Place beans in large saucepan. Add enough water to cover beans by 3 cm. Bring to boil over high heat. Boil 1 minute. Remove pan from heat; cover tightly and let stand 1 hour. (Or soak beans overnight in large bowl with enough cold water to cover by 3 cm.) Drain. Combine beans in 5 litre Dutch oven or soup kettle with chicken broth, celery, carrots, onions, and lemon zest. Bring to boil over high heat. Reduce heat to low and simmer, uncovered, until liquid is just below surface of beans and ingredients are very tender, 2.5 to 3 hours. Using slotted spoon, transfer soup solids in batches to food processor and puree. Transfer puree to large bowl and add cooking liquid. (Or puree solids and liquids together in blender. Or force soup through coarse sieve or pass it through food mill, discarding skins.) Return soup to Dutch oven. Stir in lemon juice, salt and pepper and cook over medium heat, stirring constantly, until heated through. Place lemon quarter in bottom of each bowl. Pour in soup and serve immediately. Makes about 3 litre of 10-12 servings.