Question

Over a 3 year period, waste from a population of 100,000 has been placed in a sanitary landfill with a gas recovery system. This practice is to continue into the foreseeable future, so a steady supply of gas CH4 is expected. A nearby armed forces base has a 50 oil-heated detached houses for married personnel. The homes use an average of 100 X 106 kJ of heat energy during the year and have peak demand during the coldest month 3 times the average.

(a) Will there be enough landfill gas available to heat these homes? [20 marks]

(b) Give your opinion on the advantages of using this gas for space heating? Consider the economics on recovery, storage, etc for 50 homes. Consider the environmental impacts of using and not using this gas. [6 marks]

Assumptions:

 Quantity of municipal solid waste delivered to site = 1 tonne/capita per year.

 Total gas production = 200L CH4 per kg of refuse over 30years.

 Gas recovery will be at least 15% of that generated, i.e. 30L CH4 per 1 kg of refuse.

 At least one-third of this gas will be generated within the first 5 years.

 Conversion efficiency of the gas furnaces is 75%.

 The first year the homes are connected will be the most critical since the gas available will be dependent on the 3 year supply of waste already in place. This amount of waste will therefore be used as the basis for the calculations.

 Given:

o CH4 = 810kJ/mol (combustion energy)

o PV=nRT at STP (ideal gas equation), given R= 0.0825 L.atmK-1mol-1

Answer 1

Total gravity of MSW delivered = 100000 times 3 tones = 300000tones = 300000 times 1000kg = 3 times 10^8kg total gas production = 200L Cn_4 per hg over 30 years 20L cm_4 per hg over 3 years therefore total gas produced = 20 times 3 times 10^8 = 60 times 10^8 = 6 times 10^9L gas recovery = 15% of the total gas produced = 0.15 times 6 times 10^9 = 90 times 10^7 = 9 times 10^8L given 1/3 of gas produced in 5 years therefore in 1 years gas produced = 1/15 of gas in 3 year gas produced = 1/5 of gas therefore Net gas produced = 40/5 times 10^7L = 18 times 10^7L conversion efficiency of gas to heat energy = 75% required of personal houses heat energy = 50 times 100 times 10^6 = 5 times 10^8KJ 0/4 coldest month = 5 times 3 times 10^8KJ = 15 times 10^8KJ