Trash to Gas? Move on...
For decades people seem to have tried gasification of waste. Either the economics didn't work out, the technology was finicky, or a combination of both. The failure of gasification comes as no surprise, as I looked into my kitchen trash bag this morning, seeing an incredible potpourri of materials, sizes, shapes, and consistencies.
Garbage is heterogeneous. It contains particles ranging in size from a grain of sand to a sofa. It contains materials ranging from road grit, metal, and glass with no energy value to plastics with a high energy value. The array of chemical compositions is almost endless, and some of these materials are literally nothing but water which will extinguish any hope for successful conversion pretty quickly.
What happens when the gasifier is fed a load of two dozen pumpkins, as I witnessed being unloaded at the Lancaster incinerator?
The solution to trash: mass-burn incineration with cogeneration of electricity and heat. It seems that most of the gasification proposals will be using the gas to generate electricity anyways, so why not skip the complicated gasification process?
British Airways apparently would like to convert trash into aircraft fuel - producing what they call "green airliner fuel". Good luck with that one, there's nothing more like running backwards up Mt. Second Law.
Let's let physics work in our favor - instead of burning oil and gas for space heating and making fuel from garbage, let's use trash for space heating and make fuel from oil and gas. If we want to be "green", we will let the airlines keep burning oil-derived fuel and we will heat our cities by burning garbage - NOT the other way around.
One Hyped-Up Fuel Cell
The journalists at CBS appear to have made a big stink about a little black (er...gray) box called a Bloom Box. Developed by Silicon Valley entrepreneurs, The Bloom Box is a modular fuel cell capable of producing electrical power on a small scale at high efficiency using natural gas.
The company's employees reportedly spent ten years of their lives developing this thing. I do not know what kind of innovations in materials and construction they may have developed and cannot criticize that aspect of the product. I am willing, however, to criticize the insane amount of hype which the media has generated. The articles have exploded over Web sites over the past few days, and having read some and scanned through others, some fairly outrageous claims are being made.
Essentially, they made another fuel cell - the same technology that Sir William Grove demonstrated in 1839, the same technology which Ballard Power Systems, Fuel Cell Energy, and others have been selling for decades, and the same technology in the mythical cars Honda has been advertising on TV for years now. They may have made a more robust fuel cell, cheaper fuel cell, etc. But nonetheless, it is a fuel cell.
- It does not create electricity from thin air.
- It does not necessarily use renewable resources.
- It burns NATURAL GAS, the same gas people are familiar with for heating their homes. CH4 + 2O2 -> 2H2O + CO2, the same chemical reaction which takes place in the all-familiar gas forced air furnace and gas range burner.
- There are emissions. Water and Carbon Dioxide, the same emissions that make up the majority of power plant, car, truck, and home heating exhausts. These are also the emissions of human beings, but we do not run on fossil fuels, at least not directly.
- It has the capacity of increasing efficiency through cogeneration, provided that facilities are installed for recovering the waste heat - e.g. for domestic hot water, space heating.
- It is not an energy storage device.
- It does not defy the laws of physics (specifically thermodynamics and conservation of matter/energy)
- It will not reduce "resistive" losses in energy transmission. There is resistance in gas pipes just as there is resistance in electrical wiring. It takes energy to move gas around just as it takes energy to move electricity around.
- Running the Bloom Box on methane from manure/sludge digestion, landfills, etc. is possible. So long as every last molecule of sulfur is removed beforehand, as sulfur is notorious for bringing a slow death to fuel cells and catalysts. Landfill gas will also tend to have chlorine compounds and other nastiness in it, that's why the big LFG plants use boilers and steam to generate power as it is even too harsh for diesel engines.
- Companies like eBay, Google, Walmart, etc. are willing to pay big bucks for this because using such a box to power their data centers means that they can do away with big and expensive uninterruptable power supplies and diesel generator sets required as a backup to the grid power used now.
- Using the Bloom Box to produce a "methane-like fuel" from the water and CO2 exhaust is brain dead, senseless, stupid, et. al. Turn natural gas into electricity, heat, water, and carbon dioxide, then go and reassemble it all back into natural gas. All while adhering to the Second Law of Thermo and making money at the same time? Rube Goldberg would be happy.
Currently the consumer economy requires the energy equivalent of filet mignon - electricity and liquid fuels - to survive. In my opinion, those who make the world run on 60/40 ground beef - low-grade heat and light from the sun - will be the ones who win the prize. They probably will not become rich enough however to be able to afford the $5000 office chairs of dot com fame, let alone sustain consumerism.
Urban power generation
One of the most serious problems with our current energy system is that it does not consider the idea of exergy - the amount of useful work which can be produced from a given fuel. In our buildings we use fuels to create heat for comfort purposes, diluting all of the potential to produce useful work down to essentially useless 72 degree space heat. In contrast, we burn fuel at power plants, taking the useful work (electricity) and throwing away the dilute heat. We heat our dwellings the same way we make birdhouses: by chopping prime lumber into little itty-bitty planks and wasting all of the potential to build bigger things. We make electricity the same way we build houses to live in: use the "useful" parts of the lumber and throw away all of the scraps and excess.
What if we could build a real big house, and then use the leftover scrap wood to build birdhouses? We can - it's called cogeneration and it is certainly not a new/expensive/radical concept, it is just one which we have been reluctant to implement for various reasons - cheap fossil fuels, climate (in the case of the South), dispersed population, pressure from electric utilities, etc.
In the United States, Canada, and many other countries, the penetration of district heating never really progressed as fossil fuels were always cheap and subsidized and the populations are so dispersed in suburbs as well as rural areas. It is for the most part completely impractical to run district heating to these areas. Electricity, however, can and is easily delivered to such areas.
Barring some radical change in the ways people live (e.g. moving from mega-cities and suburbs into smaller walkable cities as is promoted by James Howard Kunstler and others), it will be necessary to modify the energy infrastructure to accommodate the population distribution while maintaining a high level of exergy utilization.
...So if we can use district heating and/or small-scale building-based cogeneration in the cities, we should be concentrating thermal electrical generating plants in urban areas rather than in faraway middle-of-nowhere settings as has been the practice since the 1950s. This allows as much electrical energy to produced from the fuel as possible and then a market for the remaining heat. Natural gas is, obviously, the most desirable fuel and in most situations would be the only option barring biomass or solid waste generated within city limits.
Combined-cycle gas plants actually produce more electricity than heat, and since most urban societies today with district heating consume far less electricity (on a joule-for-joule basis - see Denmark) than heat, an electricity surplus would be available for export to rural areas. Homes and businesses here would be able to utilize electricity in place of natural gas for heating and cooling through the use of heat pumps. By converting as much natural gas energy into electricity as possible, a much better utilization of the energy is realized. Using natural gas for space heating is wasteful. I don't care if a gas furnace/boiler is "98% efficient", it is still essentially a machine that grinds steaks into hamburgers (to use Deffeyes' analogy). That is wasteful.
URBAN ENERGY CONCEPT: GENERATE POWER IN THE CITIES, SEND WHAT'S LEFT OUT TO RURAL AREAS.
Those rural buildings (as well as the urban centers, though it would be less urgent due to the wide availability of excess heat from the power plants) would also greatly benefit from solar-thermal panels, reducing the need to use electricity for dilute heating purposes whether it be through the use of resistance or a heat pump. Surpluses of electricity are unlikely to be an issue, especially if electrified transportation were to come on stream. Renewable electricity from the various hydro, wind, solar thermal, and geothermal installations would have to be introduced into the system through a transmission grid.
Climate Effects
The effectiveness of such a system would depend upon the climate; northern cities would benefit much more than southern cities due to the colder climate creating a larger demand for heat. During summer and in the South, electricity is the dominant form of energy consumption - driving all of those central air conditioning units in Sun Belt suburban homes while their occupants are at work is a serious electrical black hole. Higher ambient temperatures also reduce the efficiency of thermal power generation schemes (Lower delta T = less efficiency).
Aside from common-sense demand-side measures (e.g. white roofs in the South, passive solar buildings, ice storage, turning OFF the air conditioner when building or home is not occupied, reducing the very common OVER-cooling of buildings, etc.), summertime demand could be met with energy from large-scale solar-thermal generation and rooftop PV systems. Those systems fit the demand profile of air conditioning very nicely. Even if the urban power generation schemes continued to supply full power, the gas energy loss as dilute heat would be at most 40% and the city occupants would continue to be supplied with domestic hot water through the district heating network.
Waste heat and power plant
Aside from standard natural gas combined cycle cogeneration plants, a facility including a waste-to-energy system for solid waste treatment could be integrated to utilize the energy present in waste.
- Municipal waste residues collected from the urban population is hauled to a resource recovery facility and sorted. Residual non-recyclable portions are incinerated. It may be better suited to have multiple smaller WTE installations utilizing oscillating kilns rather than large regional plants. The smaller footprint and daily tonnage allows the plant to better fit into an urban environment.
- The heat from the incineration lines is used to produce steam at a temperature of no more than about 725 K (if I do recall correctly these are values typical of modern waste-to-energy plants).
- Natural gas is burned in a gas turbine to produce electrical energy.
- The exhaust heat from this gas turbine is used to boost the temperature of the steam from the incineration lines to higher temperatures and pressures.
- The steam is then used to drive a steam turbine to produce additional electrical energy.
- The turbine exhaust is condensed and the remaining heat dispatched to the district heating network.
- The flue gases from the incineration lines are condensed (burning high-moisture waste along with vapor from the scrubbing process) and used to preheat the incoming district heating water.
- An interesting technical question regarding flue gas cooling/condensation is the mechanical work required to force cooled flue gases up the stack. If we extract as much heat as possible from the gases, it may appear that we have increased the efficiency of the plant. However, if we then need to use more electricity to induce a draft to get the now less-buoyant gases up the stack, could the efficiency be less? Electricity to drive a fan is much more valuable than a couple hundred degrees of heat in flue gases. Removal of water, however, decreases the mass flow rate up the stack and may reduce the work needed to pump the gases. It would be interesting to see the result on the new WTE plants in Europe built with gas condensation and pencil-thin stacks.
- Wastes may be baled and stored during summer when demand for district heating is low, then burned in winter to provide additional heat.
Hybrid Power Plants
Solar thermal electricity generation is often touted as one of the most promising renewable energy solutions due to its lower cost and higher efficiency on a large scale compared with photovoltaics.
While promising, building such a plant is still expensive. I have recently gained interest in a hybrid power plant concept - using solar heat to aid in the generation of steam at existing fossil-fuel power plants. It combines the political popularity of the word hybrid with the generators and turbines already in place at existing coal-fired power plants to make a solar system which has the capacity to be cheaper and more effective (e.g. more watt-hours of solar pumped out to the grid) than a standalone plant.
It is obvious that hybrid power plants are not a long-term solution to the problem (just as hybrid cars are not), but looking at our current electrical generation mix (50% coal, 20% nuclear, 20% gas, 7% hydro, 3% everything else) it is clear that more renewable energy is necessary and has not been delivered. Anything that can make use of existing equipment and integrate renewable energy into the system is worth pursuing, especially if it can be done in a way that is cost-competitive with fossil fuels (which have most of their real costs externalized).
Such a plant would produce a constant electrical output. The amount of solar power will vary throughout the day and based upon weather conditions. Coal burn in the boiler would be modulated based upon solar radiation, keeping the amount of steam flowing to the turbine more or less constant.
SEPARATE WATER CIRCUIT IS USED FOR THE ARRAY TO AVOID HAVING TO WITHSTAND THE FEEDWATER PRESSURE. A BYPASS SYSTEM IS ALSO INCLUDED TO ALLOW COAL-ONLY OPERATION.
Advantages:
- Reduced coal burn in the boilers during sunlight hours.
- Cost of solar implementation is lower, using already-existing coal-fired plants with turbines and generators already in place.
- Utilization of equipment (turbines and generators) is higher than a solar-only plant due to constant power generation, reducing the payback period.
- Steam inlet temperatures to the turbine are higher than standalone solar, meaning lower heat rates and better electricity sales and utilization of equipment.
- Pollutant reduction from adding solar to a coal-fired plant is much greater than adding solar to a natural gas plant, as the carbon/joule ratio in coal is higher than gas.
- The absence of turbine/generator expenses means more money could be spent on collectors to capture more energy and burn less coal.
- A cheaper array may be built if it is only being used as a feedwater heater; temperatures and pressures may be much lower than in a standalone array working at the turbine inlet temperature. Large coal-fired plants require more power for feedwater heating than most commercial-scale solar plants produce in total.
Disadvantages:
- Siting of collectors arrays may be a challenge at existing power plants; transmission of the heat over long distances becomes impractical.
- May create a mentality that "coal is okay so long as it is supplemented with solar"
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