Clean Coal Technology: A Novel Process for the Combustion of Coal Using an Oxygen Carrier

全球电力供应约占大气人为碳排放总量的38%或约2，400 Mte/y（碳基），预计到2020年将超过4，000 Mte/y。在全球范围内，煤炭在世界电力生产中所占份额最大（占总交付能源的39%），其次是可再生能源（主要是水电）（20%），核能（17%），天然气（15%）和石油（10%）。2000年至2020年期间，发电量将增加约80%，煤炭发电量将保持在约39%，这是由于印度和中国煤炭开采量的增加以及美国的稳定增长。在英国，最近的白色文件仅仅设想了煤的未来，只要能够找到实质上减少其碳排放的方法，因此这需要将由煤燃烧产生的CO2或由其衍生的燃料封存在地球中。封存的成本很小（约4 -8美元/ te C），与从典型烟道气中分离CO2的成本相比（~ 100 - 200美元/te C），因此只有当CO2几乎以纯形式存在时，这种处置才接近可行性，基本上不含氮气和其他惰性气体。我们希望使用化学循环原位气化和燃烧煤的过程，以生产CO2和蒸汽作为纯产品，没有明显的N2污染。在我们提出的方案中，将有一个反应器，含有氧载体的鼓泡流化床，最有可能是在二氧化钛或氧化铝载体上的Cu基氧化物，其耐用性已由其他工作人员证明。反应器将在三个连续时段t1、t2和t3的循环中操作。在t1期间，该床将被蒸汽（或蒸汽和CO2）流化，并且煤将被稳定地进料到该床，该床的温度将为~ 800 - 1000 ℃。两个事件将发生：（1）煤将进行气化（吸热），得到含有CO和H2的合成气（加上较少量的CH 4和高级烃）：C（s）+ H2O（g）= CO（g）+ H2（g）（反应焓：+131 kJ/mol），（2）合成气将通过以下方式与周围的CuO颗粒反应以产生CO2和蒸汽：CuO（s）+ H2（g）= Cu（s）+ H2O（g）和CuO（s）+ CO（g）= CO2（g）+ Cu（s）（反应的能量分别为-86 kJ/mol和-127 kJ/mol）。铜是唯一在（2）中产生放热反应的氧化物;产生的热量超过（1）中吸热气化反应所需的热量。实际上，金属氧化物已被用于代替空气或低温产生的O2，使得燃烧产物不含N2。当然，这种系统只能在金属氧化物还原到一定程度时才能起作用。因此，在时间t1之后，煤的进料停止，并且允许床碳的剩余存量再气化和燃烧一段时间t2，直到存量足够小。在t2结束时，用空气代替蒸汽流化床一段时间t3，在此期间还原的金属氧化物载体在Cu + 0.5O2 = CuO（反应焓~ 156 kJ/mol Cu）中再生。在t3期间，来自焦化的金属氧化物或来自在t2之后剩余的残余碳存量的一些碳将被烧掉，使得将存在与再生空气一起的少量CO2释放，但这将比煤在空气中直接燃烧所排放的少得多。一旦金属氧化物已经再生，循环在t1再次开始。因此，该概念使得煤能够清洁地燃烧，与使用例如从空气中低温分离的氧气来分离CO2的其他方案相比，热效率的降低相当小。

The global supply of electricity accounts for ~38% of total anthropogenic carbon emissions to the atmosphere or ~2,400 Mte/y (carbon basis), a figure projected to exceed 4,000 Mte/y by 2020. Globally, coal generates the largest share of world electricity production (39% of total delivered energy) followed by renewables (principally hydroelectricity) (20%), nuclear (17%), gas (15%) and oil (10%). Electricity production will increase by ~80% over the period 2000 to 2020, with the fraction generated from coal remaining at ~39%, due to increased exploitation of coal in India and China and steady growth in the USA. In the U.K., the most recent White Paper only envisages a future for coal provided ways can be found materially to reduce its carbon emissions , which therfore requires the sequestration of the CO2 arising from the combustion of the coal, or fuels derived from it, in the earth. The cost of sequestration is small (~ $4-8/ te C) compared to the costs of separation of CO2 from typical flue gases (~ $100 - 200/te C)so that such disposal only approaches viability if the CO2 is available in almost pure form, largely free of nitrogen and other inert gases.We wish to use chemical looping for the in situ gasification and combustion of coal in a process to produce CO2 and steam as pure products, without significant contamination by N2. In our proposed scheme, there would be one reactor, containing a bubbling fluidised bed of oxygen carrier, most likely a Cu-based oxide on a titania or alumina support, the durability of which has been demonstrated by other workers. The reactor would be operated in a cycle of three consecutive periods, t1, t2 and t3. During t1, the bed would be fluidised by steam, (or steam and CO2) and coal would be fed steadily to the bed, the temperature of which would be ~ 800 - 1000 C. Two events would occur:(1) the coal would undergo gasification (endothermic) by the steam to yield a synthesis gas containing CO and H2 (plus smaller amounts of CH4 and higher hydrocarbons): C(s) + H2O(g) = CO(g) + H2(g) (enthalpy of reaction: +131 kJ/mol),(2) the syngas would react with the surrounding CuO particles to give CO2 and steam by: CuO(s) + H2(g) = Cu(s) + H2O(g) and CuO(s) + CO(g) = CO2(g) + Cu(s) (enthalpies of reaction -86 kJ/mol and -127 kJ/mol, respectively).Copper has the only oxides which give exothermic reactions in (2); the heat produced exceeds that needed for the endothermic gasification reaction in (1). In effect, the metal oxide has been used in place of air, or cryogenically-produced O2, so that the products of combustion do not contain N2. Of course, this system can only function down to a certain degree of reduction of the metal oxide. Thus, after time t1, the feed of coal ceases and the remaining inventory of bed carbon is allowed to gasify and combust for a further period of time, t2, until the inventory is sufficiently small. At the end of t2, the bed is fluidised by air instead of steam for a period of time, t3, during which the reduced metal oxide carrier is regenerated in Cu + 0.5O2 = CuO (enthalpy of reaction -156 kJ/mol Cu). During t3 some carbon will be burnt off, originating either from coked metal oxide or from residual carbon inventory remaining after t2, so that there would be a small release of CO2 with the regenerating air, but this would be very much less than that emitted by direct combustion of the coal in air. Once the metal oxide has been regenerated, the cycle starts again at t1. Thus, the concept enables coal to be burnt cleanly with a rather smaller reduction in thermal efficiency than is obtained with other schemes for isolating the CO2, using e.g. cryogenically-separated oxygen from air.