Planning Guide for Biogas Plants

Introduction

Failure or unsatisfactory performance of biogas units occur mostly due to planning mistakes. The consequences of such mistakes may be immediately evident or may only become apparent after several years. Thorough and careful planning is, therefore, of utmost importance to eliminate mistakes before they reach irreversible stages.

As a biogas unit is an expensive investment, it should not be erected as a temporary set-up. Therefore, determining siting criteria for the stable and the biogas plant are the important initial steps of planning.

A general problem for the planning engineer is the interference of the customer during planning. As much as the wishes and expectations of customers have to be taken into consideration, the most important task of the planner is to lay the foundation for a well functioning biogas unit. As in most cases the customer has no experience with biogas technology, the planner has to explain all the reasons for each planning step. Planners should have the courage to withdraw from the planning process, if the wishes of the customer will lead to a white elephant on the farm.

Moreover, all extension-service advice concerning agricultural biogas plants must begin with an estimation of the quantitative and qualitative energy requirements of the interested party. Then, the biogas-generating potential must be calculated on the basis of the given biomass production and compared to the energy demand. Both the energy demand and the gas-generating potential, however, are variables that cannot be accurately determined in the planning phase. Sizing the plant(digester, gasholder, etc.) is the next step in the planning process.

In the case of a family-size biogas plant intended primarily as a source of energy, implementation should only be recommended, if the plant can be expected to cover the calculated energy demand.

Information about the economic evaluation of a biogas plant can be found in the section on Costs and Benefits.

Overview

Before building a biogas plant, there are different circumstances which should be considered. For instance, the natural and agricultural conditions in the specific countries are as important as the social or the economic aspects. To consider the most important factors, we provide a checklist for the planning procedure, a planning guide and a checklist for construction of a biogas plant.

Considering a Biogas Plant

Throughout the world, a countless number of designs of biogas plants have been developed under specific climatic and socio-economic conditions. Chosing a design is essentially part of the planning process. It is, however, important to familiarize with basic design considerations before the actual planning process begins. This refers to the planning of a single biogas unit as well as to the planning of biogas-programs with a regional scope.

Physical Conditions

The performance of a biogas plant is dependent on the local conditions in terms of climate, soil conditions, the substrate for digestion and building material availability. The design must respond to these conditions. In areas with generally low temperatures, insulation and heating devices may be important. If bedrock occurs frequently, the design must avoid deep excavation work. The amount and type of substrate to be digested have a bearing on size and design of the digester and the inlet and outlet construction. The choice of design will also be based on the building materials which are available reliably and at reasonable cost.

Skills and Labor

High sophistication levels of biogas technology require high levels of skills, from the planner as well as from the constructor and user. With a high training input, skill gaps can be bridged, but the number of skilled technicians will get smaller the more intensive the training has to be. In addition, training costs compete with actual construction costs for scarce (project) resources. Higher technical sophistication also requires more expensive supervision and, possibly, higher maintenance costs. To which extent prefabricated designs are suitable depends largely on the cost of labor and transport.

Standardization

For larger biogas programs, especially when aiming at a self-supporting dissemination process, standards in dimensions, quality and pricing are essential. Standard procedures, standard drawings and forms and standardized contracts between the constructor, the planner, the provider of material and the customer avoid mistakes and misunderstandings and save time. There is, however a trade-off between the benefits of standardization and the necessity of individual, appropriate solutions.

Selection of Appropriate Design

The design selection is determined largely be the prevailing design in the region, which, in turn takes the climatic, economic and substrate specific conditions into consideration. Large plants are designed on a case-to-case basis.

Typical design criteria are:

Space: determines mainly the decision if the fermenter is above-ground or underground, if it is to be constructed as an upright cylinder or as a horizontal plant.

Existing structures may be used like a liquid manure tank, an empty hall or a steel container. To reduce costs, the planner may need to adjust the design to theses existing structures.

Minimizing costs can be an important design parameter, especially when the monetary benefits are expected to be low. In this case a flexible cover of the digester is usually the cheapest solution. Minimizing costs is often opposed to maximizing gas yield.

Available substrate determines not only the size and shape of mixing pit but the digester volume (retention time!), the heating and agitation devices. Agitation through gas injection is only feasible with homogenous substrate and a dry matter content below 5%. Mechanical agitation becomes problematic above 10% dry matter.

The Planning Guide

This guide to planning is intended to serve agricultural extension officers as a comprehensive tool for arriving at decisions concerning the suitability of locations for family-sized biogas plants. The detailed planning outline has a data column for entering the gathered information and a rating column for noting the results of evaluation.

Evaluation criteria are:

• • • • • • • • • • • • • • • • + Siting condition are favorable

  o Siting condition are unfavorable, but

  a) compensable by project activities

  b) not serious enough to cause ultimate failure

  - Siting condition are not satisfactory

Despite its detailed nature, this planning guide is only a framework within which the extension officer should proceed to conduct a careful investigation and give due consideration, however subjectively, to the individual conditions in order to arrive at a locally practical solution. By no means is this planning guide intended to relieve the agricultural extension officer of the responsibility to thoroughly familiarize himself with the on-the-spot situation and to judge the overall value of a given location on the basis of the knowledge thus gained.

Initial Situation

	Data	Rating
Addresses/project characterization Plant acronym: Address of operator/customer: Place/region/country: Indigenous proj. org./executing org.: Extension officer/advisor: General user data Household structure and number of persons: User's economic situation: Crops: types, areas, manner of cultivation: Non-agricultural activity: Household/farm income: Cultural and social characteristics of user: Problems leading to the "biogas approach" Energy-supply bottlenecks: Workload for prior source of energy: Poor soil structure/yields: Erosion/deforestation: Poor hygiene and other factors: Objectives of the measure "biogas plant" User interests: Project interests: Other interests:

Natural / Agricultural Conditions

	Data	Rating
Natural conditions Mean annual temperature: Seasonal fluctuations: Diurnal variation: Rating:		- o +
Subsoil Type of soil: Groundwater table, potable water catchment area: Rating:		- o +
Water conditions Climatic zone: Annual precipitation: Dry season (months): Distance to source of water: Rating:		- o +
Livestock inventory (useful for biogas production) Animals: kind and quantity: Type of stable: Use of dung: Persons responsible for animals: Rating:		- o +
Vegetable waste (useful for biogas production) Types and quantities: Prior use: Rating:		- o +
Fertilization Customary types and quantities of fertilizer/areas fertilized: Organic fertilizer familiar/in use: Rating:		- o +
Potential sites for biogas plant Combined stable/biogas plant possible: Distance between biogas plant and livestock stable: Distance between biogas plant and place of gas consumption: Rating:		- o +
Overall rating 1		- o +

Balancing the Energy Demand with the Biogas Production

	Data	Rating
Prior energy supply Uses, source of energy, consumption: Anticipated biogas demand (kwh/day or l/d) for cooking: for lighting: for cooling: for engines: Total gas demand a) percentage that must be provided by the biogas plant: b) desired demand coverage: Available biomass (kg/d) and potential gas production (l/d) from animal husbandry pigs: poultry: cattle: Night soil Vegetable waste (quantities and potential gas yield) 1. 2. Totals biomass and potential gas production a) easy to procure: b) less easy to procure: Balancing Gas production clearly greater than gas demand -> positive rating (+) Gas demand larger than gas production -> negative rating (-); but review of results in order regarding: a) possible reduction of gas demand by the following measures -> b) possible increase in biogas production by the following measures -> If the measures take hold: -> qualified positive rating for the plant location (o) If the measures do not take hold: -> site rating remains negative (-)
Overall rating 2		- o +

Plant Design and Construction

	Data	Rating
Selection of plant design Locally customary type of plant: Arguments in favor of floating-drum plant: Arguments in favor of fixe-dome plant: Arguments in favor of other plant(s): Type of plant chosen: Selection of site Availability of building materials Bricks/blocks/stone: Cement: Metal: Sand: Piping/fittings: Miscellaneous: Availability of gas appliances Cookers: Lamps: ... ...
Overall rating 3		- o +

Plant Operation / Maintenance / Repair

	Data	Rating
Assessment of plant operation Incidental work: Work expenditure in h: Persons responsible: Rating with regard to anticipated implementation:		- o +
Plant maintenance Maintenance-intensive components: Maintenance work by user: Maintenance work by external assistance: Rating with regard too anticipated implementation:		- o +
Plant repair Components liable to need repair: Repairs that can be made by the user: Repairs requiring external assistance: Requisite materials and spare parts: Rating with regard to expected repair services:		- o +
Overall rating 4		- o +

Economic Analysis

	Data	Rating
Time-expenditure accounting Time saved with biogas plant Time lost due to biogas plant Rating:		- o +
Microeconomic analysis Initial investment: Cost of operation/maintenance/repair: Return on investment: energy, fertilizer, otherwise: Payback time (static): Productiveness (static): Rating:		- o +
Quality factors, useful socioeconomic effects and costs Useful effects: hygiene, autonomous energy, better lighting, better working conditions, prestige: Drawbacks: need to handle night soil, negative social impact: Rating:		- o +
Overall rating 5		- o +

Social Acceptance and Potential for Dissemination

	Data	Rating
Anticipated acceptance Participation in planning and construction Integration into agricultural setting: Integration into household: Sociocultural acceptance: Rating:		- o +
Establishing a dissemination strategy Conditions for and chances of the professional-craftsman approach: Conditions for and chances of the self-help oriented approach:		- o + - o +
General conditions for dissemination Project-executing organization and its staffing: orgnaizational structure: interest and prior experience in biogas technology: Regional infrastructure for transportation: communication: material procurement: Craftsman involvement, i.e. which acitivities: minimum qualifications: tools and machines: Training for engineers, craftsman and users: Proprietary capital, subsidy/credit requirement on the part of user: craftsmen: Rating:		- o +
Overall rating 6		- o +

Further Information

• "Biogas Portal"