Distributed Hydrogen Production with Carbon Capture: A Novel Process for the Production of Hydrogen from Biomass

决策者和工业界认为，使用氢作为清洁能源载体，是缓解化石燃料向大气排放二氧化碳所引起的气候变化的一种潜在途径。氢也是目前最理想的燃料，用于当前一代实际的固体氧化物燃料电池。然而，用氢代替化石燃料需要下述两种方式生产氢气：(i)可再生资源，如生物质；或（ii）化石燃料，并捕获和长期封存由此产生的副产品二氧化碳。该提案涉及一种从生物质中生产氢的新方法，以一种清洁的形式适合直接用于燃料电池，而无需大量的气体净化。它也是一种技术，可以适用于各种规模的操作，从适合当地生物质或废物来源的小型、分布式装置（例如在发展中国家）到更大、更集中的发电站。简而言之，该过程包括：1)将CO2或CO2/蒸汽中的生物质气化为含有CO和H2.2)通过Fe2O3填充床将合成气转化为纯CO2和蒸汽流，其中发生以下不同反应：0.788 CO + 0.947 Fe3O4 = 0.788 CO2 + 3 Fe0.947O (1)0.788 H2 + 0.947 Fe3O4 = 0.788 H2O + 3 Fe0.947O (2)H2 + 3Fe2O3 = 2Fe3O4 + H2O, (3)CO + 3Fe2O3 = 2Fe3O4 + CO2。(4)在工作温度为1173 K时，热力学计算表明：(3)和(4)基本上位于有还原性气体存在的右边。因此，在充填床层中，只要它足够长，在出口处将有一个Fe2O3区域，在此之前是Fe3O4区域。而在床层入口处，[CO]和[H2]含量较高。这意味着首先由反应(3)和(4)形成的Fe3O4可以通过反应(1)和(2)进一步反应。因此，要将Fe3O4还原为Fe0.947O，需要pCO/ pCO2 > 0.49和pH2/ pH2O > 0.39，这对于典型的合成气来说是可能的。因此，在入口处将是Fe0.947O的区域。随着时间的推移，实际上会有两个锋面穿过床层：一个是Fe0.947O和Fe3O4之间的边界，另一个是靠近出口的Fe3O4和Fe2O3之间的边界。在Fe3O4 / Fe2O3前缘突破床层之前，停止合成气向填充床层的流动，以避免CO滑入出口气流。因此，这个床的出口将是一股纯二氧化碳和一些水，这些水可以被浓缩出来，如果需要的话，可以把二氧化碳封存起来。一部分二氧化碳将被回收到气化炉。3)氢气的生产。氢气将从2)中的废床中通过蒸汽产生，从而逆转反应(2)。在这种情况下，pH2/ pH2O < 0.39，因此氢会以Fe3O4的前端从入口传播，直到到达阶段2结束时留下的Fe3O4。一旦床层在3)中充分转化，向床层提供空气将其氧化回Fe2O3；产品正在消耗空气和能源。经过加热、耗尽的空气在高温下（约1273 K）离开氧化反应器，可用来提高蒸汽。5)最后，重复这个循环，重新开始向4)再生床供应合成气。因此，通过有一些这样的床，安排在一个合适的循环操作中，就有可能连续操作气化炉。假设将纯碳气化，则上述总反应为：C(s) + H2O(g) + 2.38 (0.21O2(g) + 0.79N2) = CO2(g) + H2(g) + 1.88 N2总反应中的每个产物都在一个单独的流中。

The use of hydrogen as a clean energy carrier is seen by policy makers and industry as a potential way of mitigating climate change arising from the emissions to atmosphere of CO2 arising from the use of fossil fuels. Hydrogen is also, currently, the most desirable fuel for use in the present generation of practicable solid oxide fuel cells. However, to replace fossil fuel with hydrogen requires the hydrogen to be produced either (i) from renewable resources, such as biomass, or (ii) from fossil fuels, with capture and long-term sequestration of the resulting by-product CO2 in the earth. This proposal is concerned with a novel method for the production of hydrogen from biomass, in a clean form suitable for direct use in a fuel cell without substantial gas clean-up. It is also a technique which could lend itself to operation at a range of scales / from small, distributed units, suitable for local sources of biomass or waste (such as in developing countries) to larger, more centralised power stations. Briefly, the process involves:1) The gasification of biomass in CO2 or CO2/steam to syngas containing CO and H2.2) Conversion of the syngas to a pure stream of CO2 and steam by passing it through a packed bed of Fe2O3, where the following reactions variously occur:0.788 CO + 0.947 Fe3O4 = 0.788 CO2 + 3 Fe0.947O (1)0.788 H2 + 0.947 Fe3O4 = 0.788 H2O + 3 Fe0.947O (2)H2 + 3Fe2O3 = 2Fe3O4 + H2O, (3)CO + 3Fe2O3 = 2Fe3O4 + CO2. (4)At an operating temperature of 1173 K, thermodynamic calculations show that eqs. (3) and (4) lie essentially well over to the right where reducing gases are present. Thus, in the packed bed, provided it is sufficiently long, there will be a region of Fe2O3 at the outlet, preceded by a region of Fe3O4. At the entrance to the bed, on the other hand, [CO] and [H2] are high. This means that the Fe3O4 first formed there by reactions (3) and (4) can react further by reactions (1) and (2). Thus, for the Fe3O4 to be reduced to Fe0.947O would require pCO/ pCO2 > 0.49 and pH2/ pH2O > 0.39, which is likely for a typical syngas. Accordingly, at the entrance will be a region of Fe0.947O. As time proceeds, there will, in effect be two fronts moving through the bed: one defining the boundary between Fe0.947O and Fe3O4 and one, nearer the exit, the boundary between Fe3O4 and Fe2O3. The flow of syngas to the packed bed would be stopped just before the Fe3O4 / Fe2O3 front breaks through the bed, to avoid the slip of CO into the outlet stream of gas. Accordingly, the outlet from this bed would be a stream of pure CO2 and some water, which could be condensed out, allowing sequestration of the CO2 if required. A proportion of the CO2 would be recycled to the gasifier. 3) Production of hydrogen. Hydrogen would be generated from the spent bed in 2) by passing steam through it, thus reversing reaction (2). For this to be the case, pH2/ pH2O < 0.39 and so the hydrogen would occur with a front of Fe3O4 propagating from the entrance until it reaches the Fe3O4 left at the end of stage 2).4) Regeneration of Fe2O3. Once the bed is sufficiently converted in 3), air is supplied to the bed to oxidise it back to Fe2O3; the products being depleted air and energy. The heated, depleted air leaves the oxidation reactor at high temperature (ca. 1273 K) and can be used to raise steam.5) Finally, the cycle is repeated, with the supply of syngas re-commenced to the bed regenerated in 4). Hence, by having a number of such beds, arranged in a suitable cyclic operation, it would be possible to operate the gasifier continuously.The overall reaction in the above, assuming that gasification of pure carbon were being undertaken, would be:C(s) + H2O(g) + 2.38 ( 0.21O2(g) + 0.79N2) = CO2(g) + H2(g) + 1.88 N2 Each of the products in this overall reaction would be in a separate stream.