Solid domestic waste polygon biogas as source of energy

Vostretzov S.P, OJSC “Galurgiya” Perm, Russia

Recently, much interest in alternative energy sources tends to increase, particularly, in renewable energy source-based bioenergetics. One of the above significant areas is the sector of utilization of livestock facility, sewage, dumps and solid domestic waste polygon biogas.

The solid domestic waste polygon biogas generated as a result of anaerobic decomposition of waste organic components is of great interest for any research.

The solid domestic waste dry substance weighed by 1 t is the volume to be enough for generation of 170-200 m³ of biogas with the ratio of methane (strong heat gas) specified by one half of its volume and 25-30% of its mass. Due to its properties to be similar to the ones of powerful natural (bogs) and anthropogenic (oil fields) sources, the rate of environmental impact (greenhouse effect) of landfill methane is specified by 6%. As regards to the rate of methane emission to be taken per a surface area unit (approximately 200 t/year per 1 ha), solid domestic waste polygons are the ones to advance other sources.

Due to methane combustible and explosive properties, any waste area should be degassed irrespective to the purpose of operation.

It is found out that by reason of escape, up to 20% of solid waste dry substance starting weight is lost together with biogas including 8-10% - in the course of polygon operation. Decrease of waste mass is followed with increase of waste density and dehydration, as biogas is a product generated by lightweight organics with loose structure and high humidity.

Just a half of generated biogas potential being collected and used is equal to 10% of waste utilized at a polygon. For comparison, potential level of polygon waste utilization by means of expensive waste-sorting equipment is estimated similarly. But with the waste sorted the rate of consumed energy is specified within the range of (25-30 kW*h/t of SDW) and with the biogas utilized the rate of generated energy is specified within the range of (50-60 kW*h/t of SDW).

By its capacity, 1 m³ of biogas corresponds to 0.5 m³ of natural gas. Any gas-energy potential of the polygon, where 1 million tons of solid domestic waste with 40% of humidity is disposed, may be considered as an anthropogenic field with natural gas reserved in amount of 50-60 million m³. As per one serviced inhabitant, the volume of biogas to be collected at the solid domestic waste polygon may be specified in amount of 10-15 m³ per year. With the amount of biogas to be utilized at the polygon servicing the town of 100 thousand inhabitants, the rate of energy and heat needs of any settlement inhabited by 1 thousand persons may be covered up. But this kind of anthropogenic gas field cannot be considered as of dedicated matter. The solid domestic waste polygon being a huge biochemical reactor may be considered as the human being-generated by-product and, to some extent, as habitation’s “latrine”. As compared to the natural fields, this kind of field is generally featured by unavailability of gasproof isolation facilities to be used for selection of generated gas. As a result any volume of biogas will be emitted to atmosphere polluting it.

With volume content of methane to be specified of 50%, theoretical power potential of biogas amounts to 5 kW*h/m³. With the full 100% gas volume extracted and used, the theoretical rate of any gas-energy biogas-running plant would make up 600 kW per 1 million m³/year of biogas utilized.

As compared to theoretical power potential, the rate of performance power potential of biogas utilized is as follows:

·   As fuel flue gas: 90-92%;

·   As power generation engine fuel: 35-37%;

·   As electric and heat power cogeneration (joint output) engine fuel: 75% to 87% (depending on heat utilizer engineering solution).

For example, with base model mini-heat station MTD-100/110 converted for operation on biogas, the rate of heat and electric power is generated within the ratio 110:100. In this case the rate of biogas efficiency is 75.6% and the one of performance potential - 5*0.756=3.78 kW*h/m³.

Any gas-energy facility operating on biogas by the method of cogeneration (joint output of electric and heat power) may gain the rate of electric capacity 200-220 kW and heat capacity 220-280 kW and provide 1 million m³/year of the utilized biogas. The rate of potential biogas power output shall exceed the rate of polygon needs by 2-3 times.

Biogas efficiency principally depends on seasonal and daily irregularity of power consumed. In the course of year the rate of gas generation and extraction shall amount to relatively stable value, while electric and heat loads are subject to considerable variations, both seasonally and daily. As a result, at certain periods the rate of utilized gas consumption will be insufficient for covering of peak loads, and with power consumption decreased – excessive one.

For the purpose of power supply along with cyclic and seasonal loads, the rate of effectively utilized biogas shall be specified within the ratio of 25-30% - if only the polygon performance needs would be satisfied, and 65-70% - if the polygon would be equipped with own power-consuming facility to be used for production of profitable product (e.g. heating plant). The rest volumes of collected biogas would have to be burned at the periods of load drops. In terms of the above example, the rate of any small polygon biogas underutilization energy loss would amount to 1500-2000 MW*h/year and economy loss – 2.5-3 million rubles per year. At the same time, owing to the biogas burned, considerable ecological effect is gained as a result of reduction of greenhouse gas emission (principally, methane) by 3-4 times as compared with the rate of biogas emitted in atmosphere from the waste mass. Thus, in terms of ecologically reasonability, biogas should be subject to burning out.

Cost of power generation by the method of cogeneration is twice lower than the purchase price of power mains. If biogas is used only for generation of electric power, its cost will be higher than the one of power mains and will be exceeded by 30-50%.

Even taking account of insufficiently high efficiency of collected biogas, its utilization as electric and heat output engine fuel will make it possible to considerably decrease fuel-energy resource costs (e.g. as compared with any habitation supplied from electric mains, at any polygon servicing the town inhabited by 100 thousand persons the amount of total twenty-year current cost would be reduced by 10-11 million rubles (as per 2006)). The estimated period of additional capital expenditure payback would be realized in 6-7 years.

Any project subject to biogas utilization should be developed considering the matters of biogas effective factor. The most promising activities are as follows:

·  With loads dropped, accumulation of excess gas in special-purpose gas storage facility;

·  Biogas enrichment along with generation of methane for the purpose of automotive (particularly, polygon process machinery) engine fuel.