Ok. Technically Sewage Sludge is a phrase.
The other day I was talking about cofiring. And I discovered that one of the fuels they cofire alongside coal is sewage sludge.
What is sludge?
Up to 95 percent is water. But it starts as wastewater, which is a mix of food, paper, diapers, plant mater, feces, condoms, sanitary napkins, paints, pesticides, bacteria, pathogens, pharmaceuticals, sand, metal particles, road salt, insects and gases.
I think I would prefer 100% coal.
Cofiring: the combustion of two different types of materials at the same time.
This word may not be new to many of you (or some of you) but it was to me. Or course I have mocked the idea of replacing coal with wood since burning wood from the USA creates more CO2 than coal. The DRAX post from the other day points out that even DRAX’s own study showed more CO2 from wood pellets than from coal.
And destroying forests to produce more CO2 in the atmosphere seems to me to be amazingly stupid.
So I’ve been investigating to see what kind of cofiring goes on and how much CO2 is produced. The really important terms are Total CO2 and Net CO2 and CO2 neutral.
Total CO2 refers to the gross emissions of CO2 from this power plant.
Net CO2 refers to the emissions of CO2 from the fossil fuel used in this power plant, since biomass is assumed to be CO2 neutral. Gross CO2 and net CO2 will be the same where only fossil fuel is used.
In my opinion the concept of CO2 neutral is bogus. CO2 is CO2. If you generate 600MW of power and you care about CO2 then it shouldn’t matter whether you use coal or sewage sludge or any other biomass. It should be total CO2. (Not completely true because other things are produced from coal power plants like SO2 etc but today we talk CO2)
I came across this paper: A Techno-economic assessment of the reduction of carbon dioxide emissions through the use of biomass co-combustion
The paper claims:
Using sustainably-grown biomass as the sole fuel, or co-fired with coal, is an effective way of reducing the net CO2 emissions from a combustion power plant. There may be a reduction in efficiency from the use of biomass, mainly as a result of its relatively high moisture content, and the system economics may also be adversely affected.
Notice the term net CO2 is used. Their conclusions are based on the fallacy that the CO2 produced by burning the biomass is zero. But they were nice enough (honest enough?) to show the figures for total CO2.
The table shows the result of the experiments. The one I highlighted has 4 sections:
PN1: a 600MW power plant burning 100% coal. CO2 = 759 g/kWh
PN2: a 600MW power plant burning 80% coal and 20% straw. 773 g/kWh
PN3: a 600MW power plant burning 80% coal and 20% sewage sludge. 765 g/kWh
PN4: a 600MW power plant burning 80% coal and 20% straw (reburn). 818 g/kWh
In all cases biomass+coal cofiring produces more CO2. And the CO2 numbers don’t take into account transportation of coal or biomass. So locally sourced biomass isn’t a disaster. But wood pellets from the USA produce a lot of CO2 just in transport costs.
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