W. J. Oosterkamp

Oosterkamp Oosterbeek Octooien, The Netherlands
email: willemjan@oosterkamp. org

OUTLINE

Introduction 204

Biodegradability 204

Addition of Macro — and

Micronutrients 204

Addition of Microbes 205

Addition of Enzymes 206

Pretreatments 207

Biological Pretreatment with

Enzymes 207

Chemical Pretreatment 207

Hot Water Treatment 207

Mechanical Pretreatment 207

Longer Retention Times 207

Energy Crops 207

Food Processing Residues 207

Rice Husks 207

Bagasse 207

Coffee Husks and Mucilage 208

Crop Residues 209

Spent Bedding 209

Kitchen and Garden Waste 209

Aquatic Weeds 209

Digestion Systems 211

Family-Size Biogas Plant 211

Wet Digesters 211

Scum Layer Digester 211

Solid Biomass Digester 212

Increase in Solids Content in Wet Digesters 212

Loading and Unloading of Digesters 212

Treatment of Digestate in Wet Digesters 212

Use of Methane 213

Chemical Conversion of Volatile Solids 213

Combustion 213

Gasification 213

Thermal Conversion of Volatile Solids 214

Slow Pyrolysis 214

Flash Pyrolysis 214

Discussion 214

Maximum Methane Yield 214

Nutrient Recycling 214

Soil Fertility 214

Digesters 214

Conclusions 214

References 215

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00013-9

INTRODUCTION

All-renewable energy resources are required to reduce dependency on fossil fuels from politically un­stable regions. Biomass is one such renewable energy resource. Farm and food processing residues are preferred but, where economic, energy plants can be used.

Biomass as such cannot replace fossil fuels. Such materials have to be converted into gas, liquid or elec­tricity. Biological volatilizing (anaerobic digestion) con­verts organic by-products and residues into methane and carbon dioxide, an energy source that can be used for cooking, the production of electricity and as trans­portation fuel.

In Asia there are over 10 million family-size anaerobic digestion plants utilizing manure and some straw. The biogas is used for cooking. There are significant health advantages in using biogas, compared to the local alter­native of the burning of cattle manure, leaves and wood inside the houses.

There are a few thousand centralized biogas plants in Europe that use manure with a whole range of easily digestible residues. Other biogas plants in Europe use sludge from wastewater cleanup plants. They convert the biogas into electricity and heat. Carbon dioxide is removed from the biogas in a number of recent plants; the gas is compressed and injected into the natural gas grid.

The digestate, after the production of biogas, should be used as an organic fertilizer. This will recycle the macro elements nitrogen, potassium, phosphorus and carbon to the soil. Recycling of carbon is essential for high soil productivity and will reverse the trend of lowering of crop yields (Hossain, 2001).

The energy content of the animal residues (mostly manure) produced worldwide is equivalent to an average power of 50—150 W per person (9—25EJ/a). The energy content of crop residues (mostly straw) is also 50—150 W per person (Hoogwijk et al., 2003). Worldwide energy consumption is 2.5 kW per person (500 EJ/a). Oil production worldwide is 1 kW per per­son (80 million barrels a day). Biogas from straw and manure can replace about 10—30% of the world oil pro­duction. This substitution can be doubled by the use of forest residues.

BIODEGRADABILITY

only part of it can be depolymerized into soluble com­ponents. Anaerobic digestion is a complex process that is slow compared to chemical processes. Chynoweth et al. (1987) have published on the processes involved in the anaerobic digestion of biomass. Hydrolytic bacte­ria break down the cellulose and hemicellulose into organic acids and neutral compounds. Hydrogen pro­ducing bacteria convert the acids into hydrogen. Homoacetogenic bacteria convert hydrogen into acetic acid. Methanogenic bacteria convert acetic acid into methane. A by-product in these conversions is carbon dioxide.

Anaerobic biodegradation potential assay is per­formed by mixing the material with digestate from an operating digester or by mixing the material with a defined nutrient medium according to Owen et al. (1979). The methane produced is measured at different times.

Chandler et al. (1980) made a correlation based on 15 different lingocellulosic materials.

yCH4 = a * (b — c * Zj) (13.1)

where

yCH4 is the methane yield in l/kg volatile solids (VS) a = 440 l/kg is the conversion between methane yield and VS reduction (Jerger et al., 1982). b = 0.83 fitted constant. c = 2.8 fitted constant. li is lignin fraction of the VS

This correlation gives a standard deviation of 80 l/kg VS for straws and woody biomass (Table 13.1).

A different correlation was developed for straws and woody biomass.

y CH4 = a * (1 — Zi) * (1 — e—dt) (13.2)

d = f * (1 — g * li) is exponential factor. f = 0.025 fitted constant. g = 3 fitted constant.

This correlation assumes that biodegradation can be described as a first-order process. Shielding of cellulose and hemicellulose by lignin is reflected in the exponen­tial factor. This shielding eventually breaks down. This correlation performs better with a standard deviation of 32 l/kg VS.