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

使用氢气作为清洁能源载体，被政策制定者和工业界视为缓解因使用化石燃料而向大气排放二氧化碳而产生的气候变化的潜在途径。氢也是目前实用的新一代固体氧化物燃料电池最理想的燃料。然而，用氢取代化石燃料需要(I)从可再生资源(如生物质)中生产氢气，或(Ii)从化石燃料中生产氢气，并将产生的副产品二氧化碳捕获并长期封存在地球上。这项建议涉及一种从生物质中生产氢气的新方法，这种方法以一种清洁的形式适合直接用于燃料电池，而不需要进行大量的气体净化。它也是一种技术，可以在各种规模下运行，从适合当地生物质或废物来源(如发展中国家)的小型分布式装置到更大、更集中的发电站。简而言之，该过程包括：1)在二氧化碳或二氧化碳/水蒸气中将生物质气化成含有CO和H2的合成气。2.2)通过使合成气通过Fe_2O_3填充床将其转化为纯的CO和蒸汽，其中不同地发生以下反应：0.788 CO+0.947 Fe3O4=0.788 CO2+3Fe0.947O(1)0.788 H2+0.947 Fe3O4=0.788 H2O+3Fe0.947O(2)H2+3Fe2O3=2Fe3O4+H2O，(3)CO+3Fe2O3=2Fe3O4+CO2。(4)在1173K的工作温度下，热力学计算表明方程。(3)和(4)基本上位于存在还原气体的右侧。因此，在填充床中，如果足够长，在出口处将有一个Fe2O3区域，前面有一个Fe3O4区域。另一方面，在床的入口处，[CO]和[H2]都很高。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还原到Fe0.947O将需要PCo/pCO2&gt；0.49和PH2/pH2O&gt；0.39，这很可能是典型的合成气。因此，在入口处将是Fe0.9470的区域。随着时间的推移，实际上会有两个锋面穿过床层：一个界定Fe0.947O和Fe3O4之间的边界，另一个靠近出口的地方是Fe3O4和Fe2O3之间的边界。在Fe_3O_4/Fe_2O_3前沿穿过床层之前，合成气向填充床的流动将被阻止，以避免CO滑入出口气流。因此，这个床的出口将是一股纯净的二氧化碳和一些水，这些水可以被冷凝出来，如果需要的话，可以隔离二氧化碳。一部分二氧化碳将被回收到气化炉中。3)制氢。氢气将通过蒸汽通过废床产生，从而逆转反应(2)。在这种情况下，p H2/p H2O和lt；0.39，因此氢将出现在Fe3O4的前沿，从入口处传播直到它到达阶段2)结束时留下的Fe3O4。4)Fe2O3的再生。一旦床被充分转化，空气就被提供给床，将其氧化回Fe2O3；产品会耗尽空气和能量。加热、耗尽的空气在高温下(约1273K)离开氧化反应器，可用于提高蒸汽。5)最后，循环重复，合成气重新开始供应到在4)再生的床。因此，通过以适当的循环操作布置多个这样的床，就可以连续地操作气化炉。假设进行纯碳的气化，上述总反应将是：C(S)+H2O(G)+2.38(0.21O2(G)+0.79N 2)=CO_2(G)+H_2(G)+1.88 N_2。

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.