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No, I have nothing against solar gasification. These guys have a very great idea. Because it is an endothermic process and therefore needs heat input, gasification is one process that can truly benefit from zero-carbon solar heat. Solar can provide at least some of the heat requirements, if not all. I can see the technology being applied in sunny desert areas to gasify local coal, shale, and shipped biomass for fuels production, using seawater for the process, by the way.
what I wanted to add - I think that solar should be used for drying the biomass feedstock, not the actual gasification.
I read the solar gasification article, interesting from the theoretical point, but I think there will be big challenges as the gentlemen involved with the project already pointed out. There is not enough sunshine in wet and cool areas where biomass grows best for this to work, and even in the most favorable locations such as the desert it can't function more than 10 hours a day. And then there is another problem - what to do with FT reactor tail gas. Maybe it can be stored and used to heat the reactors overnight when there is no sun...
HB, This figure (82%) is deducted from thermochemical equations which assume that water is in gaseous state; it does not account for any heats of evaporation. Likewise, energy needed for moving stuff around, fans, compressors etc is also not accounted for. Indeed, in real life the biomass has to be dried, and water needs to be evaporated to make steam for the gasification reaction. There will be substantial heat losses from the gasification reactor. Electricity is needed to power equipment, and to prepare the feedstock. All of this reduces efficiency. The good thing is that a very substantial fraction of the energy can be recovered as FT reaction heat that has the potential to cover the electricity requirements of the plant. Biomass can be air dried to a very low moisture content. I think that 60% efficiency is easily achievable by streamlined plant design, and possibly even more, up to 70%. with a 60 to 70% efficiency, 4 lbs of dry biomass gives 1 lb of hydrocarbons. As for the pollution, I think that it is no greater than from a biomass burning power station or boiler.
The 82% conversion efficiency figure for biomass to hydrocarbons via gasification/FT synthesis which they give, and which by the way I calculated a while ago even before these scientists, does not apply to lignin only. C5, C6 sugars, and any kind of biomass material can be converted to hydrocarbons with 82-83% theoretical thermal efficiency via gasification/FT synthesis. The real question is what is the highest efficiency that can practically be achieved. C6 sugars that can be fermented, like starch, can be converted to ethanol with 96-97% thermal efficiency, so brewing still remains a very efficient conversion technology.
"This is entirely consistent with all the other numbers which prove, beyond any doubt, that biofuels from higher plants are NOT going to continue BAU. The bulk of any viable program will involve efficiency and electrification." In the US, with the current US driving habits, definitely not. Not so in many other countries, even in advanced western European countries, where biofuels can cover a very substantial portion of transportation fuel without putting excessive effort in producing them. Yes, methanol as the final product can theoretically improve overall efficiency of the process. Conversion of syngas to hydrocarbons is highly exothermic (a major efficiency loss), but it is less so for the methanol synthesis. Similarly, HC's can be obtained from methanol as in Mobil MTG process, which is exothermic with part of the chemical energy of methanol being lost as the reaction heat. However, methanol synthesis require very large pressures and therefore lots of energy for compression. Furthermore, methanol synthesis is done with huge recycle ratios, with low conversion per pass. HC synthesis can be done at atmospheric pressure, and is done in a single pass over the catalyst with conversions of up to 80%.
An interesting project! @SJC It is definitely possible to improve the total yield of BtL by using external heat and hydrogen. You can do the calculations yourself. Assuming biomass consists of 49% C, 44% O, and 6% H (wt%, 1% ash), then 100 g of biomass conains 49 g carbon, 44 g oxygen, and 6 g hydrogen. From this it is possible to obtain, purely stoichiometrically, 4.1 mol CO and 4.3 mol H2 (I added 1.3 moles of water to convert all of carbon). These gases upon combustion release 530 kcal. 100 g biomass contain about 450 kcal. This way you need about 80 kcal external heat energy to convert 100 g biomass to CO and H2. Now if 3.9 mol of H2 is added from an external source, you would get 4.1 mol of CO and 8.2 mol of H2, which would yield after FT reaction 4.1 mol of (CH2)n hydrocarbons. That's about 57 g of hydrocarbons from 100 g of biomass, or 570 kg per ton! No biomass C is released as CO2 during the production process. However, considerable external heat energy (about 1/5th of the biomass feed energy), and a lot of additional hydrogen are required. Another problem is that the heat energy has to be supplied at very high temperatures, at least 1200 degC, through heat conducting wall of the gasification reactor. The heat energy could be solar thermal or even nuclear. The extra hydrogen is harder to come by, electrolysis of water is probably the most feasible option. However, these guys as well as CHOREN guys are using oxygen for gasification. This is gasification by internal heating, which inherently meens that part of the feed carbon has to be combusted to supply the energy for the endothermic gasification, and this way inevitably some of biomass carbon will end up as CO2. Internal gasification with oxygen is technologically simpler, but I personally believe that external gasification is the way to go if we want to convert as much of biomass carbon to fuel as possible.
@DaveD You cannot synthesize selectively any alcohol from synthesis gas other than methanol. There are catalysts for higher alcohols, but they give always a mixture. The pressures required are much higher than for F-T, which adds to the cost. By comparison F-T synthesis can be done even at atmospheric pressure. In my opinion this is the best general approach to convert biomass completely into clean hydrocarbon fuel. Bacteria make individual alcohols selectively such as ethanol and butanol. But these proceses are slow, and convert only a fraction of input biomass energy into the product alcohol. However, they can be viable if a suitable feedstock is available locally at low cost.
Yes, Anne, you are right. It takes about 0.5 kg of coal to make one kWh of electricity; burning of 0.5 kg of coal emits about 1 kg of CO2. So this process captures all of the carbon dioxide generated in the process. With CCS, no CO2 will be emitted from such plant.