Background:
Steam turbine generators have long been the backbone of power generation throughout the world. Steam is a needed source of heat, for making the world’s products, keeping people warm and providing food at mass scale. While steam turbine generators are acknowledged to be needed at power plants, the same is not true for industrial plants. This needs to change.
Thermodynamics:
What makes the backpressure STG so efficient? When steam is needed for another purpose, when we have no other choice but to heat water to create steam, we waste 1,000 Btu’s per pound just to change state from liquid to steam. The temperature of the fluid doesn’t change, it just changes state. Conversely, increasing the pressure and temperature of steam takes very little energy. Therefore, if we use the opportunity when steam must be produced to bump up the pressure and temperature, as much as possible or as much as commercially viable, we can use that steam for a dual purpose: power generation and steam heat. By using it twice instead of once, we dramatically increase the steam efficiency. How efficient is it? The modern standard for utility power generation is the combined cycle plant (gas turbine + steam turbine). Those facilities consume approx. 7,146 Btu/kW (per US EIA). A backpressure STG consumes approximately 3,600 Btu/kW. That means a backpressure STG consumes only ½ the amount of fuel a utility combined cycle facility consumes. To make this product even better, it is a significantly lower cost to install. All it takes is a slightly more expensive boiler (for the higher pressure), piping in parallel to a pressure reducing valve, the STG itself and a generator circuit breaker to synchronize power into the plant system. The typical cost is around $1,000/kW which is half the cost of a utility scale combined cycle plant. At ½ the cost and ½ the fuel consumption, the backpressure STG pays back at a rate of 4x a utility combined cycle power plant.
Conclusion:
Given the lower energy consumption, and the lower cost of generation, all steam plants should have a backpressure STG installed, assuming they meet the minimum criteria. Below is a generic schematic of a backpressure STG, with the minimum qualifications shown. Steam flow rates lower than 10,000 lb/hr with a pressure drop lower than 100 psi generally do not justify economically; the power generation is simply too low to merit implementation.
Carbon Reduction:
Per the U.S. Energy Information Administration (US EIA) the average CO2 emission rate is .96 pounds of CO2 per kWh produced (natural gas generation basis). If we assume that all new fossil fuel generation will be combined cycle power plants, any power produced by a backpressure STG will reduce CO2 emissions by half, or save .48 pounds of CO2 per kWh compared to a combined cycle power plant.
Example Project:
Assume a sawmill is producing 40,000 lb/hr of steam, and the kilns need 30 psig steam to dry the lumber. If the sawmill elected to increase the pressure of its steam from 30 psig to 350 psig, run that high pressure steam through the backpressure STG and exhaust to the kilns, they would produce 1,250 kW and save over 5 million pounds of CO2 per year, while saving over $500,000 per year in electricity (after accounting for the small increase in fuel cost).
Benefits of Backpressure Turbines
Thermodynamics:
It takes ~ 1,000 Btu/lb to generate
steam from liquid. And that doesn’t
change temperature or pressure.
Adding pressure/temperature is
incrementally inexpensive and energy
efficient. Using that steam twice
instead of once increases your steam
plant efficiency.
Every steam plant should have a STG.
Typical STG Equipment Cost: $1.5 – 2 million
Vaporizing Water
1,000 Btu/lb and its useless.
It needs to be bottled up and
pressurized to become useful.
Every steam plant should have a STG.
Mollier Diagram
Mollier Diagram
Mollier Diagram
Output Calculation
Steam Flow: 50,000 lb/hr
Enthalpy Diff: 105 Btu/lb
Output: 50,000 lb/hr * 105 Btu/lb = 5,250,000 Btu/hr
Conversion: 3412 Btu/kWh
5,250,000 Btu/hr ÷ 3412 Btu/kWh = 1,539 kW
1,539 kW * $.08/kWh * 8,000 hrs/year
= $985,000/year
What about the fuel cost?
Fuel Cost Calculation
105 Btu/lb * 50,000 lb/hr = 5.25 mmBtu/hr
5.25 mmBtu/hr * $4/mmBtu = $21/hr
$21/hr * 8,000 hrs/yr = $168,000/yr
Net Benefit:
$985,000 – $168,000 = $817,000/year
Unless you have excess wood fuel and its free.
Then the benefit is $985,000
Heat Rate Comparison and CO₂ Savings
Energy Information Agency:
Combined Cycle Heat Rate: 7,146 Btu/kWh
Backpressure Turbines: 3,600 Btu/kWh
* Saves ½ lb CO2 per kWh
*Compared to conventional power generation
Questions?
Bill Hunter, PE
West VP Sales and Turbine Products
bhunter@processbarron.com
206-601-0047
Download Our Informational PDF
For more details about backpressure steam turbine generators, including technical specifications, efficiency data, and economic benefits, download our informational PDF.