BTU's in a Therm?

Factual Questions
1

As I mentioned in GD, my city is protesting a natural gas fired power plant in a neighboring city. Since this is California, I think a new Powerplant is a Good Thing.

Being an engineer, I thought I would do a back of the envelope calculation to see how many homes a NGPP would represent.

I can get from kilowatt-hours to BTU’s but I cannot get from the “Therms” on my gas bill to BTU’s. Does anyone have a conversion factor?

I will argue along the lines of well, if you are so concerned about the burning of NG and our children, maybe you should be concerned about all these houses spewing out the products of NG burning, too!

2

1 Therm = 100,000 BTU

Arjuna34

3

100,000 Btu’s

4

Possible correction!

My ABB-CE “Combustion Fossil Power” book gives the following conversions:


"1 Therm, 10^5 Btu, I.T., Eur Comm. = 105.5060 MJ"
"1 Therm, US Natural Gas            = 105.4804 MJ"

So if one wants to be exact, then note this very slight difference.

5

Arrghh, answered already! Here’s a complete list of definitions on your gas bill:
http://www.okaloosagas.com/home-appl.cfm

6

Woo-hoo!! I beat Anthracite answering an energy question :slight_smile:

Although I guess she gets points for citation and exactness …

Arjuna34

7

Thanks!

That was quick!

(I figgered Anthracite would have the answer handy!)

My standard physics reference guy here at work thought that a BTU was about 10000 cubic feet of NG. Is he all wet, or is the 1 cubic foot of NG conviently 1 BTU?

8

Thanks for the exact number Anthracite, but I said that this was a Back of the Envelope kind of thing.

While I have your attention, does anyone know how efficiently a NGPP turns NG BTU’s into MW? I was going to ignore this, since your average City councilman probably would not catch it, but if someone can give me a better number…

Also in the OP, the city is concerned with the effect of the NG burning on children, not the burning of NG and children.

9

How on Earth could one not consider the plant heat rate?

And what type of gas plant? Gas boiler (unlikely)? Simple-cycle gas turbine? Combined cycle gas turbine? What type of emissions controls? This value could range from 11,000 Btu/kWh for a poor gas boiler, to 6800 Btu/kWh for a combined cycle, top-of-the-line unit without Selective Catalytic Reduction (gross basis for both numbers, BTW). There is a wide range of possible efficiencies.

10

Here are GRI “official” gross heating values for natural gas assumptions (1994, I don’t have anything newer, but close enough), depending on the region of the LDC (local distribution company):

Note: values are for Natural Gas Standard Cubic foot.


LDC Region          Gross Heating Value
Northeast           1036.1 Btu/ft[sup]3[/sup]
South Central       1028.3 Btu/ft[sup]3[/sup]
Southeast           1043.4 Btu/ft[sup]3[/sup]
Mountain             985.5 Btu/ft[sup]3[/sup]
North Central       1027.7 Btu/ft[sup]3[/sup]
Pacific             1033.0 Btu/ft[sup]3[/sup]
General US Avg.     1034.6 Btu/ft[sup]3[/sup]

Note these are only valid for the US. Sorry, other countries!

11

You only beat me because my connection is slower than an arthritic Galapagos Tortoise today. :wink:

12

Standard Temperature and Pressure for Natural Gas is typically 14.73 psia, 60 F.

13

I think I will have to use an 8x10 envelope!

according to here:
http://www.energy.ca.gov/sitingcases/nuevaazalea/

This plant is going to be a 550 MW, NG fired, combined cycle power plant. So I believe that I can use numbers at the lower end of your range, Anthracite.

14

Pick 7000-7350 Btu/kWh as a good, conservative guestimate. CA will require an SCR system for certain, and possibly a steam injection and/or low-NOx burner system.

If it was coal, I could tell you numbers to +/- 0.1% confidence. But I don’t know much about gas.

15

My estimates:

Assume Gross output 550 MW
Assume unit auxiliary power 1% (mainly due to SCR)
Therefore, unit Net output (supplied to grid) is 544.5 MW

Now, let’s assume a GPHR (Gross Plant Heat Rate) of 7200 Btu/kWh. To find Btu/hr required to get the 550 MW, we do:


7200 Btu   *   550,000 kW =      3.96x10[sup]9[/sup] Btu
--------       ----------   -----------------------
  kWh                                 hr

To find natural gas burn rate, just assume Pacific LDC company heating value (gross) of 1033 Btu/ft[sup]3[/sup], and:


         3.96x10[sup]9[/sup] Btu
    -----------------------
              hr                     3.833x10[sup]6[/sup]ft[sup]3[/sup]
----------------------------- = -------------------------
       1033 Btu                          hr
       --------
          ft[sup]3[/sup]

Or,

     1064.86 ft[sup]3[/sup]
----------------------
        sec

So to deliver 544.5 MW to the grid (customers), you ned 1064.86 ft[sup]3[/sup] per second of natural gas to the plant.