After Gutenberg

Basic components of an anaerobic-digestion system
Hansen, 2001

The decomposition of organic material (i.e., volatile solids) in the absence of oxygen is anaerobic digestion. Bio-gas (methane plus carbon dioxide, other gases, and water vapor) is a product of the decomposition, which can be separated into two stages. First anaerobic bacteria known as “acid-formers” transform the feedstock into constituent fatty acids. In the second stage, these acids are further converted into bio-gas by more specialized bacteria known as “methane formers.”

This blog recently noted two reports from the Biopact team about the conversion of biomass to bio-gas:

1. Austrians now can drive automobiles on a blend of 80% CNG (Compressed Natural Gas) and 20% bio-methane obtained from fermenting smooth meadow-grass.
2. In India, sugar producers are learning that distillery sludge, bagasse and spent wash make an excellent feedstock for anaerobic digestion.

In addition to biomass there is other feedstock that can be subjected to anaerobic digestion: offal and manure. This blog previously noted that Svensk Biogas has made use of waste from meat rendering plants to produce bio-gas, which then was upgraded for use as an alternative fuel for public transportation. More often, this blog has focused upon the conversion of manure to bio-gas.

Digesters can make use of various biodegradable materials; the more putrescible the material the higher the gas yields possible from the system. Co-digestion is combining such feedstock. Waste from the raising of livestock is the main source, to which other material is added.

Based at the University of Southern Denmark, ProBioGas is a European consortium with a goal to foster in countries belonging to the European Union, CHP (Combined Heat and Power) from the combustion of bio-gas. The 30 months project (January 2005 – June 2007) was co- financed by the ”Intelligent Energy-Europe” Programme of the European Community.

Over the past 30 years, the Danish government had supported research and development of cost efficient biogas production systems. Consequently, Denmark has become a European leader in developing biogas technologies, especially when it comes to co-digestion of different biomass materials. Intelligent Energy-Europe brought together a consortium of European institutes and companies. The purpose of ProBioGas then is to promote the concept of centralized co-digestion, because the results from Denmark proved that biogas from centralized co-digestion provided quantifiable environmental and economic benefits for agriculture, industry, energy and the overall society.

A principle of co-digestion is to reduce transport costs and emissions, i.e., make the process more efficient by using feedstock produced locally. (An additional benefit is that local jobs are created and secured.) Co-digestion means a consistent, adequate supply of feedstock from a variety of suitable organic wastes. By suitable, European advocates not only mean is the feedstock readily putrescible, but also the material is unsuitable for incineration to produce heat and power, nor is it better suited for recycling or composting.

A central collection point for waste from agriculture, food processing industries, households, etc. also benefits system efficiency. Centralized, co-digestion plants are located in the areas with a high concentration of manure, in order to minimize the transportation costs. It is unknown whether such efforts are eating their own dogfood, i.e., the city of Fresno, California asked Odyne Corporation to convert a new Autocar Refuse Collection Truck to a CNG-electric plug-in series hybrid. Perhaps, either BRC Gas Systems or Lovato has converted some garbage trucks to ProBioGas, eh?

Such infrastructure also can promote better environmental health. The ProBioGas site informs:

The concept of centralized co-digestion is based on producing biogas by anaerobic co-digestion of animal manure and slurries… Animal slurries are collected from the pre-storage tanks of several farms in special vacuum container trucks and transported to the biogas plant, where they are mixed with organic wastes, homogenized and pumped in the digester tank. The digestion process takes place at mesophilic (30-40 ºC) or thermophilic temperatures (50-55 ºC). A controlled sanitation process takes places as well, where pathogens are effectively reduced, and the contamination cycles are broken.

Research Centre Foulum is a new experimental biogas complex operated by the Faculty of Agricultural Sciences at Aarhus University.

While the principle product from co-digestion facilities is methane, for combined heat and power and / or upgraded for use as a transportation fuel, another important product is material that is safe, economical and beneficial for farmers to use as fertilizer. The ProBioGas site informs:

The digested slurry -now called digestate- is pumped from the digesters to the gas proof storage tanks, where the remaining gas production (up to 15 % of total) is recovered and joins the gas produced in the digesters. The produced biogas is used for combined heat and power generation. The digestate is transported at the storage tanks of the farmers, placed out in the fields and applied on the crops as an integrated part of the fertilization plan of each farm, replacing mineral fertilizers. The excess of digestate is sold to the crop farms in the neighborhood. This way, a redistribution of the excess of nutrients takes places in the area. Some plants are equipped with installations for liquid and fiber separation of digestate.

The authors for information at the ProBioGas site also contend that centralized co-digestion of animal manure and slurries to producing biogas is a very competitive tool in reduction of GHG emissions. When bio-gas is upgraded to Bio-CNG (Compressed Natural Gas), it has the lowest grams of carbon dioxide equivalent per kilometer traveled of any alternative fuel used for transportation.

Advocates claim that the optimized diversion of manure to bio-gas production by means of anaerobic digestion results in a negative value for production and is the only alternative fuel to do so.

CNG already is a valued transportation fuel, even though containing less energy value than other fossil fuels, since its combustion produces far fewer emissions than the combustion of diesel oil or petrol. Since natural gas has been rising in price and there are concerns about future scarcity, it may be very forward thinking of Denmark and other EU countries to develop bio-gas as an alternative.

Such development certainly is occurring elsewhere. As previously noted, over the next five years the largest garbage hauler and landfill operator in North America will spend roughly US $400 million to add facilities to 60 landfills that obtain bio-methane from anaerobic fermentation. And, in California, fuel cells running on digester gas are categorized as renewable energy sources.

PR Newswire reports that Sinoenergy would seem to be experiencing a boost in CNG equipment sales.

Sinoenergy will supply 40 gas transportation trailers to Wuhan Green Energy Transport and 50 to Xuancheng Anjie Natural Gas Transport… With the sales volume of trailers in Sinogas fluctuating between 20 to 25 CNG transport trailers during each of the past five quarters, these recent orders represent a substantial increase in overall demand.