Mller via Wikimedia Commons
Remember the one about the two guys in the woods who are seen by a
hungry bear?
They start running from the bear, and one guy says to the other guy
"Why are we running? Everyone knows you can't outrun a bear". The
other guy says "I don't need to outrun the bear, I only need to
outrun you".
Any company can find a market for their ethanol if they can make
ethanol much cheaper than the corn ethanol companies in the US or
much cheaper than the sugar cane companies in Brazil. If a company
carefully counts pennies, carefully uses energy, carefully uses
water and finds a cooperation model with incumbents in the MSW
market, then it can be quite profitable.
THE GLOBAL MSW MARKET
There are more than two billion tons of MSW produced worldwide every year, with more than 250 million tons per year produced in the USA
every year. Disposal of MSW is a thousand year old industry and
there's an efficient and well-established system for collecting it, transporting it and disposing of it. There's a steady supply of MSW year-round. People pay money for disposing of their MSW. On the
surface it looks easy - get a municipality to pay you to take the
MSW, make ethanol from it, and pay someone to put what's left over
into their waste dump. It's not that simple.
The most valuable fraction of MSW is waste paper, comprising up to
40% of MSW in developed countries but only 5% in developing
countries. But new Kraft paper pulp costs 800 per ton. Another
significant fraction of MSW is food waste, comprising up 20% of the MSW in developed countries and up to 60% in developing countries.
Note that Kraft paper and food waste that haven't been pulped aren't good biofuels feedstocks. The wood fibers in paper form a strong
mesh of 5 to 50 layers, covered in coatings. Together, this prevents enzymes from accessing the cellulose. Unpulped food waste isn't a
good feedstock either - that's why we first boil food and then chew it - to form a pulp that enzymes in the body can convert to sugars.
It isn't difficult to get paper pulp and food pulp from MSW - but
it's very difficult to do this cost-effectively for two reasons: the microorganisms in MSW and the physics of water.
Food waste is ideal for microorganisms - they thrive on the sugar
and starch in food waste, and the longer the waste sits in garbage
cans or in bags on the curb, the more these microorganisms grow
(exponentially). Decomposing food waste stinks and is full of
dangerous pathogens, so people want it to be disposed of far from
where they live. Where population densities are low, it's easy to
find a ravine to dump it into - this is how MSW has been disposed of for millenia.
Where population densities are high, it needs to be transported to
where people don't live. Even the world's largest country, Russia,
has a severe MSW problem in its large cities because it's expensive to transport MSW far away to where there's empty land and because
existing waste dumps near the large cities are filling up. Russia
has immense expanses of empty land suitable for landfills, but it's very expensive to transport millions of tons of MSW to this empty
land. Garbage trucks don't get good gas mileage, and it's expensive to transport MSW long distances.
WHY NOT INCINERATE MSW?
Municipal solid waste during combustion in a moving
grate incinerator. Public domain photo by Ole Poulsen via
Wikimedia Commons
The first thing that comes to mind for solving this problem is "Ok, let's just burn it and then truck the ashes out of town". There's
one big problem - food waste is wet and is 80% water. The physics of water are unyielding - it's very expensive to cause water to go from liquid water to water vapor. It costs about 5 per ton of water to
heat water to boiling, and another 15 per ton of water to make
water go from a liquid to water vapor, so it's expensive to burn
something that's wet. The paper and plastics in MSW burn nicely, but the wet food waste uses up a lot of the energy you get from burning mixed MSW.
No sane person would buy MSW to make money from burning it - but it is a way to get rid of the MSW, albeit inefficiently. The other
problem with burning MSW is that it gives off toxins, and these
toxins are costly to scrub. Plastics burn efficiently, but there's a small fraction of PVC (polyvinyl chloride) which is toxic when
burned. MSW also contains batteries, and these give off heavy metals when burned. To solve this, people want incinerators to be located
far, far away from where their children play, but then there's again the problem of the cost of transporting the waste.
So the technical challenge of making biofuels from MSW is to make
biofuels in populated areas without creating an environmental
nuisance. Germany is a densely populated country and uses either
incineration with stringent environmental scrubbing or anaerobic
digestion with odor removal equipment and subsequent disposal of the digestate.
WHY NOT MAKE POWER?
Toronto, Canada has people put their food waste and other organics
in blue bags which get picked up every two weeks, get pulped using
BTA pulpers, and processed with anaerobic digestion. However,
neither incineration nor anaerobic digestion are a way to make a
profit - they both run at a loss. Incineration gets rid of MSW, but the money from selling the electricity it produces is less than the cost of doing the incineration and scrubbing the gasses. Toronto
pays millions of dollars every year to dispose of their organic
wastes with anaerobic digestion. Anaerobic digestion produces 50%
methane and 50% carbon dioxide, but it's expensive to convert this
to pure methane. Even if the methane is purified, nobody will pay
enough for the methane to pay for cleaning it.
Burning the methane/CO2 mixture can produce electricity, but Canada won't pay subsidies for this electricity and it's more expensive to produce this electricity than to make electricity from natural gas.
WHAT ABOUT ANAEROBIC DIGESTERS?
The next thing that comes to mind is "Ok, let's incinerate the MSW
or use anaerobic digestion, and find a municipality that will pay us enough to make a net profit.". This is what fueled the solar
industry - governments promised subsidies, people bought expensive
solar panels, and then then governments stopped the subsidies.
Italians, Spaniards, and others have found themselves saddled with
years of payments for their solar panels without getting the
subsidies promised them. No rational company depends on subsidies
for their future profits, since these subsidies can (and do) go
away.
SO, WHAT'S THE SOLUTION?
So what's the solution to profitably making biofuels from MSW? A
solution needs to be located in neighborhoods where the existing
transfer stations are located. This minimizes transportation costs
and re-uses existing waste collection infrastructure. When the wind blows, the smell can't annoy the neighbors. It has to be profitable without subsidies, since subsidies can (and do) go away. It can't
emit any toxins into the environment - it has to be something that
people won't object to when it's located near where their children
play. It can't dump dirty water into the sewers - this is expensive.
The physics of water are at the root of a solution - using hot (but not boiling) water to simultaneously kill microorganisms, pulp food waste and pulp waste paper. Microorganisms die when heated in water at 70 C (158 F) for 30 minutes. When the microorganisms die, the
waste stops giving off odors. However, heating MSW above 70 C gives off a strong stench (hence the saying "a steaming pile of garbage").
Boiling water is expensive, so a solution needs to heat the MSW to
between 70 C and 95 C while concentrating the odors so they can be
put through an odor removal system. Food waste softens at 85 C and
above (the pectin is loosened) - drop an uncooked potato on the
ground and a cooked potato on the ground to see how the former
bounces and the latter pulps. Paper forms a pulp when subjected to
shear forces, and this is most efficient above 60 C (140 F).
The most cost-effective way to pulp waste paper is using a drum
pulper, similar to the Metso OptiSlush or the Andritz (ADRZF)
Fibreflow drum pulpers. A drum pulper is a large rotating drum,
usually at least 2.4 m (8 feet) in diameter and at least 20 m (65
feet) long. These are used all over the world to take waste paper,
add water, and lift and drop the wet waste paper at more than 10
revolutions per minute. This lifting and dropping causes shear
forces that separate the wood fibers from the wet paper. Using a
drum pulper, it costs about 10 kWh to pulp a ton of waste paper,
which costs about 0.80 per ton of waste paper at 0.08 per kWh.
A drum pulper will also pulp food waste using these same shear
forces, if the food waste is first heated to 85 C (185 F). A
temperature of 85 C will also kill all the microorganisms in MSW.
Heating MSW from 25 C (77 F) to 85 C costs about 5 per ton, and
rotating the drum pulper for an hour will cost another 2 per ton.
Separating the pulped paper and food waste from the inorganic
fraction will cost another 1 per ton (using a trommel screen and a
dewatering device).
Our solution for separating carbohydrates from MSW is based on this idea. MSW is pulped with process water in a heated drum pulper at
high consistency transforming the food waste and waste paper to a
pulp. This pulp contains sugar and starch from food waste and paper fibers from waste paper. A screen with additional process water
separates clean recyclables from the pulp. A hydropulper removes
sand, grit and glass fragments from the pulp. A dewatering device
separates paper fibers from the pulp. The remaining pulp is added to the process water. The process water is treated with alpha-amylase
enzymes to convert starch to sugar. The overflow of the process
water is a sugar and starch solution that reaches an equilibrium
concentration. No water is added in this process and commercially
available drum pulpers, screens, hydropulpers and dewatering
equipment can be used.
This produces three fractions from the MSW. The clean fraction with plastics, metals, glass and other inorganics can be separated using a materials recycling facility (MRF) to get some value from
recyclables. The sugar-water fraction and the paper fiber fraction
can be used together to produce ethanol or other products using
enzymatic hydrolysis. The remainder after enzymatic hydrolysis can
be used as a soil improver since there aren't any heavy metals in
the organic fractions.
Our solution removes the odors from the MSW by suctioning the air
from the entry to the drum pulper and treating it with commercially available odor removal equipment. Using a drum to concentrate the
odors works better than using a positive pressure in the whole
building, since buildings aren't completely airtight.
Our solution doesn't shred the MSW. Shredding MSW is noisy, the
shredders sometimes explode, and shredding MSW puts heavy metals
into the waste stream. If you shred MSW, you can't use the remaining organics after enzymatic hydrolysis for soil improvement or compost because there are strict limits on heavy metals put back into soil.
Food waste pulp and waste paper pulp don't contain heavy metals -
tests have shown heavy metals below the limits of detection.
Water treatment is an under-appreciated requirement. Garbage is
dirty, and any time you pour water over garbage, it gets very dirty.
You can't just flush this water into the sewer. World Waste
Technologies built a factory in Anaheim in 2006 to extract paper
fibers from MSW. They went out of business and sold the factory for scrap because they were producing dirty water that would have cost
more to clean than the money they'd make from the paper fibers.
Our solution uses process water that's refreshed by the 80% water in the food waste. No fresh water is used, and no waste water is
treated. The water from the food waste is eventually disposed of
with the residual organics as soil improvement or compost.
RUNNING FROM THE BEAR
The economics of biofuels are complicated, with issues like the
blend wall (too much ethanol being produced already for blending
with gasoline), RINs, etc.
At the Advanced Biofuels Markets conference last week, Philippe
Lavielle of Virdia gave a thought-provoking keynote address about
the viability of various feedstocks for making biofuels - but he
didn't mention MSW as a viable feedstock.
For sure, if a company does dumb things like using autoclaves
(converting water to steam), producing waste water, burning wet MSW, producing a low-value product like methane, or has an otherwise
complex solution full of conveyers and sorting equipment, then they won't outrun the other guy running from the bear.
