Optimization of solid domestic waste polygon biogas utilization progress

Vostrtzov S.P, Opened Joint-Stock Company “Ural Research and Engineering Halurgy Institute” (OJSC “Halurgy”), Perm

Solid domestic waste polygon biogas generated as a result of anaerobic decomposition of waste organic products is subject to interest, primarily, as an alternative source of energy.

By its power potential, 1 m³ of biogas corresponds to 0.5 m³ of natural gas. Gas-energy potential of any polygon, where 1 million of solid domestic wastes with the rate of humidity 40%, may be considered as an anthropogenic deposit with the reserves of natural gas 50-60 million m³.

With volume content of methane rated by 50%, theoretical energy potential of biogas makes up the value of 5 kW*h/m³. If total amount of extracted gas with its efficiency = 100% is utilized, theoretical capacity of any biogas gas energy plant output might make up 600 kW per 1 million m³ of utilized gas per year.

As compared to biogas theoretical potential, the performance one is rated by the values, as follows:

• As fuel oil: 90-92%;

• As electric power generated engine fuel: 35-37%;

• As electric and heat power jointly generated engine fuel: 75% to 87% (depending on heat utilizer engineering solutions). With heat mini-station applied, the rate of highest efficiency 87% may be gained at the ratio of 144:100 (heat power to electric power) and of minimal efficiency - 75% at the ratio of 100:100.

With biogas utilized as the electric and power generated engine fuel, quite an amount of fuel and energy resources may be saved and the rate of polygon operation total current cost may be reduced. The durability of additional investment payback period amounts to 6-7 years.

With biogas utilized in amount of 1million m³ per year (including its burn-down), the rate of greenhouse gas emissions may be reduced by 8.3 thousand tons of ÑÎ₂-equivalent. Within the bounds of Kioto Protocol regulation mechanisms, the rate of emission decrease, to be specified in the greenhouse gas emission quota selling market, is valued in $USD 40-50 thousand.

The rate of biogas utilization efficiency, principally, depends on irregularity of seasonal and daily energy consumption. As per the period of one year, the rate of biogas generated and extracted is of relatively stable character, while electric and thermal loads are subject to considerable seasonal and daily variations. As a result, at certain periods the rate of utilized biogas will be insufficient for covering of peak loads and with consumed power decreased – exceeded. Exceeded volumes of biogas shall be burnt down by means of flare.

For example, let us consider the polygon power consumption test summary (e.g. size of served population – 100 thousand persons). As per the rate of utilized gas consumption (155 m³/h), any gas-energy plant potential output will make up 585.8 kW, and with diesel fuel added (diesel fuel igniting dose) – 741.5 kW.

The rates of inherent polygon needs are specified by the following figures: electric energy - within the range of 44 kW (in summer, at night) to 156 kW (in winter, in the daytime) and heat – within the range of 0 (in summer) to 255 kW (in winter). With the loads to be compared to the rate of gas-energy plant output, the current energy potential is featured with its low efficiency: maximal consumption power output is rated by 55% and annual output is rated by 23%. Due to the gas-energy plant irregular load factor, more than 70% of biogas being extracted for the purpose of utilization must be flared.

Biogas utilization efficiency may be considerably increased at the expense of the gas-energy plant total and balanced loading.

The amount of electric energy to be supplied to electric mains on the basis of established tariff payments is the widely spread method used for selling of excess energy. But, with the above method applied, certain regulating acts (within the bounds of environment protection and energy saving regulations) need to be adopted for the terms making relevant companies purchase electric energy. In Russian Federation this kinds of regulations are not adopted. Besides, due to its low utilization efficiency, biogas is subject to generation of only electric energy. Quite an amount of energy that might be used for generation of electric power will be lost in the form of excess heat.

Arrangement of own energy-intensive profitable production facility (e.g. hothouse farm) may be considered as a promising variant.

If to take a small-area polygon, the total area of 4600 m² may be covered by “summer” hothouses (from May to September) and 700 m² – by “winter” hothouses (12 months) to the end of the polygon useful life.

Power is distributed by months more evenly and structured with the favorable ratio of annual heat output to annual electric output gaining the value of more 1, thanks to which heat energy overproduction excess costs may be excluded. In terms of maximum value of power input, the rate of energy potential of any gas energy plant will make up 92.8% and in terms of annual output – 63.9%; the amount of gas to be burnt will make up 33.8%.

At its 9-10 year of operation, the polygon biogas utilization energy potential will be considered as sufficient enough for arrangement of any hothouse farm. So, for the purpose of gaining favorable economic effect, the highest possible hothouse facility capacity may be accumulated only at the end of polygon useful life. But, actually, the polygon functioning cannot be terminated immediately on finishing waste reception.

Due to maximum amount of filtrate that will escape within the period of 5-7 years, treatment facilities must be operated at their full capacity. The required rate of biogas will be gained only in 1-2 years upon closing the waste polygon and the maximum one to be utilized – 1-2 years later. In 2-3 years, with the polygon being totally closed, some works related to arrangement of waterproof coating and degassing of waste mass will be continued.