What's this process configuration called?

There's a relatively common process configuration for pilot and demonstration plants that operate at least in part continuously, where process steps are isolated in a modular fashion and tankage is placed between them to store intermediates/effluent.  It's a method of design that's focused on being a modular platform for testing and investigation rather than for a manufacturing campaign, such that individual process steps/units can be devoted to different teams for testing and investigation, but it can be integrated to achieve pseudo-steady state operation if need be. It looks approximately like this:

I've seen it in pilot and demonstration plants all over the place, but what is it called?  I want to not look like an idiot when I speak at a conference.

The Efficiency Fallacy

One of the most common arguments that I hear against different types of alternative energy is that the respective technologies are inefficient. Solar and wind power are two of the most common subjects to receive this treatment. The argument implies that the low efficiency of many renewable energy processes when compared with fossil fuel processes are an indication of a correspondingly low technical maturity, or still worse, of outright impracticality.

That asinine arguments like this gain traction is a result of efficiency numbers being taken out of context and used to, in effect, compare apples to oranges in more ways than one. Efficiency measures can only be used as a basis for judging technology in the narrowest of cases. Unfortunately, I have yet to find a single case where efficiency is correctly cited in a debate about renewable energy, whether for or against.

This uniform lack of understanding and the sheer ubiquity of the efficiency fallacy makes it one of the most dangerous red herrings in arguments on renewable energy.

Gasification as a route to biofuels

Note: I am teaching a one-shot class on advanced biofuels this November. While this class is to high school kids, it will still require a lot of organized content to throw at them, so I am gathering my thoughts here.

The problem of liquid transportation fuels, in many ways, is filled with less than perfect solutions brought on by our limited development of technology. Corn ethanol is the biggest bogeyman, although these days I'm less certain that its demise is inevitable, for other reasons. The other, lesser-known equivalent is biodiesel, which is a much better fuel in terms of EROEI, GHG balance and competition with food resources. Unfortunately, the size of the resource is miniscule compared to fuel requirements of the world. While new methods of biodiesel production are out there that will make the fuel more easily and make better use of its byproducts (an area I once did research in), the fact remains that the amount of oil that can be gotten from plants and animal wastes aren't going to be making up more than 2-3% of the total liquid fuel supply, simply by virtue of the fact that there will never be enough oil to go around (and yes, I am discounting algae). Plant starch is easier to find in nature than oil, hence the scale of ethanol production from starch being much larger than biodiesel production, which can draw only from the pool of oilseed and rendered animal byproduct markets. The largest resource is, of course, lignocellulosic biomass itself, which is the feedstock of choice for all next generation biofuels that you'll see in the next few years.

So what to do with all this biomass? There has been a lot of focus on fermentation routes from the biofuels community. This is the result of a confluence of infrastructure and human capital from the ethanol industry and  amazing players in enzyme engineering. Enzyme engineering is so good these days that tough cellulosic feedstocks can actually be hydrolyzed into sugars and fermented using parts of the corn ethanol fermentation train. A great example of this is POET's Project Liberty, which will derive a great deal of its cost advantage from being built "over the fence" from a corn ethanol plant.

The other route I feel is getting much less attention is gasification. In this general category of processes, fast pyrolysis of biomass quickly turns most of it into carbon monoxide, hydrogen, and ash/char residue, and the gas is swept downstream into other uses. People have been doing fast pyrolysis for a long time. Before oil products became abundant, many chemicals were made using coal tar from pyrolysis. Steam gasification (a process using steam as a heating medium) of lignite is featuring prominently in coal-to-chemicals industry in China. For example, most of the growth in PVC-making over the past few years has been from vinyl production based on acetylene, which in turn is derived from ethylene and coal-bsed sodium carbide.

I'm cautiously optimistic about gasification as a route to biofuels. It has a few things going for it over biofuels and a few things going against it.

Indian Points

One of the biggest issues coming around in New York these days is the relicensure of the Indian Point Nuclear Reactor by the NRC. Indian Point is coming under attack by the environmental movement in NYC, and they haven't been afraid to raise the Specter of Fukushima over and over again. The very fact that this license renewal is coming at a low-water mark for nuclear energy makes it one of the more hotly contested fights in recent years.

As part of the opening salvos, the NYC Department of Environmental Protection commissioned a study by Charles River Associates, an economics and management consulting firm, and the NRDC and Riverkeeper Inc. fired back with another by Synapse Energy Economics, an energy economics consulting firm, both of Cambridge, MA. Not trusting the news reports, whose science and economics journalism pretty much sucks, I went ahead and read both of them.

How the Arizona Solar Tower Actually Would Work

Slashdot recently posted an article on a massive project, currently in the land acquisition and planning stages, to create a massive solar thermal tower in the Arizona desert using a pretty clever and unique design. Rather than using water, glycerin, or even molten sodium as a working fluid, the design is based around air and uses what amounts to greenhouses to power the plant. A field of greenhouses heating air surrounds a massive tower, with turbines in the base of the tower. If they had hired a better PR guy, the greenhouses might even be referred to as heat farms.

Unfortunately, the article does the usual bit of hand-waving about how the system actually works, claiming it's based on "temperature differences" between the hot air and the upper atmosphere. While I'm pretty sure this is, charitably speaking, not wrong, it is only a small part of a larger and more complicated concept. I'm going to endeavor to explain it: the power plant appears instead to be designed around a clever application of the stack effect.

In brief, the stack effect can be explained by looking at a static equilibrium of a stack, or, if you like, chimney, cooling tower, or any other somewhat cylindrical hollow structure full of hot gas. Hot gas, as might be predicted by every gas law you've ever seen, is less dense than cool gas. If we assume that the stack is sealed against air, then all things being equal a stack emitting gas hotter than the ambient air will have lower pressure at its base than ambient air pressure at the datum - the column of air above it is less dense. At the same time, if the stack is sufficiently high, the hot gas at the top will still have enough of a pressure differential with the air at that height to flow out. If it isn't high enough, additional energy has to be added to the gas, which is why most industrial installations have induced draught systems (with fans in the stack) or forced draft systems (fans at the air intake) to add enough pressure difference without having to build a huge stack.

Now suppose we drill a hole in the bottom of the stack. Since there is a pressure differential caused by the stack effect, air will intrude into the stack without any additional energy input.

That's basically what this plant is trying to do: take advantage of that pressure differential to run a few turbines. The tower has to be huge, to maximize the column of hot air overhead to create the maximum pressure differential at the bottom. In addition, since the turbines at the base will actually decrease the exit pressure of the stack gas stream, the stack has to be high enough that when this air reaches the top, it will have more pressure than the surrounding air. Ground-level air from the ambient area will spontaneously flow into the apertures provided on the outside of the solar heat farms, which will warm up the air so that when it flows into the stack it is about as dense as whatever prompted the original driving force.

I think this is a really cool idea. The article mentioned that it would be able to operate under most weather conditions. I believe this, since you can probably take one or more of the turbines offline to give a smaller pressure differential at the base between turbine intake and exit to adjust for varying heat input. I'm pretty certain the only thing necessary to start it up is the provision of an initial hot gas stream in the stack, probably from flaring some natural gas.

Two things bother me though. The first is the expense: a 200MW power plant that can only operate during daylight hours is costing $750 million to build. For that price, you could build a 1 GW top-of-the-line supercritical water-based coal plant. Sure you'd need to buy the fuel, but coal is not all that expensive. I don't have any information about this deal they have with the SoCal Power Authority, so I can't say much else, but it definitely looks to be a little shaky.

The second bit that bothers me is that the Slashdot posting mentioned that food could be grown in the greenhouse if a water source could be found. This was not mentioned in the article, thankfully, because it is a really fucking stupid idea. I'll grant that you might use only the outer parts of the greenhouse solar heat farming complex for growing crops - no one in their right mind would want 90degC air for their growing environment. But even excepting that ambient temperatures in Arizona are already hot enough for most plants. It's the lack of water and soil that really hurts. So why bother growing in a greenhouse if you've already got the temperatures you need?

And let's just suppose that this design is copied and put somewhere that doesn't have soil or water problems. You're still going to need a whole honking lot of water. This system is designed not just to heat, but to circulate. A conventional greenhouse retains water by being a relatively closed system. An open one would literally evaporate all of your water away by continually replacing your hot, humid air with dry air from the outside, which would proceed to warm up, suck up moisture, and leave. Recovery of the water couldn't be done unless you either liked it salty (through a salt dehumidifier) or wanted to make your entire power plant pointless (by cooling the air so water condenses). Growing crops in an environment like that would be insane without unlimited water. And besides, making this plant in a dry area has a secondary benefit: avoiding corrosion. Why'd the designers want to give that up?