation reduces losses through the skid system because of the use of water cool pipes used at higher temperature. If steam is not needed by the plant then an ORC (adapted for such temperatures around 200 deg C) can be installed on the steam circuit to produce electricity. This installation has the benefit of being easily and safely operable especially with high variability of the losses because of the constant temperature brought by the water phase change. Most of the time, this technology is not installed because of long payback, and the energy contained in exhaust gases is wasted.

An electricity production system is possible to recover energy from exhaust gases. Depending on the heat source temperature, either a ‘water-steam cycle’ (with low efficiency furnace) or an ORC (with better efficiency) are available. However, most of the time those technologies are not installed because of their long payback. This situation can have another solution. This solution combines heat from the skid cooling loop operated at higher pressure and temperature so as to produce a mixture of steam and water at around 215 deg C in a closed loop and heat from exhaust gases. The two heat sources are recovered separately thanks to organic heat fluid loops and then combined to form a common heat source.

The heat fluctuation from the exhaust gases (temperature and volume are modified) in case of furnace power variations (production or product variations) are balanced because of the constant temperature of the heat coming from the skid cooling system. Thus operation of the system is easy and makes the global heat source more stable especially with high fluctuations.

It is possible that the reheat furnace production can fluctuate in few minutes, which affects the heat content of exhaust gases entering the WHR system. The ORC is a rather flexible system which can accommodate such variations upto a certain point. An ORC can typically operate down to 30 % of its nominal capacity, and automatically shuts down when the heat input goes below that threshold. However, the economic aspect is affected as electricity production also decreases as well.

Heat storage solutions can be adapted to daily variations are becoming available for industrial applications and can be used in combination with an ORC to flatten its production. Oil is, for instance, is appropriate heat storage medium at that temperature level. Economic benefits need to be assessed on a case by case basis.

Waste heat from solid streams

In addition to waste heat losses from off gases, solid streams and cooling water are sources of additional sensible heat losses. Solid products and byproducts with significant waste heat losses include hot coke, hot sinter, BF slag, BOF slag, cast steel, and hot rolled steel. Though the heat from solid streams are often more difficult to recover, the heat losses are high. The sensible heat loss from coke is recovered in some plants coke dry quenching (CDQ) as an alternative to wet quenching. CDQ involves catching incandescent coke in a specially designed bucket, which is discharged into the CDQ vessel. An inert gas such as nitrogen passes over the coke and recovers its sensible heat. The hot gas is then passed through a waste heat boiler. Energy saving is in the range of 0.2 million to 0.25 million kcal per ton of coke. There have also been attempts to recover heat from other solid flows via radiant heat boilers. This was unsuccessful for BF and BOF slag, but has been commercialized for recovering heat from cast steel in a few locations in Japan and Germany.

Another option for reducing heat losses from cast steel is hot charging, in which cast products are charged to the reheating furnace while still hot. Hot charging can save about 0.12 million kcal per ton. Sensible heat loss from hot rolled steel can also be partially recovered by using water cooling. Since the final temperature of the cooling water is generally low (around 80 deg C), it can be upgraded for other heating applications with a heat pump.